Insects The Yearbook of Agriculture J952 For sale by the Superintendent of Documents, Washington 25, D. C. Price $2.50 Insects THE YEARBOOK OF AGRICULTURE $»- t3«*:ae=»r=»iSTi: United States Department of Agriculture Washington, D. C UNITED STATES GOVERNMENT PRINTING OFFICE Digitized by the Internet Archive in 2010 http://www.archive.org/details/insectsyearbookoOOunit The Yearbook Committee Bureau of Entomology and Plant f. c. bishopp, chairman Quarantine G. J. HAEUSSLER H. L. HALLER W. L. POPHAM B. A. PORTER E. R. SASSCER J. S. WADE Bureau of Animal Industry benjamin schwartz Bureau of Plant Industry, Soils, and karl s. quisenberry Agricultural Engineering Office of Experiment Stations e. r. mcgovran Office of Information Alfred stefferud, editor Foreword THIS PRACTICAL BOOK GIVES farmers and many other persons a great deal of information about the useful insects, as well as the harmful ones which are estimated to cost us four billion dollars a year. It is a timely book. In helping us combat our insect enemies it helps us produce more food, feed, fiber, and wood, all of which we need more than ever before. It is also a disturbing book — and that, to me, is one of its virtues. Although the science of entomology has made great progress in the past two decades, the problems caused by insects seem to be bigger than ever. We have more insect pests, although we have better insecticides to use against them and better ways to fight them. Effective though our quarantines are against for- eign pests, some of them are slipping through and require vigorous attention. Many aspects need to be considered in the control of insects. We must stop the destruction of our crops and forests, but the insecticides we use must leave no dangerous residues on foods, destroy no beneficial wildlife, and do no damage to our soils. We thought we had some of the problems solved when we got such good results from the new insecticides. DDT, for example, made medical history in 1 943 and 1 944 when an outbreak of typhus in Naples was controlled in a few weeks by its use. Entomologists hoped then that DDT could end all insect-borne diseases and even eradicate the house fly. In less than a decade, however, DDT was found to be a failure against the body louse in Korea, and the specter of typhus hung over that area. DDT and the insecticides substi- tuted for it failed to control mosquitoes in some places. In 1952 the house fly was no longer controlled in many places by any of the residual-type insecti- cides in use, and it seemed likely that other pests (those of agricultural, as well as medical, importance) in time would develop resistance. The answer, like the challenge, is clear. We dare not think of any knowledge — least of all knowledge of living things — as static, fixed, or finished. We need to push on to new horizons of thinking and investigation and, reaching them, see newer horizons. We need a longer view in research and an appreciation that it can have two goals: First, practical, everyday results that can be expressed in terms of definite methods, tools, and advice, and, second, fundamental, basic knowledge, on which the applied science rests. A book like this and the long research that made it possible exemplify the first goal. But if we are to progress further in this vital work, we need to keep the second goal always before us, remembering that science and knowledge are ever-growing and ever-changing. Charles F. Brannan, Secretary of Agriculture. Preface INTO THIS YEARBOOK HAVE GONE the results of nearly 100 years of the study of insects. The Bureau of Ento- mology and Plant Quarantine, which was responsible in large measure for the book, traces its origins that far back. The century has seen great changes in farming methods, the intensiveness and extent of agriculture, transporta- tion, and crops. All have affected profoundly our relationships with insects. We hope this Yearbook will be a contribution to the general understanding of those relationships and to the efficiency and well-being of American farming and living. Insects takes its place in the new Yearbook series that began in 1936 and has dealt successively with plant and animal genetics, soils, nutrition, eco- nomics, climate, livestock diseases, developments in agricultural sciences, grass, trees, and the processing of farm products. Some of those volumes can be bought from the Superintendent of Documents, Government Printing Office, Washington 25, D. C. He will quote the prices on request. No person in the Department of Agriculture has copies for general distribution. Some of them are out of print — that is, they cannot be bought from the Government Printing Office. (They are available in nearly every public library in the country, however, and used copies are not very hard to come by. ) Sometimes we are asked why we do not reprint the old books. We give several reasons. Although the information in them remains basically correct, recent scientific developments would make certain revisions necessary. Even small changes and additions very likely would mean new plates for many pages, and the cost of the second edition might be the same as that of the first. Also, many subjects of great importance to farmers and other citizens are waiting to be treated in Yearbooks. Among them, for example, are plant diseases, marketing of farm goods, the farm home, the small farm, and water. We look upon the published and the projected Yearbooks all together as an inclusive, authoritative agricultural library. We select the Yearbook subjects (two or three years in advance) on the basis of need and interest, as indicated in communications and comments from farmers and others, as well as the availability of research findings and writers. We try to avoid duplicating material to which farmers have easy access elsewhere. A number of persons contributed greatly to this book. C. F. W. Muesebeck and A. M. Vance, of the Bureau of Entomology and Plant Quarantine, gave valuable advice and help on a number of technical matters. Arthur D. Cushman, also of that Bureau, made most of the color illustra- tions, many of the line drawings, and the end papers. Edwin Way Teale took seven of the eight photographs. The eighth was supplied by Frank M. Carpenter. Alfred Stefferud, Editor of the Yearbook. Contents The Yearbook Committee, v Charles F. Brannan Foreword, vii Alfred Stefferud Preface, rx Introducing the Insects Curtis W. Sabrosky How Many Insects Are There? i Edwin Way Teale Oddities of the Insect World, 8 Fossil Insects, 14 How Insects Live, 20 Frank M. Carpenter E. O. Essig Frank H. Babers John J. Pratt, Jr. Charles T. Brues Life Processes of Insects, 30 How Insects Choose Their Food Plants, 37 How To Know an Insect C. F. W. Muesebeck What Kind of Insect Is It? 43 C. F. W. Muesebeck Progress in Insect Classification, 56 Clarence E. Mickel Values of Insect Collections, 60 Paul W. Oman How To Collect and Preserve Insects for Study, 65 Insects as Helpers F. C. Bishopp Insect Friends of Man, 79 Honey Bees as Agents of Pollination, 88 Pollination by Native Insects, 107 Breeding Bees, 122 George H. Vansell W. H. Griggs George E. Bohart Otto Mackensen William C. Roberts Frank E. Todd S. E. McGregor James K. Holloway C. B. Huffaker Insecticides and Bees, 131 Insects To Control a Weed, 135 Page xi Insects as Destroyers G. J. Haeussler F. C. Bishop p Cornelius B. Philip Gerard Dikmans A. O. Foster C. D. Stein L. T. Giltner Everett E. Wehr John T. Lucher L. D. Christenson Floyd F. Smith J. G. Leach Clay Lyle R. C. Roark H. L. Haller John J. Pratt, Jr. Frank H. Babers C. V. Bowen S. A. Hall R. H. Carter Louis Feinstein P. J. Chapman L. A. Riehl G. W. Pearce W. N. Sullivan R. A. Fulton Alfred H. Yeomans E. J. Newcomer W. E. Westlake B. ]. Landis Kenneth Messenger W. L. Popham Page xii Losses Caused by Insects, 141 Carriers of Human Diseases, 147 Carriers of Animal Diseases, 161 Insects and Helminths, 169 Insects and the Plant Viruses, 1 79 Insects. Bacteria, and Fungi, 191 The Nature of Insecticides Can Insects Be Eradicated? 197 How Insecticides Are Developed, 200 How Insecticides Are Mixed, 202 How Insecticides Poison Insects, 205 The Organic Insecticides, 209 The Inorganic Insecticides, 218 Insecticides From Plants, 222 Oil Sprays for Fruit Trees, 229 Aerosols and Insects, 240 Applying Insecticides Using Insecticides Effectively, 245 From o to 5,000 in 34 Years, 250 /. S. Yuill D. A. Isler George D. Childress Howard Ingerson Frank Irons T. E. Bronson Earl D. Anderson F. C. Bishopp John L. Horsfall R. D. Radeleff R. C. Bushland H. V. Claborn Victor R. Boswell B. A. Porter J. E. Fahey Allen B. Lemmon W. G. Reed P. B. Dunbar B. A. Porter W. N. Bruce W. V. King Research on Aerial Spraying, 252 Machines for Applying Insecticides, 258 Choosing and Using Hand Equipment, 262 Warnings as to Insecticides The Safe Use of Insecticides, 271 Toxicity to Livestock, 276 Residues, Soils, and Plants, 284 Residues on Fruits and Vegetables, 297 State Pesticide Laws, 302 The Federal Act of 1947,310 Insecticides and the Pure Food Law, 314 Resistance to Insecticides Insects Are Harder To Kill, 317 Insecticides and Flies, 320 Mosquitoes and DDT, 327 Fumigants Robert D. Chishclm Nature and Uses of Fumigants, 33 1 Fumigating Soils and Plants, 340 R. T. Cotton Fumigating Stored Foodstuffs, 345 Randall Latta M. C. Lane Quarantines Ralph B. Swain George G. Becker Herbert J. Conkle E. A. Burns A. P. Messenger How Insects Gain Entry, 350 An Agricultural "Ellis Island", 355 Our Domestic Quarantines, 360 Inspection in Transit, 365 Inspection at Terminals, 371 Page xiii Other Controls Barnard D. Burks C. P. Clausen Edward A. Steinhaus Ira M. Hawley A. C. Baker Henry H. Richardson Howard Baker T. E. Hienton Alfred H. Yeomans Harlow B. Mills C. M. Packard John H. Martin W. A. Baker O. R. Mathews J. J. Davis G. J. Haeussler R. W. Leiby Lea S. Hitchner Ed. M. Searls M. P. Jones Helen S oilers L. S. Henderson Harry H. Stage E. F. Knipling C. F. Rainwater R. C. Gaines L. F. Curl R. W. White K. P. Ewing Page xiv Insects, Enemies of Insects, 373 Parasites and Predators, 380 Infectious Diseases of Insects, 388 Milky Diseases of Beetles, 394 The Vapor-Heat Process, 401 Gold Treatment of Fruits, 404 Traps Have Some Value, 406 Radiant Energy and Insects, 41 1 Weather and Climate, 422 Resistant Crops, the Ideal Way, 429 Good Farming Helps Control Insects, 437 Economic Entomology Milestones in Entomology, 441 Surveys of Insect Pests, 444 The Insecticide Industry, 450 The Industrial Entomologist, 455 Extension Work in Entomology, 457 Entomologists in Washington, 462 Insects, Man, and Homes Household Insects, 469 Mosquitoes, 476 The Control of Insects Affecting Man, 486 Insects on Cotton Progress in Research on Cotton Insects, 497 The Boll Weevil, 501 The Pink Bollworm, 505 The Bollworm, 5 1 1 R. L. Wallis W. A. Shands B. J. Landis R. A. Roberts T. A. Brindley Joseph C. Chamberlin John E. Dudley, Jr. William C. Cook J. R. Douglass William C. Cook Walter Carter P. A. Hoidale Howard Baker Charles H. Hadley Walter E. Fleming William Middleton Timothy C. Cronin C. M. Packard J. R. Parker Claude Wakeland J. R. Parker R. A. Roberts Claude Wakeland Wm. G. Bradley D. J. CafJrey R. T. Cotton Wallace Ashby Insects and Vegetables The Potato Psyllid, 515 Potato Aphids, 519 Sweetpotato Weevil, 527 The Pea Weevil, 530 The Pea Aphid, 538 The Beet Leafhopper, 544 Insects on Fruit The Oriental Fruit Fly, 551 The Mexican Fruit Fly, 559 Spider Mites, Insects, and DDT, 562 The Japanese Beetle, 567 Off Limits for Beetles, 574 Insects on Field Crops Cereal and Forage Insects, 581 Grasshoppers, 595 The Mormon Cricket, 605 White-Fringed Beetle, 608 The Chinch Bug, 6 1 1 The European Corn Borer, 614 Insects That Attack Tobacco, 621 Insect Pests of Stored Grains and Seed, 629 Pests on Ornamentals C. A. Weigel R. A. St. George Insect Pests of Flowers and Shrubs, 640 Floyd F. Smith Spider Mites and Resistance, 652 Page xv Gaines W. Eddy E. F. Knipling W. G. Bruce Ernest W. Laake Irwin H. Roberts Curtis May White ford L. Baker R. C. Brown F. P. Keen John M. Corliss Livestock and Insects Flies on Livestock, 657 Ticks, Lice, Sheep Keds, Mites, 662 Screw-worms, 666 Cattle Grubs, 672 Forests, Trees, and Pests Insects and Spread of Forest-Tree Diseases, 677 The Spruce Budworm, 683 Bark Beetles in Forests, 688 The Gypsy Moth, 694 Insects and Wildlife Oliver B. Cope Insects and the Lower Vertebrates, 699 Some Insect Pests of Wildlife, 708 E. R. Kalmbach Birds, Beasts, and Bugs, 724 /. P. Linduska Arthur W. Lindquist Ina L. Hawes ]. S. Wade David G. Hall R. H. Nelson Ralph W. Sherman Bibliography and Appendix A Selected List of Publications, 732 How To Get Further Information on Insects, 737 Conversion Tables and Equivalents, 743 Summary of Federal Plant Regulator)- Legislation, 747 Insecticides, 748 Index, 751 Some Important Insects Color Plates, following the index. Page xvi Some Important Insects COLOR PLATE i Boll Weevil ii Bollworm iii Cotton Aphid iv Cotton Fleahopper V Cotton Leafworm vi Pink Bollworm vii Spider Mites viii Japanese Beetle ix Effect of Milky Disease on Japanese Beetle Grubs X Oriental Fruit Moth xi Apple Maggot xii Codling Moth xiii Plum Curculio xiv Two-Spotted Spider Mite, European Red Mite XV San Jose Scale xvi Gypsy Moth xvii Elm Leaf Beetle xviii Ips Bark Beetles xix Old House Borer XX Subterranean Termites xxi Carpenter Ants xxii Red-Headed Pine Sawfly xxiii Roaches xxiv Cattle Grub XXV Horn Fly xxvi Irrigation-Water Mosquitoes xxvii Screw-worm xxviii Fabric Pests xxix House Ants XXX Pantry Pests xxxi Hornworms on Tomato xxxii Mexican Bean Beetle xxxiii Colorado Potato Beetle Page xvii 970134' COLOR PLATE XXXIV XXXV xxxvi xxxvii xxxviii xxxix xl xli xlii xliii xliv xlv xlvi xlvii xlviii xlix 1 li lii liii liv lv lvi lvii lviii lix Ix lxi lxii lxiii lxiv Ixv lxvi lxvii lxviii lxix lxx lxxi lxxii Page xviii Harlequin Bug Gladiolus Thrips Striped Cucumber Beetle Potato Leafhopper Imported Cabbageworm Squash Vine Borer Tomato Fruitworm Sweetpotato Weevil Seed-Corn Maggot Pea Weevil Beet Leafhopper Pacific Coast Wireworm Tuber Flea Beetle Onion Thrips Clay-Backed Cutworm Pea Aphid Citrus Mealybug Pickleworm Alkali Bees Tobacco Hornworm and a Natural Enemy Praying Mantid Squash Bug, Tricopoda Pennipes (a Parasite) Stink Bugs Corn Earworm Fall Armyworm European Corn Borer White-Fringed Beetle Alfalfa Weevil Lygus Bugs Chinch Bug White Grubs Wheat Stem Sawfly Hessian Fly Mormon Cricket Wheat Jointworm Velvetbean Caterpillar Potato Leafhopper on Alfalfa Lesser Migratory Grasshopper Differential Grasshopper Insects Introducing the Insects How Many Insects Are There? Curtis W. Sabrosky When people ask, "How many in- sects are there?" they usually want answers to two different questions: How many kinds of insects are there? What is the total number of individual insects in the world ? An honest answer to both is: Nobody knows exactly. The number of kinds, or species, is so great that entomologists cannot keep an accurate count, except for small groups. The number of kinds that have already been described and named is estimated by various scien- tists at 625,000 to 1,500,000. No one can even guess when the big tally will be finished. For such huge groups as beetles and flies, an exact count may never be possible, although generally the numbers of the smaller groups can be tallied more accurately. Workers in the division of insect identification of the Department of Agriculture estimate that by the end of 1948 approximately 686,000 differ- ent species of insects had been de- scribed and named for the entire world. In addition were some 9,000 species of ticks and mites, which are not true insects but look like insects to the lay person. About two-fifths of the known kinds of insects are beetles. Moths and but- terflies, ants, bees, wasps, and true flies comprise another two-fifths. For North America, north of Mex- ico, the latest figures show nearly 82,- 500 kinds of insects, plus 2,613 kinds of ticks and mites. Just as for the world, beetles far outnumber other kinds of insect life, with ants, bees and wasps, and the true flies having a good share. The moths ana butter- flies, which run second to beetles in the world as a whole, are in fourth place in our area, with 10,300 species. The true bugs are not far behind, with 8,700 species. The remaining 5,400 species belong to the other 19 orders. Not all 82,500 kinds live in the same locality or even in the same region. The mountains and the plains, the great swamps of the Everglades and peaks of the Sierras, the deserts of the Southwest and the northern forests — each has its own particular insects. Some kinds live only on the very top of a mountain or two. Others are found in many States. How many species can we expect to find in any one State? For most States we have no totals. A few tabulations, made in various years, are available: Total insects Flies Connecticut 8, 869 1, 565 Michigan 3, 233 New England 3, 304 New Jersey 10, 385 1 , 66 1 New York 15, 449 3,615 North Carolina n, 094 2, 1 1 1 From them we can deduce that States of average topography, climate, and vegetation might have 10,000 to 15,000 kinds ; there might be fewer spe- cies in the smaller States and more in the larger ones that have wide ranges of growing seasons, types of plants, ele- vation, and so on. I How many insects are injurious to man? Entomologists estimated some years ago that approximately 6,500 spe- cies of insects in the United States were important enough to be called public enemies. Today the number is prob- ably closer to 10,000. How do the numbers of insects com- pare with those of other animals? In current books on zoology, estimates of the total number of described species of animals range from 823,000 to 1,- 1 15,000. If the number of kinds of in- sects is between 625,000 and 900,000, probably 70 to 80 percent of all the known kinds of animals are insects. That proportion has held quite steady in the estimates of many zoologists for the past century or more. The starting point of our modern system of naming animals is 1758. In that year the names, pedigrees, and descriptions of all the animals then known were printed in one book of only 824 pages, the Systema Naturae by the great Swedish naturalist, Caro- lus Linnaeus. He listed 4,379 kinds of animals, of which 1,937 were insects. From that beginning, knowledge has expanded greatly as scientists explored the lesser known parts of the earth from pole to pole and the crannies of the better known places, their own back yards. Within 100 years, nearly 100,000 kinds of insects had been identified. By 1900 the total was about 300,000. It has more than doubled since then. Each year now about 6,000 or 7,000 kinds of insects are described and named for the first time. Today a mere list of the scientific names of the known insects (based on a conservative estimate, of the total number) , without one word of de- scription or anything else, in a book with two columns to a page and print fine enough for 100 lines in each column, would fill a volume of 3,300 pages. To say it in another way: If the names were printed one to a line in an 8-page, 8-column newspaper of average size, without headlines and pictures, more than 8 weeks, including Sundays, would be needed to print Yearbook of Agriculture 1952 only the names of the insects that are already known in the world. What is the real total? So far, we have been considering the number of different species of insects that have been described and named. But how many kinds would there be if all were known and named? No one can say for sure, but the question has pro- voked a good deal of speculation. Re- cent guesses vary from 2,500,000 to 10,000,000 different kinds. Maybe there are not quite so many as some people think, however. For example, a listing in 1949 of the ter- mites of the world recognized 1,717 distinct species, even though some pre- vious estimates ranged as high as 2,600 species. For North America, north of Mexico, there are 41 distinct species, compared to 59 in earlier lists, because further study showed that some pro- posed names applied only to subspecies or color varieties or were simply syn- onyms, that is, duplication of names for the same species. That experience in a small and intensively studied group may be repeated to an even greater extent in some of the larger groups. Even so, many really new and hitherto unknown kinds of insects are being found and described every day somewhere in the world, and their number should far exceed any decrease caused by duplication of names. The final roll call may be far short of 10 million but it seems sure to be some- where in the millions. The number of individual insects, the second part of our question; is a tremendous problem in itself. No one dares to guess the answer for the world, or a country, or a State. Even for smaller areas, such as acres or square miles, any figures are only approxima- tions based on square-foot samples or similar measures. In any given area, the population of insects will not only depend on such things as the soil and the plants, but it will vary from season to season and even from one minute to the next. Still, samples will give us How Many Insects Are There? some ideas of the normal population. Sometimes insects break out of their usual population by swarming or mi- grating or by sudden bursts of thou- sands or millions of individuals that cover sidewalks or lay waste mile after mile of grain fields or strip leaves from thousands of trees. Then we can make special counts or estimates of the size of the crowd. Many other figures are also available for such concentrations of insects as occur in beehives, ant nests, and termite colonies. Let us look at a few of the many facts that are known about the numbers of insects. Great reproductive capacity is com- mon among insects. One example : In one summer season from April to Au- gust, the descendants of one pair of house flies, if all lived and reproduced normally, would make a total of 191,- 000,000,000,000,000,000. But fortu- nately reproduction usually does not go on at full speed. Other insects, birds, diseases, insecticides, and weather take a toll of the eggs that are laid and the young that are born. Many calculations have been made for aphids, or plant-lice, because they have many generations in a season. Glenn W. Herrick found that the cab- bage aphid, which had an average of 41 young per female, had 16 genera- tions between March 31 and October 2 in New York State. If all lived, the descendants of one female aphid would amount to 1,560,000,000,000,000,000,- 000,000 by the end of the season. Such related kinds as the melon aphid or the cotton aphid will have twice as many offspring per female and more generations per year in the South. Not all kinds of insects are so pro- lific. Some have very few young, some have only one family each year, and some take years to grow from egg to adult. But even such insects, if common enough, may be very numerous. Con- sider a slow breeder like the famous periodical cicada, or 17-year locust; the swarms that result when one of its broods emerges from the ground after a 17-year childhood will always be re- membered by those who have seen them. As many as 40,000 cicadas may emerge from the ground under a large- sized tree. Sometimes the emergence holes are so close together that 84 of them can be counted in a square foot of soil surface. Some insects lay eggs continuously over long periods. Especially is that true among the social insects, those that are organized into societies such as nests, hives, or colonies. Ant queens have been known to lay as many as 340 eggs a day. Honey bee queens can lay 1,500 to 2,000 eggs a day. Termites, the so-called white ants, hold the record : The queen is a specialized machine for turning out eggs day after day. Alfred E. Emerson, an authority on termites, has stated that a capacity of 6,000 to 7,000 eggs a day is not unusual for specialized termite queens, which may live from 15 to 50 years. Many years ago, in four different queens of an East African termite, Macrotermes bellicosus, Karl Escher- ich observed an egg-laying rate of one egg every 2 seconds, or 43,000 a day. We do not know, of course, how long eggs are laid at such record rates. Under natural conditions the daily number may vary a good deal. But in large colonies and under good con- ditions, egg production is certainly a highly developed big business. A remarkable method of reproduc- tion in some insects is polyembryony, a process whereby two or more young result from a single egg. In its simplest form, one egg divides into two, just as identical twins originate in the higher animals. But some insects do not stop there. The parts of the original egg may keep on dividing. In some species as many as 1,500 to 2,500 insects finally result from a single egg. L. O. Howard, in his book The Insect Menace, said he found that nearly 3,000 small para- sitic wasps emerged from a single caterpillar in which probably no more than a dozen eggs had been laid. Polyembryony occurs in parasitic in- sects, a fact of obvious importance to man when he uses them to fight his insect battles for him. Swarms or outbreaks of insects — the spurts or surges of numbers that at- tract attention — are the natural result of such potential powers of reproduc- tion. They may be a normal part of the life of the insect, such as mating flights or swarms of honey bees. Or they may occur when something hap- pens to tip the balance of nature and give a head start to some insect with great powers of reproduction. Probably everyone has seen such a swarm or outbreak — a great flight of mayflies, whose dead bodies wash up on the shores of lakes in large wind- rows, the swarming of honey bees, the flights of ants and termites, the migra- tions of locusts and butterflies, army- worms, periodical cicadas, or chinch bugs on the march into corn fields. A fantastic number of individual insects might be in such a mass outbreak, and their damage could be almost beyond belief — whether the earth is scorched by swarms of locusts and grasshoppers or the destruction caused by less con- spicuous insects. In Canada in 1919 and 1920, for example, an outbreak of the spruce budworm destroyed a vol- ume of wood said to be equal to a 40- year supply for all the pulp mills then operating in Canada. Tremendous swarms of locusts, such as described in the Bible as a plague on the Children of Israel in Egypt, are reported in many parts of the world. We have figures for outbreaks in Africa and the Near East. For the Moroccan locust, workers found as many as 6,000 egg pods per square yard, with an aver- age of 30 to 35 eggs in a pod. During a campaign against migratory locusts in western Turkey, collectors gathered 430 tons of eggs and 1,200 tons of lo- custs in 3 months. The most spectacular examples in the United States are the migrating swarms of Rocky Mountain grass- hoppers in the Great Plains in the 1870's. The locusts are said to have left fields as barren as if they had been burned over. Only holes in the ground showed where plants had been. Trees were stripped of their leaves and green Yearbook of Agriculture 1952 bark. One observer in Nebraska re- corded that one of the invading swarms of locusts averaged a half mile in height and was 100 miles wide and 300 miles long. In places the column, seen through field glasses and measured by surveying instruments, was nearly a mile high. With an estimate of 27 lo- custs per cubic yard, he figured nearly 28 million per cubic mile. He said the swarm was as thick as that for at least 6 hours and moved at least 5 miles an hour. He calculated that more than 124 billion locusts were on the move in that one migration. Not always is the occurrence of large numbers of insects harmful. In the mountains of California and elsewhere, lady beetles (or ladybirds) overwinter in masses in sheltered places. Two men working together can sometimes col- lect from 50 to 100 pounds of beetles in a day. Judging by the average weight of each beetle, one can figure that such collections contain 1,200,000 to 2,400,000 beetles. It is thus possible to gather large numbers of these in- sect-eating beetles and later release them in places where they will attack insects that are feeding on crops. Migrations of butterflies are espe- cially striking. Millions of butterflies may fly for days and as far as 2,000 miles, and the migrating swarm may be several hundred miles in width. Such flights apparently are more com- mon in other parts of the world, but some have been recorded in the United States. The monarch butterfly (or common milkweed butterfly) is a regu- lar commuter. Each fall, individuals of this species fly south, and some of them may make the return trip of 1,000 miles or so the following spring. In Texas in the summer of 1921, C. H. Gable and W. A. Baker recorded a migration of snout butterflies, Liby- theana bachmanii, which were so nu- merous that an average of about 1,- 250,000 of them per minute flew across a front 250 miles wide. At the main observation point the migration con- tinued at the same level of intensity for 18 days. How Many Insects Are There? The normal population of in- sects, not counting swarms or unusual increases, has been studied for some situations and some species. The best figures we have are for insects living in the soil, probably because it is easier to get practically complete samples of the population. Even so, the data are hard to compare because the studies are so different: Different kinds of soil or time of year, samples taken down to different depths, and treated in different ways that might or might not find such small things as mites and springtails. Because those two kinds far outnumber all other animals in most soils and forest litter, a small difference in technique could make a difference of millions per acre in the number of insects reported. Studies of grassland insects in Eng- land, in which the top 9 to 12 inches of soil was examined, disclosed totals for insects and mites that ranged up to several hundred million per acre. Even for specific kinds of insects, the estimates may be unbelievably high. For example, certain wireworms, such as the larvae of Agriotes beetles, have been found in numbers calculated to be from 3 million to 25 million per acre. In most of the reports, mites and springtails formed two-thirds or more of the total; in some, the number of springtails was nine-tenths of the total for insects. The population of arthropods (jointed-legged invertebrate animals — insects, mites, centipedes, and such) in the forest litter and humus also has been studied. From samples taken to a depth of 5 inches in oak and pine stands on stony clay and sandy soils in North Carolina, A. S. Pearse calcu- lated that there were approximately 1 24 million animals per acre. Of these, nearly 90 million were mites, 28 mil- lion springtails, and 4.5 million other insects. In a scrub oak area in Pennsyl- vania with apparently a richer forest litter, C. H. Hoffmann and his co- workers found an average of 9,759 arthropods per square foot of surface in 2 inches of litter and 1 inch of humus. That figures out to 425 million per acre. As in Pearse's study, the mites were the most abundant kind of ani- mal, averaging 294 million per acre. Springtails averaged 1 1 9 million, with only 1 1 million for all other arthro- pods. The number per acre is an esti- mate based on the average of square- foot samples. It may be smaller in some parts of an area and much larger in others. A census of colonies of social in- sects is easy compared to the difficulties of counting or estimating the general insect population. Many figures have been published for ants, termites, bees, and wasps, some being actual counts and some estimates based on samples. Ants differ greatly in the size of their colonies, from small nests with a dozen workers to large and populous nests with several hundred thousand. E. A. Andrews calculated that an ant colony in Jamaica had 630,000 individ- uals, nine-tenths of them workers. Large nests of Formica in Europe are generally agreed to contain an average of 150,000 to 200,000 ants. In a 10- acre study area in Maryland, E. N. Cory and Elizabeth Haviland found 73 mounds of various sizes of the Alle- gheny mound ant. In two mounds studied, they found 41,000 and 238,000 ants. From these figures, and the ap- proximate relation between size of mound and number of ants, they cal- culated an average of about 27 ants for every square foot of the 10 acres. Colonies of termites vary in size as much as ant nests do. Some have a few hundred individuals at most, but others may have several million. The colonies are relatively small in the United States, and a nest with a quarter of a million termites is a very large one. The records for size go to the tropical species, especially those that build large nests in the soil. Alfred E. Emer- son found 3 million termites in a colony of the South American Nasuti- termes surinamensis. F. G. Holdaway and his colleagues recorded from 750,- 000 to 1,806,000 termites in several Yearbook of Agriculture 1952 Known Species of Insects and Other Animals Group Common names Insecta Insects Other Arthropoda Spiders, centipedes, crawfish, etc . . Mollusca Clams and other shellfish, snails . . , Chordata Mammals, birds, fish, reptiles, etc. All other animals Sponges, corals, worms, etc Total . Estimate by Metcalj and Flint (*939> '95') Estimate by Ross (194V 640, 000 goo, 000 73, 500 80, 000 50, 000 80, 000 60, 000 38, 000 62, 500 47, 000 916, 000 1 , 115, 000 Number of Described Species of Insects, Ticks, and Mites at the End of 1948 Order Common names Anoplura Sucking lice (true lice) Coleoptera Beetles, weevils, twisted-winged in- sects. Collembola Springtails Corrodentia Booklice, barklice Dermaptera Earwigs Diptera Flies, mosquitoes, gnats Embioptera Embiids Ephemeroptera Mayflies Hemiptera True bugs and Homoptera (cicadas, leaf hoppers, aphids, scale insects). Hymenoptera Ants, bees, wasps Isoptera Termites ("white ants") Lepidoptera Butterflies and moths Mallophaga Biting lice (bird lice) Mecoptera Scorpionflies Neuroptera Lacewings, ant-lions, dobsonflies .... Odonata Dragonflies, damselflies Orthoptera Grasshoppers, crickets, roaches, man- tids, katydids. Plecoptera Stoneflies Protura Siphonaptera Fleas Thysanoptera Thrips Thysanura Bristletails, "silverfish" Trichoptera Caddisflies Zoraptera Total. Acarina. Ticks . Mites North America, north 0/ World Mexico 250 62 277, 000 26, 676 2, 000 3H 1, 100 120 1, 100 18 85, 000 16, 700 H9 8 1,500 550 55, 000 8,742 103, 000 i4,528 1,717 41 1 1 2, 000 10, 300 2,675 318 35° 66 4,670 338 4,870 412 22, 500 1,015 1,49° 340 90 29 1, 100 238 3, 170 606 700 50 4,45o 921 19 2 685, 900 82, 394 440 "3 8, 700 2,500 mounds of N. cxitiosus in Australia. Honey bees have long been the sub- ject of insect censuses. Jan Swammer- dam in 1737 counted the cells and bees of three Dutch straw hives. In 1740 Rene de Reaumur counted 43,008 bees in a large swarm. Strong colonies in modern beehives contain about 55,000 bees. As many as 30,000 may leave a hive in a swarm. How Many Insects Are There? A good colony with a vigorous queen should produce about 200,000 bees in a year. The normal egg production during the lifetime of a queen bee has been estimated to be as high as 1,500,- 000, but probably it does not usually exceed 500,000. Some wild species of bees may also have large colonies. In the South American stingless bees (Trigona), 50,000 to 100,000 individuals may be in a single nest. The largest known nest of a tropical bee, Trigona postica, had 27 combs with about 64,000 cells and 70,000 to 80,000 adult bees. The social wasps and hornets have rather small colonies. The largest nests range from a few hundred individuals to several thousand. How many insects are there? And how many kinds of insects? Maybe we shall never know. But wherever we go and whether we see them or not, we are surrounded by countless millions of insects. Every forest, every field, every back yard, every roadway is a gigantic insect zoo. A wide world of endless variety and interest is open to all who will do a little investigating on their own. Curtis W. Sabrosky has been a specialist in Diptera (flies) in the divi- sion of insect identification of the Bu- reau of Entomology and Plant Quar- antine since 1946. He was instructor and assistant professor of entomology for 8 years in Michigan State College and has served with the United States Public Health Service on studies of malaria mosquitoes. He is the author of numerous publications on the classi- fication of flies and on rules governing the scientific names of animals. For further reading on insect popula- tions, Mr. Sabrosky recommends Malcolm Burr's The Insect Legion, published by James Nisbet & Co., London, in 1939; C. B. Williams' The Migration of Butterflies, Oliver and Boyd, Edinburgh, 1930; and the following articles in periodicals: E. A. Andrews: Populations of Ant Mounds, Quarterly Review of Biology, volume 4, pages 248-257. 1929. F. S. Bodenheimer : Population Problems House fly. of Social Insects, Biological Reviews of the Cambridge Philosophical Society, volume 12, pages 393-430- 1937. Royal N. Chapman, Kenneth M. King, Alfred E. Emerson, Samuel A. Graham, and Harry S. Smith: Symposium on Insect Populations, Ecological Monographs, vol- ume 9, pages 259-320. July 1939- E. N. Cory and E. E. Haviland: Popu- lation Studies of Formica exsectoides Forel, Annals of the Entomological Society of America, volume 31, pages 50-57. March 1938- C. H. Gable and W . A. Baker: Notes on a Migration of Libythea bachmanni Kirtl., Canadian Entomologist, volume 54, pages 265-266. 1922. Glenn W. Herrick: The "Ponderable" Substance of Aphids, Entomological News, volume 37, pages 207-210. July 1926. C. H. Hoffmann, H. K. Townes, H. H. Swift, and R. I. Sailer: Field Studies on the Effects of Airplane Applications of DDT on Forest Invertebrates, Ecological Mono- graphs, volume 19, pages 1-46. January J949- F. G. Holdaway, F. J. Gay, and T. Greaves: The Termite Population of a Mound Colony of Eutermes exitiosus Hill, Journal of the Council for Scien- tific and Industrial Research, volume 8, pages 42-46. 1935. Z. P. Metcalf: How Many Insects Are There in the World? Entomological News, volume 51, pages 219-222. October 1940. A. S. Pearse: Observations on the Mi- crofauna of the Duke Forest, Ecological Monographs, volume 16, pages 127-150. April 1946. Oddities of the Insect World Edwin Way Teale Nineteen centuries ago, when Pliny the Elder was writing his natural his- tory in Rome, men believed that insects were creatures without blood, that but- terfly eggs were drops of solidified dew, that echoes killed honey bees, and that gold was mined in the mountains north of India by a giant ant "the color of a cat and as large as an Egyptian wolf." "This gold," Pliny assured his readers, "is extracted in the winter and is taken by the Indians during the heats of summer while the ants are compelled by the excessive warmth to hide them- selves in their holes. Still, however, on being aroused by catching the scent of the Indians, they sally forth and fre- quently tear them to pieces, though provided with the swiftest camels for the purpose of flight, so great is their fleetness, combined with their ferocity and their passion for gold." Today, nobody credits Pliny's story of wolf-size ants with a passion for gold any more than they believe in his orien- tal locusts that grew to such size that their hind legs were dried and used for saws. These traveler's tales, the prod- uct of imagination or misunderstand- ing, have been long discredited. Imagi- nary wonders, in fact, are less needed in dealing with the insects than with any other group of living creatures. The truth is odd and dramatic enough. In 1857, when Alfred Russel Wallace landed on the Kei Islands of the Malay Archipelago to collect natural-history specimens, he soon noticed that each time he entered a deep damp forest glade he found the air filled with a fra- grance that reminded him of attar of roses. For a long time he tried to trace the perfume to flowers. Finally he dis- covered that its source was not a flower 8 but a beetle, the green and purple and yellow tiger beetle, Therates labiatus. It inhabited the damp and gloomy glades and fed mainly on insects that visited the flowers. Its perfume, Wal- lace concluded, aided it in attracting small nectar gatherers to the spot. At least three species of oriental praying mantids use color instead of perfume to aid them in securing their food. These insects, like the mantid na- tive to the southern part of the United States, imprison their prey within the spined traps formed by their forelegs. By having parts of their bodies ex- panded into thin plates which are brightly tinted on the under side, the oriental insects resemble flowers on the bushes where they hunt. When climb- ing to a favorable position, the mantid keeps the bright-colored under sides of the plates hidden. However, when it finds itself among flowers to its liking, it turns the colored plates uppermost and remains motionless until a victim alights close by. One British naturalist reports seeing a mantid in India climb laboriously to the tips of three branches before it found flowers in bloom. On the first two times, when it found buds, it slowly retraced its steps and began again. Attaining the flowers, it took its posi- tion among them and exposed the under side of its pink, petallike plates. Some oriental mantids have plates that are blue, some mauve, some purple. Still others have pure white plates, that have a surface that is glistening and waxy, like the petals of real flowers. In a number of instances, the or- thoptera of the Tropics are ingeniously camouflaged by nature to escape the notice of their enemies. For example, the long-horned grasshopper, Metapro- sagoga insignis, possesses wings which not only resemble leaves but which are equipped with irregular patches that look as though the leaf tissue had been eaten away by an insect, leaving only a network of veins visible. Another tropical leaf -grasshopper has brownish wings that suggest dried leaves. The resemblance is heightened by the fact Oddities of the Insect World that markings near their extremities give the impression that they are cracked or torn. Then there is a mantid of the Orient, Brancsikia aeroplana, which has curled-up brownish edges to its wings, thus heightening their re- semblance to dry brown leaves. On the wings of a katydid from Venezuela, which William Beebe once showed me, imitation dewdrops and fungus spots increased the effectiveness of the in- sect's camouflage. Probably the most famous camou- flaged insect in the world is Kallima, the dead-leaf butterfly of the Far East. In The Malay Archipelago, Alfred Russel Wallace tells of his first meet- ing with this remarkable butterfly. At the time he encountered it he was col- lecting in Sumatra, beating the bushes for insects and examining his net care- fully for poisonous snakes, which were often dislodged from the branches, before extracting the insects he had caught. "When on the wing," he writes of the dead-leaf butterfly, "it is very con- spicuous. The species is not uncommon in dry woods and thickets and I often endeavored to catch it without success, for, after flying a short distance, it would enter a bush among dry or dead leaves and however carefully I crept up to the spot where the butterfly set- tled and though I lost sight of it for some time, I at length discovered that it was close before my eyes but that in its position of repose it so closely re- sembled a dead leaf attached to a twig as almost certainly to deceive the eye even when gazing full upon it. "A very closely allied species, Kal- lima inachis, inhabits India where it is very common. No two are alike but all the variations correspond to those of dead leaves. Every tint of yellow, ash, brown, and red is found here and in many specimens there occur patches and spots formed of small black dots, so closely resembling the way in which minute fungi grow on leaves that it is almost impossible at first not to believe that fungi have grown on the butter- flies themselves!" Walkingstick insects, in the Tropics, also present some amazingly realistic instances of insect camouflage. One of the most remarkable bears the scien- tific name of Achrioptera spinosissima. About half a foot in length, its green and brown body is decorated with spines that are tinted bright red like thorns. The insect looks for all the world like a broken piece of briar mov- ing along on six legs. Another tropical walkingstick, Palophus reyi, is almost a foot long. The outer skin of its body is roughened into an amazingly close approximation of dry bark on a dead twig. Such resemblances benefit the insect by making it inconspicuous amid its surroundings. But what benefit those brownie bugs of the insect world, the Membracidae, obtain from the fantas- tic adornments they possess is often difficult to see. Again, it is in the Trop- ics that the most spectacular examples are found. Nature seems to have run riot, designing oddities just for the sake of originality. In some species of treehoppers, the prothorax is drawn out into hornlike adornments; in oth- ers, it rises in a high, curving crown; in others, it forms spears or balls. Of- tentimes these are brightly colored. While American treehoppers are less extravagantly formed than those in the Tropics, some species are among our oddest-appearing insects. All are small, and the strangeness of their forms fre- quently is unappreciated without the aid of a magnifying glass. When Charles Darwin was crossing the Atlantic in 1832, at the start of his famous voyage in the Beagle, the ship dropped anchor at desolate St. Paul's Island, 540 miles from the coast of South America. "Not a single plant," Darwin writes, "not even a lichen, grows on this islet ; yet it is inhabited by several insects and spiders." Most of them were parasites on the boobies and other sea birds that landed on the bar- ren rocks and one was a small brown moth belonging to a genus that feeds on feathers. The bleak cluster of volcanic rocks 10 that form St. Paul's Island is but one of many strange places where insects are able to survive. Oceanic water striders skate over the waves hundreds of miles from shore. They lay their eggs on floating sea-bird feathers and other bits of refuse and often live their whole lives without ever seeing land. In Ecua- dor, butterflies are found among the crags of the Andes 16,500 feet above sea level, while explorers, scaling the flanks of the Himalayas, have encoun- tered a praying mantid almost as high. Snow-white and blind insects live deep beneath the earth's surface in caverns. Springtails skip across snow- banks during February thaws in North- ern States. Certain flies breed in the brine of the Great Salt Lake and a number of insects make their homes in the dangerous confines of insectivorous pitcher plants. One curious little larva spends its early days swimming about in pools of petroleum, breathing through a tiny tube which it thrusts above the surface. And another insect is able to live in the mud of hot springs where the water reaches a temperature of 1200 F. At the opposite extreme is the so- called ice-bug, or alpine rock crawler, which inhabits cold mountain recesses, usually at elevations from 5,400 to 8,600 feet above sea level. It prefers temperatures of about 38 ° F., tempera- tures at which most insects are dor- mant. If the mercury rises to 8o°, the ice-bug seems to suffer heat prostra- tion. Two insects that spend their early days under curious conditions are fa- miliar to most parts of the United States. They are the rat-tailed maggot and the froghopper, the immature form of the Cercopidae. The former inhabits stagnant water, caught in knotholes, or other waste fluids. It feeds on the bottom and breathes air through an extensible tube that forms its tail. Thus, like a diver obtaining oxygen through an air hose while working on sea bottom, the fly larva is able to remain submerged as long as it desires. Yearbook of Agriculture 1952 By surrounding itself with bubbles, the little froghopper produces its own climate. In spring and summer, small masses of froth often appear on grass stems and weeds. They are the foam castles of the cercopids. A kind of bi- cycle pump, formed of overlapping plates beneath its abdomen, which pro- vide a chamber into which air is drawn and expelled, permits the insect to pro- duce bubbles in excess sap, which it has sucked from the plant. Within this bubble mass, sheltered from the direct rays of the sun and kept moist by the foam, the immature insect spends its early days. For millions of years, it has been employing its own primitive form of air conditioning. One of the classic studies of the French entomologist, J. Henri Fabre, concerned the aerial journey of the wingless larva of the oil-beetle. Hatch- ing from eggs deposited by the female insect close to flowering plants, the mi- nute larvae slowly ascend the stems and lurk among the petals until a wild bee alights in search of pollen or nectar. Quickly the young beetle attaches itself to the hair on the bee's back and goes sailing through the air as a passenger when the winged insect flies back to its nest. Here the larva lets go. It has found its proper home, a place where it will be supplied with ample food until it transforms into an adult beetle. Not all larvae attach themselves to the right insects, but enough do to carry on the species by means of this ingenious stratagem. Even more remarkable is the se- quence of seemingly unrelated events that transport to their destination the eggs of the human bot fly. The female fly makes no effort to lay her eggs on the ultimate victim. Instead, she visits swampy lowlands where mosquitoes are emerging. There, she overtakes a mosquito, grasps it, and swiftly deposits minute eggs on the under side of its abdomen. Then she releases it and flies away. Her work is done. The mos- quito— or, at least, some of the mosqui- toes thus burdened with bot fly eggs — eventually lands on a human being. Oddities of the Insect World The eggs are on the under side of the insect where they come in contact with the skin of its victim. Thus heated, the eggs hatch while the mosquito is suck- ing blood and the tiny larvae burrow into the skin of their unwilling host. Eggs of human bot fly attached to a mosquito. Another instinctive stratagem is em- ployed by an ant queen found in Tunis. She alights, after the nuptial flight in which she is fertilized by a male of her own species, near the nest of a larger species of ant. Workers seize her and drag her into the underground cham- bers. There she takes refuge on the back of the queen and remains unmo- lested. Using her opportunity, she eventually decapitates the rightful queen and is accepted as the new queen by the workers. Her eggs develop workers of her own species and, in the end, the colony is made up of the smaller ants. William Morton Wheeler, in his Ants, Their Structure, Development and Behavior, tells of a carnivorous butterfly larva that lives in the nests of an Australian ant where it feeds upon the young. An especially tough outer shell protects it from attacks by adult ants. In Queensland this remark- able butterfly, Liphyra brassolis, was studied by F. P. Dodd in the early years of the present century. The adult but- terfly, emerging in the nest, is covered by fugitive scales, which save its life because the loose scales come off in the mandibles of the ants. In describing his observations, Dodd writes: II "Directly the ants encounter the scales they are in trouble. They fasten on to their feet and impede their move- ments, or, if their antennae or mandi- bles come in contact with any part of the butterfly, the scales adhere thereto, so that the ant is soon in a bad way and has quite enough to do in attempting to free herself of her incumbrances without taking any further interest in the butterfly. It is exceedingly ludi- crous to observe the ants endeavoring to free themselves, their legs move awk- wardly and their mandibles open and close in evident annoyance and per- plexity, and they are also much con- cerned about the state of their an- tennae, for the obnoxious scales will not be shaken off, and they seem to become very low-spirited." A number of insect oddities reveal their peculiar characteristics in defend- ing themselves against attack. The blister beetle of southern Europe is equipped with a caustic fluid that pro- tects it from its enemies. In olden times, such insects were ground up to form blistering ointments and plasters. A number of common insects, such as the familiar lady beetle, have weak places at the joints of their legs, which rupture to let out drops of disagreeable fluid when they are attacked. The mon- arch butterfly, noted for its seasonal migrations, is said to possess blood dis- agreeable to birds, thus reducing its chances of being attacked. As the polecat, among animals, relies on an offensive smell to repel its enemies, so a number of insects pro- tect themselves by exuding disagree- able odors. Stink bugs are familiar to everyone. The lacewing, a pale-green, filmy little insect with golden eyes that lays stalked eggs from which hatch the aphis-lions that devour hordes of plant- lice, is another skunk of the insect world. Handle one and the disagree- able fluid it exudes clings to your hands for hours. Incidentally, the plant-lice that are preyed upon by the immature lace- wings are reported to employ a sur- prising method of defense occasionally 12 when they are approached by the sickle-shaped, sucking jaws of an aphis- lion. The plump aphids produce, in addition to honeydew, a waxy secre- tion that collects at the ends of two tubes projecting backward from their abdomens. Before its enemy can use its jaws, an aphid sometimes will back quickly toward it, pushing the waxy blobs into its face. This sticky material halts the attack while the aphis-lion stops to clean away the wax. The bluish-backed bombardier bee- tle, Brachinus fumans, gains time by a different ruse. When it is pursued by an enemy, it emits a little cloud of offensive gas. This gas attack takes the pursuer by surprise. It stops and the momentary pause is often sufficient to permit the beetle to escape. These are active forms of defense. Other insects employ passive forms. They feign death to escape death. Otto Plath, in his Bumblebees and Their Ways, records an instance in which a robber fly was fighting with a bumble bee in a glass jar. Getting the worst of the battle, the fly suddenly fell on its back as though stung to death. It lay there, apparently lifeless, until Plath shook both insects out of the jar. Then the "dead" robber fly sprang into the air and darted away. Ambush bugs, lady beetles, monarch butterflies, and a long list of other insect opossums feign death. Some walkingsticks will become rigid and apparently lifeless when alarmed. In one instance, a walk- ingstick feigned death for 6 hours, remaining as rigid as a twig all during that time. Giants and dwarfs among the in- sects cover a wide range. The great at- las moth of India, with a wingspread of a foot; an East Indian walkingstick 15 inches in length; the Hercules beetle of Africa, which drones over the countryside at evening with a sound like an approaching airplane — those are some of the giants. Among the pigmies are the microlepidoptera, the minute beetles so small they can liter- ally creep through the eye of a needle, and the fairy flies, which are built on Yearbook of Agriculture 1952 such a miniature scale that, although they are perfect in all their parts, they measure only one-hundredth of an inch from head to tail. In addition to oddities of size among the insects, there are innumerable odd- ities of form. Near the pyramids in Egypt early entomologists discovered a singular ant-lion with a slender and elongated neck. Its caliper jaws seem held at the end of an outstretched arm. This pipestem neck, in many instances, is far longer than the rest of the insect's body. It has been suggested that this lengthy neck permits the insect to se- cure its prey in deep crevices. A fold- ing, extensible lip, which reaches out like a straightened arm to grasp under- water victims, is a feature of the head of every dragonfly nymph. At the end of the lip are grasping hooks, by means of which the nymph pulls its captive back into its mouth. Enormous forelegs, more than twice the length of the rest of its body, are the characteristic of a black wood beetle discovered by Alfred Russel Wallace in the Moluccas. This beetle, Euchirus longimanus, covers a space of 8 inches with all its legs extended. Another insect curiosity of the Malay Archipelago is an antlered fly. Various species have protuberances on their heads that suggest the horns of deer, elk, and moose in miniature. Even more remarkable is a stalk- eyed fly of South Africa, Diopsis api- calis. Like the hammerhead shark, it has its eyes extending out from the sides of its head. The stalks to which they are attached, however, are drawn out to such surprising length that the measurement from eye to eye is one- third more than the length of the body from head to tail. An abdomen that has amazing pow- ers of distention is a characteristic of the nymph of the bloodsucking Rhod- mus. In a few minutes, one of these nymphs can distend itself with blood up to 12 times its original weight. As the huge meal is digested, the abdomen contracts smoothly like a deflating bal- loon. Similarly, the abdomens of the Oddities of the Insect World honey ants of the Southwest possess the ability to expand enormously. Cer- tain members of the colony act as stor- age vessels for the honeydew gathered by the workers. They never leave the nest. With abdomens so swollen they cannot walk, they cling to the roof of their underground chamber, regurgi- tating food to the workers when it is needed. Various other ants must be num- bered among the insect oddities. In Ant-Hill Odyssey, William M. Mann tells of collecting a species that is known to Brazilian natives as "The Terrible Ant." Fully an inch in length, it is said to produce a serious fever by its sting. A hundred years ago, when Henry W. Bates was collecting in the Amazon basin, he encountered villages that had been deserted because of an invasion of fire ants. These small red insects have stings like red-hot needles. Then there are the army ants that march in long lines in the jungle, the slave-making ants that raid other col- onies for pupae, the tree ant of India, Oecophylla smaragdina, that uses its larvae as a means of sewing leaves to- gether into a nest, passing the silk- producing grubs back and forth from one leaf edge to another to provide a solid bond. Within these leaf sheds, the ants keep smaller insects that produce honeydew, the sweet fluid upon which the ants feed. Honeydew is so universally relished by ants that it has been described as their "national dish." Other insects have a taste for varied and often sur- prising things. That goat of the insect world, the drug-store beetle, is known to consume 45 different substances, in- cluding the poisons aconite and bella- donna. Other beetles feed on cigarettes, mustard plasters, and red pepper. Ants have shown themselves resistant to cy- anide. Termites are able to digest cellu- lose in wood because of the aid of mi- nute organisms within their intestines. In the case of some insects, a reduced diet slows down growth. Some wood- boring grubs, such as those of the cer- ambycid beetles, sometimes live in 970134° — 52 3 13 house timbers or furniture for years after they have been put in place. In one instance, an adult beetle emerged from a porch post that had been stand- ing for 20 years. The dried timber lacks the nutritive qualities of the living tree and the growth of the grub is arrested, so long periods pass before it reaches maturity. Underground, the nymph of the periodical cicada spends more than a decade and a half tunneling through darkness in the soil before it emerges into its brief life as an adult. In the mating and reproduction ac- tivities of the insects, we find some of the strangest habits of all. The death- watch beetle, that stand-by of ghost stories laid in old castles, bumps its head on the top of its wooden tunnel to send a kind of telegraphic message to its mate. To attract the attention of the females at mating time, the males of certain flies blow shining little bubbles of froth. Some chalcidflies which parasitize caterpillars, have the faculty of laying self-multiplying eggs. More than 2,000 larvae may be pro- duced by the depositing of a single chal- cid egg in the body of a victim. During the lifetime of a termite queen, in the Tropics, as many as 10 million eggs may come from the insect's bloated body. L. C. Miall, in The Natural History of Aquatic Insects, tells of a minute fly found in England under the bark of poplar, willow, and beech trees. It pro- duces viviparously small larvae "which escape by tearing open the body of their parent and in turn produce other larvae after the same fashion." These seem fantastic creatures and bizarre habits. But to one who views with fresh eyes the old, taken-for- granted, commonplace habits of even the most familiar insects — the everyday butterflies and grasshoppers and ants we see about us — there is in the events of their lives much that is a source of astonishment and wonder. A century ago, this amazing strangeness of the familiar insects was eloquently ex- pressed in describing the metamor- phosis of a moth in the early pages of the pioneer entomology by William Kirby and William Spence. "Were a naturalist to announce to the world," they write, "the discovery of an animal which first existed in the form of a serpent; which then pene- trated into the earth, and weaving a shroud of pure silk of the finest texture, contracted itself within this covering into a body without external mouth or limbs, and resembling, more than any- thing else, an Egyptian mummy; and which, lastly after remaining in this state without food and without mo- tion . . . should at the end of that pe- riod burst its silken cerements, struggle through its earthly covering and start into day a winged bird— what think you would be the sensation excited by this strange piece of intelligence? After the first doubts of its truth were dis- pelled, what astonishment would suc- ceed! Amongst the learned, what sur- mises!— what investigations! Even the most torpid would flock to the sight of such a prodigy." Edwin Way Teale is a past presi- dent of the New York Entomological Society and the author of numerous books on insects, including Grassroot Jungles, The Boys' Book of Insects, Near Horizons, The Golden Throng, and North With the Spring. His books have appeared in British, Spanish, French, Swedish, Finnish, and Braille editions. Near Horizons was awarded the John Burroughs Medal for distin- guished nature writing. In 1949, Mr. Teale edited a one-volume omnibus of the writings of Fabre, entitled The Insect World of J. Henri Fabre. =>> Stenomema canadense, a common mayfly. 14 Fossil Insects Frank M. Carpenter Written in the rocks of Colorado, Kansas, Oklahoma, and many other places is the story of insects in the ages before man appeared on earth. The insects were trapped, caught in mud or sticky resin, and thereby left a permanent record — as did dinosaur, mollusk, and plants — that broadens our knowledge of their evolution. About 1 2,000 species of fossil insects have been described. Countless thou- sands of specimens have been collected. Fossil insects are not found in as many deposits or localities as most other invertebrates. Like other organ- isms, insects are preserved as fossils by a sequence of events that results in their burial in a suitable medium. Im- mediate burial is necessary to preserve the whole insect; otherwise the body parts soften and fall apart, and only the wings remain. The wings decom- pose more slowly and therefore can be preserved under less favorable condi- tions. That is the reason why many specimens of fossil insects consist of wings alone. When conditions were good for preserving insects, large num- bers of fossils usually occur. An example of such abundance is provided by the Tertiary shales at Flo- rissant, Colo., which have yielded up- wards of 60,000 specimens. The shale originated about 40 million years ago in a shallow lake, extending into sev- eral narrow valleys and rimmed by granitic hills. Several neighboring vol- canoes frequently erupted and scat- tered ashes and debris over a wide area. Whatever insects were flying or were being blown over the lake at those times were forced into the water by the falling ashes and were promptly buried. Fossil insects have been found at Fossil Insects nearly 150 localities in various parts of the world. About nine-tenths of the specimens have been collected at 1 2 of these deposits. The remainder has come from less productive rocks. Some of the latter are important because of their geological position, however. One of them is the Commentry shales of cen- tral France. These were deposited by a deep fresh-water lake, which existed during the Upper Carboniferous pe- riod some 250 million years ago. About 1,500 specimens have been found in the shales. They are well preserved and are almost the oldest insects known. Another deposit, notable for the abundance of fossils as well as their ages, is the Elmo limestone in eastern Kansas. The rock, fine-grained and nearly white, was deposited by a shal- low fresh-water lake inhabited by. aquatic insects, crustaceans, and small king crabs. A collector who carefully breaks the limestone, after it has been dug up and dried, may get as many as 50 good insects a day. Most of the fos- sils are strikingly well preserved. Some show even the coloration and minute hairs on the wings. About 10,000 speci- mens so far have been collected there. A similar but more extensive lime- stone formation was discovered in 1 940 in northeastern Oklahoma. It origi- nated in a shallow, saline lake, barren of life except for algae and bivalve crustaceans (Conchostraca) . Most of the insects preserved there were pre- sumably carried to the lake by floods. The lithographic limestone of Ba- varia, famous for such fossil vertebrates as the flying reptiles and the earliest birds, is not nearly so important for its insects. Several thousand specimens have been found there, but fewer than one-tenth of them are well preserved. The richest of all deposits is the Bal- tic amber from Germany. The mate- rial is itself the fossil resin from an extinct pine tree (Pinites succinifera) . The Amber Pine Forest existed for several million years during the early Tertiary period, and extended from about the site of Bornholm and Riigen in the south to that of the White Sea and Ural River in the east. The north- ern and western borders are uncertain because those regions are covered by the ocean. At any rate, the local ac- cumulation of the amber along the coast of East Prussia is the result of the washing out of the flooded forest. In- sects and other small invertebrates, which were caught in the resin on the tree trunks, are preserved in great de- tail and perfection. At least 150,000 insects have been found in the amber. The earliest geological record of the insects is still uncertain. Fragments of small arthropods, which have been re- covered in a Devonian chert in Scot- land, have been determined by some entomologists as Collembola (spring- tails), but the identity will remain doubtful until more is known about them. The oldest unquestionable in- sects have been found in rocks of early Upper Carboniferous age, about 250 million years ago. Only three of these fossils are known — one each from Czechoslovakia, Germany, and Penn- sylvania— and each consists of a single wing. Whatever else may be inferred from the specimens, it is certain that insects with fully developed wings ex- isted then. Insects are much more abundantly represented in the later Upper Carbon- iferous rocks than in the earlier ones, so that we have at least a working knowledge of the insect fauna of the time. Six orders of insects have been recognized, all but one of them extinct. The most interesting was the Palaeo- dictyoptera, which were of medium size and resembled mayflies. Since some of the Palaeodictyoptera were more generalized than any of the other winged insects known, the group as a whole is usually considered to be the ancestral stock from which all other winged insects have been evolved. As far as we know, all species of the order had a pair of membranous lobes on the first thoracic segment. The lobes ap- pear to be homologous with the func- tional wings of the other two thoracic segments and are regarded as indicat- ing the steps by which functional wings i6 arose. Unfortunately nothing is known about the immature stages of the Pa- laeodictyoptera. The order reached its maximum development in the Carbon- iferous period but persisted through the Permian period. The most spectacular insects of the Carboniferous and Permian were the Protodonata. They resembled dragon- flies. Their chewing mouth parts were powerful, and their legs, like those of true dragonflies, were covered with strong spines. They were undoubtedly predaceous, catching their victims in flight and devouring them while rest- ing on tree ferns or other ancient plants. All of the Protodonata were large and some were veritable giants, having a wing expanse of 30 inches and a body length of 15 inches. Specimens of such large species have been found in rocks in France, Kansas, and Okla- homa. Since birds and other flying vertebrates did not exist at that time, these huge insects presumably ruled the air. Their nymphs have not been found, but they were probably aquatic and like those of true dragonflies or damselflies. The only living order or group of families of insects known to have ex- isted in Carboniferous time is the Blat- tidae, or cockroaches. Their remains make up a high percentage of insects of that period, but that is probably due partly to the favorable conditions pre- vailing in the Carboniferous swamps that produced the deposits. Some for- mations of that period, such as the coal beds of Pennsylvania, have yielded no insects except roaches. The average size of the Carboniferous roaches was somewhat greater than that of living species, but none of the fossil forms ex- ceeds in size certain living species of the Tropics. The difference between the ancient roaches, existing some 250 million years ago, and those of today is exceedingly slight, involving chiefly position of wing veins. By the beginning of the Permian pe- riod, about 50 million years after the appearance of the first insects, a marked change had taken place in in- Y ear book of Agriculture 1952 Geological Ages of Existing Orders of Insects Name of order Earliest geological record Collembola Devonian [?]. Entotrophi Middle Tertiary. Thysanura Jurassic. Odonata Lower Permian. Ephemeroptera . . . Lower Permian. Plecoptera Upper Permian. Orthoptera Triassic. Orthoptera Upper Carbonif- (Blattidae). erous. Isoptera Lower Tertiary. Dermaptera Jurassic. Embioptera Lower Tertiary. Corrodentia Lower Permian. Mallophaga [No fossils known.] Hemiptera Lower Permian. Anoplura Quarternary. Thysanoptera .... Upper Permian. Mecoptera Lower Permian. Neuroptera Lower Permian. Trichoptera Jurassic. Diptera Jurassic. Siphonaptera Lower Tertiary. Lepidoptera Lower Tertiary. Coleoptera Upper Permian. Strepsiptera Lower Tertiary. Hymenoptera .... Jurassic. sects. Although the several extinct orders which arose in the Carbonifer- ous still existed, several living orders besides the roaches were represented. Along with the giant dragonflies were minute barklice, only one-eighth of an inch across the wings. Altogether, the lower Permian insect fauna was very diverse — more so, in fact, than any other insect fauna known. There was about equal representation of the ex- tinct orders of the Carboniferous and relatively specialized existing orders. Also adding to this diversity were sev- eral other extinct orders, known only from Permian strata. One of them, the Protelytroptera, included beetle-like in- sects, having well-developed elytra, but they were closely related to the roaches and had 410 affinities with the Coleop- tera. The living orders that appeared in early Permian time include such types as the Odonata (dragonflies), Ephemeroptera (mayflies), Corroden- tia (barklice), Hemiptera (bugs), Neuroptera (lacewings), and Mecop- tera (scorpionflies) . The lacewings and scorpionflies are especially noteworthy because the living species have com- Fossil Insects plete metamorphosis. Coleoptera and Plecoptera are first found in late Per- mian strata, but they probably existed earlier in the period. With the beginning of the Mesozoic era, the insect fauna changed even more markedly. In fact, the contrast between the archaic fauna of the Per- mian and the relatively modern one of the Triassic is as great as that between the faunas of the Triassic and the Re- cent periods. None of the extinct orders remained after the beginning of the Mesozoic, but a few living families oc- cur in Triassic strata. Among the nota- ble insects of that period were certain Australian species related to the Or- thoptera, which had a large stridula- tory apparatus on the wing. This con- stitutes the earliest record of sound production in the insects. Because at the time these insects lived there were no birds or other vertebrates that pro- duce the ordinary animal sounds of for- ests or woodlands, it is quite possible that these stridulating insects and their relatives were the noisiest creatures then in existence. The Jurassic insect fauna was much like that of the Triassic except that more existing families occur. In fact the appearance of this fauna is so mod- ern that if we had a collection of Jurassic species pinned in the usual way, it would not look very different from our present-day collections, ex- cept that there would probably be no flower insects, such as the bees and syrphid flies. This is a great contrast to the condition of the vertebrate fauna of the time, which included the dino- saurs, flying reptiles, and toothed birds. By the beginning of the next period, the Cretaceous, the flowering plants had become established and in all probability the types of insects asso- ciated with these plants promptly fol- lowed. Unfortunately our knowledge of the Cretaceous insects is insignifi- cant because of the lack of adequate specimens. Early Tertiary strata have yielded a higher percentage of living genera than the Jurassic, especially of flies, beetles, 17 Golden-eye lacewing. Plathemis lydia, a dragonfly. dragonflies, and true bugs. The insects in the Baltic amber, which is now re- garded as of early Tertiary age, are especially important in enabling accu- rate comparisons with living genera and species. Studies of families of amber insects have shown that the amount of evolution that has taken place since the early Tertiary has var- ied for different families. The Baltic amber ant fauna, for example, includes 43 genera, of which 24, or 55 percent, still exist, whereas all but one of the genera of bees in the amber are extinct. In this connection, it is noteworthy that William Morton Wheeler, who made an extensive study of the amber ants, found eight species of them which he could not distinguish from living spe- cies. Furthermore, he also found that the social habits of the amber ants were about as highly organized as those of the living forms, with caste differen- tiation, polymorphic workers, and even Yearbook of Agriculture 1952 Table of Geologic Periods Era Period Cenozoic (age of mammals and ("Quaternary, man) \ Tertiary . . . I Cretaceous . Jurassic . . . . Triassic . Paleozoic (age of invertebrates and primitive vertebrates) fPermian Carbon iferous< T " |_Lower . Devonian Silurian Ordovician .Cambrian Approximate time [in millions of years Duration of Since bt %in- period ning off eriod I 69 1 70 5° 120 35 155 35 190 25 215 35 250 5° 300 50 35° 40 3Qo 90 480 70 550 the association with plant-lice. Since this was fully 50 million years ago, be- fore the time when most existing fam- ilies of mammals were evolved, it is apparent that the social organization of ants is a much older one than ours. Although Tertiary insects do not contribute so much to our understand- ing of insect evolution as the older fos- sils, they have given information about changes in geographical distribution of the genera and families since the early Tertiary. Many genera and families which have been found in the Baltic amber are now entirely absent from Europe, and some are known to occur only on such distant land masses as Australia and South America. The same is true of Tertiary insects of other parts of the world. The Florissant shales (middle Tertiary) in Colorado have yielded several species of lace- wings of the family Osmylidae, a group now absent from North America. Hun- dreds of instances of this sort can be cited. The significance of such changes in distribution is not clear and will not be until more evidence has been ac- cumulated and correlated with the fos- sil record of other groups of animals and plants. A detailed study of the geological history of the insects, which I have only sketched, yields evidence of certain progressive changes in structure and development which confirm conclu- sions on insect evolution reached by morphological and embryological in- vestigations. Although this is still a highly controversial subject, we have enough evidence at hand, derived from these three sources, to indicate the main steps in insect evolution. There is, however, no fossil evidence bearing on the question of insect ori- gin; the oldest insects known show no transition to other arthropods. On the other hand, morphological and embry- ological studies carried out mainly since 1935 have pointed to the prob- able origin of the insects from some terrestrial arthropod, related to the ex- isting Symphyla. The time of that ori- gin is pure conjecture, but judging from the fossil record we can only con- clude it was at least as far back as the Lower Carboniferous (Mississippian) . Morphological studies of existing in- sects prove that the first true insects were wingless (Apterygota) , like the Thysanura (silverfish) and Entotrophi, combining the generalized characteris- tics of both of these groups. The development of winged insects (Pterygota) was the first great evolu- tionary step within the insect line. The origin of wings is by no means clear; they probably were developed from lateral flaps, like those on the first tho- racic segment of the Palaeodictyoptera. These primitive flying insects, termed the Palaeoptera and exemplified by the Odonata, Ephemeroptera, and several extinct orders, were unable to flex their wings over the abdomen at rest. The second main evolutionary Fossil Insects change was the development of an articulation that made it possible for the wings to be held over the abdomen when the insect was not in flight. All living Pterygota except the Odonata and Ephemeroptera belong in this category, which is termed the Neop- tera. The acquisition of this wing-flex- ing mechanism was an important change, for it enabled the insects, in adult as well as the immature stages, to hide in debris or under stones or logs. The first neopterous types had a simple or direct type of postembryonic de- velopment and are usually termed the hemimetabolous Neoptera. The third main evolutionary step was the attainment of the more com- plex type of metamorphosis, with larval and pupal stages, resulting in the holo- metabolous Neoptera. The fossil record of the insects, though incomplete, has given us a gen- eral idea of the time of occurrence of the three events. The existence of two orders of insects ( Mecoptera and Neu- roptera) with complete metamorphosis in lower Permian rocks can only mean that this step was attained at least by late Upper Carboniferous time. Simi- larly, the presence of species with wing- flexing abilities in the early Upper Car- boniferous shows that the hemime- tabolous Neoptera arose in the Lower Carboniferous. Unfortunately, since no insects have been found in strata older than those of the Upper Carboniferous period, we have no actual record of the existence of Palaeoptera before these Neoptera; nor, for that matter, is there any Paleozoic record of the Aptery- gota. Because all evidence derived from other sources indicates the primitive nature of these two categories, how- ever, we can infer that the Palaeoptera preceded the Neoptera, and therefore that they existed in the early part of the Lower Carboniferous. Similarly, we can infer that the Apterygota, which must have preceded them, arose still earlier in the Lower Carbonifer- ous or, more likely, in the Devonian. The conclusion to be drawn from the record, at any rate, is that all three of 19 the main steps in insect evolution took place before the end of the Car- boniferous period, about 250 million years ago. Nothing nearly so important has happened to the insects since then. Another contribution that fossil in- sects have made to our understanding of the evolution of the group pertains to the progressive increase in the rel- ative numbers of species having com- plete metamorphosis in the geological periods since the lower Permian. Start- ing from the beginning of the Permian, during which only about 10 percent of the known species had complete meta- morphosis, there has been an increase up to 88 percent at the present time. The most rapid change ( 10 to 40 per- cent) seems to have taken place in the interval of the Permian period. Al- though there is a possibility that such a marked shift in the insect population did actually take place in that time, the more probable explanation is that complete metamorphosis arose further back in the geologic time than the lower Permian, and that the change was more gradual. Those are two examples of the way in which the study of fossils has con- tributed to our understanding of insect evolution. There is every indication that the insects have been as numerous on earth as they are now for at least the time since the Jurassic period, about 150 million years; and also that the insect fauna of our time is but a small part of the total parade of insect life that has lived on the earth during the past 250 million years. It is not sur- prising, therefore, that our understand- ing of insect evolution depends to a large extent on a knowledge of this extinct population. Frank M. Carpenter is professor of entomology, Alexander Agassiz pro- fessor of zoology, and curator of fossil insects in the Museum of Comparative Zoology at Harvard University. He joined the staff of Harvard in 1932 and has done research on fossil insects, insect evolution, and the taxonomy of Mecopttra and Neuroptera. How Insects Live E. O. Essig For some 250 million years insects have been able to flourish on land and water, in arctic barrens and tropic jungles, in deserts and grassy prairies because they have developed special and wonderful adaptations to meet all the varied conditions of this earth. Not just a few but literally thousands of species, representing practically every order, live together in nearly every ecological niche. They have survived so long without being greatly altered in size and form or reduced in numbers. In direct com- petition with all other higher forms of life on land, they stand supreme in numbers of species and individuals. Only some lower microscopic forms like bacteria may outnumber them. It is easier to describe the remark- able adaptations of insects than it is to explain the reasons for them. It is dif- ficult to know whether the body struc- tures and complicated life histories or the environmental factors were the most important. Many other factors may have entered into the long, slow process of change and adjustment. Consider first as a factor of survival the dominant position of the female insect. She is the foundation, the per- petuation, the multiplication of the species. In many species she is the all; males do not exist. The termite queens of the fungus-feeding tropical species may live for a half century. Certain Australian queen termites may lay as many as 360 eggs in an hour, 8,640 in a day, 3,153,000 in a year. She may continue without interruption for 25 to 50 years. Parthenogenesis, or reproduction without fertilization of the female, is not unusual among insects. The most common examples are among the aphids. Certain species have in their life cycle both parthenogenetic and sexual forms. In the tropical and warmer regions no males appear, how- ever, and the females continue year after year producing only their own sex. A similar phenomenon occurs among coccids, or scale insects, and among many weevils, sawflies, gall wasps, bethylid wasps, certain bees, and hymenopterous parasites. Among honey bees, unfertilized females may produce only males. In certain para- sites, one species may produce only males and another only females. A South African race of the common honey bee is said to produce partheno- genetically not only males and workers but even queens. Some ants also repro- duce by unfertilized eggs. Thus we find that parthenogenetic reproduction re- sults only in males in many of the in- sects although females are not uncom- mon, while among aphids and coccids only females are produced. Polymorphism is a condition in which there are adults of two or more distinct forms of the same sex. There also are polymorphic larval forms. To illustrate, in the aphid genus Periphyl- lus there are as many as 17 distinct recognizable forms, some of which are so different from the others that they have been mistaken for separate spe- cies. Among social insects, especially among termites and ants, polymorph- ism reaches its peak in the insect world. As many as 12 distinct series of castes and forms occur in the species of Eutermes. Among the many qualities of insect fitness are the hard, elastic, tough ex- oskeleton with its powers of renewal and its resistance to corrosive chem- icals; the many protective devices, such as rugosities, hairs, spines, and scales, as well as the folded wings; the many legs; the ability to lose and even to regenerate certain appendages without greatly interfering with life and repro- ductive processes; the protective color- ation and devices for mimicry; the ex- cretion of protective waxes, resins, and 20 How Insects Live offensive glandular materials; poison- ous body fluids and gases; stinging hairs and other devices; the specially constructed living quarters in plant tissues, in water, soil, debris; the en- veloping and protecting cases of wood, earth, waxes, paper; the webs, cocoons, spittle, nests, galls; the internal para- sitic habits on other hosts; the innum- erable other means of protection and of escaping natural enemies through the complicated processes of develop- ment ; the methods of escape by protec- tive coloration and mimicry, death feigning, jumping, snapping, and flight; the aggressiveness exemplified by ants, mosquitoes, bees, wasps; the ability to bite and sting; the ability to reproduce in such numbers as to over- come almost every opposing factor, even including larger animals and human beings. Those are the weapons with which insects counter man's inroads on for- ests and native vegetation and other habitats of insects, the cultivation of onetime grasslands, the draining of aquatic breeding places, and the de- vices of chemical and biological con- trol. That is why insects are rarely ex- terminated. In fact, many species, benefited by the immense acreages of special farm crops, have actually in- creased because of man's agricultural activities. As I said, insects generally have oc- cupied only land surfaces and fresh- water areas. On land, they occupy all areas except the permanent arctic and antarctic icecaps. They live in the soil, in fresh and brackish water, and in and on all conceivable kinds of animal and plant materials. Insects, even the aquatic forms, are always associated with the flora and fauna of every re- gion. The associations may be some- what detrimental to plant and other animal life, or they may be mutually beneficial to both the hosts and the insects; despite the damage by locusts, armyworms, weevils, and other serious pests, plants have not been greatly hampered by them. On the other hand, it is difficult to ascertain to what degree 21 plants may be benefited by insects. Assuredly, we do know that the polli- nation of many plants can only be accomplished by certain ants, bees, but- terflies, moths, flies, beetles, and other insects. Not all insects are plant feeders. Fleas, lice, gnats, midges, mosquitoes, and some bugs, beetles, thrips, neurop- terons, strepsipterons, ants, wasps, and parasitic forms prey on animals. As carriers of diseases to man and to do- mestic and wild animals, they are of great concern and have spread death and misery over the world through the ages. The varying conditions of topogra- phy, climate, and other forms of life with which they are associated and upon which they subsist undoubtedly also have influenced their variability of structure and habits. Although the ancestors of insects are thought to have been aquatic, many entomologists be- lieve insects first evolved on land. An- other general belief is that insects did not arise until after plants appeared on earth. R. J. Tillyard has said that the earliest fossils have been found in the lower part of the Upper Carbonif- erous period in North America, and that at a somewhat higher horizon, in the upper part of the Upper Carbonif- erous, insects occurred abundantly both in North America and in Europe. These insects were aquatic and terres- trial. If insects were ever oceanic, they did not remain so because up to now, at least, they have never invaded the ocean to any extent. Few, if any, truly oceanic aquatic insects feed, for in- stance, on the abundance of aquatic plants along the shores of all the great oceans. This warehouse of plant food appears not to have been the objective of insect adaptability. In fresh water, insects are quite at home and often develop in tremendous numbers, as, for example, the mayflies in certain of the Great Lakes. The abundance of dragonflies, stoneflies, caddisflies, water bugs, water beetles, mosquitoes, gnats, and other insects testify to their perfect conformity to an 22 aquatic life. Many of the land and aerial forms also have remarkable aquatic adaptations, such as walking on water and swimming with the wings that ordinarily they use for flight. An example is certain minute hymenop- terous species, which parasitize the eggs of water insects. Probably many more years of study will be needed to explain all of the complex adaptations and interrela- tions of insects to other insects and ani- mals, plants, and lower organisms. The more general relationships have been known for many years, but only in re- cent times has the relationship of in- sects to viruses and other micro-organ- isms been revealed. Small size is not the only important asset of insects in their struggle to sur- vive. Their varied methods of locomo- tion are admirably adapted to their needs. The younger stages of all insects and all stages of many species are wing- less. Thus early in life they learn to walk, crawl, run, jump, and hop. Ex- cept among the more primitive wing- less forms, flying is attained in the ma- ture stages. In some, like aphids, wing- less (apterous) and winged (alate) forms appear in the same generations. Then the alates are the dispersing and migrating forms, and the apterae re- main more or less fixed in the immedi- ate surroundings of their birth. Among many other insects, only one of the sexes (the male in some coccids and aphids, for example) may be winged. Throughout practically the whole in- sect group we find both the winged and wingless condition in the adult forms, as exemplified by such well-developed groups as ants, parasitic hymenoptera, flies, moths, and beetles. In the lower orders, Protura, Thysanura, Aptera, and Collembola, all stages are wing- less. Nevertheless, representatives of nearly all of them occur throughout the whole world. Among the higher in- sects, all members of the orders Mallo- phaga, Anoplura, and Siphonaptera are wholly apterous. In nearly all the other orders, wingless adults occur. The remarkable thing is the phe- Yearbook of Agriculture 1952 nomenal degree of variation from any definite standard in their anatomical, physiological, and ecological constitu- tion. Practically all insects walk or crawl. Six legs enable the adults to move along at a rather even and rapid rate. Certain of the running forms, such as silverfish, cockroaches, psocids, bird lice, bugs, many beetles, flies, and ants may move very fast. The larvae may be legless (apodous) or have either the normal three pairs of legs, or these and additional prolegs, as among caterpillars, sawflies, and horntails. Larvae of weevils, flies, and other members of the order Diptera and some other specialized parasitic forms are apodous. Many of the weevil larvae appear to get about as easily as caterpillars; for them, pseudopods serve as legs. Certain running insects, like silver- fish, cockroaches, and ants, travel almost like lightning and are gone in a flash. Other forms move with a slow and often wobbly gait. Insects that are more or less attached to the host plant, like wingless aphids, move about slowly and depend upon the winged forms for dispersal. Such forms as the female armored scales can crawl only in the first stage of development. Males eventually de- velop into fully legged and also winged adults, which seek out and mate with the immobile females. Many insects jump. Those that do usually also walk and run. Jumping may be a means of locomotion and of escaping from enemies, as exemplified by springtails, crickets, grasshoppers, leafhoppers, fleas, and others. Aquatic insects are efficient swim- mers. They may maneuver with all their legs or with only the hind legs as oars. Dragonfly nymphs expel water through the terminal abdominal open- ing with such force as to propel them forward, slowly or in a rapid darting motion. Whirligig beetles swim under water and also gyrate in rapid, fantas- tic movements on the surface. Several families of bugs, including water How Insects- Live striders and many gnats and flies, run rapidly on the water. Many sliders and other arthropods can do that, too. Small size and light weight keep in- sects from flying as fast as some birds, but in maneuverability insects prob- ably exceed all other animals.* Only adults have wings. There may be only a single pair in th'e true flies, in certain mayflies, and in male coccids or scale insects^ but ordinarily there are two pairs. They, are membranous with simple or complicated venation and are naked or covered with hairs and scales. The scales and hairs may be beautifully colored and arranged in more or less definite color patterns, as in the butter- flies, moths, dragonflies, flies, caddis- flies, and others. In the whiteflies and the coniopterygids, they are covered with a white, powdery wax. At rest, the wings may remain ex- tended at right angles to the body, as in the dragonflies and many true flies, but they are more often folded together above the dorsum or flat over the back or somewhat around the body. The flight of insects is remarkable almost beyond belief even though rarely have we noted closely the lei- surely floating of butterflies, the tireless movements of honey bees from flower to flower, the aimless whirl of grass- hoppers on a hot day, the determined buzzing of mosquitoes, flies, and gnats. The plump and lazy-looking bot fly is believed to be the fastest flier and may attain a speed of 40 or 50 miles an hour; the large dragonfly is about the most agile flier of the animal kingdom. These so-called hawkers capture their insect prey on the wing and remain almost constantly in the air, going for- ward and backward, remaining poised in a fixed position, and then darting faster than the eye can follow- — only to reappear in almost the same spot. They have been observed great distances from water during migration flights. Hummingbird moths get their name from their resemblance to humming- birds and their habit of collecting food from flowers in a manner so like that of the real hummingbirds that they are 23 often mistaken for them. While the adults are beneficial, because, by means of' thei? excessively long rostrum, they pollinate many flowers which cannot be fertilized by any other natural means, yet the large caterpillars, like the tomato and tobacco worms and other species, do considerable damage to crops. Many species migrate long dis- tances on the wing. Locusts have be- come an international problem because they may breed in one country and migrate to devastate another country often 100 miles or more away. Butter- flies and moths and many other in- sects have regular yearly seasonal migrations. Most insects have dispersal flights. Often during the spring, summer, and fall certain species take to wing, and the air may be filled with flying thrips, aphids, termites, crickets, beetles, flies, ants, and others. The flights may be for relatively short distances and for only a few hours or days, but altogether the insects may gradually move across vast areas of cultivated and natural vegetation in seeking new feeding grounds. Such flights follow a rather definite pattern year after year. There are, for example, the regular flights of insects that winter over as adults and young in the southern parts of the United States and Mexico and move northward as spring and summer ad- vance. Therefore there may be an even and sometimes excessive and rapid dis- tribution of a given species to all the available hosts in a large region. Insects may be carried in other ways. Winds and air currents pick up the minute wingless and winged forms alike and carry them far. Such fragile forms as aphids, scale insects, thrips, and tiny caterpillars may have means of supplementing their weak flight by spinning waxy filaments and silken threads or balloons, which increase their buoyancy and carry them long distances. An example is the winged individuals of a spruce aphid, Lachnus piceae, which have been observed in 24 large numbers on the fresh snow of the icecap of Northeast Land, Spits- bergen, 800 miles from the Kola Penin- sula, Russia, from which the insects were believed to have been carried. This aphid is often abundant in the coniferous forests of northern Russia and is carried by strong winds into regions in which it cannot exist nat- urally. It may also be carried by more favorable air currents to new and de- sirable forested areas. Insects may also be transported by water, host animals, birds, and other insects. Quarantine laws cannot completely prevent the un- usual natural migrations and move- ments of insects, which have been go- ing on over long periods of time. Wing beats of insects, according to L. E. Chadwick, vary with the species and may be as rapid as 350 strokes a second. Even among the different in- dividuals of a species the wing beats, as in Drosophila, may vary from 9,000 per minute to about 13,000. Wing beats for the large yellow swallowtail butter- fly average about 6 a minute, for the sphinx moth 90, for the honey bee from 160 to 220, for the bumble bee 240, and for the house fly about 160. Many insects have complicated life histories. In fact, the entire life cycle of most species is still imper- fectly known. Even the common insect pests that have been under constant observation for years may present bio- logical puzzles. As we advance in our understanding of some of the more complex life histories of species, such as the parasitic and social forms, the more are we forced to go back and re- view the exact development of com- mon forms that have been generally considered to be simple and thoroughly understood. In that way, many new and unsuspected facts have been learned that were overlooked by previ- ous students and investigators. The entomologist regards every in- dividual insect he encounters as only one phase of a simple or complex stage in the development of the insect from birth to adult. Between the beginning Yearbook of Agriculture 1952 and end of this cycle, few (3 or 4) or many (6 to 17) different stages or types of individuals may appear. To know these for more than a few groups is be- yond the capacity and experience of most entomologists. It is possible to recognize all the types of individuals in a genus or possibly even in more than one family, however. Let us consider briefly the more gen- eralized types of transformation in cer- tain of the representative orders. Primitive or ametabola (unchange- able) : In this type there is little difference in the general appearance of the vari- ous stages throughout the entire life of the insect. The condition is evident in certain so-called primitive insects such as silverfish, bristletails, springtails, campodids, and japygids. Metabola (change or metamor- phosis) : 1. Paurometabola (small or slight change) — having gradual or direct metamorphosis, in which anatomically the various stages look much alike, but there are usually marked changes in color and size and often in the acqui- sition of wings. Representatives are earwigs, grasshoppers, katydids, crick- ets, phasmids, mantids, cockroaches, termites, booklice, barklice, embiids, thrips, sucking animal lice, biting bird lice, and bugs and their relatives (ful- gorids, delphacids, cicadids, treehop- pers, leafhoppers, spittlebugs, psyllids, aleyrodids or whiteflies, plant-lice or aphids, phylloxeras, and chermes) . 2. Hemimetabola (incomplete changes) — in which the aquatic young or nymphs differ from the adults by having accessory organs for aquatic res- piration while the free-living adults are winged ( mayflies, dragonflies, dam- selflies, stoneflies, and salmonflies) . 3. Holometabola (complete change) — metamorphosis in higher in- sects, which pass through complete and complex changes having larvae, pupae, and adult stages ( dobsonflies, dusty- wings, lacewings, ant-lions, owlflies, mantispids, raphidiids, scorpionflies, caddisflies, moths, butterflies, beetles, How Insects Live weevils, ants, bees, wasps, gall wasps, horntails, parasitic hymenopterons, flies, and fleas) . Just how these simple and compli- cated methods of development affect the physiology and instincts of insects is not fully known, but, according to William Morton Wheeler, the insects, like ants, that have the most compli- cated metamorphoses and most highly developed reflexes manifest "in addi- tion to these reflexes . . . more com- plicated trains of behavior, the so- called instincts; and both these and the reflexes may be affected with a certain modifiability or plasticity which, in its highest manifestations, has been called intelligence." It is interesting to ask and difficult to answer a question often brought up: "Do insects display intelligence?" Al- though entomologists and biologists disagree on the point, it is important to remember that such a phenomenon as instinct, bordering on intelligence, ex- ists among insects and must exert a considerable influence upon their at- tainment of their present conspicuous status in the life of the world. T. D. A. Cockerell, an authority on bees, discussing bumble bees, re- marked: "Although we have empha- sized the dominance of instinct in the affairs of these insects, it must be ad- mitted that ordinary memory and what we must call intelligence have a part." The late William Morton Wheeler, an authority on ants, said this : "Many attempts have been made to define instinct, but it is evident that none of these could be completely suc- cessful, because instinct transcends in- telligence and has its mainspring in the depths of the life process itself. Perhaps as good a formal definition as I am able to give is the following: An in- stinct is a more or less complicated activity manifested by an organism which is acting, first, as a whole rather than as a part; second, as the represent- ative of a species rather than as an individual; third, without previous ex- perience; and fourth, with an end or purpose of which it has no knowledge. 25 This definition will satisfy the person of scholastic mind, but to the biologist it is a mass of obscurities; for it is cer- tain that the man lives not who can tell where the whole begins and the part leaves off in a living organism, or can frame a satisfactory definition of a living individual or a species; and the intellect abdicates when it is called upon to grasp an activity that is un- consciously purposeful." Then, too, there are so many in- teresting facts concerning the devel- opmental patterns of growth of both lower and higher categories of insects that it is almost impossible to grasp more than a few fragments as to what it all means. The course of the life history of a simple insect, such as a maple aphid of the genus Periphyllus, which gives rise to at least 1 7 different kinds of progeny from egg to true sex- ual males and females, is only one example. The phenomenon is further complicated by the fact that only the eggs survive the winters and every year the full complement of different types of individuals regularly appear and disappear in cyclic order. In contrast to this somewhat lowly member of the insect world, there are extremely highly developed life cycles and interrelations among the members of different castes with their guests, ectoparasites and endoparasites, and with plants, with other food-producing insects, like aphids and coccids, and with all the environmental factors sur- rounding each species. Other activities of ants, such as slave making, and the toleration and uses of permanent social parasites, all indicate the ability of these insects to hold their own and to thrive along with literally a million other species of insects in what appears to be a world already overrun with their own kind. Aside from their remarkable ana- tomical and physiological characters, insects have developed special traits, devices, and means of protection, such as running, jumping, snapping, flying, swimming, death feigning, motionless attitudes, and protective mimicry to 26 their surroundings and of many natural objects. They also protect themselves by spinning silken webs and cocoons; by constructing earthen and other types of coverings, cells, or nests; by excret- ing wax or protective and often offen- sive fluids and gases; by a covering of stinging hairs ; or by using stings for de- fense or for procuring food for their young. Eggs are protected by inserting them into plant tissues, such as wood, fruit, or seeds ; by placing them in cells or cases in the soil; or in the nests of aggressive and protecting ants. Larvae protect themselves by working as leaf miners, gall makers, or as parasites in or on other insects or higher animals. Many insects are luminous. Wonder- ful indeed are the fireflies, which are beetles belonging to the family Lampy- ridae; the fire beetles belonging to the elaterid genus, Pyrophorus; beetles be- longing to the families Lycidae, Phen- godidae, Drilidae, Cantharidae, Ca- rabidae (Physodera) ; the springtail genera Anurida, Achorutes, and Ony- chiurus; and certain midges and gnats, including the remarkable cave-inhabit- ing New Zealand glowworm. The larvae of the North Carolina fungus gnat, Platyura jultoni, glow at either end, and the adults of the European midge, Chironomus plumosus, are lu- minous. Luminescence appears to be associated with insects living in damp or moist locations or during the rainy seasons and may also be associated with mating reactions. The New Zealand glowworm is unique. R. B. Goldschmidt wrote: "[Its] very successful adaptation cannot function without all of the following acquisitions : ( i ) The tendency of the larva to live in dark, moist places. (2) The development of a continuously shining organ of luminescence out of theMalpighian tubules. (3) The ability to build a tent. (4) The ability to build the trapping threads and to insert on them the special sticky droplets. (5) The carnivorous habit with all its phys- iological adjustments. (6) The ability to choose the proper habitat where the prey breeds in large quantities. (7) The Yearbook of Agriculture 1952 instincts needed for the feeding process. (8) The adaptation of the entire cycle to the ecological features." Such a com- plicated and specialized life with its adaptations must have required a very long time for development. In order to survive the heat of sum- mer and cold of winter and unusual periods of drought, insects can assume an inactive condition for short or long periods. The periods usually conform to ordinary seasonal conditions, but some extremes occur during the reg- ular growing periods of spring and summer. Some insects, like aphids, may have as many as 10 or more generations a year; cicadas may require 2 or 3 to 17 years to develop a single generation. Aphids usually live exposed on the plant and reproduce in almost unbe- lievable numbers. Conversely, their rather close relatives, the cicadas, re- quire much longer periods of develop- ment, and this may be made possible because the larval period is spent in the soil where much greater protection is afforded from natural enemies and from unfavorable weather. The term "aestivation" refers par- ticularly to quiescence and cessation of growth and development during the summer. Usually during this period the insect may not be fully fed. Examples of aestivating insects occur in many of the higher orders. One of the most re- markable cases is the aphid belonging to the genus Periphyllus. The insect overwinters as an egg, which hatches in the spring. After two generations of apterous parthenogenetic females and a third generation of similar apterae and alates, tiny wingless individuals, called dimorphs, also appear. The di- morphs continue to appear along with three or more succeeding generations of normal apterous and winged individ- uals. The dimorphs are flat and disc- like and so small as to be barely visible to the unaided eye. They settle on the surfaces of the leaves and are so closely appressed and so near the color of their immediate surroundings that they often are overlooked even by entomol- How Insects Live ogists studying the other forms of the species. From the end of May to nearly the middle of August, there is no no- ticeable growth in size or change in their appearance. By then, they are the only surviving form of the aphid. Then, with the approach of fall, the most suitable time for going into aestivation, these minute dimorphs begin to grow and finally assume normal size and form. When fully mature they give birth to apterous and alate partheno- genetic forms. These in turn give rise to the sexuales, which disappear after laying the overwintering eggs. The im- portant thing in this life history is the fact that the minute, delicate, aestivat- ing dimorph is the only survivor of the species during the midsummer period. Many weevils, including the vege- table weevil, introduced into the west coast of North America from South America, aestivate in California. The Colorado potato beetle aestivates dur- ing the dry season in the Tropics and hibernates in central and northern United States and in Europe. Some insects can live in a quiescent condition for long periods in various stages of their development. I have re- ferred to the 1 7-year larval and pupal periods of the cicada. A South Ameri- can root-infesting coccid, Margarodcs vitium, is reported to have lived 17 years without food in the dry, unnat- ural condition of an insect collec- tion. Adults of the wood-infesting Buprestis aurulenta have remained in structural timbers for 10 to 26 years before finally emerging. Aestivation is particularly adapted to insects living in arid and desert areas where the summers are dry and hot. During the more unfavorable periods many insects hide themselves in the soil at suitable depths and under all avail- able protective objects. In these areas many other animals feed upon them, but the remaining insects survive de- spite all obstacles. Hibernation means passing the win- ter in an inactive and quiescent condi- tion. In this phase the insect, in what- ever stage of development, may or may 27 not be fully fed, but nevertheless is in condition to withstand the rigors of the cold season in the temperate and arctic regions. Hibernation is almost uni- versal among insects in those areas. The winter may be spent as an egg, in which the tiny first-stage larva may often be fully developed by late sum- mer or fall and awaiting the coming of spring to emerge. It may be spent as a larva, as an active nymph, as a pupa, or as an adult. Because insects can withstand very low temperatures, they normally suffer no serious losses during the winter within their normal range. Hibernation by rather fragile forms like mosquitoes and butterflies in sheltered, relatively dry places out of doors there- fore is quite common and successful. Somewhat more difficult to compre- hend is the successful hibernation of butterflies in low shrubbery, where they may be completely enveloped in snow and ice for 3 to 4 months. Methods of hibernation are varied. Some insects prepare for hibernation by constructing such shelters as silken nests or cocoons. Others seek hiding places above or below ground which may be suitable for the condition of the particular insect. Insects are highly efficient in using every available source of food. They consume every kind of plant product, including the entire living and dead plant from root to top. Plants that may be poisonous to other animals or to some insects are acceptable to other kinds of insects. Insects feed on all animals, including insects themselves. Only the stripped bones devoid of all digestible matter resist their hunger. How, therefore, can plant and ani- mal life exist on the earth and escape destruction by all the multitudinous insect destroyers? The answer lies with the insects themselves. Among all the varied species, they maintain a reason- able balance, which permits normal and sometimes even excessive popu- lations to survive, but at a low enough level so that plants continue to propa- gate in what we may consider to be a normal manner. Of course this so- 28 called balance may have been deter- mined between insects and other re- lated natural factors ages ago. Think of what would happen if all insects were completely wiped out! The inter- relations between the plant-feeding in- sects and the predacious and parasitic forms are exceedingly intricate and un- relenting. Parasitism by other insects invades every species of insect — the plant and animal feeders and even the parasitic ones themselves. All forms of the host from egg to adult are subject to de- struction by predators and parasites. The degree of parasitism or hyperpara- sitism may be fourfold or more. The development of many young from a single egg or embryo also occurs among the parasitic forms: Not only is there competition among several species of parasites within a single host, there- fore, but there is a struggle for survival among many individuals from a single egg of the same species within the body of a single host. Some of the relationships be- tween insects and plants have become so complicated that in most instances neither insect nor plant could long exist apart. Similar relationships exist between insects and other animals and between certain insects and others of their own kind, yet their basic existence depends on the vegetable kingdom. Although millions of insects derive their subsist- ence from plants, they seem not to in- terfere greatly with the natural de- velopment of the plant world. It is true that in special situations insects might even exterminate a species of plant in a given location, and we know too well that they are responsible for tremen- dous losses to crops almost everywhere. However, it appears that plants have actually occupied as much of the earth's surface as is possible despite their insect dependents. The ability to create a specialized type and supply of food is not uncom- mon among many groups of insects. Such modifications may affect other Yearbook of Agriculture 1952 animal and especially insect hosts, as in the case of parasites, but are more often conspicuous in plant hosts. Chief among these latter are galls, produced by the dipterous gall midges or gall flies belonging to the family Itonididae, and the gall wasps of the hymenopterous family Cynipidae. Members of other orders, including certain species of thrips, lace bugs, psyllids, aphids, coc- cids, beetles, weevils, sawflies, trypetid flies, and possibly still others also pro- duce plant galls in which the larvae develop. The galls formed by these var- ious insects may also be inhabited by many other kinds of insects that feed upon the gall itself and by predators and parasites that prey upon all the various insects associated with the galls. Complicated biological relation- ships thus are associated with insect galls. The development of galls of dif- ferent insects may vary somewhat, but generally it appears to be caused by ex- cretions of the developing larvae or nymphs and to follow a more or less definite pattern for a given species or group of closely related species. Thus, the shape, vestiture, sculpturing, and color may be characteristic of a species or variety and may thus aid in recog- nizing them. The galls of a Chinese aphid, Mela- phis chinensis, are artificially reared on Rhus semialata in China in commercial quantities as a source of dye and tannin and for medical purposes. The host plants are carefully cultivated so as to enable the aphids to produce a maxi- mum crop of galls. Quantities of the galls have been shipped to the United States and other countries. Host specificity is illustrated among the cynipid gall flies. In general, about 90 percent of the galls are produced on species of oaks, 5 percent on species of roses, and 5 percent on different genera of the composites. Among the aphids and coccids, a species is usually associated with a distinct genus or even species of plant. Certain insects cultivate specialized types of plants, especially fungi. This type of propagation attains its highest How Insects Live development among subterranean and mound-building termites, which also have highly developed caste systems. Their termitaria may be entirely under- ground or they may extend to the sur- face or rise many feet above it, in which case they are formed of thick, earthen walls hardened by the salivary excre- tions used in their construction. The termitaria may vary from a foot in height and diameter for some species to great mounds, pillars, or chimneys 15 to 30 feet high and almost as great in diameter. The fungus gardens are distributed throughout the central por- tions of the mound in a somewhat irregular manner. Termitaria of these types are constructed by rather small tropical or subtropical termites that reach their highest development in tropical Africa, South America, south India, and Australia. The termites are nocturnal foragers and prey on various types of vegetation, which is com- minuted and mixed with excreta. This forms the food for rearing the fungi upon which the termites subsist. Leaf-cutting ants of the tribe Attii also cultivate fungi in much the same way as termites but feed only on the fungus hyphae. The so-called ant gar- dens of the Amazonian ants in the genera Azteca and Camponotus are prepared and planted and the crop utilized for food. Harvester ants play an important part in the accidental distribution of seeds by collecting, carrying, and stor- ing them for food in their nests. As INSECTS HAVE BEEN UPON this earth millions of years longer than human beings it is to be expected that they have acquired specializations and adaptations not wholly understood by us. The degree of development among insects is extremely variable and diffi- cult to measure by human standards. There has been much speculation con- cerning the faculties of insects. It is well known that many species of ants, bees, and wasps, especially those that have communal tendencies, show a high degree of differentiation and effi- 970134°— 52 4 29 ciency in organization and labor. They may be likened to living machines mo- tivated by some unexplainable power defined as instinct, if not powers of reasoning and intelligence. Yet insects display many remarkable traits not wholly understood by man. The high degree of organization and caste systems of social insects (termites, ants, bees, wasps) have been investi- gated by entomologists, but there is still a great deal to be learned about them. Much has been written about the highly specialized attainments of ants, in particular, concerning their ability to build nests; their social organiza- tions, caste systems, and slave making; their means of communication; meth- ods of collecting, storing, and growing food ; their art of defense and selection and procedure in producing queens; and their maintenance and tolerance of an assemblage of nurses, guests, satellites, commensals, paupers, scav- engers, kidnappers, murderers, and as- sassins somewhat after the pattern of our human society. E. O. Essig is professor of entomol- ogy, entomologist in the experiment station, and former chairman of the di- vision of entomology and parasitology in the College of Agriculture, Univer- sity of California, Berkeley. He has been a member of that institution since igi4- Following graduation from Po- mona College in igog, he has devoted most of his life to agriculture and espe- cially to entomology, on which subject he has written more than 1,000 papers and four books. His specialty in that field is the taxonomic and economic study of aphids, which insects have re- cently come into prominence in agri- culture because they are among the worst insect vectors of plant virus diseases. Readers can find more information on how insects live in W. V. Balduf's The Bi- onomics of Entomophagous Coleoptera, published by John S. Swift Company in 1935; R- ]■ Tillyard's The Insects of Aus- tralia and New Zealand, Angus and Rob- ertson (Sydney, Australia) , 1926. Life Processes of Insects Frank H. Babers, John J. Pratt, Jr. A study of insect physiology can tell us a great deal about the phenomenon of life and help entomologists in their fight against insect pests. Of especial interest is nutrition, for the digestive systems of insects are as diverse as the insects themselves and the kinds of food they eat. When all the evidence on that complex subject is in we will be that much closer to the solution to some mysteries that still confront the biologist, physiologist, and biochemist. Some species eat almost anything, but some have a restricted diet. Some have been given more than one com- mon name because they customarily feed on more than one plant: Boll- worm, tomato fruitworm, and corn earworm, for instance, are one insect. The influence of diet on the growth is illustrated by the honey bee. Larvae that are to become queens are fed on a diet of royal jelly. Other larvae des- tined to become workers are fed on royal jelly for only 2 days and for the rest of their larval life receive honey and pollen. Several factors or vitamins are necessary for normal growth. The fat- soluble factors so important in mam- malian physiology, except for choles- terol, apparently are not required by a number of insects. The water-soluble factors do play an important role. Most species need the B vitamins. Vitamin C does not seem to be required, but at least one insect, the cockroach, synthe- sizes vitamin C. Symbionts are bacteria that are transmitted hereditarily from parent insect to offspring. Apparently these bacteria are essential in the nu- trition of many insects. Sometimes the insects provide specialized structures, called mycetocytes, for the bacteria to live in. A vinegar fly, Drosophila melano- gaster, has been reared under sterile conditions on a definite chemical me- dium. It is the first multicelled organ- ism to be raised in the absence of micro-organisms on a diet whose chem- ical composition was exactly known. Certain insects, such as mealworms, require little water because they can derive metabolic water from carbo- hydrates. The ability to utilize sugars varies considerably. Mannose is used by blow flies and vinegar flies but not by bees. Only aphids are able to use arabinose. Vinegar flies can survive for long pe- riods on a diet of pure sucrose, raffi- nose, or melezitose. To grow, insects must have proteins or their equivalent. Some mature in- sects can survive a long time on a pro- tein-free diet, but they either undergo no further development in their adult stages or utilize food materials already stored in the body. Certain amino acids, the building blocks of proteins, seem to be essential for proper growth and development; the German cock- roach requires at least five, valine, tryptophane, histidine, arginine, and cystine. Ectoparasites such as lice seem to develop better on vitamin-deficient rabbits than on well-fed ones. When human volunteers were fed for several months on a diet deficient in certain vitamins and then infested with lice, however, the lice developed just as well as they did on humans that had a com- plete diet. On the other hand, it seems true that various insects often develop better and in greater numbers on plants with nutritional deficiencies than on well-fed plants. Powder-post beetles cannot digest cellulose. If allowed to choose among pieces of oak sapwood of different starch content, the female almost always chooses the wood with the highest starch content in which to lay her eggs. Metabolism is the sum of all the chemical and physical processes by 30 Life Processes of Insects which living organized substance is produced and maintained. The subject obviously is complex and in only a few instances has the metabolism of an in- gested food been followed completely. What happens to the blood pigment hemoglobin after it is ingested by bloodsucking arthropods has been in- vestigated. In most of the insects stud- ied the bulk of hemoglobin seems to be broken down in the gut to hematin, which is then excreted unchanged. In mosquitoes and fleas, no pigment seems to be absorbed. In all the other forms, pigment in varying amounts is ab- sorbed and circulates in the hemo- lymph. In the louse the absorbed pig- ment is further broken down to the bile pigment, biliverdin, and in other species bilirubin is also found. During metamorphosis, the period during which the insect changes from an immature stage to an adult, the de- hydrogenase enzyme activity in the blow fly falls rapidly at first, reaches a minimum at about the halfway point of the pupal period, then rises rapidly and continuously until metamorphosis is completed. The acidity of the pupal fluid follows a somewhat different course, becoming strongly acid soon after the beginning of metamorphosis, and reaches a maximum at about the same time the dehydrogenase activity is lowest. The acidity then decreases until the time for emergence, when the fluids are almost neutral. In the Japa- nese beetle, the changes in fat and gly- cogen content during metamorphosis may indicate that the insect synthesized glycogen from fat. The metabolism of iodine by vinegar flies was studied by the use of radio- active iodine (I121) . When it was fed to larvae, the iodine was concentrated mostly in the protein of the skeletal parts of the larvae. If the pupae formed from larvae fed radioactive iodine were removed from the food before emer- gence, the adult insects did not contain radioactive material. The amount of oxygen consumed by tissues during metabolism is an indica- tion of the activity of the metabolic 31 processes. The oxygen consumption of cockroach muscle is about the same as that of pigeon-breast muscle, which heretofore has been considered the most active tissue known. Besides the usual waste products of metabolism, many insects excrete ma- terials like wax and silk, which they use for various purposes. Other sub- stances, such as the fetid material ex- creted by stink bugs, are used for pro- tection. Still others, such as the venom of the wasp, are used in obtaining food. Radioactive amino acids .have been injected into the giant silkworm and apparently radioactive silk was ob- tained. The studies will help explain the chemical structure of silk. The naturalist Athanasius Kircher in 1643 recommended music as an antidote for tarantula bites. Different treatments are used today for insect bites, but often they are no more effec- tive than Kircher's. We know little about the nature of insect venom. In some ants it is formic acid; in others, toxic protein. Bee venom is made up of several toxic constituents, the chief of which is apitoxin. When it is injected by the sting of the bee, enzymes in the toxin cause a breakdown of cell proto- plasm and the liberation of histamine. It is this chemical that is responsible for many of the symptoms of bee sting. Since early times bee venom often has been recommended for the treatment of arthritis, neuritis, and rheumatism. Another mystery is the nature of the salivary gland secretion of various mos- quitoes and flies. A toxic arrow poison used by the Bushmen of the Kalahari Desert in South Africa is obtained from the larva of the beetle Diamphidia locusta. Insects do not have blood vessels. The circulating fluid flows freely throughout the body cavity except while it is being moved by the dorsal vessel or heart. It corresponds to both blood and lymph and is called hemo- lymph. In some insects it is clear and colorless. In others it is yellow or green. The volume varies greatly between spe- cies and individuals of one species. The 32 hemolymph does not contain respira- tory pigments such as hemoglobin or hemocyanin. Many analyses of hemo- lymph have been made, but the func- tion of only a few of the many compo- nents has been determined. Insect hemolymph contains more free amino acids than does human blood, which averages about 6 milli- grams per hundred milliliters. Insect blood may contain as high as 385 mil- ligrams per hundred milliliters. At least 24 compounds with the chemical properties of amino acids that occur free in the hemolymph of insects have been identified by the use of paper chromatographic methods. Several of them have not been identified, yet as constituents of proteins. In most insects the hemolymph con- tains a much higher percentage of po- tassium than does mammalian blood. Among phytophagous, or plant-feed- ing insects, the sodium-potassium ratio is less than 1 ; among carnivorous in- sects, the ratio is greater than 1 . Some species of insects apparently have some sort of sodium-potassium regulatory system, because the ratio in the body fluid is not dependent on the ratio in rfood. In the silkworm larva, the sodium concentration in the body fluid seems to be in simple diffusion equilibrium with ingested sodium. Silkworm pupae and adults contain almost no sodium. It- therefore must be selectively ex- creted. Insect hemolymph contains a num- ber of cells, or hemocytes. Their most obvious activity corresponds to that of the leucocytes, or white blood corpus- cles, of the vertebrates in that they ingest any small particles of solid mat- ter set free in the blood. Ten classes and 32 types of cells have been found in the blood of the southern army- worm, and 8 classes and 23 types of cells in the blood of the mealworm. When removed from the insect, the hemolymph of some species clots rap- idly and in others more slowly or not at all. The process of coagulation is not comparable to that of mammalian blood and varies between insect species. Yearbook of Agriculture 1952 Hemolymph from Japanese beetle grubs coagulates by a gelation of the plasma, while that from the wax moth coagulates by agglutination of the cells. The coagulation of the hemolymph from these two species may be greatly retarded by exposing the larvae to sub- lethal intensities of ultrasonic waves. None of the chemicals normally used to prevent the clotting of mammalian blood has a similar effect on insect blood. Twenty or more species of insects have developed resistance to insecti- cides following exposure to insecticides under natural conditions. Resistant strains have been developed in the lab- oratory by exposing many insects to concentrations of insecticide that killed 90 percent of them. Eggs from the sur- vivors were used to maintain a colony. The process was repeated with each generation. In a short time the off- spring showed considerable tolerance for the insecticide used in the selective process and also, usually, for many chemically unrelated compounds. The control of the wild resistant in- sects has become a serious problem — DDT, after a few years of use, often has failed to control- house flies and mosquitoes. Apparently no external differences exist between susceptible and insecticide-resistant flies. Scien- tists have tried to find out whether there are physiological differences. They have yet found no significant dif- ference in vigor between susceptible and resistant strains : Resistance is not due simply to the failure of the insecti- cide to penetrate the cuticle of the insect, because the insects are also re- sistant when the insecticide is injected directly into the body cavity. The enzyme cholinesterase rapidly destroys actylcholine, a chemical of im- portant function in the transmission of nerve impulses across the nerve-cell junctions of several animal species. In insects the role of cholinesterase has not- been determined, but because of the high concentration of the enzyme in insect nerve tissue, it is of interest in Life Processes of Insects the physiology of resistance to insecti- cides. The cholinesterase activity of the heads of resistant flies is less than that of normal flies. DDT applied exter- nally to house flies apparently is first absorbed and then metabolized. One DDT-resistant strain was more able to metabolize DDT than were normal flies. By the use of large amounts of piperonyl cyclonene, the conversion of DDT to the metabolic product by the resistant flies was largely prevented. Another strain of DDT-resistant flies also rapidly metabolized DDT that had been absorbed. The main product of metabolism was DDE ( 1,1-dichloro- 2,2-bis-(jfr-chlorophenyl) ethylene). A small amount of DDA (bis-(/;-chloro- phenyl) acetic acid) was also identi- fied. Only small amounts of the metab- olites were excreted and large amounts were retained in the body. In contrast to that strain, the normal, or DDT- susceptible, flies could metabolize only a negligible amount of DDT in 24 hours; neither DDE nor DDA was a product of the metabolism. Cytochrome oxidase, an enzyme found in cells, is of great importance in the metabolic processes. The activity of the enzyme was much greater in one strain of resistant flies than in a normal strain. We do not know whether the physio- logical differences between resistant and susceptible strains are due to the resistance. They may be variations in strain, due to differences in rearing procedures or some other factor. The rate of loss of tolerance to in- secticides following cessation of ex- posure also seems to vary tremendously between strains. Some strains, whose resistance was developed either in the field or laboratory, revert rapidly to nonresistant insects. Others retain re- sistance over many generations once they have acquired it. Resistance seems to be inherited, but the method of genetic transmission is still in doubt. Colors of insects are as varied as those of the rainbow and are frequently due to complex mixtures of pigments. 33 Most of the insect pigments were once thought to be simple end products of metabolism without physiological func- tion. We now know that this is not always correct. Before the physiological function of the pigments was estab- lished, many were of commercial im- portance. Carminic acid from the cochineal insect was used as a wool dye from early times until azo dyes were discovered. The term melanin is loosely applied to denote what is probably a group of pigments with varying composition. The pigments appear to be derived from tyrosine, an amino acid, by a series of enzymatic reactions. The dark- ening of insect blood on exposure to air is also generally due to melanin forma- tion; the blood does not darken in the insect because of the inhibition of the enzymes by some unknown factor, pos- sibly by a low oxidation-reduction po- tential. The biochemistry of pigment formation has thrown considerable light on the action of genes, the units in the chromosomes that carry the hereditary characters. Evidence has been presented that the process of me- lanin formation in mammalian tissues follows a similar pattern to that of in- sects. The presence of hemoglobin in in- sects is an interesting point because it does not ordinarily function in its tra- ditional role of oxygen carrier. In midge larvae (Tendipedidae) , such a function seems doubtful, even when the oxygen tension is reduced greatly. The active group of the hemoglobin found in bot fly (Gasterophilus) larvae is the same as that in the hemoglobin of horse blood, but the protein part of the complex is different. Its molecular weight is about 34,000, compared to about 67,000 for human hemoglobin. Its affinity for carbon monoxide is much less than horse hemoglobin, but it has a high affinity for oxygen. Its functional significance, however, is obscure. The metabolism of chlorophyll, the green pigment in plants, has been studied in the silkworm, potato beetle, 34 sq.uash bug,, and a few other species. Many of the pigments in the squash bug are the results of the breakdown of chlorophyll-, the site of breakdown apparently being the ventriculus, or functional stomach. Insects do not always produce their own pigments. Sometimes the colors are the result of hereditary symbiotic bacteria found in insect tissues. The wing pigments of the Pieridae, a common family of butterflies, were first studied by F. G. Hopkins in 1889. He concluded that they were waste meta- bolic products whose only physiolog- ical function is ornamental. The pig- ments mostly are now classified chem- ically as pterines. We have evidence that they are not just ornamental. The pterine ring may be considered a de- rivative of riboflavin, which is usually present in the Malpighian tubes of in- sects, for example. Thus there is a pos- sible connection with the metabolism of vitamin B2. The pterine ring is also found in the folic acid molecule, which has been shown to be an essential me- tabolite for the larva of the yellow- fever mosquito and the mealworm. The walkingstick Dixippus is dark- colored at night and light-colored by day. Its color may be changed by illu- minating the insect at night or placing it in a dark place by day. The color of other species, such as the cabbage- worm, Pieris brassicae, is influenced by the illumination of the larva as it rests before pupation. Hormones are chemical substances produced in an organ and liberated into the blood stream. Other organs then become excited by this hormone and functional activity results. Much light has been shed on many phases of invertebrate endocrinology, which sci- ence includes the hormones, since Ste- fan Kopec of Jagellonian University, Poland, in 1922, demonstrated the effect of a hormone on insect develop- ment. In insects, periods of great cell activity are followed by a process called molting during which time the insect sheds its old skin and is fitted with a new and larger one. Kopec showed Yearbook of Agriculture 1952 that the molting process is controlled by hormones that are species-non- specific. Blood taken from an insect at the proper time and injected into an insect of another species will cause molting, although the injected insect is not normally ready for the process. The hormones, which have so many varied functions, are secreted- by at least 1 1 organs in insects. Physiological functions which are hormone-regu- lated are often influenced by tempera- ture and humidity or other environ- mental factors. It has not been determined whether sex hormones comparable to those found in the vertebrates are present in insects. There are present in insects physiologically active substances that participate in the development of he- reditary characters. Because of their similarity to hormones, they are called gene hormones. Developmental hormones in the im- mature stages are not always found in similar organs in all species, but they are secreted by at least three glands, all located in the head of the insect: The corpus allatum ; the ring gland, or Weismann's ring; and some glandlike cells of the brain. In adult insects, one or more hormones secreted by the corpus allatum are important in repro- duction. Color changes are also appar- ently due to the action of hormones. Some hormones cause metamorpho- sis. By the injection of material from larvae ready to pupate, very young larvae have been made to undergo metamorphosis. But if a large larva is divided into two sections by ligaturing about 1 2 hours before time for normal pupation, only the forward half will pupate, although the rear half will re- main alive for many days. In the Cecropia silkworm, as in many other insects, metamorphosis is interrupted by a long diapause period that begins soon after the formation of the pupa. Diapause is characterized by cessation of cellular growth. It appar- ently is caused by the destruction of enzymes involved in the cytochrome system. Following the liberation of a Life Processes of Insects growth hormone by cells in the pupal brain and the stimulation by this hor- mone of the prothoracic gland, the cytochrome system again begins to function and cell growth occurs. The outer covering of insects, the integument, is both skeleton and skin. As in all arthropods, the integument consists of an epidermis, one cell layer in thickness, and a hard cuticular mem- brane. During their intermittent pe- riods of activity, the epidermal cells secrete the circular membrane, com- monly called the cuticle, over the sur- face of the animal. Entomologists used to believe that the characteristic com- ponent of cuticle was chitin, a hard, insoluble compound found in varying proportions in most insect cuticles. It was thought that the chitin formed a framework in the interstices in which other components of the cuticle were deposited. Scientists now believe that proteins rather than chitin are the fundamental constituents of cuticle. It might be (as suggested by the interpre- tation of X-ray diffraction data) that the cuticle consists of alternating layers of protein and chitin. The cuticle may be rigid, flexible, or elastic. It also is waterproof; it has to be, because the integument keeps a proper water balance in the insect. The epicuticle, the thin outer membrane that is the most important in water- proofing the cuticle, is a complex struc- ture of several layers. The first to be deposited is the innermost or cuticulin layer, believed to consist of a lipopro- tein, which perhaps is denatured, con- densed, and finally tanned along with other proteins present in the outer lay- ers. Then a thick viscous fluid is dis- charged, and on top of that a wax layer. The wax layer is then topped with a hard cement layer, which is thought to consist of tanned proteins combined with lipids. The cement layer is secreted by some of the dermal glands whose openings are scattered over the surface of the integument. Throughout the cuticle, running ver- tically from the cells, are the pore canals, of unknown function. In the 35 cuticle of flesh fly larvae (Sarcophagi- dae), 15,000 of these were found per square millimeter. The cuticle cannot grow and, in the rigid parts of the insect, cannot be stretched. As the insect grows, there- fore, the cuticle is shed and is replaced by a larger cuticle. That process, molt- ing, follows a period of great cell activ- ity. When first laid down, the new cuti- cle is soft and often colorless, but it rapidly hardens and assumes its normal color. The blow fly has been a favorite ex- perimental insect because its larval cuticle is not shed before pupation, but rather is converted into the hard pupa- rium. In the flesh fly {Sarcophaga barbata) the formation of the hard puparium from the soft larval cuticle is by the following process: Phenols are oxidized enzymatically by polyphenol oxidase to orthoquinone. The ortho- quinone combines with the protein present and hardens it by a tanning process, during which the integument, which may have been colorless for a short period after the molt, becomes colored brown or black. The colors due to tanning, however, are not the basis for the brilliant iridescent or metallic colors of some insects. Such colors are due usually to the interference in the reflection of light from the multiple thin plates or scales that some insects have. The shell, or chorion, of an insect's egg is like cuticle in many ways, but is even more complex. The shell of the egg of the assassin bug, Rhodnius pro- lixus, consists of seven layers, none of them waterproof. A cement layer is added to the outside of the egg when it is laid. Waterproofing is effected by a thin wax layer on the inside of the chorion, similar to the one that water- proofs the cuticle of most adult forms. The wax is secreted by the maturing egg and is attached securely to the innermost layer of the chorion. The other layers are modifications of vari- ous proteinlike materials somewhat like those in the cuticle. The development of high-speed 36 cameras, with which many exposures per second are possible, and cathode- ray oscilloscopes, by which small changes in electrical potential can be accurately recorded, has aided the study of the physiology of insect flight. When certain insects are held so that their feet are in contact with a movable platform, the insect will rest quietly. If the platform is removed, the insect moves its wings as in flight and many experiments can be done while the insect is actually suspended in air under simulated flight conditions. If small electrodes are inserted among the flight muscles, potential changes can be measured and correlated with wing movement. Some butterflies move their wings at a leisurely 5 beats a sec- ond, but certain midges attain about 1,000 wing beats a second. The vinegar fly is capable of flights lasting up to 2 hours. At the start, wing beats are about 300 per second but at the end, when fatigue becomes evident, they are about 100 a second. Among the insects with slow frequencies of wing beats, the wing movements are com- pletely synchronous with nerve im- pulses, but when the frequency of wing beat increases there is no synchrony. Insects, like the vertebrates, have highly developed, specialized sensory receptors that can be stimulated by chemicals. The chemical senses of in- sects may be roughly classified as taste, smell, and the common chemical sense of vertebrates whereby response is made to such irritants as ammonia and chlorine. The structure of the organs of taste and smell of insects differ greatly from that of the vertebrates, but a striking similarity exists in the physiological behavior toward many compounds and in the way in which stimulation is brought about. As among the vertebrates, however, the distinc- tion between taste and smell is based on unsatisfactory evidence. We cannot yet relegate either taste or smell in in- sects to specific areas of the body; areas of contact chemoreceptors have now been found on the mouth parts, tarsal leg segments, antennae, and oviposi- Yearbook of Agriculture 1952 tors of various species, although the actual organs are not always known. In seeking materials that will attract and repel insects, research workers have investigated the mechanical re- sponse of insects to thousands of com- pounds that vaporize at body tempera- tures. Many of the compounds are syn- thetic; many are natural materials of unknown composition. One such is a substance secreted by the female gypsy moth. It will attract male gypsy moths over long distances. The method by which the worker honey bees inform other bees of the location of a new food supply has been described by Karl von Frisch, of the University of Munich. It has long been known that worker bees returning to the hive often performed a kind of dance on the comb, but the reason for the dance was obscure. Von Frisch found that by the direction and dura- tion of their movements, during the dance, the worker bees transmitted to other workers the direction and dis- tance from the hive to the new-found food. He observed the antics in the darkness of the hive by the use of red light, to which the bees are insensitive. He found that he could predict the distance to about 100 yards. Direction was accurate to about 30. The system worked for any distance up to about 3.7 miles. For direction, the bees use the sun as an orienting point. They also are apparently sensitive to polar- ized light, which they can use to get their bearings, because they can fly accurately whether or not the sun is visible. Dr. von Frisch's discoveries, like others we have discussed, throw new light into the mysteries of nature. More such discoveries will come. They will give us a better understanding of insect physiology, of better controls of insects, and, indeed, of all life processes, in- cluding our own. Frank H. Babers, a biochemist, is in charge of a project that deals with the mode of action of insecticides and physiology of insects. He is a grad- uate of the University of Florida and Princeton University. He joined the Department of Agriculture in 1936. From 1946 to 1948 he was in charge of the chemical section of the Orlando, Fla., laboratory of the Bureau of Ento- mology and Plant (Quarantine. John J. Pratt, Jr., joined the re- search staff of the Bureau of Ento- mology and Plant Quarantine upon receiving his doctor's degree from Cor- nell University in 1948. His work concerns the study of the mode of action of insecticides, the development of resistance to insecticides by insects, and insect physiology. Suggested for further reference are Bee Venom Therapy; Bee Venom, Its Nature, and Its Effect on Arthritic and Rheumatoid Conditions, by Bodog F. Beck, D. Appleton- Century Co., New York, /Q35;Physiologie de l'lnsects, by Remy Chauvin, Institute Na- tional de la Recherchie Agronomique, Paris, 1949; Bees; Their Vision, Chemical Senses, and Language, by Karl von Frisch, Cornell University Press, Ithaca, N. Y., 1950; Bib- liography of Animal Venoms, by R. W. Har- mon and C. B. Pollard, University of Florida Press, Gainesville, 194.8; The Principles of Insect Physiology, by V . B. Wigglesworth, Methuen and Company, London, 1950; and the following articles in periodicals: V. G. Dethier and L. E. Chadwick: Chemoreception in Insects, Physiological Reviews, volume 28, pages 220-254. 1948. Hubert and Mabel Frings: The Loci of Contact Chemoreceptors in Insects — a Re- view With New Evidence, American Mid- land Naturalist, volume 41, pages 602-658. 1949- Aaron Bunsen Lerner and Thomas B. Fitz patrick: Biochemistry of Melanin For- mation, Physiological Reviews, volume 30, pages 91-125. 1950. William Trager: Insect Nutrition, Biolog- ical Reviews of the Cambridge Philosophical Society, volume 22, pages 148-iyy. 1947. V. B. Wigglesworth: The Fate of Haemo- globin in Rhodinius Prolixus (Hemiptera) and Other Blood-Sucking Arthropods, Pro- ceedings of the Royal Society (London) , series B, volume 131 , pages 313—339, 1943; and The Insect Cuticle, Biological Reviews of the Cambridge Philosophical Society, vol- ume 23, pages 408-451. 1948. Carroll M. Williams: Biochemical Mech- anisms in Insect Growth and Metamorpho- sis, Federation Proceedings, volume 10, pages 546-552, 1950. J. Franklin Yeager: The Blood Picture of the Southern Armyworm (Prodenia Eri- dania), Journal of Agricultural Research, volume yi, pages 1-40, 1945. How Insects Choose Their Food Plants Charles T. Brues All forms of animal life need organic materials in order to exist, grow, and reproduce. Some subsist on living, dead, or decaying plants. Others get the foods they require from living or dead animals. Many kinds, including some of the insects, live on a mixed diet of both plant and animal materials. Civilized man has almost endless variety in his diet: Bacteria, yeasts, fungi, roots, berries, fruits, and foliage of plants furnish vegetable food ; he eats the flesh of many invertebrate animals such as crustaceans and mollusks, although fish, birds, and mammals commonly furnish his main protein requirements. No insect selects food in such variety, but a few insects are omnivorous in the sense that they may consume many kinds of plant and animal materials. Most of the more specialized kinds restrict their diet to a limited range — particularly the forms that develop as parasites within the bodies of host animals, which almost invariably are other insects. Such parasitic ones, which are called entomophagous parasites, generally are very specific in the selection of their hosts. They usually lay their eggs on or directly within the body of the host insect and continue from generation to generation to confine their attacks to the same species of hosts. Predatory insects, which capture liv- ing prey just as do the carnivorous birds and mammals, confine their diet to animals smaller or less active than themselves. Quite frequently they also select particular kinds of prey: Some consistently capture aphids, some de- vour caterpillars, some feed on scale insects, and a few are addicted to a diet of snails. On the other hand, groups 37 3» like the praying mantids and ant-lions accept and relish a wide range of flesh. Because predators subsist largely on other insects, they depend mainly on the vegetarian kinds, which are the most abundant source of suitable prey. Parasitic and predatory insects re- duce the abundance of plant-eating in- sect life. Nevertheless, under the condi- tions that have prevailed in nature for millions of years, their influence has not kept the vast hordes of vegetarian in- sects from maintaining populations at a high level. Neither has it curtailed their evolutionary differentiation, be- cause they have developed innumer- able adaptations in structure and in habits to their environment. Some of the most striking features in this respect relate to the instinctive behavior that determines the selection of food plants. Farmers always have known that many species of insects feed only on a particular crop or series of crops. They appear season after season and evince an unvarying predilection for the plants that nourished their forebears. There is great variation in the number and variety of food plants they select, but there is a fixity of purpose in their behavior that is far beyond their die- tary requirements. In a search for the causes underlying such selection, we shall consider mainly those species — about half of the living species of insects — which feed on the flowering plants, particularly those of economic importance, since we have more accurate knowledge of them than we do of most of the insects that are associated with wild plants. Common in home and market gar- dens are cabbage, cauliflower, radish, kohlrabi, brussels sprouts, turnips, and collards. These members of the family Cruciferae have a pungent odor and taste because of the presence of chem- icals known as mustard oils, which the tissues of the plants secrete. The chem- icals attract a series of generally unre- lated insects to the plants, on which they may lay their eggs. Thus, the cabbage butterfly seeks out the cabbage patch in the garden to de- Yearbook of Agriculture 1952 posit its eggs. The caterpillars that hatch from the eggs eat and grow to maturity on the plants selected by the parent butterfly. If they are placed on other plants to which they are not ac- customed, they go on a hunger strike, doggedly refusing to eat, and finally perish miserably in the midst of plenty. Only if sap of the food plant or mus- tard oils are smeared on the strange foliage will they recover their appetites and resume feeding. There is a close correlation between the choice made by the butterfly and the fondness of its caterpillar offspring for the kind of food that has been chosen for them. In some insects, the adult and larval stages feed on the same plants, but the adult cabbage butterfly, like other but- terflies and moths, sucks the nectar of various flowers and the laying of its eggs on the larval food plant is not a response to its adult appetite. Any fail- ure of the butterfly to select plants ac- ceptable to its offspring would spell disaster, because the young caterpillars cannot go foraging in search of plants other than those upon which they find themselves. Similar peculiarities pre- vail among the great variety of diverse insects that restrict their feeding to specific plants. The sense of smell in adult insects is so much more acute than that of hu- mans that we cannot appreciate its ac- tion. In the developmental, growing stages of the higher insects, such as caterpillars and grubs, it is far less acute but equally discriminative, and it is commonly associated with the re- fusal of any food that lacks the specific stimulus to which their olfactory appa- ratus is attuned. It is as if a human would eat corn pone only, or cabbage, or onions, or cottage cheese, and never venture a baked potato, hot dog, or ice- cream cone to vary the monotony. Another example of the association of insects with specific kinds of host plants is the Colorado potato beetle, which spread northward from its na- tive home in Mexico, following its native food plant, a common weed of the potato family. Now widely distrib- How Insects Choose Their Food Plants 39 uted in the United States, it confines its feeding almost entirely to the foliage of the potato plant. Sometimes it ap- pears on tomato and eggplant, which are related members of the family Sola- naceae. Grow a few potatoes in the garden, and the potato beetles will find them sooner or later — sooner if your neighbors harbor them, and later if a long journey is required. The Mexican bean beetle feeds only on the foliage of various sorts of gar- den beans, cowpeas, soybeans, and re- lated legumes. In recent decades it has extended its range into the north- ern parts of the United States; wher- ever it goes, it always seeks out beans. It is hard for humans to appreciate this point, for we cannot perceive any- thing special about the odor of potato or bean foliage. The bean beetle is a black sheep of the large family of lady beetles, whose other representatives, eminently pred- atory, feed voraciously on plant-lice and scale insects, both as larvae and adults. This small group may have be- come vegetarians in the geological past, for it has had time since to spread around the world and to develop a number of species, each restricted to special plants, such as members of the cotton family in Africa, potatoes in the Orient, and legumes in Europe. Another American species, the squash beetle, feeds on the foliage of native gourd vines and on several other garden cucurbits. The squash beetle has never reverted to a meat diet; it may well be that its vegetarian habits rep- resent a sudden shift of instinct com- parable to the structural mutations that occur sporadically in nature or as the result of experimental techniques. Of the wild plants, consider the milk- weed, which has a milky sap, or latex. The familiar monarch butterfly always lays its eggs on milkweed, which is the only food that its caterpillars will ac- cept. Also on the leaves of milkweed are commonly seen rather large, black- spotted red beetles, which eat the foli- age as adults and bore in the roots as larval grubs. Like the monarch butter- fly, they are addicted to milkweed and would be starved out of existence with- out it. In the scheme of nature, they are fortunately provided for by the mother insect, whose unvarying in- stinct leads her to lay her eggs in the proper site. An insect that restricts its feeding to a single species of plant is the boll weevil. The larvae of this snout beetle burrow within the flower buds and immature bolls of cotton. It is native to the New World Tropics, whence cotton came into cultivation, and has extended its range into the cotton fields of our Southern States since the begin- ning of the present century. Thorough search has failed to find any other ac- ceptable food plants, which is a very striking point because another insect, a caterpillar known as the bollworm, similarly bores into the green bolls of cotton but also likes other succulent fruits and vegetables. It can be so abundant in the ripening ears of corn as to rank as a major pest of corn. Another pest of corn, the European corn borer, now widely naturalized in our country, is a still more general feeder. It bores into all above-ground parts of the plant. It does not stop at that, however, as it appears equally fond of many plants as diverse as dahlias, smartweed, and hemp. We can group the insects I have mentioned and others as well into three categories. The first group includes the feeders that exercise little choice, de- pending largely on availability, abun- dance, texture of foliage, succulence, and the like. Nearly all of them have preferred food plants, however. Thus, the gypsy moth caterpillar feeds on the leaves of a variety of deciduous forest trees, but it is most abundant on oak and on birch, avoids ash, and refuses chestnut, while the older caterpillars in a pinch will consume even tough pine needles after their jaws have become big and strong enough to cope with such material. The common cecropia moth seems to prefer willow leaves, but its diet includes a great variety of our common deciduous trees and shrubs. 4o Many grasshoppers range over a wide variety of low plants. Such insects are known as polyphagous because they ac- cept plants in considerable variety. The second group, the insects that restrict their feeding to a small and dis- crete number of usually similar plants, are termed oligophagous. No clear-cut line can be drawn to separate them from the polyphagous forms, but they obviously represent a distinct speciali- zation in food selection, especially when their food plants have some char- acteristics in common, which we can demonstrate through our own senses or by laboratory methods. Members of the most highly special- ized series, the third group, are referred to as monophagous; that is, they are restricted to a single species of food plant. They are comparatively few in number; indeed, some entomologists believe that none exists in the strictest sense. But to all intents and purposes the boll weevil, whose habits have been minutely studied, falls into this cate- gory, and several other insects appear to be just as precise in their tastes. All in all, the vegetarian insects form a vast series in which more or less indis- criminate choice of food becomes more and more restricted and sometime may reach a stage of absolute dependence on a single species of plant. Such a succession appears to be an evolutionary process, but by no means is it a single progression of changes, as the restriction of food plants appears time and time again in unrelated groups. Rather, it is the indication of an inherent tendency in insects (un- doubtedly engendered by their delicate sense of smell) that leads them to live in a world of odors. Up until a few decades ago plau- sibly enough it was customary to attrib- ute the unerring selection of food plants by oligophagous insects to a sixth botanical sense that enabled them to recognize the natural relation- ships of plants without recourse to treatises on systematic botany. Such a supposition clarifies the be- havior of some of the insects I have Yearbook of Agriculture 1952 mentioned, but it has flaws. An ex- ample is the cabbage butterfly, which commonly restricts its feeding to plants of the cabbage family although its caterpillars sometimes appear on other dissimilar and unrelated plants. One is the nasturtium. The explanation is that nasturtium leaves have the same pun- gent odor and taste due to an essential oil similar to that in cabbage. A Dutch entomologist, E. VerschafFelt, who studied the behavior of the butterflies and caterpillars with reference to mus- tard oils, concluded that the presence of those chemicals was the factor that determined their choice. Other ento- mologists, particularly Vincent G. De- thier, have extended such studies to a large number of other insects and their food plants, and have found that some specific chemical (or more than one in combination) commonly forms the tie that binds them to an invariably con- stant diet. It is evident now that odors, recog- nizable to the acute and discriminative chemical sense of the insects, are the main factors involved in the selection of food plants by oligophagous insects. But knowledge of the multitude of chemical substances elaborated by plants is still too fragmentary to permit any broad generalization. Some species of less fastidious tastes will accept con- siderable variety, but may evince a dis- like for some chemical to which they are not accustomed, even when it is combined with one that is highly at- tractive when not thus contaminated. That is not the whole story, however. We must approach some other con- siderations with caution, as their mean- ing is not yet clear. They relate more directly to instinctive behavior. Among the aphids, or plant-lice, there occurs quite commonly an alter- nation of generations, whereby the aphids migrate during the course of the seasonal cycle from one kind of plant to another and then return to the original host plant after the period of winter dormancy. These aphids fre- quently have a small series of accept- able plants, and one or both of the How Insects Choose Their Food Plants 4i alternate food plants may not include more than a single botanical species. Early in the summer, several genera- tions of wingless females rapidly suc- ceed one another on the summer food plant, followed by a generation of winged females, which then migrate to another species of plant known as the alternate food plant. There they give rise to a generation of aphids of both sexes, which produce eggs that over- winter. The next spring these give rise to a brood of winged females that migrate back to the summer food plant, after which the cycle repeats itself. With some minor variations, that is the fundamental pattern — two very different food plants are selected alter- nately. Thus the migrating aphids are conditioned to two diverse plants at different times, and we cannot attrib- ute their attraction to a single specific chemical stimulus. Other factors obvi- ously enter the picture, but they can- not be singled out further than to note that the appearance of the winged mi- grants and sexual phases is correlated with the season. This is a phenomenon similar to photoperiodism in plants, where vegetative growth and flowering are often closely related to seasonal variations in the duration of the day- light period. We cannot state definitely whether chemical changes in the plants may elicit a differential response by the aphids as the season progresses. More surprising still is the relation- ship that exists among some gall-mak- ing insects. Many diverse insects induce the formation of abnormal growths or of highly modified specific structures. They are known as galls and are de- veloped by the plant under some stimu- lus from the insects that lay their eggs in the tissues of the plant and undergo their growth feeding within the de- veloping gall. The nature of the stim- ulus is not yet understood, but it is highly specific, as the galls produced by each species are always alike in form and structure. The gall insects are quite uniformly oligophagous or monophagous. One group, the gall wasps, are small, wasp- like insects in some hundreds of species. Nearly all of them produce galls on oaks. They are highly specific for par- ticular kinds of oaks — each lays its eggs in some restricted part of the tree, whether leaves, twigs, buds, or roots. Furthermore, some species of gall wasps undergo an alternation of gen- erations, whereby one generation in- duces galls on some aerial part of the plant and the next goes underground to induce a root gall, returning in the succeeding generation again to the pre- vious location above ground. In the gall wasps, the restriction of choice to particular food plants is sim- ilar to the one I outlined among the aphids, but the matter is further com- plicated by the fact that the response of the plant is an essential requirement for the maintenance of the relationship. So we see that an insect's selection of food plants depends primarily on an acute and discriminative chemical sense, which enables it to recognize by smell and taste many essential oils and other less pungent substances in partic- ular plants. As the presence of each such chemical is usually confined to some natural group of plants, they are the ones to which the insects are at- tracted. When the same chemical at- tractant appears sporadically in unre- lated plants, they also may be chosen. This basic conception is supported by observations of the behavior of insects in nature and by the application of some experimental techniques. It ex- plains the puzzling "botanical sense." It has already opened up a promising field for the study of attractants and repellents that should have great prac- tical value. The insect's selective appetite, so far as we can see at present, is purely an instinct to do thus and so, whether or no. Such instincts are innate and unalterable attributes of all insect be- havior which excite our wonderment and captivate our curiosity because we can go no further than to catalog their manifestations. Some matters relating to the correla- tion that exists between the larval ap- 42 petite and the consistent choice of ac- ceptable food plants on which to lay the eggs are amenable to analytical treatment. Even in insects that shift to another type of food when they reach the adult reproductive stage, we may assign some form of memory or nostal- gia to account for the return of the gravid butterfly to a cabbage head after a round of sipping the nectar of sweetly scented flowers. That may seem a bit farfetched, but it is obvious that once it has been incorporated into the sphere of instinct, an identical response will inevitably be called forth. At this stage, any transgressions will be quickly eliminated through the most rigorous action of natural selection whereby any butterfly that failed to select a proper food plant is unable to pass on to pos- terity her careless or vacillating tend- encies. That such aberrations of in- stinct do occur, although rarely, is at- tested by actual observations of insects under natural conditions. Any such mutation of instinct" may conceivably persist if compatible with' the appetite of the larva and capable of weathering the competitive pressure imposed by the living environment. In yet another way may memory have a part in the differential choice of food plants by insects that normally accept a variety of host plants-. Where we cannot detect chemical attractants in common, it appears that memory of the larval food might lead the adult to prefer it to other acceptable plants. There are many cases where such strains, races, or clones appear to be 'established in nature. Experimental proof so far has been inconclusive, but it seems probable that when varied food plants are readily acceptable such strains do exist. We might even com- pare them to those racial or geographic components of our human population that consistently evince a preferential fondness for cabbage, garlic, red pep- per, baked beans, macaroni, curry, or some other item of food. Charles T. Brues is professor of entomology emeritus in Harvard Uni- Yearbook of Agriculture 1952 versity, where he taught and engaged in research on various phases of ento- mology for 37 years. He has devoted much time to studies on the food habits of insects, a subject on which he has published extensively. A graduate of the University of Texas, he served as field agent for the Bureau of Ento- mology and later as curator at the Mil- waukee Public Museum, before joining the biological staff of Harvard in igog. For accounts of the life histories and food habits of some of the insects mentioned in his article, Dr. Brues suggests his article in Psyche, Food- Preferences of the Colorado Potato Beetle, volume 47, pages 38—43, 1940; and the following publications of the Department of Agriculture: Bulletin 250, Food Plants of the Gipsy Moth in America, by F. H. Mosher, issued in 1915; Farmer's Bulletin 1548, The Eu- ropean Corn Borer, by D. J. Caffrey and L. H. Worthley, 1922; Technical Bulletin 77, The Host Plants of the European Corn Borer in New England, by Benjamin E. Hodgson, 1928; and' Bureau of Entomology Bulletins 50, The Cotton Bollworm, by A. L. Quaintance and C. T. Brues, and 51, The Mexican Cotton Boll Weevil, by W. D. Hunter and- W. E. Hinds, both issued in 1905- For information about the factors con- cerned in food selection he suggests his ar- ticles, The Selection of Food-Plants by In- sects, With Special Reference to Lepidop- terous Larvae, in the American Naturalist, volume 54, pages 313-332, 1920; The Spec- ificity of Food-Plants in the Evolution of Phytophagous Insects, American Naturalist, volume 58, pages 127-144, 1924; Aberrant Feeding Behavior Among Insects and Its Bearing on the Development of Specialized Food Habits, Quarterly Review of Biology, volume 11, pages 305-319, 1936; and ar- ticles by: V. G. Dethier: Gustation and Olfaction in Lepidopterous Larvae, Biological Bulle- tin, volume 72, pages 7-23, 1937; and Chemical Factors Determining- the Choice of Food Plants by Papilio Larvae, American Naturalist, volume 75, pages 61—73, 1941. M. Raucourt and B. Trouvelot: Les prin- cipes constituants de la pomme de terre et le Doryphore, Annates des Epiphyties et de Phytogenetique, volume 2, pages 51-98. I936- Also recommended for further reading are Insect Dietary: An Account of the Food Habits of Insects, by Dr. Brues, published by the Harvard University Press, 1946, and Chemical Insect Attractants and Repellents, by Vincent G. Dethier, The Blakiston Co., Philadelphia, 1947. How To Know an Insect What Kind of Insect Is It? C. F. W. Muesebeck The Animal Kingdom is made up of a number of major divisions, or phyla. One of them, the Chordata, includes man and the other mammals, birds, reptiles, fish — in fact, all the verte- brates, the creatures that have back- bones. By far the largest division from the standpoint of the number of different kinds, or species, it comprises is the Arthropoda. At least 80 percent of all known animals are arthropods. This phylum comprises invertebrate (back- boneless) animals that have a seg- mented body, jointed appendages, and a hard outer covering, or exoskeleton. It is in turn divided into a number of groups called classes, each of which differs in some fundamental charac- teristics from the others. One of these classes, known as Hexapoda, or In- secta, contains all the insects. Various members of other classes of Arthro- poda, especially such organisms as mites, ticks, spiders, scorpions, milli- pedes, centipedes, and sowbugs, how- ever, are so commonly regarded as in- sects that it seems advisable to indicate the basic distinctions between these several classes in a simple key. A key is based on the process of elimination. In the key that follows, for example, one considers ( as in entry number 1 ) the number of legs of the creature he wishes to identify. If it has five or more pairs, he consults entry 2 (as given at the right) ; if it has three or four pairs, he skips to entry 4. And so on. Key to the Principal Classes of Arthropoda 1. With five or more pairs of legs 2 With three or four pairs of legs 4 2. Body wormlike; head not merged with the thorax and provided with one pair of antennae or with none 3 Body not wormlike; head merged with the thorax and provided with two pairs of antennae Crustacea (crabs, lobsters, shrimp, sowbugs, etc.) (fig- ure 1, next page) 3. Body segments each with only one pair of legs Chilopoda (centipedes) (figure 2) Most of the body segments each with two pairs of legs Diplopoda (millipedes) (figure 3) 4. Body composed of two main divisions, the cephalothorax (fused head and thorax) and abdomen; four pairs of jointed legs ; wings and antennae lacking Arachnida (spiders (figure 4), scorpions (figure 5), mites, ticks, etc.) Body composed of three main di- visions, the head, thorax, and ab- domen; only three pairs of jointed legs ; wings usually, antennae al- ways, present Insecta (all insects) For purposes of orderly classification and to facilitate identification, each of these classes is divided into a number of orders, an order is broken down into families, a family is divided into genera, and each genus is comDOsed of related 43 44 Yearbook of Agriculture 1952 Sowbug. Millipede. Centipede. Scorpion. Spider. / - ** S. *• %►. *** . To ' "% A froghopper producing its protecting mass of bubbles. Within, it is sheltered from the direct rays of the sun and kept moist by the loam. The lace'ving is one of the polecats of the insect world. It exudes a disagreeable smelling fluid when it is touched. mw j^v MM0& \ *v After 17 years of tunneling in the darkness of the earth, a periodical cicada nymph begins transforming into the adult cicada. A carpenter ant obtaining honeydew from the nymphs of the treehopper, Entylia sinuata. One of the American walkingstick insects. In the Tropics, such insects assume their largest and their oddest forms. 3$ The dragonfly. The nymphs of these insects live under water and are equipped with extensible lower lips that can shoot out to grasp prey. %• './• Hfc Face of a robber fly, which sometimes feigns death to escape its enemies. The foregoing photographs are by Edwin Way Teale. (See page 8.) '*A.- S? ^ •X , . ' It This dragonfly, preserved in the limestone of Solnhofen, Bavaria, lived millions of years ago. (See page 14.) What Kind of Insect Is It? species. Thousands of species of insects have thus far been described. Obvi- ously each species must have a distinct name, and because many species are cosmopolitan the same name must be used for the same species everywhere. When the present system of naming animals was established about 200 years ago, most scientific books were written in Latin, and Latin was con- sidered the universal language of science. The scientific names of ani- mals, therefore, are in Latin or in Latin form. The name of each species con- sists of two words, the name of the genus to which the species belongs and a word, often an adjective, that stands for the species. The generic name begins with a capital letter; the specific (i. e., the species) name is written in lower case and may be followed by the name, or an abbreviation of the name, of the person who originally proposed the scientific name and described the species. Thus, the name of the house fly is written Musca domestica L. The "L." is an abbreviation for Carolus Linnaeus, the Swedish scientist who described this species. Under the International Rules of Zoological Nomenclature, a generic name may not be duplicated anywhere in the animal kingdom. The same spe- cific name may be used repeatedly but only for one species in any one genus. The rules assure a distinctive name for each kind, or species, of animal and make it possible to record information about any species under a designation that will be universally understood. As shown in the foregoing key to the major classes of arthropods, insects have only three pairs of legs, never more. This is the most distinctive char- acteristic of the class Insecta. Insects also have three separate body divisions, head, thorax, and abdomen. There is always one pair of antennae. Wings are usually present. When they occur, wings alone will serve im- mediately to identify an arthropod as an insect, for they are found in no 970134°— 52 5 45 other class of this phylum. Many in- sects, however, are wingless. In all the major orders, some wingless forms occur. In a few of the smaller orders, such as the Thysanura (silverfish), Collembola (springtails) , Siphonap- tera (fleas) , Mallophaga (biting lice) , and Anoplura (sucking lice), all the species are wingless. The winged or wingless condition, the texture or cov- ering of the wings, their shape, their number (whether two or four), the manner in which they are held when at rest, and the peculiarities of their system of veins furnish characters that help one recognize a given in- sect as belonging to a particular or- der. Examples of wings of different kinds are noted in the sketches of in- sects that are used to illustrate the key to the principal insect groups. Other structures, in which significant differ- ences occur that are useful in the defi- nition of orders or families, are the mouth parts and the antennae. For the identification of genera, and eventually of species, a great array of characteristics must be studied, includ- ing minute details of sculpture, ar- rangements of hairs or bristles, shape and proportional measurements of var- ious parts of the body and appendages, and even details of the reproductive organs that can be demonstrated only by preparation on microscope slides after dissection. Details of every kind that tend to be distinctive of group or species need to be used and the range of variation in all of them must be determined. Many of the insects commonly col- lected or observed doing damage are in the immature stages. During this pe- riod they may bear no resemblance whatever to the adults of the same species. The members of the Orthop- tera (grasshoppers, roaches, crickets, mantids) and the Hemiptera (bugs), among others, develop by gradual change after hatching from the eggs, and the young are similar in general form to the adults, differing principally in size and in the lack of wings. The young of other large orders, however, 46 Yearbook of Agriculture 1952 Antenna Hind leg Foreleg Middle leg such as the Coleoptera (beetles), Dip- tera (flies), Lepidoptera (moths and butterflies) and Hymenoptera (saw- flies, bees, wasps, and ants) , are wholly unlike the adults. The eggs hatch into larvae, some of which are commonly called grubs, maggots, or caterpillars. These represent the feeding stage, dur- ing which all growth occurs. When de- velopment is complete, the larva changes into a pupa, which is the rest- ing, inactive stage, and then the re- markable transformation to the adult insect takes place. Most of the com- monly encountered larvae belong to the four orders just mentioned and may be distinguished by the following characteristics: The larvae of moths and butterflies have a pair of jointed legs on each of the first three body segments and, in addition, short, fleshy, unjointed legs (called pro legs) on some of the other segments. The head is distinctly set off. Beetle larvae resemble those of the moths and butterflies in usually hav- ing a pair of jointed legs on each of the first three body segments and also in the distinct head, but they are at once distinguished by lacking prolegs. The maggots of flies are completely legless, the body tapers noticeably toward the anterior end, and the head is usually not distinctly set off from the rest of the body. Larvae of sawflies (a section of the What Kind of Insect Is It? order Hymenoptcra) are often mis- taken for those of moths and butterflies because they are provided with both jointed legs and prolegs and are found in similar situations, but they may be recognized by the presence on each side of the head of a dark-colored eyespot, which is lacking in larvae of the other group. They also are usually not hairy whereas larvae of Lepidoptera are often conspicuously so. Adult insects, even those of the same order, often differ so much in appear- ance that they are not thought to be related, and it is impossible to construct a key by which every insect may be cor- 47 rectly placed. The simplified key that follows, however, will aid one with lit- tle knowledge of insects to recognize the more common types in the adult stage. Often even such group recogni- tion will suffice to indicate what should be done in a practical case that seems to demand prompt action. It should be easily possible, for example, for any- one with no entomological training to distinguish an ant from a termite. Most persons consider termites to be a type of ant, distinguishable only by an expert, but actually termites and ants belong in widely separated orders and structurally are quite unlike. Key to Major Groups Containing Common Insects 1. Wings present, the front wings often 4. Wings with a network of veins, in- in the form of hard, leathery or eluding many cross veins .... horny, wing covers 2 Order Ephemeroptera (may- Wings absent or represented only by flies), in part (figure 6, be- minute pads 43 low) 2. With only one pair of wings, these Wings with very few longitudinal always membranous 3 veins and no cross veins .... With two pairs of wings, the front Hemiptera, in part (males of pair often represented by hard scale insects or Coccidae) v/ing covers beneath which the 5. Antenna very short, usually three- hind wings are concealed in re- segmented, the last segment the pose 8 longest and provided with a con- 3. End of abdomen with two or three spicuous, long bristle or with a slender but conspicuous, back- number of rings or annulations . . wardly projecting filaments ... 4 Antenna longer and composed of End of abdomen without such fila- many segments; body generally ments slender; wings narrow Order Diptera (mosquitoes, (Midges, crane flies, mosqui- midges, flies) 5 toes) (figure 7) Mosquito. 48 Yearbook of Agriculture 1952 \ / S Horse fly. Last segment of antenna ringed or annulated and without a conspic- uous long bristle (arista) at base (Bee flies, robber flies, horse flies) (figure 8) Last segment of antenna with a long bristle (arista) on upper side at base 7 Calypter (scalelike structure behind base of wing) large and conspicu- ous; mesonotal suture complete; larger flies Calypterate Muscsidea (house flies, blow flies, flesh flies and their relatives) (figure 9) Calypter small and inconspicuous; mesonotal suture incomplete; mostly small flies, much smaller than house fly Acalypterate Muscoidea (eye gnats, pomace or vinegar flies, fruit flies) (figure io) Front wings horny, rigid, opaque, without veins, meeting in a line over middle of body and conceal- ing the membranous hind wings . 9 Front wings usually membranous J 1 Blow fly. Pomace fly. although often covered with scales or hairs; if leathery, with the veins distinct, and not meeting along a line over middle of body 16 9. Tip of abdomen with a pair of prominent forceps-like append- ages; front wings (wing covers) very short Order Dermaptera (earwigs) (figure 1 1 ) Tip of abdomen without such ap- pendages; front wings (wing covers) usually covering most or all of abdomen although some- times short Order Coleoptera (beetles) . 10 10. Front of head produced into a beak Family Curculionidae (wee- vils) (figure 12) Front of head not produced into a beak n 1 1. Wing covers very short, leaving last five or more of the abdominal segments exposed Family Staphylinidae (rove beetles) (figure 13) Weevil. Rove beetle. Earwig. What Kind of Insect Is It? 49 Lady beetle. Larder beetle. May beetle. Wing covers extending to or near tip of abdomen, rarely leaving three segments exposed 12 12. Antennae enlarged toward tips ... 13 Antennae not enlarged toward tips, slender, sometimes longer than the body 15 13. Antennae lamellate (segments com- posing the club in the form of leaflike plates) Family Scarabaeidae (May beetles, Japanese beetle, rose chafer, etc. ) (figure 14) Antennae not lamellate 14 14. Tarsi apparently three-segmented . Family Coccinellidae (lady beetles) (figure 15) Tarsi four-segmented Family Dermestidae (larder beetles, carpet beetles) (fig- ure 16) 15. Tarsi five-segmented, the third seg- ment not enlarged; antennae much shorter than body Family Carabidae (ground beetles) (figure 17) Tarsi apparently four-segmented, the third segment greatly en- larged and deeply cleft conceal- ing the very small fourth segment; antennae usually longer than body Family Cerambycidae (long- horned beetles) (figure 18) 16. Front wings more or less leathery or parchment-like 17 Wings membranous 23 1 7. Mouth parts in the form of a pierc- ing and sucking beak Order Hemiptera, in part . . 18 Mouth parts fitted for chewing . . . Order Orthoptera 19 18. Front wings leathery only at base, the apical third or more ab- ruptly membranous and over- lapping; beak arising from front part of head Suborder Heteroptera (true bugs) (figure 19) Front wings of same thickness throughout and usually sloping rooflike over the body Suborder Homoptera, in part (leafhoppers and their al- lies) (figure 20) Ground beetle. Stink bug. Longhorned beetle. 50 Yearbook of Agriculture 1952 Leafhopper. Cockroach. Praying mantid. Cricket. Grasshopper. Katydid. 19. All legs slender, similar in form; body flattened from above . . . Family Blattidae (roaches) (figure 2 1 ) Either the front legs or the hind legs greatly modified and very differ- ent in form from the others . . . 20. Front legs greatly enlarged and spined, fitted for seizing and hold- ing prey; prothorax slender, in form of a long neck; head broad and capable of unusually free movement Family Mantidae (mantids) (figure 22 ) Front legs normal; hind legs fitted for jumping, the femora much enlarged 21 21. Antennae short, much shorter than body Family Acrididae (grasshop- pers) (figure 23) Antennae longer than the body . . 22 22. Tarsi four-segmented Family Locustidae (long- horned grasshoppers, katy- dids, etc.) (figure 24) Tarsi three-segmented Family Gryllidae (crickets) (figure 25) What Kind of Insect Is It? 23. Wings covered with minute over- lapping scales, often in beautiful color patterns Order Lepidoptera (moths and butterflies) 24 Wings not covered with scales ... 26 24. Antennae usually threadlike or feathery, not enlarged at tips; wings, in repose, held rooflike over body; body very hairy. Mostly night-flying insects . . . Suborder Heterocera (moths) (figure 26) Antennae enlarged at tips; wings, in repose, usually held in a vertical position, or the forewings erect and the hind wings more or less horizontal; body not especially hairy. Mostly day-flying .... Suborder Rhopalocera (but- terflies and skippers) .... 25 25. Extreme tips of antennae recurved or hooked Family Hesperiidae (skippers) (figure 27) Extreme tips of antennae knobbed . Family Papilionidae and allies (butterflies) (figure 28) 26. Wings very narrow, bladelike and fringed with long bristles ; tarsus ending in a large bladderlike structure Order Thysanoptera (thrips) (figure 29) Wings not bladelike; tarsus without such a bladderlike structure ... 27 27. Mouth parts in the form of a beak fitted for piercing and sucking . . Hemiptera, in part 28 Mouth parts fitted for chewing . . . 29 28. Front wings lacelike, horizontal and overlapping in repose ; small, flattened insects Family Tingidae (lace bugs) (figure 30) Front wings not lacelike, usually sloping and not overlapping in repose Families Cicadidae (cicadas) (figure 31 ), Aphidae (plant- lice) (figure 32), and their relatives 51 Moth. Skipper. Thrips. Lace bug. Cicada. 52 Yearbook of Agriculture 1952 Aph d. Damselfly. Termite. 36 Stonefly. Dragonfly. Lacewing. 29. Wings with numerous longitudinal veins and many cross veins form- ing a network 30 Wings with few cross veins and usu- ally with few longitudinal veins, not net-veined 35 30. Antennae very short and inconspic- uous, composed of few segments . 3 1 Antennae conspicuous, composed of many segments 33 31. Hind wings very small; tip of abdo- men with two or three long fila- ments extending backward . . . Order Ephemeroptera, in part (mayflies) Front and hind wings of about equal size; abdomen without terminal filaments Order Odonata 32 32. Front and hind wings similar in shape, slender at bases; wings, in repose, held in a vertical posi- tion over abdomen Suborder Zoraptera (damsel- flies) (figure 33) Hind wing much broadened at base; wings, in repose, horizontal, ex- tended outward Suborder Anisoptera (dragon- flies) (figure 34) 33. Wing veins mostly membranous and faint; front and hind wings of same size and shape; tarsi four- segmented Order Isoptera (termites) (figure 35, winged form) Wing veins strongly developed . . . 34. Tarsi two- or three-segmented . . . Order Plecoptera (stoneflies) (figure 36) Tarsi five-segmented Order Neuroptera (lacewings (figure 37) dobsonflies (fig- ure 38), etc.) 34 What Kind of Insect Is It? 53 Caddisfly. Dobsonfly. Chalcidfly. Ant. 35. Tarsi five-segmented 36 38. Tarsi two- or three-segmented . . . Order Corrodentia (psocids) 36. Wings covered with fine long hair and held rooflike over abdomen, in response Order Trichoptera (caddis- flies) (figure 39) Wings transparent, not covered with long hairs, not held rooflike over abdomen in repose 39. Order Hymenoptera 37 37. Abdomen broadly joined to the thorax Suborder Symphyta (sawflies, wood wasps) (figure 40) Abdomen more or less constricted at base 38 Petiole of abdomen (basal part by which abdomen is attached to thorax) composed of a single ver- tical platelike segment or of two narrow segments that are con- spicuously set off from the re- mainder of the abdomen Family Formicidae (ants) (fig- ure 41, winged form) Petiole of abdomen not as above . . 39 Front wing without a stigma (a more or less triangular, opaque, often discolored spot behind mid- dle of front margin) Superfamily Chalcidoidea, etc. (chalcidflies and their rela- tives) (figure 42) Front wing with a stigma 40 54 Honey bee. Hornet. Yearbook of Agriculture 1952 40. Body hairy, the hairs branched; first segment of tarsus often greatly broadened and fitted for gather- ing pollen Superfamily Apoidea (bees) (figure 43) Body usually not so hairy, the hairs not branched 41 41. Wings folded lengthwise when in repose Family Vespidae (wasps and hornets) (figure 44) Wings not folded lengthwise when in repose 42 42. Antennae usually long and slender, composed of many segments; fe- male usually with a projecting ovipositor Superfamily Ichneumonoidea (ichneumonflies) (figure 45) Ichneumonfly. Silverfish. What Kind of Insect Is It? Antennae short, composed of 12 or 13 segments; female without a projecting ovipositor Superfamily Sphecoidea (thread-waisted wasps) (fig- ure 46) 43. Tip of abdomen with two or three long appendages directed back- ward 44 Tip of abdomen without such ap- pendages 45 44. Abdominal appendages thick, rigid, in the form of forceps Order Dermaptera, in part (earwigs) Abdominal appendages delicate, flexible, antenna-like Order Thysanura (silverfish, etc.) (figure 47) 45. Tarsus composed of only one to three segments 46 Tarsus composed of four or five segments 50 46. Antennae conspicuous, projecting in front of head 47 Antennae very short, inconspicuous, not projecting in front of head . 49 47. Antennae composed of three to six segments 48 Antennae with more than six seg- ments ; very tiny insects that some- times occur by the thousands in damp houses Corrodentia (psocids) (figure 48) 48. Mouth parts in the form of a dis- tinct beak; body greatly flat- tened Order Hemiptera, Family Ci- micidae (bed bugs) (figure 49) Mouth parts not in the form of a beak; body not flattened .... Order Collembola (spring- tails) (figure 50) 49. With biting mouth parts Order Mallophaga (biting lice) (figure 51) With piercing and sucking mouth parts Order Anoplura (sucking lice) (figure 52) 50. Antennae prominent 51 Antennae inconspicuous, not pro- jecting _ 52 51. Body noticeably constricted at base of abdomen, antennae elbowed, the basal segment very long; tarsus five-segmented Order Hymenoptera, Family Formicidae (ants) (figure 53, wingless form) Body not constricted at base of abdomen ; antennae not elbowed, basal segment short; tarsus four- segmented Order Isoptera (termites) (figure 54, wingless form) 55 Termite 52. Body strongly compressed from the sides; abdomen distinctly seg- mented; coxae very large and strongly flattened; legs fitted for jumping . Order Siphonaptera (fleas) (figure 55) Body not compressed; abdomen not distinctly segmented; legs not fitted for jumping Order Diptera, in part; wing- less forms (sheep-tick and its relatives) Progress in Insect Classification C. F. W. Muesebeck The accurate identification of an insect is the key to all past recorded experience with that species. Without it, costly mistakes may be made in the application of control measures, in- effective or unjust quarantine prac- tices may be instituted, or much work may be unnecessarily duplicated. If there were only a few hundred, or even a few thousand, different kinds of in- sects it would not be very difficult for an entomologist to learn to recognize them all and to call their names — but nearly 700,000 different kinds have been described and named, and it is estimated that at least twice that num- ber remain to be identified. Obviously it is quite hopeless there- fore to determine what a given insect really is without the help of some or- derly arrangement or classification of all the known kinds. To be sure, a com- paratively small number of common and distinctive insects will always be readily recognizable without special aids, but the vast majority can only be identified by the skillful use of keys, descriptions, and other guides that re- sult from the painstaking research of many specialists. It is this research in classification that makes definite iden- tification possible. How accurate and complete the identification will be depends on how thorough and critical the research has been. Classification of living things is an effort to interpret nature. It attempts to bring together the kinds that are alike and closely related and to sepa- rate those that are unlike and unre- lated. The earliest classifications of insects were based largely on habits and habitats and on certain gross ana- tomical features that contribute to de- 56 fine the facies, or general aspects, of the different kinds. They were trial classifications and were naturally ex- tremely artificial. They brought to- gether things that were in no sense re- lated and separated widely forms that belonged close together. With the growth of knowledge about insects and the rapid increase in the number of known kinds, however, the search for new characters usable in the develop- ment of more satisfactory classifica- tions was intensified. Methods and concepts are improving steadily; the result is that insect taxonomists grad- ually are producing in their classifica- tions an interpretation of insect life that is much closer to existing facts than any of the arrangements previ- ously developed. In the efforts to attain the goal of having classifications in accord with the natural relationships of insects, one has to take into account not only ana- tomical characteristics but also facts pertaining to the physiology, biology, distribution, ecology, and sometimes cytology, of the species. Sometimes, in- deed, anatomical distinctions are lack- ing or at least are not evident, although conspicuous differences have been ob- served in the life habits of the insect populations in question — differences in time of appearance, food preferences, method of hibernation, or even in reac- tion to certain insecticides. Such facts, when known, are usually indicators of fundamental distinctions between forms that at first appeared to be identical. They suggest that a re- study of series of specimens may reveal structural differences formerly over- looked, and often it does. That was so with the screw-worm flies, the Euro- pean spruce sawflies, and the Califor- nia red scale and the yellow scale of citrus, among others. The two screw- worm flies were long regarded as a single species that fed sometimes as a true parasite on warm-blooded ani- mals, including man, and sometimes as a scavenger upon dead animals. These differences in feeding habits led tax- onomists to a comparative study of flies Progress in Insect Classification reared from larvae of the two habit types, and to the discovery that the flies from the two sources could, after all, be distinguished on the basis of anatomical differences and repre- sented two entirely different species. That information resulted in an abrupt change in control procedure against the parasitic form. With the spruce sawflies it was a cytological study that established the distinctness of two species long regarded as one. In the case of the red and yellow scales of citrus, differences in susceptibility to attack by certain parasites and dif- ferences in location on infested trees eventually led to the discovery of struc- tural differences by which the two species could be identified. By the utilization, then, of all avail- able information, taxonomists are at- tempting to correlate behavior and other life characteristics of insects with anatomical features, since it has come to be realized that only in this way can sound, natural classifications be de- veloped. In the formation of keys, which are the guides to identification and* which reflect the judgment of taxonomists with respect to relation- ships, however, it is necessary to de- pend on the use of physical character- istics of the insects. Only those are always definitely determinable from the specimens themselves. Therefore the principal efforts of the research taxonomist are necessarily directed to- ward the search for physical peculi- arities, however small, that seem likely to be relatively constant and more or less distinctive. That that is no simple task must be obvious from the enor- mous number of known kinds and the continued addition to that number of 10,000 or more new species annually. A key that will infallibly lead the user to the correct name for a given insect can rarely be constructed even for a comparatively small group of insects. Few characteristics are absolutely fixed, and the extent and direction of the variation are themselves extremely var- iable. Bracon hebetor, an abundant and widely distributed parasite of cer- 57 tain pests of stored products, ranges in color from completely yellow to wholly black. On the other hand, all the moths of all the species in the genus Rupela are without exception entirely white. One individual may be five times as large as another fully matured specimen of the same species. In another species, per- haps a closely related one, the speci- mens may be of rather uniform size. Details of sculpture or of wing vena- tion may be strikingly constant or may vary widely. It is important to deter- mine the range of variability in each instance, but that requires large num- bers of specimens and these are not always to be had. Seldom, therefore, can a character that is employed in a key be considered absolute, and no key can be regarded as more than a tem- porary guide to identification. It will inevitably require modification, or even complete recasting, as knowledge of the particular group involved, in- creases; its usefulness will depend to no small extent on the aptitude, experi- ence, and perhaps even intuition of the user. Dimorphism among adult insects of the same species is a common phe- nomenon. It is often a cause of serious difficulty in the development of classi- fications. The winged sex forms in termites and ants, for example, bear little resemblance to the wingless work- ers of the same species; in the mutillid wasps, which are popularly and inac- curately known as velvet ants, the winged males are so unlike the wing- less females that their identity can only be established by biological associa- tion. That is true also of the canker- worm moths, in which the female is a grublike egg sac but the male is a nor- mal winged moth. Such striking caste or sexual dimorphism occurs in vari- ous sections of all the major insect groups. Furthermore, since the biologi- cal association of conspicuously differ- ent castes or sexes of the same species may be difficult and slow, it sometimes happens that the male and female of a single species are long treated as two 5« distinct things and are known under different names. Only field observa- tions or biological studies can establish the facts in such cases. More often than not it is in the larval stage that an insect is destructive. Be- cause it is harmless and is seldom seen, the adult may be unknown to the grower whose crop is being damaged. In order that the right control meas- ures may be applied, the insect has to be identified in its larval stage. For practical reasons, then, it has become essential, in the case of various insect groups, to supplement classifications founded on adults with keys to the larvae. The development of such keys is particularly difficult and slow be- cause the identity of the larvae must first be definitely established. Other- wise, however good the key might be, it would not lead the user to the avail- able information on habits and control of the pest. The name originally pro- posed for the adult insect is the clue, and the larva must be identifiable by the same name. For definite association of the larva with the adult, however, field or laboratory studies must usually be conducted, and these often demand facilities not readily available. Accord- ingly, the number of different kinds of insects for which this type of associa- tion has been worked out is very small and grows slowly. Even in the Lepi- doptera (moths and butterflies) and the Coleoptera (beetles), where more work on immature forms has been done than in the other major groups of in- sects, fewer than 3 percent of the de- scribed species are known in the larval stage. Since about 19 10, however, a great deal of progress has been made in this field of taxonomic work, and because emphasis has naturally been placed on the injurious species, most of the major pests are now identifiable in the immature stages. Although, as I have indicated, the normal course is to base names on the adults and to develop the original and principal classifications from adult characteristics, there is one conspicuous exception. That involves the important Yearbook of Agriculture 1952 group of plant pests comprising the family Aleyrodidae, the members of which are known as whitefiies. They are tiny insects that are not collected abundantly in the adult stage but are commonly seen fixed on leaves of in- fested plants when they are in the pupal stage. The family, which con- tains such devastating pests as the citrus blackfly, was long ignored, and most of the present knowledge con- cerning its classification has been ac- cumulated during the past 50 years, the first significant and basic work being done by American taxonomists. Good characters upon which to base a classi- fication were discovered in the pupae, and nearly the whole classification of the whitefiies is founded on this stage. In fact, it is rarely possible to identify adults in this group because adults have been definitely associated with the im- mature forms in only a few instances. In keeping with its growing com- plexity, taxonomy has gradually be- come increasingly specialized, until now a worker usually confines himself to a single limited field, as, for ex- ample, aphids, or fleas, ants, biting lice, cutworm moths, leafhoppers, scale insects, termites, thrips, weevils, cer- tain sections of the wasps or bees, grass- hoppers, gall flies, or mosquitoes. Such specialization is essential for thoroughly competent and authorita- tive work in the identification and classification of insects. Even to keep abreast of the taxonomic literature in any one of the restricted fields cited is a time-consuming task, for taxon- omy— the description, nomenclature, and orderly classification of organ- isms— is international, and a taxono- mist must take into account everything that is published in his specialty throughout the world. Furthermore, new approaches and more refined techniques must be sought continu- ally in dealing with problems involv- ing the classification of so complex a group of variable and evolving organ- isms as the insects have been found to be. Final decision as to identity often rests on features of certain internal Progress in Insect Classification structures that must be dissected out and mounted on slides after more or less elaborate preparatorial treatment. Indeed, such meticulous preparation of slides, which usually involves stain- ing the tissues, is now a routine pre- requisite for the study and identifica- tion of the whole insects of many groups, including aphids, whiteflies, scale insects, thrips, fleas, and lice. The very recognition of the diffi- culties and complexities that have been outlined here is itself evidence of significant progress in classification re- search. More and more emphasis is being placed on fundamentals, and greater caution is practiced in making identifications. Today the taxonomists tend increasingly not to venture spe- cific identifications in groups that have not been thoroughly studied and re- vised, although a generation or two ago, when the problems involved were not so well understood, determina- tions, often inaccurate, were freely made in the same groups. With the increase in knowledge about any group of insects, identification has grown more difficult but it has, at the same time, become more accurate and pre- cise. The recent history of taxonomic re- search in the mosquitoes is a good ex- ample of the progress that is being made as the result of intensive, special- ized study of one family of insects. At the same time it indicates rather clearly how vast the task of the insect tax- onomist is. From the time of the dis- covery of the transmission of malaria, yellow fever, and dengue by mosqui- toes, this family, wjiich now contains approximately 2,000 species, was stud- ied actively. It soon became one of the best known insect groups of com- parable size. The basic classification, after going through a period of great instability in the first two decades of the twentieth century, had reached a high degree of stability, thanks largely to the efforts of H. G. Dyar in the United States and F. W. Edwards in England. This taxonomy was based on both adult and larval characters, which 59 greatly enhanced its strength. Large revisionary works had been published for the mosquito faunas of most of the regions of the world, and it seemed possible to make definite determina- tions rather readily for mosquitoes from anywhere. With the outbreak of the Second World War, when rapid recognition of the mosquitoes encoun- tered in remote parts of the world was important, however, it became evident that much of what had been done on the classification of this family was out of date and that many species could not be determined satisfactorily. Under that demand, intensive study of the family was undertaken by many taxonomists, and comprehensive keys were prepared to the anophelines (malaria vectors) of the world, as well as keys to other mosquitoes of medical importance occurring in certain cru- cial areas. The presence of military entomologists in the war theaters made possible the extensive and careful col- lection of specimens, many of the adults being individually reared and associated with larval and pupal forms. Such material was studied in the field laboratories and in the museums where the specimens were finally deposited; a flood of papers describing new species and revising genera and species groups resulted. Much was learned about the taxonomic relationships of species, and many new characters usable for dis- tinguishing the different species were discovered. The study of mosquitoes in the pupal stage was given a great im- petus, and the results are proving fruit- ful in the continuing attempts to improve the classification of the family. Although the gaps in the knowledge of mosquitoes are being filled in rapidly, however, much remains to be done. That is even truer of other insect groups, including the scale insects, ants, fleas, lice, aphids, grasshoppers, and certain small families of moths and beetles which have been rather inten- sively studied because of their conspic- uous economic importance. If this, then, is the situation in the relatively small groups that have received special attention because of their unusual importance to man's wel- fare, it must be evident that an im- mense amount of work will need to be done before the many larger groups that have had comparatively little study are thoroughly investigated and classified. Thus, in 200 years since Linnaeus, during which time the number of known species of insects has increased from fewer than 2,000 to approxi- mately 700,000, insect classification has become an elaborate and complex activity. At first it consisted essentially of the mere sorting of specimens into a series of figurative pigeonholes on the basis of differences that were often purely superficial. Gradually it has had to take into consideration many factors that have increased immeasur- ably the difficulty of the work. These include variation in all its aspects, di- morphism, the correlation of biologi- cal characteristics with structural pe- culiarities insofar as knowledge of biology will allow, and the identifica- tion of immature insects. The last vir- tually is a separate field in itself, be- cause it is concerned with forms utterly unlike the adults with which they belong. Insect classification is now recog- nized as a task that is never finished. Adjustments or complete revisions of the classifications of all groups become necessary as more new species are dis- covered and new information is ac- cumulated about those already known. C. F. W. Muesebeck, who received his academic training in entomology at Cornell University before joining the Bureau of Entomology in igi6, has been in charge of the division of insect identification since 1935. In recognition of his service, the Depart- ment of Agriculture awarded him a distinguished service medal in /Q5/. Before he joined the Department, Mr. Muesebeck spent several years in Eu- rope in search of specific parasites of some injurious insect pests that had been introduced into the United States. Values of Insect Collections Clarence E. Michel The pleasure and challenge of taking part in one vital scientific activity can be his who makes a collection of insects. He starts for the fun of it, the joy in the endless variety of form, color, behavior, and universality of insects. Before long he wants to know the cor- rect scientific names of the specimens he has and to expand his collection to include examples of other species. His interest grows with his collection and both may attain considerable size. Whatever his age and schooling he is a scientist then, one of a group whose work has great economic value to farmers and everybody else. He will discover the basic value of a collection of correctly identified speci- mens— that the correct scientific name of a species is the key to all published information about that species, its habits, and the damage or good it does. He will also discover that there is still a great deal to be learned about all insects and that his own careful obser- vations are of value in adding to the store of knowledge about them. His collection may be small, from twenty to several hundred specimens, with examples of the orders and prin- cipal families of insects. Or it may in time embrace thousands of specimens and be restricted to certain groups of insects, such as a family or a genus. He may prefer to make what we call a general collection. His aim then is to accumulate representative specimens of the common insects in his own neighborhood so that he can enjoy their beauty of color and form or use them to learn to recognize the insects or simply to satisfy his instinct for collect- ing. The scope and size of his collection will depend on him and the breadth of 60 Values of Insect Collections his interest. He may limit himself to specimens he finds in his own back yard or he may include those of his town or county. In any event, as he attempts to iden- tify the specimens he has collected, he will discover much about the methods of science — the need for proper mount- ing and preservation of specimens, lest parts of a specimen be damaged and lost and thereby make impossible its correct identification, the need for the minute examination of the specimen as it is being identified, the difference be- tween learning for its own sake and learning with a practical application in mind (in this instance the control of harmful insects), and the scientist's deep concern for orderly classification and naming. He will do well to get all the stages of life of the insects he collects, par- ticularly if his aim is to know all the pests of plants, animals, stored prod- ucts or buildings around his home. Before long, he will gain information on where, when, and how insects live and the names of the orders and fam- ilies of the specimens he has collected. Such details he will get from reference works, including the chapter "What Kind of Insect Is It?" on page 43. Anyone can make such collections. He must have some technical knowl- edge, but that he can acquire as his collection is made and grows. Mount- ing equipment and supplies, storage boxes for insects, and a few reference books will require some monetary out- lay, but the amount will not be ex- cessive. If he owns this Yearbook he will have many of the facts he wishes to know, but if he seriously expects to identify insect specimens he will re- quire some additional technical works. Collecting insects is as inexpensive a hobby as anyone can have. His collec- tion may or may not have scientific value, but it can have tremendous per- sonal value to him: It teaches him a great deal about the insect world and about all living things. The chances are that anybody, boy or man or girl or woman, who makes a general collec- 970134°— 52 6 61 tion gets an intense interest in insects that will last his lifetime and expand into other branches of natural science. Another kind of collection is the one a biologist or entomologist may make on his own. Occasionally a person with training in biology becomes interested in the problems of the classification of a limited group of insects, such as a family or genus. He may do research on the classification of the group and, if he is not associated with some ento- mological research institute, he may have his personal collection of the group. Actually, some professional en- tomologists own private collections of the group of insects in which they are interested and on which they conduct taxonomic research. They take more pains than begin- ners do with mounting and preserving specimens: Each specimen bears a label stating the locality where the specimen was collected, the date, the name of the collector, and any other biological information that can be printed on a small label. Each specimen also bears a second label giving the correct scientific name, the name of the scientist who made the identifica- tion, and the year the identification was made. The first or locality label is important — no specimen has scientific value without it. The owners of such collections often are competent en- tomologists and their colleagues regard their work highly. The results of their researches are published in the profes- sional journals. It follows, therefore, that private collections may be of great scientific value; in fact, they may be of as great scientific value as any pro- fessional collection in a museum or entomological research institute. The owners may describe new species and genera of insects and the specimens from which they make their descrip- tions become valuable as reference specimens. When an entomologist, amateur or professional, publishes the results of his research in a professional journal, his description of a new species and the data regarding the specimens on 62 which it is based become public prop- erty, but the specimens themselves are still in private hands. Often the publi- cation is adequate for the identifica- tion of specimens collected thereafter, but sometimes an accurate identifica- tion cannot be made without reference to the original specimens. In such cases it may be necessary that an entomolo- gist other than the original describer examine the specimens from which the description was made. It is vital then that the original specimens are prop- erly taken care of and that they be available for examination by compe- tent investigators. Collections built up, financed, and taken care of by an individual should remain his property as long as he is engaged in research, of course, but collections of scientific value belong to the entomological research world as soon as their usefulness to the owner is finished. Many collections are given or sold to research institutions, where they are cared for and maintained for the use of all entomologists, but it has hap- pened that fine collections have been so neglected that the specimens have be- come damaged or lost. One example is the collection of the famous American entomologist, Thomas Say, which upon his death in 1834 was lost or destroyed by pests — one of the most valuable of all the early insect collections was lost because no provision was made for its care. Professional insect collections are maintained by institutions for ref- erence, research, and teaching. The reference collection is system- atically arranged so that any series of specimens representing a single species can be consulted to verify the identifi- cation of new specimens. The number of specimens in a series need not be large, although it should include speci- mens of both sexes, of the immature stages, and of the injury caused by the insect of economic importance. Be- cause of the differences in methods of preservation, adult specimens often are maintained separately from immature Yearbook of Agriculture 1952 specimens, and both may be main- tained separately from the specimens of insect injury. The scope of the reference collection depends on the institution sponsoring it. A city, county, or State institution may often elect to limit the collection to the insects found within its bound- aries, but large museums and a few universities maintain reference collec- tions that are almost world-wide in scope. When a specimen is sent to the in- stitution for identification, it can be compared with already named mate- rial by a specialist who is intimately familiar with the group, and a decision made as to whether it is the species it is thought to be. The name must be correct, because all published infor- mation regarding the insect is indexed under the scientific name ; once that is available, all the known facts about the insect can be assembled in a short time, and, if the insect is doing or can do damage to crops or trees, men who are working to control it know with what sort of thing they have to deal. Thus- an extensive reference collection is of inestimable value in the correct naming of new specimens and is in- dispensable to Federal and State agen- cies charged with the duty of research on the control of injurious insects. Because hundreds of thousands of species have been described, a com- plete representative collection of all species would be next to impossible. No collection in the world includes representatives of all of the described species. Some important collections contain a few thousand, but some con- tain specimens of many thousands of species. A considerable number of specimens submitted for identification can be routinely identified without difficulty. There still remain, however, thousands of undescribed species of insects, and many which have been so poorly de- scribed, or are so little known, that identification of specimens is of great technical difficulty. A research collec- tion is necessary to study and solve such Values of Insect Collections problems of classification, and it differs from a reference collection in the num- ber of specimens of each species in- cluded— the research collection hav- ing long series of specimens of a species whenever possible, while the reference collection will have only a few. Thus research collections often have a tre- mendous amount of unidentified ma- terial which, because of technical diffi- culties, can only be identified after much study and research. Obviously, extensive reference and research col- lections require the services of a specially trained curatorial staff, the larger collections requiring a corre- spondingly large staff. The number and variety of species of insects make their systematic ar- rangement or classification a highly technical procedure. The scientific study of the similarities and differences among species of animals is known as taxonomy; the systematic arrangement of the species based on such studies is known as their classification. As new and undescribed species of insects are discovered and studied, the classifica- tion is bound to become more difficult and complex. The naming and iden- tification of insect specimens conse- quently depends on how well the group has been studied and described. Some groups are well known and can be identified easily, but many are poorly known and their identification is diffi- cult and laborious. There is a great need therefore for taxonomic research on them to facilitate their identifica- tion. The reserves of unidentified in- sect specimens in research collections provide a reservoir of material that can be drawn upon when opportunity offers for the intensive study of some insect group. Reference and research collections help the entomologist who works to reduce the economic injuries of in- sect pests. They also are useful to other biological scientists. Animal physiol- ogists, animal ecologists, geneticists, plant pathologists, cytologists, and others carry on research in which they use insects as experimental animals, or 63 insects impose themselves upon the ex- periments in some way or other. It is necessary for scientific accuracy that these scientists know the correct names of the insects with which they are deal- ing. Usually only the insect taxonomist can supply this information, and to do that efficiently he must have an ade- quate research and reference collection at his disposal. When an insect taxonomist describes a new species, he customarily draws up the description from a single specimen. When he does that, he calls the speci- men a holotype. If he has available a specimen of the other sex and has to give it a separate description from the holotype, he describes and designates it as an allotype. If the taxonomist has a series of specimens at the time he makes the description, all the remain- ing specimens are designated as para- types. His purpose in all this is to pro- vide an indisputable specimen of the species which he intends to describe. The apparent structural differences between some insect species are ex- ceedingly slight. It has happened often that a man has described a new species from a series of several specimens only to have some other person years later find that the series consisted of several species. Then the question arises: To which species in the series does the name belong which the original author gave? It is to prevent situations such as this that the practice of designating a holotype has come into being. Types fholotypes, allotypes, and para types) have served this purpose so well that insect taxonomists have come to de- pend on them. They have great scien- tific value and should be well cared for. In insect taxonomy no specimen is of any greater value than the holotype. Most reference and research collec- tions include a considerable number of type specimens and the more type specimens an insect collection contains, the greater its value. Primarily then, the reference and research collections are of the greatest value to the insect taxonomist. They are the indispensable material with 64 which he works. They are of value to workers in the future who find in them a reservoir of material for study and comparison. Indirectly they are of great importance to the public, be- cause all the information the public receives about insects is associated with the name. If the name is incor- rect the public will be misinformed, and misinformation about insects may result in economic loss, in personal discomfort, in sickness, and even loss of life. Among the large research and refer- ence collections in the United States and Canada are the National Museum in Washington, D. C; Canadian Na- tional Collection in Ottawa. Ontario: American Museum of Natural History, New York: Museum of Comparative Zoology. Cambridge, Mass.; Academy of Natural Sciences of Philadelphia: Illinois Natural History Survey, Ur- bana; California Academy of Sciences, San Francisco; Carnegie Museum, Pittsburgh; Chicago Museum of Nat- ural History; Cornell University, Ithaca. N. Y. ; University of Minne- sota, St. Paul; Ohio State University. Columbus; University of Kansas, Lawrence; and the University Mu- seum. University of Michigan, Ann Arbor. The universities or colleges that have no reference or research collec- tions often have collections that are used in courses in entomology and zoology. Teachers have found it need- ful to have enough specimens so that each student can have a specimen to study and examine under a micro- scope. A student can learn the orders and families of insects only by han- dling specimens and attempting to identify them; he must make a collec- tion of his own or the teacher must furnish the specimens, both adult and immature, and samples of the injury the insect has caused. Sometimes, as a substitute or if the teaching is informal, the specimens are arranged in a per- manent exhibit for use by more than one individual. Yearbook of Agriculture 1952 Another type of teaching collection is that arranged by extension workers and other entomologists who find small portable exhibits useful in acquainting the public with insect pests. Sometimes- — when a private collec- tion is offered for sale, say. or an in- ventory has to be made of an institu- tional collection — it is necessary to put a monetary value on a collection. Some of the factors then involved are: The actual cost of obtaining specimens, in- cluding the collector's time and ex- penses, or the purchase price; the cost of the technical materials used in prep- aration of the specimens, such as pins, labels, trays, cork, storage cabinets, microscope slides, and chemicals: the labor in preparing the specimens; the number of holotype or type specimens in the collection: the rareness of the specimens; and the physical condition of the collection. No financial estimate, though, can place a value on the use- fulness of collections that permit ready identification and so promptly unlock the store of existing knowledge about any species; neither can a financial estimate adequately assess the educa- tional, cultural, and scientific values of collections. Those values are indeed high. Clarence E. Mickel is chief of the division of entomology and economic zoology in the University of Minnesota, which he joined in 1922. In 1930 he studied in Europe on a fellowship. He zvas secretary-treasurer of the Ento- mological Society of America in 1936- 44 and President of the society in 1944-45. He was chosen President of the International Great Plains Confer- ence of Entomologists in 194.6. Panorpa rufescens, a scorpionfly. How To Collect and Preserve Insects for Study Paul W. Oman The equipment used in collecting in- sects is simple and inexpensive. The average collector usually will need only a few items : Nets, killing bottles, suc- tion bottle, tweezers, scissors, small brushes, and insect pins. Many collec- tors prefer to make most of their own equipment even though most items may be purchased from commercial supply companies. The insect net is essentially a cloth bag hung from a loop that is attached to a handle. The size, shape, and ma- terial of the net depend on its use. The beating net (fig. i) must be strong enough to stand rough use. A handle of straight-grain hickory or ash, such as a hoe handle, fitted at one end with a metal ferrule (fig. i, C) about an inch in diameter to hold the wire loop in place, is recommended. The handle should be about i% inches in diameter and 3/2 to 4.5/2 feet long. The wire loop (fig. 1, A) should be of No. 12 steel wire (0.189 incn in diameter) , although even heavier wire is sometimes preferred. After the loop is shaped, it can be tempered so that it will spring back into shape if it is bent when it is used. For the bag, 6-ounce drill, heavy muslin, or light canvas is recom- mended. It may be made as shown in figure 1, D. The four lobes form the rounded bottom of the bag when sewed together. The details of the double- thickness hem as it hangs on the wire loop are shown in figure 1, E. This type of construction is advisable be- cause that part of the bag gets the most wear. For a lightweight bag, the entire top band may be made of a stout mate- rial and the bag sewed to it. The final step is to complete the bag by sewing together the two ends of the material and the margins of the cut lobes. This beating net is not satisfactory for the capture of moths, butterflies, flies, wasps, and other swift-flying or fragile insects. For them, the nets de- scribed in the next three paragraphs are useful. The general-purpose net should have a loop 1 2 inches in diameter and a bag of unbleached muslin or of coarse or medium-mesh brussels. It should be tapered more toward the bottom than the beating net, but it should not come to a point. The handle need not be so stout as that for the beating net. The butterfly net is like the general- purpose net, but the bag is of good- quality marquisette or fine netting, and the handle is a little longer and of lighter weight. This net is also useful in capturing dragonflies and other large-winged insects. The fly net should have a loop 8 inches in diameter and a bag of me- dium-mesh brussels or fine netting. The handle should be short and light. The wire loop need not be so heavy as that for the beating net. This net is also good for collecting bees and wasps. The aquatic net, for collecting in- sects that live in or on water or on aquatic plants, should not have a cir- cular loop, but should be either square (with the handle attached to one cor- ner) or about semicircular (with the side opposite the handle straight) . The bag should be shallow (about as deep as the length of the straight side in the semicircular net) and should be made of heavy scrim with a canvas band for the wire loop. The bag for any of the nets I have described may be made of silk bolting cloth, which is durable and has meshes of various sizes but is more expensive. Nylon may also be used. The bag for any net, excepting the water net, should be long enough so that the tip may be flipped over the rim of the wire loop to form a pocket from which the netted insects will not escape. 65 66 Yearbook of Agriculture 1952 1. The construction of a beating net: A, Steel wire loop 15 inches in diameter; B, end of net handle showing grooves and holes into which the arms of the wire loop fit; C, net handle with metal ferrule to hold net in place; D, how to cut a single piece of cloth to make a round-bottom bag; E, details of top part of net fitted over a section of the wire loop. Nets should be kept dry. A wet net damages the specimens and dampness causes the fabric to rot quickly. Aquatic nets should be thoroughly dried after A killing bottle may be made from any fairly heavy glass jar or vial with a wide mouth. The collector should have several bottles of various sizes (fig. 2, A, B). Empty pickle jars, olive jars, and the like will furnish an assortment of larger bottles. Smaller ones may be made from test tubes or shell vials i to 1/2 inches in diameter. These should be supplied with tight- fitting corks. Figure 2, C illustrates a convenient adaptation of a screw cap for a jar to keep bees, grasshoppers, and other lively insects from escaping from the killing bottle when it is opened for putting in other specimens. This cap is made by soldering an in- complete metal cone to a screw cap with the top cut out. A metal tube about 1 inch in diameter is then sol- dered inside the cone. Calcium cyanide, potassium cya- nide, or sodium cyanide may be used as the killing agent in the bottle. Wrap some granular cyanide ( a heaping tea- spoonful for small bottles, larger amounts for large bottles) in cellu- cotton, or place it in a "nest" in cellu- cotton or a little cloth bag, and put this in the bottom of the bottle. Over this place a plug of several layers of cellu- cotton or a layer of dry sawdust. If the bottle is more than 1 J/2 inches in diameter, a quarter-inch layer of plas- ter of paris should be poured in and allowed to harden for a few hours before the bottle is corked. If the bottle is a small one, several disks of clean blotting paper, cut to fit the bottle snugly, may be used in place of the plaster of paris. Cyanide is a deadly poison and should be handled with great care. All bottles should be conspicuously labeled poison and should be kept away from persons who do not realize the deadli- ness of the chemical. The bottom of a cyanide bottle should be taped so that How To Collect and Preserve Insects for Study 67 if the bottle is broken the cyanide will not be scattered about. To make a killing bottle in which to use ethyl acetate (acetic ether), pour a half inch or more of plaster of paris into the bottom of a suitable jar or vial, allow it to set, and dry it thoroughly in an oven. After the plaster of paris is completely dry, saturate it with ethyl acetate, pouring off any excess fluid. The killing bottle is then ready for use and will last for months if kept tightly corjsed. When it becomes ineffective it can be dried in the oven and re- charged. Insects may be preserved in such bottles for an indefinite time with- out becoming brittle if they receive an occasional moistening with ethyl ace- tate. Ethyl acetate is relatively easy to obtain, and the killing bottles have the advantage of being comparatively safe to use. The killing bottle will last longer and give better results if the following simple rules are observed : 1. Before using the cyanide bottle, put in a few strips of soft paper, such as ordinary toilet paper. This will help keep the bottle dry and will prevent the specimens from mutilating one an- other. Change these strips whenever they become soiled or slightly moist. Wipe out the bottle if it becomes moist. 2. Keep a special bottle for moths and butterflies. The scales from these insects will stick to other insects and spoil them. 3. Never mix small or delicate insects with large insects like grasshoppers and large beetles. Beetles are hard to kill Plaster of paris Cyanide in celliicollon } Disks of blotting paper Cyanide in cellncotton A B 2. Killing bottles: A, Large, wide-mouth cyanide bottle for large insects; B, vial-type cyanide bottle for small insects; C, screw-cap top for large cyanide bottle showing a con- venient arrangement to prevent the escape of active specimens. 68 Yearbook of Agriculture 1952 3. Aspirator, or suction bottle: A, Vial-type aspirator assembled; B, details of stopper assemblage for vial-type aspirator, showing outlet tube flush with surface of stopper; C, attachment for collecting tiny insects with an ordinary aspirator; D, body of tube-type aspirator; E, details of construction to convert an aspirator to the blow type. and must be left in the killing bottle longer than most other insects. 4. Never overload a bottle. Always remove insects from it as soon as they are dead. 5. Discard or recharge bottles that no longer kill quickly. Dispose of the contents of old cyanide bottles by burn- ing or burying. Many insects should not be killed in a killing bottle but should be placed in 70 percent alcohol or some other fluid". These insects I discuss in more detail later. For them the collector should have a supply of small homeopathic vials of various sizes with corks to fit. He can get them at drug stores. The suction bottle, or aspirator, is a convenient device for collecting small insects from the beating net or beating cloth or directly from under stones, bark, and such. Its construction is rather simple. For the type illustrated in figure 3, A, the following materials are needed : A glass vial 1 to 1 J/2 inches in diameter and about 4 J/2 inches long; a rubber stopper with two holes in it; two pieces of metal or plastic tubing, one about J4 inch in diameter and 10 inches long, the other slightly larger and 4 or 5 inches long; a piece of rub- ber tubing about 3 feet long and big enough to slip onto the larger of the metal or plastic tubes; and a small piece of bolting cloth or fine-mesh wire screen. The metal tubes should fit snugly in the holes in the rubber stopper. The bolting cloth should be fastened over the end of the larger metal tube to keep the insects from being sucked into the mouth. If wire screen is used it may be soldered to the end of the tube. Glass tubing may be used, but it has the dis- advantage of breaking easily. The length and size of the tubing and the degree of the bends may be adapted to the user's convenience. When the aspirator is assembled, place the end of the rubber tubing in the mouth, aim the longer tube of the aspirator at a small insect, and suck sharply. The air current will pull the How To Collect and Preserve Insects for Study 69 insect into the vial. With a little prac- tice it is possible to collect small insects much more quickly and in better con- dition this way than by almost any other method. A convenient attachment for collect- ing thrips, small flies, tiny beetles, and other minute insects normally killed in liquid is illustrated in figure 3, C. A piece of fine-mesh bolting cloth, in- serted in the glass tubing near the large end, keeps the tiny insects from going on into the aspirator. They can then be blown out into the vial of liquid in which they are to be preserved. Some collectors prefer the tube-type aspirator, the body of which is illus- trated in figure 3, D. Either the tube- type or the vial-type aspirator may be converted to a blow-type collecting bottle by substituting for the shorter tube, to which the rubber tubing is at- tached, the attachment illustrated in figure 3, E. This piece of equipment makes use of an air current to create a partial vacuum; with it in use in the assembled aspirator the same result is obtained by blowing instead of sucking through the rubber tubing. This type of attachment is essential if the aspi- rator is to be used to collect insects that emit disagreeable odors. Many insects spend all or part of their lives in ground litter and leaf- mold. They cannot be captured by ordinary collecting methods. Because they are too active to be caught by hand or feign death when disturbed, a sifter should be used. Almost any container with a wire- mesh bottom will serve as a sifter. The size of the meshes in the screen will depend upon the size of the insects sought. For general purposes a screen with eight meshes to the inch will be satisfactory. The screen may be fas- tened to a wooden frame to make a box-shaped sifter, or it may be attached to a wire hoop, which is then sewed to one end of a cloth sleeve about 12 inches in diameter. In the latter type of sifter it is convenient to have a wire hoop of the same size at the other end of the cloth sleeve to hold it open. Place the leafmold or ground litter in the sifter and shake it gently over a piece of white oilcloth spread flat on the ground. As the insects fall into the cloth they may be easily captured with an aspirator or tweezers. Many insects feign death and are not easily seen until they move, so the debris on the cloth should not be discarded too quickly. The sifter is especially useful for collecting in winter. The collector who wishes to get large numbers of the small insects that are usually found in ground litter will find it advantageous to construct a separator (usually called a Berlese funnel by entomologists) for use in- stead of the sifter. Fundamentally, the separator consists of a funnel over which a sieve containing leafmold or other litter may be placed. The funnel leads into a receptacle containing a liquid preservative, into which the insects fall when driven from the ma- terial in the sieve by the progressive drying with a light bulb or some other source of mild heat. Collecting around lights, especially on warm, humid nights, frequently permits the collector to obtain in abun- dance insects that are captured rarely or not at all by other methods. The use of light traps as a means of obtain- ing insects for the collection is not rec- ommended because specimens are too frequently damaged. Insects for the collection should be selected and cap- tured by attending the light continu- ously while it is in operation. Although any reasonably bright light will serve, more insects are at- tracted to blue lights than to other kinds. A convenient method of collect- ing at a light is to hang up a white sheet so that the light shines upon it; the lower edge is turned up to form a trough into which some of the insects will fall. The specimens are collected as they come to the sheet. Many insects may also be collected around street lights and lighted store windows. Baits of many kinds are valuable aids to the collector. One of the best 7o known uses for baits is in sugaring for moths. For sugaring, make a mixture of molasses or Brown sugar, a little asa- foetida, and stale beer or fermenting fruit juices, and daub it on tree trunks along a route that can be conveniently visited with a lantern or flashlight. As with light collecting, this method is most productive on warm, humid nights. The bait should be applied about dusk and may be visited at in- tervals all that night and frequently will be found to be attractive to in- sects on succeeding nights. Insects that are attracted to sweet substances or decaying meat may be captured in simple jar traps. Bait the jar (an olive bottle or a fruit jar will do) with an appropriate bait and bury it with the open top flush with the sur- face of the ground. It is frequently desirable to set these traps under loose boards or stones lying on the ground. An assortment of tweezers and brushes should be available as an aid in collecting and handling the speci- mens after they are dead. Such equip- ment may be purchased at small cost from most biological supply houses. A few small camel's-hair brushes, sizes o to 2, are handy for picking up small in- sects that might be crushed if handled with tweezers. Moisten the tip of the brush on the tongue or in the liquid preservative, touch the specimen with the brush, and you can transfer it safely to the collecting vial. Rearing is one of the best methods of obtaining good specimens. It has the added advantage of permitting obser- vations on the life history of the species and enables the collector to get ex- amples of the various immature stages. To rear specimens successfully, the natural conditions under which the immature insects were found should be simulated as closely as possible in the rearing cages. Insects that feed on liv- ing plants may be caged over potted plants or fed frequently with fresh ma- terial from their host plant. With a little ingenuity a suitable cage can be Yearbook of Agriculture 1952 prepared. The important thing is to have it tight enough to keep the in- sects in and yet provide for sufficient ventilation so that the container will not sweat. Some loose, slightly moist soil or sand and ground litter should be provided in case the insect is one that pupates in or on the ground. In- sects that feed on decaying animal matter should also have the cage pro- vided with slightly moist soil or sand. Insects that infest seeds and those that cause plant galls may be reared merely by enclosing the seeds or galls in a tight container. Such material should not be permitted to become too dry; neither should it be kept moist, else the material and the specimens will mold. It is a good plan to insert the open end of a glass vial through a hole in the container; then, if the con- tainer is dark, when the specimens emerge they will be attracted to the light, enter the vial, and can be easily removed and killed. Tiny parasitic wasps may be reared from their hosts in this manner. A cardboard ice-cream container is excellent for this type of rearing. Adult moths, butterflies, beetles, and many other insects may be obtained by collecting chrysalids or pupae and cag- ing them until the specimens emerge. In this way the best specimens of moths and butterflies may be secured. Always permit the reared specimen to harden and color completely before killing it, but do not leave it in the cage so long that it will damage itself in trying to escape. Cages should always be placed where they will be safe from ants. Bark and wood are often infested by boring insects, such as beetles. Often these insects can be collected during the winter, the period of effective field collecting being thus extended. If they are placed in glass or metal containers, excellent specimens of the adults may be obtained. The method of killing and pre- serving to be used depends upon the kind of insects involved. No one meth- How To Collect and Preserve Insects for Study od is satisfactory for all specimens. Frequently it is desirable to kill in liquid any specimens that will later be pinned. The best general liquid killing and preserving agent, which should al- ways be used unless some other pre- servative is especially recommended, is 70 to 75 percent grain (ethyl) alco- hol. Formalin, which is frequently used as a preservative for biological speci- mens, is not recommended as a pre- servative for insects because it hardens the tissues and makes the specimens difficult to prepare for study. In the discussion that follows, alcohol, unless otherwise indicated, means 70 to 75 percent grain alcohol. Detailed instructions for killing and preserving the various kinds of insects are given later, but if the material is not readily recognized, the following rule of thumb may be followed. Use alcohol to kill ants, aphids, beetles, bugs, fleas, lice, mayflies, silver- fish, springtails, and termites. Use a killing bottle for bees, butter- flies, crickets, damselflies, dragonflies, flies, grasshoppers, moths, roaches, and wasps. Use boiling water to kill insect lar- vae, such as cutworms, grubs, and mag- gots, and transfer the specimens to alcohol after a few minutes. Insects killed in alcohol but later mounted dry should first be dehydrated in 100 percent alcohol (200 proof, also called absolute alcohol) . That takes 1 to 24 hours, depending on the size of the specimens. They should then be degreased in xylene (xylol) or benzene (benzol) . This requires about the same length of time as the dehydration. They should be dried and mounted. Specimens killed dry, in a killing bottle, and containing considerable fatty tissue, should be degreased before being mounted. Soak the specimens in a bath of commercial sulfuric ether until the fluid ceases to become yellow from the dissolved oils; change the fluid if necessary. Complete degreasing may take a day to a week, depending on the size and number of specimens, their fat content, and the volume of 71 ether used. A wad of absorbent tissue or filter paper should be placed in the bottom of the container to absorb waste that accumulates and might otherwise cling to the specimens. Ether is highly inflammable and must be used with great care. Other solvents are chloroform, benzene, xy- lene, and diethyl carbonate. If chloro- form is used, the specimens must be held submerged by a wire screen. After being degreased, specimens should be transferred to a clean pad of absorbent tissue and their appendages arranged. When they are dry enough they may be mounted. Specimens that contain little fatty tissue may be mounted without further preparation. Pinned specimens that have become greasy because of the decomposition of body fats may be de- greased by being put in an ether or chloroform bath for a few hours. The following outline gives in- structions for killing and preserving the commoner types of insects and indi- cates the usual method of mounting for study. Anoplura (sucking lice) : Kill and preserve in alcohol. Mount on slides. Coleoptera (beetles) : Kill in alcohol or ethyl acetate vapor. Mount on pins. Collembola (springtails) : Kill and preserve in alcohol. Mount on slides. Corrodentia (booklice) : Kill and preserve in alcohol. Dermaptera (earwigs) : Kill in cya- nide, ethyl acetate vapor, or alcohol. Mount on pins. Diptera (flies) : Kill in cyanide, ex- cept minute forms, such as eye gnats and fungus gnats, which may be killed in alcohol. Mount on pins. Ephemeroptera (mayflies) : Kill and preserve in alcohol. Hemiptera (true bugs and their al- lies) : Kill in cyanide, ethyl acetate vapor, or alcohol, except the immature stages, aphids, scale insects, and Aley- rodidae (whiteflies) . Mount on pins. Nymphs should be killed in alcohol and mounted on pins. Aphids should be killed in alcohol and mounted on slides. 72 Scale insects and whiteflies on host ma- terial should be preserved dry, but if they are not on host material they should be preserved in alcohol. Mount on slides. Hymenoptera (bees, wasps, ants, etc.) : Kill in cyanide, except ants, gall wasps, and small parasitic forms, which may be killed in alcohol. Mount on pins. Isoptera (termites) : Kill and pre- serve in alcohol. Lepidoptera (moths and butter- flies) : Kill in cyanide. Mount on pins. Mallophaga (biting lice) : Kill in alcohol. Mount on slides. Mecoptera (scorpionflies) : Kill in cyanide. Mount on pins. Neuroptera ( lacewings, ant-lions, etc.) : Kill in cyanide. Mount on pins. Odonata (dragonflies) : Kill in cya- nide. Mount on pins. Orthoptera (grasshoppers, crickets, roaches) : Kill in cyanide. Mount on pins. Plecoptera (stoneflies) : Kill and preserve in alcohol. Siphonaptera (fleas) : Kill in alco- hol. Mount on slides. Thysanoptera (thrips) : Kill in a liquid made of 8 parts 95 percent alco- hol, 5 parts distilled water, 1 part gly- cerin, and 1 part glacial acetic acid. Mount on slides. Thysanura (silverfish and their al- lies) : Kill and preserve in alcohol. Trichoptera (caddisflies) : Kill in cyanide. Mount on pins. Larvae of insects should be killed in boiling water and allowed to remain in the water from 1 to 5 minutes accord- ing to size, then preserved in alcohol. Centipedes, millipedes, mites, spi- ders, ticks, and other small arthropods should be killed and preserved in alco- hol. The smaller forms are usually mounted on slides. It is frequently impracticable to mount all collected specimens soon after they are killed, and some method of caring for them so they will not be broken must be used. Specimens col- lected in liquid may be preserved in Yearbook of Agriculture 1952 4. Method of folding a rectangular piece of paper to form a triangular envelope for large-winged insects: A, Correct shape of unfolded paper, showing where the folds should be made and the sequence of the first three folds; B, "triangle" almost com- pletely folded, showing correct position of the enclosed butterfly. 5. Illustration of right and wrong methods of pinning: A, Correct height and position of specimen; B, insect too low on the pin; C, insect tilted on the pin. it indefinitely without injury, the only precaution being to keep plenty of fluid in the container. Specimens killed in the ethyl acetate bottle and intended for the ether bath may also be pre- served indefinitely in a container with just enough ethyl acetate to keep them from drying. Specimens that are killed in cyanide and are to be mounted without further treatment will soon become dry and brittle. They should be placed in paper pill boxes between layers of cellucotton cut to fit the box and packed tightly enough so that the specimens will not shift about, but not pressed down enough to flatten or distort them. Cot- ton should not be used, as legs and How To Collect and Preserve Insects for Study 73 6. Examples of correct pinning methods for common insects; the black spots show where the pins should go. A, Grasshopper and related Orthoptera, showing how wings should be spread; B, side view of a grasshopper, showing position of legs and antennae; C, a stink bug, an example of the order Hemiptera, showing method of pinning large bugs; D, a bee, order Hymenoptera, to show where bees, wasps, and flies should be pinned; E, a May beetle, order Coleoptera, showing method of pinning beetles; F, G, butterfly and moth, order Lepidoptera, showing location of pin and position of wings and antennae. antennae catch on the fibers and are apt to be broken off. Medium-size and small Lepidoptera should be packed one specimen to a layer. Large Lepi- doptera, Odonata, and other insects with large wings and relatively small bodies should be placed in envelopes or folded "triangles" (fig. 4), which may then be packed between layers of cellucotton. Specimens are mounted to facili- tate handling and study. Their value increases with the convenience with which they may be examined. Some insects, such as scale insects, aphids, lice, thrips, and other minute forms, can be satisfactorily studied only after they are mounted on a microscope slide. The proper preparation of slide mounts is a task requiring consider- able equipment and experience, and slide preparations should not be at- tempted without the aid of specific in- structions, which are usually different for different groups of insects. For the usual larger insects, standard pinning practices have been developed, de- signed to avoid injury to the specimens and to expedite study. Medium and large insects should be pinned vertically through the body. Figure 5 illustrates some right and wrong pinning practices. The height of the specimen on the pin will depend somewhat on its size. There should be enough room at the top of the pin so that it may be handled without letting the fingers touch the specimen (fig. 5, A), but it should not be so low that proper labels cannot be placed beneath the specimen. 74 Yearbook of Agriculture 1952 7. Portion of a spreading board, showing construction of the board and steps in the process of spreading the wings and arranging the abdomen and antennae of a butterfly, order Lepidoptera. The standard methods for pinning the commoner kinds of insects are : i. Grasshoppers, katydids, etc. : Pin through the back part of the thorax to the right of the middle line (fig. 6, A) . 2. Stink bugs and other large He- miptera : Pin through the scutellum to the right of the middle line (fig. 6, C) . 3. Bees, wasps, and flies: Pin through the thorax between or a little behind the bases of the forewings and to the right of the middle line (fig. 6, D). 4. Beetles: Pin through the right wing cover near the base (fig. 6, E) . 5. Moths, butterflies, dragonflies, and damselflies : Pin through the mid- dle line of the thorax at the thickest point or between or a little behind the bases of the forewings (fig. 6, F, G). Before the specimen is permitted to dry (or after being thoroughly relaxed if already dried) the legs, wings, and antennae should be properly arranged so they are visible for study, as shown in figure 6. With many insects, such as beetles, bugs, flies, and bees, it is only necessary to arrange the legs and an- tennae and they will stay in place. It is usually necessary to pin specimens of grasshoppers close to the edge of a box so that other pins to hold the legs in place may be thrust into the sides of the box at various angles. With some specimens, such as wasps and long- legged flies and bugs, the legs and ab- How To Collect and Preserve Insects for Study domen may be kept in place until dry by pushing a piece of stiff paper up on the pin beneath them. Moths, butterflies, and sometimes grasshoppers, dragonflies, and cicadas, should have the wings on one or both sides spread. A spreading board such as shown in figure 7 is useful for this purpose. The collector will find it advanta- geous to have several boards with the middle grooves of different widths to accommodate insects of various sizes, but for general purposes a board made from the following materials will be satisfactory : 1 . A hardwood base, J4 by 4 by 12 inches. 2. Two hardwood end pieces, l/z by 3/4. by 4 inches. 3. Two softwood top pieces, % by lYs by 12 inches. 4. One flat strip of cork, ^4 by 1 by 1 1 inches. When assembled as illustrated, the softwood top pieces leave a groove ^4 inch wide. On the under side of these, a cork strip is glued so that it covers the space between the top pieces. Specimens must be thoroughly re- laxed for spreading; otherwise they will be broken. Figure 7 shows the wings on the left side of the specimen spread in the proper manner. The first step in spreading the wings, after pin- ning the specimen in the groove at the proper height, is shown on the right side of the board in figure 7. To com- plete the process, hold the strip of semitransparent paper covering the wings gently with the fingers of one hand and pull the wings forward with an insect pin until the. hind margin of the forewing is at right angles to the body of the insect. The hind wing should then be brought forward until its front margin is just under the hind margin of the forewing. Pin both wings in place with plenty of pins arranged around them, not through them. The abdomen and antennae should also be held in place by pins. The paper strips holding the wings in place should be of fairly thin, not stiff, paper. 75 Specimens should be left on the spreading board until thoroughly dry. For large insects this requires 2 or 3 weeks. Smaller specimens will dry in less time. During this time they should be stored in pestproof containers. Do not forget the collection-data label, which should be associated with the specimen at all times. Small insects that cannot be pinned directly through the body with regular insect pins should be mounted on card points or on special pins known as minuten nadeln. Card points are slender triangles of paper. These are pinned through the broad end with a regular insect pin ( No. 2 or 3 ) , and the specimen is glued to the point, as illustrated in figure 8, A. Card points may be cut with scissors from a strip of paper % inch wide, but a punch, obtainable from supply houses, makes better and more uniform points. A good-quality linen ledger paper should be used; "sub- stance 36" is recommended. Ordinary glue is not recommended for fasten- ing the specimen to the point because it tends to become brittle. Some of the clear acetate cellulose cements, such as Ambroid, which may be purchased in small amounts at variety stores, are more satisfactory. An adequate supply may be made by dissolving a trans- parent resin toothbrush handle in a small amount of banana oil (amyl acetate). Pure white shellac is also fairly satisfactory. Whatever adhesive is used, it should not be permitted to get so thick that it "strings," and only a small amount should be used. To mount most insects, the tip of the card point should be bent down at a slight angle so that when the insect is in an upright position the bent tip of the point fits against the side of the insect (fig. 8, B). Only a very small part of the point should be bent; a little practice will make it easy to judge how much of the point should be bent and at what angle to fit the particular specimen that is being mounted. Most insects that are mounted on points y6 Yearbook of Agriculture 1952 Hyper a ' pos r j ca (GylO Oef, L.L.B. I9»2 8. Double mounts for small insects: A, Position of card point and labels on the pin; B, details of attachment of specimen to card point; C, small moth, order Lepidoptera, pinned with a "minuten nadeln" to a block of pith on a regular insect pin; D, a mosquito, order Diptera, pinned with a "minuten nadeln" to a block of cork on a regular insect pin; E, method of attaching an inflated larva to a regular insect pin by twisting fine wire around a block of cork. should be attached by the right side, although there are a few exceptions to this rule. A convenient method is to arrange the insects on their backs or left sides with their heads toward the worker; then, with the pin held in the left hand, touch a bit of adhesive to the bent point and apply it to the right side of the insect. If the tip of the point can be slipped between the body of the insect and an adjacent leg, a stronger mount will result. The insect should be attached to the point by the side of the thorax, not by the wing, abdomen, or head. Some insects, too heavy to be held on the point by the adhesive and not large enough to be pinned with regular pins, may be attached to card points by puncturing the right side at the place where the card point would nor- mally be placed and inserting in this puncture the tip of an unbent card point with a little adhesive on it. For puncturing specimens, a needle ground to make a small, sharp scalpel is best. Minuten nadeln are very small steel pins without heads. They are used to pin small insects on a piece of cork or pith, which is then pinned on a regular insect pin, as illustrated in figure 8, C, D. They should never be used for hard-bodied insects (beetles, bugs). As with direct pinning, insects mounted on double mounts should be prepared according to standard prac- tices. For the commoner groups these are: i. Beetles, bugs, leafhoppers, etc.: Mount on card points with the tip bent down and attached to the right side of the specimen (fig. 8, A, B) . 2. Small parasitic wasps: Mount on unbent card points with the adhesive applied to the left side of the specimen and the feet toward the pin. How To Collect and Preserve Insects for Study 3. Small moths: Mount on minuten nadeln thrust through the middle of the thorax from above and with the abdomen of the specimen toward the insect pin (fig. 8, C). 4. Small flies and mosquitoes: Pin with minuten nadeln through the side of the thorax with the right side of the specimen toward the insect pin (fig. 8, D). Some workers prefer small flies fastened directly to regular insect pins by a bit of adhesive applied to the right side of the specimen. Insects that have dried after be- ing killed in a cyanide bottle must be relaxed before they are mounted. This can easily be done in a relaxing jar made as follows: Into a wide-mouth jar or can with a tight cover put an inch or two of clean sand ; saturate the sand with water to which a few drops of phenol (carbolic acid) have been added to keep mold from growing; cover the sand with a piece or two of cardboard cut to fit the jar, and it is ready for use. Specimens must not come in direct contact with the water and should not be left in the relaxer too long or they will be spoiled. From 1 to 3 days is usually enough. A relaxer should not be left where it will get too warm, or it will sweat on the inside. Temporary labels giving essential information as to date and place of collection should be attached to speci- mens during preparation and mount- ing. Before they are put away in the collection, they should be given perma- nent labels, placed on the pin or in the vial. These labels are small, and the data on them must be restricted to the most important information. Addi- tional information about the specimen or specimens may be kept in field notes, associated with the proper material by means of lot numbers or some other convenient system. When specimens are sent for identification they should always be accompanied by all available information. The following information should be given on the label or labels for each 970134°— 52 7 77 specimen: Locality (usually a place shown on a good map) ; the day, month, and year when collected; the name of the collector; and, if known, the host, food plant, or material attacked. Permanent labels should be on good-quality paper, heavy enough so that it will stay flat when the labels are cut out, of a texture that it will not come loose on the pin, and with a sur- face that can be written on with a fine pen. The ink should be permanent and should not run if the labels are placed in jars containing liquid preservative. The size of the pin labels will depend somewhat on the insects for which they are intended. Very small labels, neces- sary for small specimens mounted on points, are not suitable for large moths, butterflies, cicadas, etc., because they cannot be easily read when pinned be- low these large-bodied insects. Large labels, suitable for the larger insects, take up too much room in the collec- tion if used for small specimens. Labels printed with 4-point type or diamond type will be found suitable for most purposes. Labels may also be made any size by printing a few of them in strips in large type, having an etching made at the desired reduction, and printing the desired number of labels from the etching. Labels should be attached so that they are balanced with the mounted specimen. Figure 8, A illustrates how to pin labels for specimens mounted on points; for pinned specimens the long axis of the label should coincide with the long axis of the specimen, and the left margin of the label should be to- ward the head of the specimen. The label may be run up on the pin to the desired height by using the pinning block; the middle step will usually give about the right height. Standard equipment for housing the collection assures uniformity of containers when additions are neces- sary. It is obtainable from any of sev- eral reliable supply houses. 78 Material preserved in liquid need receive no attention other than re- placement of preservative and corks. Vials should be examined periodically to be sure the specimens do not become dry. Small vials may be stored in racks in such a way that the corks are not in constant contact with the liquid; this also expedites arrangement and ex- amination of the material. Vials that cannot be inspected frequently should have the corks replaced with cotton plugs and be placed upside down in a jar large enough to hold several vials, and the jar partially filled with the preservative. Pinned specimens should be housed ,in pestproof boxes. Standard insect boxes, called Schmitt boxes, are rec- ommended. If other boxes, such as cork-lined cigar boxes, are used, they must be examined frequently for evi- dence of pest damage and fumigated periodically. Even pestproof boxes should be fumigated occasionally, lest a pest gain entrance and damage all the specimens. Most entomological in- stitutions store their collections in glass- top drawers fitted with cork-lined trays of various sizes which can be shifted and arranged without the ne- cessity of repinning specimens. A few simple precautions against museum pests, such as carpet beetles, are a necessary part of the care of ma- terial not preserved in liquid. Naphtha- lene, in the form of ordinary moth balls or flakes, is inexpensive and satisfactory as a repellent, but it will not kill pests once they have gained access to the collection. To kill pests it is necessary to use some fumigant such as paradi- chlorobenzene (PDB), carbon disul- fide, ethylene dichloride, or carbon tetrachloride. Carbon disulfide is prob- ably the most widely used and is effec- tive, but it is inflammable and explosive when mixed with air in certain pro- portions, it has an unpleasant odor, and it will stain insect boxes. A small amount of naphthalene or paradichlorobenzene may be included in each box of specimens, either in a cloth bag or a small box with a perfo- Y ear book of Agriculture 1952 rated top firmly pinned in the corner. Naphthalene in the form of moth balls may be pinned in the box by attaching the ball to an ordinary pin. To do this, heat the head of the pin, force it into the moth ball, and permit it to cool. Liquid fumigants may be used without the danger of staining the boxes by saturating a cotton plug and placing it in a short, wide-mouthed vial pinned in the corner. Adult insects intended for a col- lection or submitted for identification (to Federal, State, or county ento- mological authorities, for example) should not be shipped alive without a permit from the United States Depart- ment of Agriculture. Pupae or larvae sent for rearing should be enclosed in tight containers, such as tin salve boxes or mailing cases. Pupae preferably should be packed loosely in moist (but not wet) moss. Larvae should be packed with enough food material to last until they arrive at the destination. Bulky insects, or pieces of host plants bearing insects such as scale in- sects, should be partly or completely dried before being placed in a con- tainer or should be packed in a con- tainer that will permit drying to con- tinue after closure. Mounted insects should be firmly pinned in a box securely lined with cork or some other suitable material. Vials should be wrapped separately in strong paper and then packed in a mailing case or strong box with cotton or cellucotton around them. Do not put loose naphthalene or paradichlorobenzene in either pill boxes or insect boxes that are being shipped. Never send insects in ordi- nary envelopes. Paul W. Oman received his aca- demic training at the University of Kansas and the George Washington University. He joined the division of insect identification of the Bureau of Entomology and Plant Quarantine in IQ30. He began a period of military duty in ig^o. Insects as Helpers Insect Friends of Man F. C. Bishop p We must spend some time in our gardens watching insects at work to ap- preciate how they cooperate in giving us food, flowers, and comfort and to know that insects are not all bad. Some insects improve soil. Air pene- trates the soil through the burrows of ants, grubs, beetles, and wild bees. These burrowing hordes also bring earth to the surface from the deeper soil layers and thus aid in improving its physical condition and in burying decaying vegetable matter. The grubs, or larvae, of many wood-inhabiting beetles, ants, termites, and minute in- sects (like the springtails) are con- stantly at work, tearing to pieces leaves, twigs, and trunks of fallen trees so that they may be returned to the soil to pro- vide nutrients for other plant growth. Insects hasten the decay of animal bodies and their return to the soil. Thus they figure in the endless cycle that in- volves all life. Not that the insects en- gaged in soil-forming activities are wholly beneficial. Some, like white grubs and cicadas, in their young stages may damage plants by feeding on the roots and (as adults) by attacking the stems, twigs, leaves, or fruit. Others, such as blow fly maggots, after they have done their work of carrion dis- posal and soil penetration may become disease-bearing flies. Some other helpful insects we call predators and parasites. The predators are the lions and tigers of the insect world. Some devour a large part or all of their prey. Others, such as the ant-lions, merely suck the body fluids. The predatory insects of greatest economic importance are the dragon- flies, damselflies, aphis-lions, ground beetles, lady beetles, and syrphid flies. Among the many other predators are the ant-lions or doodle-bugs, robber flies, snipe flies, tiger beetles, and wasps and ants. Dragonflies and damselflies are interesting and familiar. There are about 2,000 known species, 300 of which occur in the United States. The gauzy-winged, brilliantly colored crea- tures called dragonflies, devil's-darn- ing-needles, or mosquito hawks live around ponds, lakes, and swamps. Their enormous eyes, made up of as many as 20,000 sight units, or facets, occupy a large part of the head and are so curved as to permit the insect to see in all directions at once. A network of veins covers the two pairs of large, rigidly extended wings. Its highly developed eyes and speedy flight enable the dragonfly to catch in flight the mosquitoes and other small insects that are its only food. In flight the legs form a sort of basket into which the small insects are scooped. The dragonfly, while still on the wing, promptly devours the insects with its stout jaws, which work sidewise. Among our dragonflies is the big green darner, Anax Junius. The dragonflies are fast fliers and may travel far. Some of the larger spe- 79 8o cies commonly hunt several miles from their breeding grounds. They migrate long distances when swamps dry up. Migrations from Australia to Tasma- nia, 200 miles away, have been recorded. The damselflies are smaller and more delicate than the dragonflies, flit about more leisurely, and fold the wings on the back when at rest. They prey on small, soft-bodied insects. The young of dragonflies and dam- selflies, known as nymphs or naiads, destroy mosquitoes and other insects in the water. These strange-looking crea- tures live among the debris of stones on the bottom of streams and ponds. They have an odd, jointed extension of the under lip, or labium, which folds over the mouth parts but can be suddenly extended to grasp prey with its two powerful hooks. The tiny naiads usually grow to full size, 1 to 2 inches long, in several months, but some species may spend 3 or 4 years in this stage. When it is grown, the naiad crawls out of the water on a stick or stone. When it has dried off, the skin splits down the back, and the head, thorax, netted wings, legs, and finally the long abdomen are drawn out. Soon the beautiful wings are spread, the metallic colors appear, and the new predatory life begins. The aphis-lions are among the most helpful insects of prey. There are 15 families in this group of nerve- winged insects. All are predaceous. Among them are the dobsonflies; the ant-lions, or doodle-bugs; and the aphis-lions, or golden-eyed lacewings. The aphis-lions are in gardens every- where. They destroy many kinds of de- structive insects, the eggs of many caterpillars, all stages of plant-feeding mites, scale insects, aphids, and mealy- bugs. Aphis-lions are the young, or larvae, of delicate, gauzy-winged insects with rather long antennae and beautiful golden eyes. These lacewings often are seen crawling about on the leaves or flying rather clumsily from plant to Yearbook of Agriculture 1952 plant. The many species have similar habits and general appearance. Some are pale green. Others are brownish. The adult lacewing usually lives 4 to 6 weeks. In that time the female may lay several hundred eggs. To keep the ravenous little aphis- Enallagma exsulans, a damselfly. lion that first hatches from devouring its brothers and sisters before they hatch ( and perhaps to give protection from other enemies), the mother lace- wing lays each oval egg on the top of a delicate stalk projecting from the sur- face of a leaf or twig. The incubation period is 6 to 14 days. The larvae are odd, grayish-brownish creatures. They have a rather broad abdomen and con- spicuous curved jaws, which extend forward from the head. With its pin- cerlike jaws the larva seizes its prey and sucks out its body juices. When the larvae attain full growth, in 2 or 3 weeks, they spin oval, yellow- ish-white pea-sized cocoons on a leaf. The larva in its spinning operations tops off each cocoon with a circular cap, which the pupa pushes off when it is ready to become an adult. The change to the adult stage takes 1 to 3 weeks in warm weather. Praying mantids are odd-looking relatives of the grasshoppers. The name comes from the attitude they assume as they rest on twigs or stalk their prey. The Chinese mantis is 4 inches long Insect Friends of Man and can capture, hold, and devour large insects. Since it came into the United States about 1896, it has spread through much of the East. Like all members of its family, it lives on insects in its nymphal and adult stages. The mantid is cannibalistic. The female de- vours the male with which she mates and often eats her own young. The eggs, laid in rather large masses, are firmly attached to twigs of trees. Each mass contains 50 to 400 eggs. A female often deposits 3 to 6 masses. Winter is passed in the egg stage. There is usually only one generation a year. The young resemble the adults except that they have no wings. Lady beetles have habits that are anything but ladylike. Both the young and adult beetles kill and greedily eat various soft-bodied insects. Most famil- iar are the bright reddish-yellow spe- cies, which has black spots on the wing covers, or elytra, and the black species, which has red spots. Less well known are the numerous minute black species. Not many persons associate the rather clumsy-looking dark-colored larvae with the bright-colored adults. Neither do gardeners, familiar with the Mexi- can bean beetle and the squash beetle and their depredations, recognize them as lady beetles gone astray. Many of the lady beetles are native to the United States. Their combined action in destroying the eggs and young of de- structive aphids, scales, and other soft- bodied plant-feeding insects is of great value to those who raise crops and flowers. Sometimes they are called lady birds, as in the old rhyme : "Lady bird, lady bird! Fly away home! Your house is on fire, your children do roam." The eggs of lady beetles are oval and yellow or orange. They are laid in small masses, usually on the under side of leaves, and hatch in a few days. The young larva, with its six long legs and tubercle-covered body, starts in search for soft insects. It devours one aphid after another. In about 20 days it be- comes full-grown and is about one- fourth inch long. It then attaches it- self to a leaf or stem by the tip of its abdomen, draws itself up, and pupates. The cast skin often remains more or less over the pupa. The adult splits the pupal skin and crawls forth to make further inroads on the fast-multiplying aphids. Some species congregate in great masses in the fall and spend the winter in that way in some protected place. The vedalia, the small, reddish- brown Australian lady beetle, has done yeoman service against the cottony- cushion scale on fruit trees in the United States, Hawaii, New Zealand, and other countries into which it was imported to do just that. We tell more about the vedalia on page 380. Syrphid flies help in the pollination of crops. The sluglike larvae of many species are effective killers of various plant pests, especially aphids. The flies usually are brightly colored. Some have banded bodies and buzz loudly in fly- ing, so they are often mistaken for bees. The eggs are laid on the leaves near aphid colonies. Even the newly hatched larvae capture and destroy aphids. Adults and larvae of many other groups of the true flies prey upon other insects and are of value in reducing pest damage. The parasitic insects are less spec- tacular in their work than the preda- tory ones but are more interesting and helpful to man. Several groups of in- sects contain species that are parasitic on other insects. The most abundant and important of these are two-winged flies and the wasps. Parasites attack insects of all types in all stages of development. The host is not killed at once. Usually the larva of the parasite enters the body of its host and feeds on its tissues until it is nearly grown ; then the host dies. The parasite may then pupate within the dead body or emerge and pupate on or nearby the remains of the host insect. Tachinid flies resemble large, bristly house flies. The many species prey on a wide variety of insects, especially cater- 82 pillars. The flies are seen frequently about flowers, feeding on the nectar. Most of the species lay eggs, but some deposit maggots. The eggs are usually attached to the skin of the host. On hatching, the maggot penetrates the skin. A caterpillar may be killed by a single fly larva, or it may serve as host for a dozen or more. Some species lay their eggs on the soil, and newly hatched maggots seek a host, penetrate its body, and develop within it. The troublesome European earwig is heavily parasitized by a fly of this type. Other species oviposit on the leaves of plants. When a caterpillar eats the leaf, the small eggs are swal- lowed, the maggots hatch, bore through the wall of the digestive tract, and develop in the body cavity. Compsilura concinnata, a fly im- ported from Europe to combat the gypsy moth and brown-tail moth, in- serts its young into the caterpillar. The fly has been found to develop in the larvae of about i oo different species of destructive caterpillars, which it checks effectively. The flesh flies are a large family. Some are small and some are rather large and gray. They have varied habits. Some are parasites of warm- blooded animals. Others are scaven- gers. Many are parasitic on many kinds of insects, some of which are serious crop pests. All flesh flies deposit living young. The grasshopper maggot, a parasite of grasshoppers, is a member of this family. The adult fly emerges in spring from the soil where it has spent the winter as a pupa, soon mates, and be- gins depositing its maggots on grass- hoppers, usually while the grasshopper is in flight. The fly darts at a hopper in the air and attaches one of its minute sticky maggots to its host. The larva bores in; when it is fully developed, the host dies. The large maggot then crawls out and enters the soil to pupate. Wasps feed mostly on other insects. The yellow-jackets eat vegetable mat- ter, such as overripe fruit, and soft- bodied insects, the juice of which they Yearbook of Agriculture 1952 feed to their young. Both yellow-jack- ets and the larger wasps, reddish to mahogany in color and known as Polistes, kill such destructive caterpil- lars as the corn earworm and army- worm. The yellow-jackets build large, globular, enclosed paper nests on build- ings, in trees or shrubs, or in under- ground cavities. The Polistes build flat, open nests in similar situations. They can be a nuisance about houses because they sting viciously when they are mo- lested. The benefits derived from the predacious habits of the Polistes out- weigh their objectionable traits. These wasps are social. Their fam- ilies are made up of males, females, and sterile workers. Usually the ferti- lized females of Polistes pass the win- ter in protected places like attics while the yellow-jackets overwinter in pro- tected places out of doors. In the spring they start a small paper nest, lay eggs in its cells, and rear a small number of workers, which continue to build more cells and largely take over the care of the young. During a season a Polistes nest may become 6 to 8 inches in di- ameter and house several hundred wasps; the yellow-jacket family may reach several thousand. The mud daubers, thread-waisted wasps, and digger wasps are not social. The mud daubers construct nests of mud in buildings or other protected places and store them with soft-bodied insects or spiders, upon which the young feed. The other two groups of wasps I mentioned make individual nests in the soil or in logs and store them with insects or spiders. In this group are the so-called tarantula killers and horse guards. Horse guards do much good by catching horse flies, horn flies, and stable flics on livestock. The parasitic wasps help man by combatting destructive insects of prac- tically all kinds. Like other parasitic and predatory insects, however, they do not confine themselves to injurious insects. Some direct their attack against other parasitic insects and are called secondary parasites. The appearance, host relations, and Insect Friends of Man other habits of the parasitic wasps are varied beyond the possibility of gen- eralizing about them. Typically, the adults have four wings, usually clear, with various types of veins. The body color is mostly brown or black. The Lysiphlebus testaceipes ovipositing in an aphid. wasps differ greatly in size. Some are so small that several may develop with- in an insect egg no larger than a pin- head. Others have a body length of 2 inches or more. Among the parasitic wasps are di- verse types of reproduction and host relations. Females of some species in several families reproduce generation after generation without males. Others have both sexes in certain generations. In general, virgin females produce female offspring. Among the wasplike parasites two to a dozen or more in- dividuals may develop from a single egg — a phenomenon known as polyem- bryony. It occurs in several families of this group. Their reproductive capacity often is enormous. A number of species deposit several hundred eggs a day and lay a total of 1,000 to 1,500. In some species the developmental cycle may be completed in 5 to 10 days. If suit- able hosts are present, therefore, the number of offspring of a single female might reach millions in one season. Certain species of parasitic wasps attack only one species of insect or are restricted to closely related species as hosts. Many, though, will attack a great variety of hosts. Some species of par- asites lay their eggs in the egg of the host, and the larvae do not complete development until the host has reached larval maturity or has pupated. 83 Lysiphlebus testaceipes, a useful and readily observed parasite, is a slender but industrious little insect that de- stroys millions of aphids. It becomes very active on sunny days. Then it scurries about among the aphids on a leaf and stops here and there to tap an aphid with its antennae. Afterwards, it thrusts its ovipositor into the aphid with a quick motion and deposits an egg within. The aphid shows no ill effects for about 3 days, when it stops reproducing. Soon the rapidly devel- oping parasite larva devours the vital organs of the aphid. The minute egg parasites are ex- tremely numerous and of great eco- nomic importance. One of these, Tri- chogramma minutum, which destroys the eggs of many of our most injurious pests, such as the cotton leafworm, boll- worm, codling moth, and sugarcane borer, has been propagated and re- leased by the millions in infested fields and groves. There is doubt, however, as to the degree of control these parasites can achieve. Insects are indispensable as polli- nizers of plants. Many insects serve us in this way — thrips, butterflies, ants. beetles, flies, wasps, and bees. The chapters that follow give details of this vital subject, but it is hardly Trichogramma minutum female stinging a moth egg and placing its own egg within it. 84 amiss to give some of the main points here, too. Some 50 seed and fruit crops depend on honey bees or yield more satisfac- torily because of their presence. Some, such as red and white clover, onions, most varieties of apples, sweet cherries, and plums, would be barren without insect pollinators. A strong colony of honey bees may contain 60,000 or more workers. An estimated 37,000 loads of nectar are required to make a pound of honey; the bees in a colony, each making 10 field trips a day, would visit 300,000 flowers a day. Thus honey bees are more important in fertilizing crops than in producing honey — even though 200 million pounds of honey and 4 mil- lion pounds of wax are produced each year in the United States. Beeswax, extensively used in indus- try and the arts, is secreted as thin scales or flakes by glands on the under side of the abdomen of the worker bee. The bee uses wax to make the comb, in which honey is stored and the young reared. The artistry and engineering ability of the bee can be appreciated by noting the perfection of the hexag- onal cells and the evenness of their delicate walls in a section of comb honey. In every colony there are three forms, the queen or female, drones or males, and workers. The workers, im- perfectly developed females, are most numerous and do all the work. The drones are somewhat larger than the workers, devoid of stings, and few in number. They appear most plentifully in the early summer at swarming time, after which the workers drive them out of the hives. The queen is much larger than the worker bees and her sole duty is to lay eggs. During the 2 or 3 years of her existence she may lay as many as a million eggs. These are placed in the bottom of newly cleaned and polished cells and hatch into minute white, leg- less grubs, or larvae, in 3 days. The cells are capped by the workers, and the larvae spin their cocoons and Yearbook of Agriculture 1952 pupate. In this quiet stage the larva transforms to the winged insect in about 1 2 days. Thus the development from egg to adult takes 2 1 days. Drones require 24 days and queens only 16. The worker bee, on reaching maturity, cuts out the cell cap and crawls out. For a time it is relatively inactive. Then it becomes a nurse or house bee and helps care for its sisters. Later it takes up the work as a field bee. Some field bees gather nectar. Others collect pollen, which is tucked into the pollen baskets formed of hairs on the outside of the tibia of the hind legs. The place of bees in agriculture is coming to be recognized, and more at- tention is being given to the use of honey bees in the replacement of wild bees as crop pollinizers. Intensive cul- tivation of land and the general use of insecticides are rapidly eliminating native pollinating insects. Increasing the number of hives of honey bees and the numerical strength of those colo- nies is a means of overcoming this defi- ciency in our agriculture. An example of the unusual rela- tions of insects to plants — the delicate balance between plants and their pol- linators— is that of the minute fig wasp and the Smyrna fig. The fig is a fleshy, hollow, pear-shaped growth, which contains hundreds of minute flowers lining the interior surfaces of a cavity. The cavity has a tiny opening at the apex, the free end. The Smyrna fig produces only female flowers and no pollen. Before 1900 the Smyrna figs produced in the United States were inferior to those grown in Asia Minor. An investigation revealed that this was due to the absence in California of the minute, wasplike insects that serve as pollinators of the Asiatic figs, their sole agent of pollination. The insects de- velop in wild inedible figs known as caprifigs, which produce only male flowers with an abundance of pollen and which are the parent stock of our edible figs. The male insects are wing- less and never leave the fruit in which they develop. They find a female still Insect Friends of Man in a gall-like formation within the fig, puncture this cell, and fertilize the fe- male. She then gnaws her way out and in escaping from the fig becomes cov- ered with pollen. She is winged and flies about seeking a place to lay her eggs. She enters Smyrna figs as well as caprifigs if they grow near each other. The Smyrna figs are not suitable for the development of the fig wasp, but pollen from her body accomplishes fer- tilization of the fig flowers and the development of a delicious fruit. Repeated efforts to introduce the fig wasps from Asia Minor into California were finally successful. Recently these wasps began causing trouble by carry- ing a disease, brown rot, from the wild figs to the Smyrnas. This was met by rearing the fig wasps by millions in in- cubators free from the disease and liberating them in the fig orchards. Most scale insects are injurious because they suck the juices from many of our cultivated plants. Some, how- ever, have been turned to our benefit. The lac insect, Laccifer lacca, is one. It lives on trees of the fig family, com- monly in the East Indies, Malay, and India. The minute young lac insect, or crawler, finds a suitable place on a twig, or branch, inserts its beak into the plant tissue, grows, and secretes a resin- ous material, which ultimately covers it. The thousands of crawlers settle side by side, and the resinous secretion builds up around them and completely encases the twig. Most of the crawlers develop in about 3 months into fe- males, which occupy small cavities in the resinous mass and from which they never escape. The males emerge and fertilize the females through the small openings which extend to the surface of the encrustation. As the eggs de- velop in the body of the female, she assumes a saclike, bright-red appear- ance. The red pigment is the source of the lac dye of commerce. The female dies, the eggs hatch, the crawlers escape and move to a nearby unin- fested part of the twig, and the process is repeated. . 85 The largest yields of lac and dye are obtained by harvesting the infested twigs while the females are still living. That is done twice a year, about June and November. The encrusted twigs are known as stick lac. About 40 mil- lion pounds of the material are har- vested each year. The stick lac is ground, largely in crude mortars. The resulting granular lac is called seed lac; the fine particles are molded into toys and ornaments, and the wood is used for fuel. The seed lac is then washed, melted, spread out in a thin layer, and dried, thus forming the shel- lac of commerce. Many people of In- dia depend upon the lac industry for a living. The red dye from the lac insects is little used today. It is made by evapo- rating the water in which the seed lac is washed. A dye formerly widely used in in- dustry, known as cochineal, is made from the dried, pulverized bodies of an insect related to the lac insect. It lives on a cactus or pricklypear. Cochineal is used mainly in cosmetics, as a color- ing for beverages, and in decorating cakes and pastries. It used to be prized as a dye for textiles because of its per- manence. Cochineal is produced mainly in Honduras, the Canary Islands, and Mexico. The insects are kept over win- ter on cactus plants in houses. In spring the females are transferred to cacti out- doors and can be harvested in 3 months. About 70,000 insects are re- quired to make a pound of dye. Galls are peculiar growths pro- duced by a number of insects. They usually damage somewhat the plants they attack, but some kinds are used as a source of dyes and tanning mate- rials and for medicines. The Aleppo gall, or gallnut, pro- duced by a wasplike insect on several species of oaks in western Asia and east- ern Europe, has been used for centuries as a tonic, astringent, and antidote for certain poisons. The early Greeks used it for dyeing wool, mohair, and skins. 86 Other galls have been used for dyeing fabrics and as a tattoo dye. The Aleppo gall is used for preparing a permanent type of ink. It has been specified in formulas for ink by the United States Treasury and the Bank of England. Silk originates in the spittle of an insect. In China and Japan, thousands of families care for silkworms as a part of their daily activities during the sum- mer months. The silk industry began in China, where the source of silk was kept a secret for more than 2,000 years. At- tempts to take silkworm eggs out of the country were punishable by death. A few eggs were smuggled out of China about A. D. 555 and taken to Constan- tinople. Since that time commercial production has sprung up in some of the warmer countries, but the industry has been confined largely to China, Ja- pan, India, and the Mediterranean region. Sericulture has been attempted in the United States and interest in it is considerable. Silkworms can be raised and mulberry trees grown successfully here, but a tremendous amount of hand labor is involved and Americans must compete with the low labor costs in China, Japan, and India. Silk also must now compete in price with syn- thetic fibers, which can be produced at relatively low cost. Men in the Depart- ment of Agriculture conducted experi- ments with silk culture in 1884-91 and 1902-8. That work and many commer- cial undertakings in different parts of the country proved the impracticabil- ity of silk culture in the United States. The silkworm is the larva, or cater- pillar, of the moth Bombyx mori. Man has taken care of it so long that it has become thoroughly domesticated. The ashy-white moth has a fat body and a wing expanse of about 2 inches. It takes no food and seldom attempts to fly. After mating, the female deposits 300 to 400 round, yellow eggs, which soon become gray or lilac and paler as hatching time approaches. At summer temperatures, the eggs Yearbook of Agriculture 1952 hatch in 10 days. The larval stage re- quires 30 to 40 days, during which four molts occur. The baby worms are one-eighth inch long, and the full- grown caterpillar is fully 3 inches long. It is grayish or creamy in color and hairless. It has a hump behind the head and a spinelike horn at the tail. When full-grown, the larvae become restless and, if they are given a suitable place, such as dried brushy plants, they soon begin to spin their cocoons. The opera- tion takes about 3 days of constant motions of the head from side to side at the rate of about 65 a minute. The cocoon is formed from a secretion from two large glands that extend along the inside of the body and open through a common duct on the lower lip. As the clear viscous fluid is exposed to the air it hardens into the fine silk fiber. The filament forming a cocoon is continuous and ranges in length from 800 to 1,200 yards. The cocoons are oval and vary in color, according to strain or race, from white to a beautiful golden yellow. The larva pupates within the co- coon. In about 2 weeks the moth es- capes through an opening in the end of the cocoon. The cocoons from which the moths emerge are called pierced cocoons. They are of low value because they cannot be reeled, but they are carded and made into thread. For rearing moths, the cocoons are usually strung on a thread and hung in a cool, dark place until the moths emerge. The males and females are then put on cheesecloth, where they mate and where the eggs are deposited and adhere lightly to the cloth. The race of silkworms most com- monly used produces only one genera- tion of worms a year, but other races produce two and still others produce several. The eggs are held in cold stor- age until they are to be hatched. An ounce of eggs will produce 30,000 to 35,000 worms, which will yield 100 to 1 20 pounds of fresh cocoons. The co- coons produce 10 to 12 pounds of raw silk. Mulberry leaves are used almost en- Insect Friends of Man tirely as food for silkworms. The white mulberry, Morus alba, is the preferred species. Foliage of Osage-orange has been used as a substitute. Lettuce leaves are sometimes used when the larvae are small. The rearing of silkworms is labori- ous. The larvae are kept in a rearing house on trays in constant shade at a temperature between 65 ° and 78 ° F. They are first fed on chopped mulberry leaves supplied about eight times a day. After 4 or 5 days, fresh leaves are put in a tray with bobbinet bottom. On it is placed the tray that contains the larvae. They soon crawl up onto the fresh food. As they grow, the larvae are transferred frequently to fresh leaves on clean trays. They consume a surprising quantity of leaves, which must always be dry. Wet leaves or other adverse conditions favor the develop- ment of certain diseases which often take a heavy toll of the silkworms. For reeling silk, the cocoons are gathered about 8 days after spinning begins, and the pupae are killed, usually with heat, and thoroughly dried. They are assorted and are ready for reeling. Reeling also involves much hand work, although recently devel- oped reels work largely automatically. The cement holding the fibers together is loosened by putting the cocoons into boiling water. After the loose strands have been removed by a revolving brush the cocoons are put in warm water and the filament from four or five of them is caught up and twisted into a thread which is wound on a reel. This raw silk is removed from the reel in 2-ounce hanks, which are weighed and baled. Insects do not feed entirely on plants we are interested in growing. Many kinds feed on weeds. Certain species of this sort have been intro- duced with beneficial results into re- gions where some plant has become a serious nuisance. The control of pricklypear by in- sects in Australia is an example of what can be accomplished in this way at 87 low cost. About 1787 cactus plants were taken to Australia by Capt. Arthur Phillip for culturing cochineal insects for dye. Various species of cacti escaped later from gardens so that by 1925 some 20 different kinds were found growing wild. In the absence of natural enemies the pricklypears spread rapidly. By 1925 about 60 mil- lion acres were affected, half of it so densely covered as to make the land useless. Australia established a Common- wealth Prickly Pear Board in 1920 and sent entomologists to America, the original home of these cacti, to study the insect enemies and methods of rearing and shipping them to Aus- tralia. This work was continued in North and South America in coopera- tion with American entomologists from 1920 to 1937, during which time more than a half million insects of 50 different species were dispatched to Australia. Several were successfully es- tablished, including cochineal insects, a large plant bug, a moth borer, and a spider mite. The insects checked the new growth of cactus and reduced the density of the plants so that some grass was returning. It was not until 1930, however, when 3 billion eggs of a moth, Cactoblastis cactorum, from Ar- gentina had been released throughout the territory, that the hope of control- ling the pest began to be realized. Seven years after the first introduction of this moth the last dense growth of pricklypear was destroyed and the land reclaimed and opened to settle- ment and livestock production. The total cost was about £168,600, or a fraction of a penny an acre — a modest figure as compared with £10 per acre for the much less satisfactory chemical and mechanical procedure previously used. F. C. Bishopp, a native of Colorado, has been conducting or directing re- search in the Bureau of Entomology and Plant Quarantine on insects since ig04- Since 1Q41 he has been assistant chief of the Bureau. Honey Bees as Agents of Pollination George H. Vansell, W. H. Griggs Plants have sexes somewhat as ani- mals do. Many plants carry both the male and female elements on the same individual. Other plants have the sex organs in separate plants — that is, a plant may be strictly male or female. In any case, pollen from the male part of the plant must come in contact with the female element if seed is to result. Reproduction is the sole function of a flower. In a typical flower the essen- tial female parts, regardless of their variable form and number, are ovary, style, and stigma. The ovary, the basal part, becomes the fruit. The style is a column of tissue arising from the top of the ovary. The expanded or other- wise modified tip of the style is the stigma. In many plants the surface of the stigma has a sticky secretion to which the pollen adheres. In a typical male part of the flower, the anther simply produces pollen, which is the functional male sex element. Pollination is the transfer of pollen from the anther to the stigma or the distribution of pollen. Pollination must be accomplished before fertilization (the union of the male germ cell, con- tained in the pollen grains, with the female germ cell or egg in the ovary) and eventual reproduction can take place. If pollen is transferred from an anther to the stigma of the same flower or to the stigma of another flower on the same plant, self-pollination is said to have taken place. The transfer of pollen from an anther to the stigma of a flower of another individual plant is spoken of as cross-pollination. Those are botanical definitions and do not consider the varietal factor, which is of such great importance to fruit produc- tion. Self-pollination, as used in fruit production, also includes the transfer of pollen from the anthers of a flower of one variety to the stigma of a flower of the same variety. Cross-pollination, in the horticultural sense, refers to the transfer of pollen from the flower of one variety to a flower of a different variety. Charles Darwin, the English natu- ralist, concluded from his observations and exhaustive experiments with many plant families that plants resulting from cross-pollination generally had greater vigor, weight, and height and produced flowers earlier than those resulting from self-pollination. It has since been shown that the advantages of cross-pollination and the disadvan- tages of self-pollination are not always so decisive as Darwin supposed. Never- theless, there is a long list of species and varieties of plants that are self-sterile and require cross-pollination; the ad- vantages of hybrid vigor in some mod- ern cropping practices also is well established. Darwin listed several ways in which plants are constructed to avoid self-pol- lination and insure cross-pollination: i . By the separation of the sexes, in which staminate and pistillate flowers are borne on separate plants, as in the hemp, willow, holly, and date. 2. By a difference in the time of maturity of the pollen and stigma in the same flower, as in the red clover, beet, plantain, and avocado. 3. By special mechanical contriv- ances that prevent self-pollination or that favor insect pollination, as in many orchids, legumes, and mints. 4. By producing different forms of flowers on the same plant with differ- ent lengths of stamens and pistils, as in the Chinese primrose. 5. By complete or partial sterility of the flowers to their own pollen or the prepotency of pollen from another in- dividual or variety over the plant's own pollen, as in lobelia, mignonette, mul- lein, and many varieties of apple, pear, cherry, plum, and almond. No difficulty is encountered in the Honey Bees as Agents of Pollination 89 Honey bee on comb. pollination and fertilization of some self-fertile plants because both sexual elements develop so close together that pollen is directly deposited on the stigma. Wheat illustrates such a situa- tion— it is self-pollinating and at the same time self-fertile. Self-fertile plants exist, however, which are wholly or partly incapable of fertilizing them- selves without the aid of a transferring agent. Cantaloup is a prime example of this condition. Other plants cannot fertilize them- selves following self-pollination by a transferring agent even though both sexual parts mature at the same time. Varieties of sweet cherry and almond illustrate this situation; pollen from some other variety is required to effect fertilization. Not only are all the vari- eties of these fruits self-unfruitful; there are instances as well of interin- compatible varieties. Varieties of certain fruits, such as the apple, pear, and plum, produce some fruit as a response to self-pollina- tion but not enough for a profitable crop. Such varieties are said to be par- tially self-fruitful. In such instances agents for the transfer of pollen be- tween different varieties are essential for commercial production. Even in many cases of self-fruitful- ness, like the French prune, the activity of insects on the blossoms greatly in- creases fruit production through bet- ter pollen distribution. French prune trees, enclosed in tents, gave sets of 19.0 percent when bees were present and 0.34 percent when bees were absent. Similar results were obtained when trees of the self-fruitful sour cherry Montmorency were caged in Michigan. It is perhaps significant that because California has such a large population of honey bees the yield of fruit, lint, and seed from many plants, such as plums, cotton, alfalfa, Ladino clover, onions, carrots, cantaloups, lima beans, and white mustard, often is outstand- ing. A number of agents may be neces- sary for the distribution of pollen from plant to plant, because some pollens are dry and light and others are moist and heavy. The commonest agents of pollen distribution are gravity, wind, and insects. For example, corn pollen drops from the tassel to the silks. Date palm pollen is a fine dust, which floats away like fog. Pine pollen, with its bladderlike wings, is readily carried by wind. Deciduous fruit pollens are rather gummy and generally must be transferred by insects. The huge pollen grain of cotton is thickly dotted with sticky fluid, which makes it far too heavy to be carried by the wind but adapts it well to sticking to the hairs that cover the bodies of pollinating in- sects. Pollination may sometimes also be accomplished by rain, birds, and artificial means devised by man. Even among many plants designated botanically as pollinated by wind and gravity, insects are sometimes a factor in the collection and distribution of pollen. For example, the pollens from bee colonies often contain liberal quantities from corn, oak, pine, wal- nut, ryegrass, Sudangrass, Canary 9o Island palm, date palm, juniper, cy- press, elm, or redwood. Few of those plants, if any, produce visible nectar for the attraction of insects to the male blossom parts; therefore they are prob- ably visited only by the pollen-collect- ing insects. Fruits develop in many plants with- out pollination and fertilization or the subsequent development of seeds. A number of our cultivated plants regu- larly bear such parthenocarpic — seed- less— fruits. Among them are seedless raisin grapes, English forcing cucum- bers, navel oranges, bananas, pine- apples, and some varieties of pears, figs, and Japanese persimmons. Sometimes mere pollination without subsequent fertilization may be sufficient to start fruit development. The application of synthetic plant hormones to the flowers and leaves has also been found to stimulate parthenocarpy in tomatoes, Smyrna figs, holly, pears, and others. Native wild bees (bumble bees, leaf-cutting bees, alkali bees, carpenter bees) are specially adapted for gather- ing pollen and nectar from flowers. Many other insects also do so — some beetles, flies, moths, thrips. In fact, any of the thousands of insects that visit flowers purposely or accidentally can be agents for carrying pollen grains from the anther to the stigma. But of all of them the most important by far is the honey bee, Apis mellifera, whose existence depends on pollen and nectar from plants. We estimate that bees ac- complish more than 80 percent of the pollination by insects. Yields of fruits and legumes and vegetable seed often have been doubled or trebled simply by providing adequate numbers of bees. The honey bee was introduced into the United States from Europe. Unlike the native wild bees, it is a colonial insect throughout the year and there- fore is available in force at any sea- son. Semidomestication in man-made hives makes it available for placement wherever needed for pollination serv- Yearbook of Agriculture 1952 ice. (The native bumble bee is also colonial in summer, but only the queen survives the winter to establish a new colony in spring.) Honey bees have a complete meta- morphosis— they pass through egg, lar- va, pupa, and adult stages. Each colony has three types of individuals, the queen, a handful of drones, and many thousands of workers. The queen bee is the true female whose primary func- tion is egg production. The drone, or male bee, has no function except to provide sperm when a young queen is mated. That done, the workers may drive him out of the hive. The mature workers are not sexual forms, although in the egg and early larval stages there is no difference between them and a queen. The kind of food and care given them causes them to develop into workers or queens. It is the worker that is familiar to all as the proverbial busy bee in orchards, gardens, and fields. Honey bees require carbohydrates, proteins, fat, vitamins, and other ele- ments for food. The carbohydrates are derived largely from the plant nectar, which is a liquid containing three kinds of sugars. Water is carried into the hive in quantity from various sources. The other requirements are met largely by plant pollens. Bees also sometimes seek out salt and possibly other minerals. Pollen primarily is utilized for rear- ing and maturing young bees through the larval and early-adult phases. The food of the mature field bees is largely honey or the nectar from which honey is elaborated. In seeking those foods from blossoms, a bee inadvertently ful- fills the required distribution of pollen so necessary to reproduction in plants. An individual bee usually visits one plant species to collect either pollen or nectar — a fortunate provision of nature, because a pollen from one species is not effective in completely fertilizing another kind of plant. For instance, pear pollen is of no use in setting fruit on a plum tree. Various investigators watched in- dividual bees and found that each one Honey Bees as Agents of Pollination visited only a small area to collect a load of pollen or nectar. After the trip back to the hive, the bee repeatedly re- turned to the same area. G. Bonnier one day marked all bees working on a strip of buckwheat, 3 by 16 feet in area, and found only marked bees there the next day. A. Minder- houd studied an area where bees were working clover, dandelion, and other plants. After marking all bees on areas of approximately a square yard, he re- corded their movements on squared paper. The bees repeatedly returned to the same square or within a radius of 10 yards of it. C. N. Buzzard noted that bees working Cotoneaster horizon- talis covered an area of only 2 square yards. From 15 observations of marked bees during the next 5 days, he con- cluded that the same bees returned to the same bush and strayed only where the branches intertwined. Sardar Singh noted the honey bee's liking for a small area of alsike clover, birdsfoot trefoil, aster, dandelion, goldenrod, white sweetclover, and apple in New York State. Some in- dividual bees devoted whole visits to a single apple tree and returned for later visits to the same tree. Other bees worked between adjacent trees. The ratio of observed exchanges of bees be- tween trees 10 feet apart to trees more than 15 feet apart worked' out at 2:1. Five bees (out of 66 observed) ram- bled over three to five trees. In England, C. G. Butler, E. P. Jef- free, and H. Kalmus found that visita- tion areas on Epilobium were usually less than 5 yards across. C. R. Rib- bands showed that foraging bees work- ing in a garden with five different sorts of flowers usually attached themselves to a particular area of the most suitable crop found. The size of the foraging area varied considerably. He noted that bees changed their attachment from a pollen crop to a nectar crop, but never vice versa. Karl von Frisch has reported in sev- eral publications that the scout bees, after locating a source of pollen or nec- tar, transmit their knowledge to other 91 bees through an intricate system of "dances" within the colony. Since foraging bees usually attach themselves to one species, bee pellets usually contain but one kind of pollen grain. However. A. D. Betts in Eng- land analyzed 915 pollen loads and found 3 percent were mixtures. Onlv seven of the loads showed distinct seg- regation of the grains into two separate areas. When pellets from trapped pol- len were sorted out in the western United States, only a few segregated mixtures were apparent among many thousands examined. It is presumed that the exhaustion of a source with advancing time of day may be a factor in obtaining segre- gated mixtures. For example, an orch- ard morningglory blossom opens at sunrise and closes before midday dur- ing bright, warm weather. On the other hand, redmaids (Calandrinia ciliata) unfold in late forenoon and are at their height during the heat of late afternoon. In this case a shift by a partially loaded bee from morning- glory to redmaids would be expected. Colonies in the same apiary evidently choose different pollen sources or visit different areas because the trapped supply from one colony is frequently unlike that from another. The abundance of pollen or its state of exhaustion in a source may greatly influence the rate of collection. Many bees have been seen to get full loads of pollen from one pistachio catkin. The number of visits reported to dandelion heads for a pollen load varied from 8 to 100 when dandelions were scarce and colonies numerous. Ribbands re- ported 47 foraging trips by 1 bee to Shirley poppy flowers during 1 day — indicating easy and rapid collection. The legs of a honey bee are modified for handling pollen. An eye brush oc- curs on the inner surface of the front tibia. The large first tarsal joint is cov- ered with long unbranched hairs, form- ing a body brush. At the base of the first tarsal joint is an instrument for cleaning the antennae. The hind legs of the worker bear the organs with 92 which it transports two large loads of pollen from the flower to the hive. The inner surface of the large basal seg- ment of the hind tarsus is covered with sharp, stiff spines closely arranged in transverse rows. They are particularly employed for taking the pollen from the middle tarsi and holding it until it is transferred to the pollen basket. As a bee pushes in among the anthers of a flower even for nectar, the body becomes literally covered with pollen grains. Nectar is the sweet fluid secreted in plant blossoms. Its function presum- ably is to attract insects to the flower- ing parts. When an insect gathers nectar from a blossom, its body be- comes coated with pollen grains, which are transferred later to other blossoms. Nectars of different plants have a variable sugar content and aroma. Such factors result in visitation of dif- ferent kinds of insects to the various plants. The blossom nectar of firethorn (Pyracantha) evidently has low attrac- tiveness to honey bees although blow flies feed on it greedily. The peculiar odor may influence both insects — that is, repel the one and attract the other. Dermestid beetles are common on yar- row blossoms, but a honey bee is seldom seen on them. The sugar concentration of the nec- tar is an important factor in plant visi- tation by bees. The nectar of oranges has about 16 percent sugar as the petals unfold. During a humid day, when the nectar continues at that con- centration or is diluted with fog mois- ture along the southern California coastal belt, the honey bee displays little interest in it. Then they busily gather the scanty mustard nectar, which has a much higher sugar con- tent. When the day is dry enough for evaporation of water to about 25 per- cent sugar, they collect the orange nec- tar. Before concentration increases to 40 percent, bees become so numerous that the blossoms are sucked dry. Then as humidity conditions change back to give little evaporation, bee activity on orange blossoms practically ceases. Yearbook of Agriculture 1952 At Davis, Calif., apricot nectar may be completely ignored for several days while honey bees are busy working almond blossoms. Under such condi- tions the apricot nectar contained less than 10 percent sugar when the almond had 35 percent or more. Dur- ing a dry north wind, the apricot nec- tar lost water rapidly and then it, too, was collected by many bees. Since sugar concentration of nectar affects bee activity, it likewise influ- ences the pollination potential. In plum varieties a wide variation in the quality and quantity of nectar has been found, and the bees have been observed to prefer some plum varieties over others. Eight varieties growing in the same orchard at Davis had nectar averages ranging from 10 to 28 per- cent sugar. Three other varieties had no collectible nectar. The Kelsey, a notably shy bearer at Davis, was one of these. A pollen grain is somewhat like a plant seed in that it germinates to send out a rootlike projection, which is called a pollen tube. On the stigma of a flower the tube penetrates down the style into the ovary, where union with the egg cell occurs. Each seed pro- duced requires one pollen-grain germi- nation tube. In a many-seeded fruit like a watermelon, numerous pollen grains must be applied to the stigma. Fertilization could be, effected in an almond blossom with one pollen grain. Wide differences exist in the length of the style of flowers and in the time for growth of the pollen tube down it. An alfalfa pollen tube grows down the pistil and enters the ovule in a day. In some oaks the time is almost a year in growing one-eighth inch. With Indian corn the distance from the stigma at the end of the corn silk to the attach- ment of the silk to the young corn grain may be a foot or more, yet fertil- ization is effected within a few days. In the latter case the large pollen grain carries much starchy food material which makes possible the great length to which the tube grows. Honey Bees as Agents of Pollination The pollen grains of various plants vary greatly in size, shape, and surface structure. Some flowers produce an abundance of pollen which is fully ex- posed. Others produce only a scanty amount which is tightly enclosed. These differences undoubtedly affect its collection by bees. The food value also varies. Some pollen grains are evi- dently too sticky for easy manipulation and others are too dry. In any event, trapped pollen supplies indicate the bee's preference for some types. For example, a trap operated in a Cali- fornia cotton field failed to yield pollen from the cotton although it was abun- dant on the plants. In an almond or- chard a large supply of almost pure almond pollen was readily trapped in spite of wildflower sources. Under a condition of scarcity, bees will even collect a pollen substitute like dry mash from a chicken feeder. Also, in a green- house they have been observed to col- lect tomato and beet pollens, which are ordinarily ignored outdoors. The rather omnivorous habit of honey bees of col- lecting pollen from so many plants, especially under stress for a supply, is radically different from that of many other bees, which require a special kind of pollen. To a plant breeder the variety of pollen and the resulting seed within the species is a salient factor, as it is also to a grower in the case of certain self- unfruitful varieties. In many fruits, like apples, a nearly full complement of seeds is needed to produce a shapely product. To commercial cotton pro- ducers the lint is the chief thing, but there again the production of seed is important for two reasons — the pro- duction of normal lint depends upon fertile seeds, and such seeds are the source of cottonseed oil. Producers of package bees make use of the early and abundant supplies of pollen in deciduous fruit orchards. Where that trade is established, many colonies are regularly rented to or- chardists. The possibility of getting much honey from deciduous fruit blos- soms is limited by their early blossom- 970134° — 52 8 93 ing. The quality of this honey also is generally low; for example, almond honey is bitter and prune honey readily ferments. On the other hand, several of the summer-blossoming species of legumes, including alsike clover, white clover, sweetclover, hairy vetch, birdsfoot tre- foil, and alfalfa, are sources of high- quality honeys. Red clover is usually not even considered a source of honey in the United States. Except for alfalfa, all these legumes are medium to good sources of pollen. The chief supply of pollinators in the country is maintained by the beekeep- ing industry. In the past its size and ex- tent have been based largely on the ability of the beekeeper to make a liv- ing from the production and sale of honey. There are about 6 million col- onies in the United States — one-third are in the South, one-third west of the Mississippi, and the rest in the North- east. Properly distributed, the supply of pollinators probably would be ade- quate to meet the needs of agriculture. In areas having concentrated plantings of specialty crops, such as deciduous fruits and small-seeded legumes, the growers must draw upon the bee in- dustry to meet their great requirements for pollinators during blossoming time. Maintaining the industry in a healthy condition is essential to our agricul- tural economy. A hive and all its bees can be moved readily by screening the entrance after the field workers come in for the night. It should be taken at least a mile and a half from the old location — other- wise many of the bees would return to the former location and thus be lost. During hot weather a ventilating wire screen may have to be substituted for the regular cover. Commercial bee- keepers frequently move colonies with- out closing the entrance, but the novice should not attempt to do so. After a colony is moved, the bees reorient themselves by flying close to the hive until they become familiar with their new surroundings. The habit may be made use of in pollina- 94 tion practice. Bees brought into an orchard at blooming time tend to work the nearby trees first, but often it may be necessary to move the colonies in ahead of the bloom. For example, an almond orchard in the West may be almost impassable from the beginning of the winter rains to the end of blos- soming in early spring. Bees are ordi- narily placed in such orchards follow- ing harvest in the fall because colonies are moved from the river bottoms and the mountains at that time. Throughout the country in decidu- ous-fruit areas, especially where ap- ples, sweet cherries, and plums are grown, it is common practice to rent commercial bees for pollination during blooming time. Many colonies going to fruit orchards are of local origin but some long hauls are necessary. For ex- ample, bees are moved long distances to Wenatchee, Wash., Hood River, Oreg., the Shenandoah Valley in Vir- ginia, and the apple districts of Penn- sylvania. The colonies frequently come from the neighboring States. Developments in the small-seeded legume industry since 1945 have shown the advantage of providing more bees than are normally present. Colonies are rented and often brought in from dis- tant places. To the seed district around Delta, Utah, with about 30,000 acres in alfalfa, 10,000 to 15,000 colonies are brought each year. Beekeeping in the area is not feasible the year around, and most of the colonies are removed to southern California after the alfalfa season, a distance of 500 miles or more. Some colonies were moved more than 1,000 miles from southern California to Colorado for alfalfa-seed pollination in 1950. A similar situation occurs in the clover-seed area of Jefferson County in Oregon, where a large acre- age is grown and some 15,000 colonies were moved in in 1950. Many of the colonies came from the Sacramento Valley in California, 500 miles away. A substantial income, either from a good honey crop or from rental fees, is required to take care of the high cost of such moves. Yearbook of Agriculture 1952 Bees are often moved by truck in lots of approximately 1 00 two-story hives — perhaps 4 million potential pollinators. The truck is loaded in the evening and goes nonstop to its destination. On ar- rival the colonies are placed in or near the fields. Growers of cucumbers and some other plants in greenhouses have found it necessary to supply bees for pollina- tion. The business is rather extensive. In the production of some hybrid seeds, cages supplied with bees are in use. Colonies depreciate rapidly in both greenhouses and cages, and frequent replacement is necessary. When a beekeeper engages his bees for pollination service he faces special problems: Extra expenses for moving and caring for the colonies, shortage of feed, and insecticidal poisoning. Ordi- narily he can expect no honey in or- chards, and little or no surplus honey is obtained by the large number of col- onies frequently used for alfalfa. It may even be necessary to feed the col- onies. The grower should make sure that enough bees are provided during the blooming period to produce the maximum set of fruit or seed. Bee-collected pollen pellets can be obtained readily with a pollen-collect- ing trap consisting of a screen grid which scrapes pollen from the legs of bees as they go through it to enter their hives. Quantities of pollen of apple, al- mond, cherry, plum, and pear have been collected in that way at the Cali- fornia Agricultural Experiment Sta- tion. During favorable periods the yields per trap approached 2 pounds daily. More than 50 pounds have been obtained from one colony in a year. Bees add substances to the pollen to form it into pellets. Sugar — nectar or honey — is one such substance, as shown by the higher content of sugar in bee- collected pollen than in hand-collected pollen. The viability of deciduous-fruit pollen from freshly gathered bee pellets is high, but at room temperature it rapidly loses its ability to germinate. Pollen taken from the anthers by hand, Honey Bees as Agents of Pollination however, remains viable for a longer period. The exact reason for the dif- ference is not known. In testing the viability of bee-pellet pollen, one can enhance germination by dispersing the pellets in a 15-percent cane sugar sirup immediately before plating on the agar medium. Smearing the dispersed grains on the plate sepa- rates them so that they may be readily counted in determining the percentage of germination. Storage at low temperature greatly prolongs the viable life of the pollen from the pellets of honey bees as well as hand-collected pollen. Bee-collected apple pollen, removed from the trap at 30-minute intervals, frozen with dry ice, and placed in a freezing compart- ment in 1949, remained highly viable for a year. In the spring of 1950 the pollen was used to hand-pollinate apple blossoms in the orchard from which it was collected in 1949. Subse- quently fruits were set and matured. This demonstration of stability should assure future progress in the use of bee- collected pollen in artificial pollination. Some beekeepers trap bee-collected pollen and dry and store it for feeding their bees when pollen is scarce. The trapped pollen is sometimes shipped from areas with mild winters to North- ern States where long winters make supplementary feeding necessary. Per- haps bee-collected pollen might also be valuable for medicinal purposes or as a source of vitamins. Honey contains numerous pollen grains — the age-old belief that honey is healthful may have its basis in that fact. The pollen pellets might also be beneficial in the diet of baby chicks. We believe that these and other potential uses should be investi- gated. Controlled artificial pollina- tion has been used for many years in plant-breeding work and in studies to determine the pollination requirements of our cultivated plants. The usual practice is to emasculate the blossoms of the mother variety, apply the de- sired pollen to the stigma, and bag the 95 flowers to exclude insects which might bring unwanted pollen. Many superior varieties of fruit, vegetable, and field crops have been developed in this way. The method also has been used in breeding varieties of forest trees, par- ticularly pines. Commercial hand-pollination of dates has been practiced since ancient times. The cluster of male or stami- nate flowers — the spadix — is removed from the male date palm and shaken gently over the flower clusters of the female tree. L. H. MacDaniels and A. J. Hein- icke in 1929 suggested using hand- pollination as a temporary expedient in apple orchards consisting of solid blocks of self-unfruitful varieties. They also thought such pollination might be worth while in years of un- favorable weather conditions even in apple orchards well provided with pol- linizing varieties. Later MacDaniels put the method on a commercial basis and developed carriers for diluting the pollen for more economical use. About that time the pollination problem had become acute in the apple-producing sections of Washing- ton because of the heavy spray pro- gram, which practically eliminated pollinating insects, and the removal of less profitable varieties in favor of Delicious and Winesaps, both of which are self-unfruitful. Winesap produces mostly nonviable pollen and is there- fore an ineffective pollinizer for De- licious. Washington apple growers quickly adopted the methods suggested by MacDaniels and Heinicke. By 1937 hundreds of acres of Delicious orchards were hand-pollinated in the Wenat- chee and Yakima districts. More re- cently the trend has been to provide means for cross-pollination by insects by grafting over some Winesap and De- licious trees to pollinizing varieties, in- terplanting pollinator trees, and pro- viding more bees. Most growers now make sure that their new plantings contain adequate pollinizing varieties. Pollen for hand-pollination is ob- 96 tained by gathering flowers from a va- riety known to be a good pollinizer for the variety in question and removing the anthers by rubbing the flowers over 8-mesh hardware cloth. Blossoms should be gathered when most of them are in the balloon stage, just before the petals open. The pollen can be cured by holding the anthers in shallow trays at room temperature for about 2 days. The cured pollen should be placed in bottles stoppered with cotton and held in a dry, cool place until it is used. Materials such as lycopodium spores, wheat flour, cornstarch, egg albumen, and powdered milk have been used to dilute the pollen in order to reduce the cost of hand-pollination. Lycopodium spores generally have proved to be the most satisfactory carrier, although tests have shown definite promise for pow- dered milk and egg albumen, which cost less than lycopodium. To hand-pollinate an apple tree in good bloom, the usual practice is to touch the stigmas of one flower in every fourth or fifth cluster. A size 4 pig-hair brush, the rubber end of a pencil, a cork, or the bare finger can be used. The pollen should be applied 1 to 3 days after the flowers open because the flowers are no longer receptive to polli- nation when the tips of the styles turn brown. A skilled operator can pollinate a 20-year-old apple tree in about an hour. It would take much longer to hand-pollinate a fruit tree like the sweet cherry, in which a much higher percentage of the blossoms must set fruit in order to give a commercial crop. Regardless of how the pollen is ob- tained or how much it is diluted, hand- pollination is laborious and costly — especially in light of the relatively low cost of pollination by bees. One argu- ment in favor of hand cross-pollination is that it will greatly reduce the amount of fruit thinning needed to obtain good size where natural pollinating agents are not available. According to John C. Snyder, however, the idea of reducing thinning costs does not justify the per- manent use of hand-pollination as a Yearbook of Agriculture 1952 substitute for pollinizer varieties and honey bees. Many labor-saving methods for ap- plying pollen have been developed and tested. Dust and liquid mixtures of pollen have been applied from air- planes and conventional spray and dusting equipment. Bombs and shot- gun shells containing pollen have been used for rapid distribution of pollen. Under controlled experimental tests conducted by R. M. Bullock and F. L. Overley, however, these rapid methods have failed to give significant increases in fruit set. They concluded that hand- pollination is the most satisfactory method of artificial pollination. A semiartificial method of cross-pol- lination involves pollen dispensers de- signed to force honey bees to pass through prepared pollen as they leave the hive. The idea is that the bees will pick up the pollen and spread it through the orchard. Overley and W. J. O'Neill tested two types of pollen dispensers and reported that their value was questionable. During 1951 we supplied hives of bees with pollen dispensers for almond and sweet cherry trees, which were caged to exclude outside insects. Only a few fruits were set on the trees even though the dispensers were kept sup- plied with viable pollen throughout the blooming period and the bees actively worked the blossoms. Satisfactory fruit sets were obtained on branches of the trees to which the same pollen was ap- plied by hand. A person collecting pollen by hand can usually gather only enough green anthers from fruit blossoms in an 8-hour day to produce 3 to 5 ounces of cured pollen. But bee-collected pollen can be obtained readily in almost un- limited quantities by the use of pollen traps. The pollen pellets of the com- mon fruit species are readily distin- guished by their color. Although all bees of a colony do not visit the same species, one can get a nearly pure sam- ple by careful selection of the time and place of trapping. Honey Bees as Agents of Pollination Experiments at Davis, Calif., during 1948 showed that the percentage of viability of freshly trapped pellet-pol- len is approximately the same as that of hand-collected pollen. It also gave fruit sets that compared favorably with those effected by hand-collected pollen when "it was applied by hand with small brushes. Diluting the bee- pollen with an equal amount of lyco- podium spores hardly reduced fruit set. Although viability of this pollen is rapidly lost at room temperature, it can be maintained for several days in ordinary cold storage at about 320 F., and for a much longer period at ex- tremely low temperatures. If handled properly, therefore, the pollen in freshly trapped pollen pellets of honey bees may serve as well as hand-col- lected pollen in hand-pollination. Various methods of rapid applica- tion of the pellet-pollen have been tested. The pellets have been dispersed in water and in salt and sugar solu- tions. The resulting mixtures have been sprayed on almonds, sweet cher- ries, plums, apples, and pears. The var- ious pellet-pollens also have been ap- plied as dusts after they were mixed with various carrying powders. Thus far commercial fruit sets have not been obtained from such rapid methods of application. J. C. Kremer suggested that bee-collected pollen from early blooming apple varieties could be mixed with lycopodium spores and stored under dry conditions at 34 ° to 36 °. The mixture could then be used later in pollen dispensers inserted at the entrance of bee colonies for the cross-pollination of varieties that blos- som late in the spring. This method failed to give satisfactory fruit sets when tested on caged almond and sweet cherry trees at Davis during the blossoming period of 1951. There is much more to getting re- sults in pollination than an adequate supply of bees. The blossoms must be attractive to the pollen distributors for either pollen or nectar. For best results the specific blossoms must be more at- 97 tractive than their competitors. For the self-unfruitful varieties compatible sources of pollen must be at hand. In producing seed from male-sterile vari- eties the same is true. The varieties providing pollen and those needing it must flower at the same time. Even after all other factors are taken into account, bad weather can cause failure by preventing insect activity. Some of those factors are brought out in the following paragraphs about some plants that need the help of insects for pollination. Many commercial fruit varieties are propagated asexual ly. From the view- point of pollination, therefore, an orchard of a single variety is one tree, so to speak. Self-unfruitfulness in such a case creates a pollination problem that requires special consideration in the planting of an orchard. The almond is an interesting ex- ample of self-unfruitfulness. Almonds were planted in California in 1853, but the yields of the early orchards were low and variable. The eventual failure of the first plantings was due largely to a lack of knowledge of polli- nation requirements and other factors of successful culture. California had 100,000 acres of almonds in 1952. Ac- cording to W. P. Tufts, the pollination problem with the almond was recog- nized and recorded as early as 1885, when A. T. Hatch of Suisun noted that Languedoc trees growing near seed- lings always produced heavier crops than those planted in solid blocks — the only plausible explanation for the many instances of crop failure was lack of cross-pollination. Tufts' early studies indicated the self-incompatibil- ity of some varieties. Later investiga- tions showed that all varieties were self-unfruitful and that a few pairs of varieties were interincompatible. For example, Nonpareil and I. X. L., Languedoc and Texas, and Jordanolo and Harpareil are interincompatible pairs. Combinations of those varieties therefore should be planted without putting a pollinizer with them. 98 The eventual recommendation was to plant the proper varieties with spe- cific reference to pollination require- ments. Almonds may bloom from the end of January to the end of March. They may be classed as early or later in the time of blossoming. The following list gives varieties in the usual sequence of blooming from the earliest to the latest (Nonpareil is included in both groups because it occupies a position about midway) : Early — Harriott, Jordanolo, Jordan, Ne Plus Ultra, Harpareil, King, California, Lewelling, I. X. L., Peerless, Princess, Nonpareil. Late — Nonpareil, Drake, Eureka, Languedoc, Texas, Reams. Except in the instances we noted of the interincompatibility, any varieties listed as early or late will usually serve as a satisfactory pollinizer for any other variety in the same list. The blossoming periods of very early blooming varieties as Harriott and Jordanolo, however, may not overlap sufficiently in some seasons to insure adequate cross-polli- nation with Nonpareil. Many almond growers have planted only Jordanolo and Nonpareil because of their greater commercial value. From a pollination standpoint, however, that combination is often poor. Because almond trees blossom early when the weather may be too cool for maximum insect activity, more bees and more trees of the pollinizing vari- ety are needed than for later blossom- ing fruits. There should be at least one row of pollinizers for every three rows of the main variety. In adverse seasons it would pay to have two rows of the main variety and then two rows of the pollinizer. Two or three strong colonies of bees should be supplied per acre. Almost all varieties of the Euro- pean pear are self-unfruitful. A few (Doyenne du Cornice, Flemish Beauty, Beurre Hardy, Howell) are usually self-fruitful, but even they generally will produce better crops when they are cross-pollinated. In some localities in California, Bartlett, Colonel Wilder, Yearbook of Agriculture 1952 Beurre d'Anjou, Seckel, and Beurre Clairgeau may range from partly to completely self-fruitful in some years. The Bartlett (or Williams' Bon Chretien) is a widely grown variety. California had more than 36,000 acres of Bartletts in 1952. It is self-unfruitful in the East. Bartlett is said to be usually self-fruitful under interior valley and coastal conditions in California, but should not be planted without polli- nizers in the Sierra Nevada foothills. Hand cross-pollination has given greatly increased fruit sets over self- pollination or open (natural) pollina- tion, regardless of location or whether the trees were planted in solid blocks or provided with pollinizers. Except for a few very early and late blossoming varieties, the blooming pe- riods of most of the commonly grown pears overlap well enough for cross- pollination. Bartlett has a long, mid- season blooming period that overlaps those of nearly all the other important varieties with the possible exception of the very early Le Conte, Forelle, Kief- fer, and Clairgeau. Winter Nelis has proved to be the most satisfactory pollinizer for Bartlett under most conditions in California. In the East and Northwest, Bartlett blooms several days before Winter Nelis. In California, however, follow- ing the warmest winters, the blossom- ing period of the Winter Nelis may be past before the Bartlett blossoms have opened. Nectar-collecting honey bees usually prefer flowers of other plants to those of pears. That undoubtedly is because some pear flowers provide a relatively small amount of nectar, low in sugar concentration. Bees do work pear blos- soms for pollen, which most varieties produce in abundance. If the concen- tration of bees is great enough, there- fore, effective cross-pollination will undoubtedly result in orchards that have enough pollinizing varieties. Orchardists who desire a heavier fruit set probably should provide two or three colonies of bees instead of one, as is usually recommended. Honey Bees as Agents of Pollination Nearly all of the commercially im- portant pears in the United States pro- duce viable pollen and will effectively cross-pollinate each other. Only the combination of Bartlett and Seckel has proved to be interincompatible. Other unfruitful combinations have been re- ported among closely related varieties. Several European varieties produce mostly nonviable pollen and therefore cannot be used as pollinizers. Cherries are of three groups — sweet cherries, sour or pie cherries, and Duke cherries, which are hybrids of the other two. Most of the sweet cherries grown in the United States are produced in the Pacific Coast States. California pro- duces the bulk of the crop. Small com- mercial plantings of sour cherries are in western Oregon and Washington, although the main production is in the Northeast. Duke varieties are of little commercial importance anywhere in the country. All varieties of sweet cherries are self-unfruitful and must be cross-pol- linated for satisfactory yields. Not all combinations of varieties are fruitful. Examples of variety combinations that are interincompatible and therefore will not produce crops when planted together (unless other effective pol- linizing varieties are provided) are: Early Purple and Rockport; Advance and Rockport; Windsor and Abun- dance; Napoleon (Royal Ann), Bing, and Lambert; Black Tartarian, Knight's Early Black, and Early Rivers. (Some strains of Black Tar- tarian may be interfruitful with Knight's Early Black and Early Rivers.) All important sweet cherry varieties produce good, viable pollen. Most va- riety combinations should be inter- fruitful therefore if their blooming pe- riods overlap enough. As to pollination, there are evidently different strains of certain cherry vari- eties, perhaps because seedlings that now exist are so similar to the original varieties that thev cannot be distin- 99 guished from their parents. One should therefore select trees of strains that he knows can fertilize the desired variety. Varieties bloom at different times. One should select varieties that have overlapping blooming periods and are interfruitful. The average blooming period for most sweet cherries is about 2 weeks. Weather conditions just be- fore and during bloom markedly influ- ence the length of the period of bloom as well as the dates of blooming, but varieties keep approximately the same order of blooming each season. A list of most of the varieties grown in Cali- fornia in order of earliness of blossom- ing is: Early — Burbank, Chapman, California Advance, Black Heart, Knight's Early Black, Early Purple Guigne, Black Republican, Black Tar- tarian. Late — Napoleon (Royal Ann), Windsor, Parkhill, Early Rivers, Rock- port, Bing, Pontiac, Abundance, Bush Tartarian, Noir de Schmidt, Giant, Lambert, Saylor, Long Stem Bing, Gil Peck, Deacon. The blossoming periods of the vari- eties within each group will usually co- incide well enough for effective cross- pollination. The blossoming periods of Black Tartarian and Black Republican generally overlap well enough with those in the late group for satisfactory cross-transfer of pollen. In the East the blooming periods of the main varieties usually coincide well enough for cross-pollination. In general, only sweet cherries should be planted for cross-pollination of sweet cherries. Sour cherries usually bloom too late to be satisfactory pollin- izers for sweet cherries, and the per- centages of fruit set are low. Duke cherries are unsatisfactory pollinizers for sweet cherries, although the bloom- ing periods of the Duke cherries coin- cide with those of the late sweet cherries. The commercially important vari- eties of sour cherries (Early Richmond, Montmorency, Dyehouse, and the Mo- rello group) are self- fruitful if enough pollinizing insects are available. Better crops can be expected, however, if a 100 sour cherry orchard contains more than one variety. Almost any variety of sour cherry will serve as an effective pollinizer for the other sour varieties. The later blooming sweet cherries will also satisfactorily cross-pollinate the sour varieties if their blooming periods overlap enough. The pollen of Duke cherries usually does not give satisfac- tory fruit sets on sour cherries. Some of the Duke varieties, such as Royal Duke and May Duke, may be partly self-fruitful, but cross-pollina- tion is essential for commercial crops. Duke cherries will generally set heavier crops when cross-pollinated by either sweet or sour cherries than when other Duke varieties are used as pollinizers for them. The pollen of Duke cherries gives low percentages of germination in laboratory tests. The low order of viability of their pollen undoubtedly explains why Duke cherries serve as poor pollinizers for sweet and sour cherries as well as other Dukes. Sweet cherries in the late-blossoming group make satisfactory pollinizers for such earlier blossoming Duke cherries as Olivet. Reine Hortense, and May Duke. But the sour cherry varieties may best serve as pollinizers for the later blooming Dukes, Late Duke, Royal Duke, and Abesse d'Oignies. Keeping boxes of honey bees in the home apple orchard was a common practice even before the development of the movable frame beehive. The need for cross-pollination was not ap- preciated, however, until the growers started standardizing and limiting their orchards to a few varieties. Undoubt- edly the decline in the activity of native wild pollinating insects also was a factor. A few varieties (Baldwin, Early Harvest, Grimes Golden, Oldenburg, Rome Beauty, Wealthy, Yellow Trans- parent, Yellow Newtown) are con- sidered to be self-fruitful in certain favorable locations. Some others (Ben Davis, Esopus Spitzenburg, Golden Delicious, Jonathan, Red Astrachan, Wagener, and York Imperial) produce Yearbook of Agriculture 1952 varying amounts of a commercial crop when self -pollinated. It is generally agreed, however, that all varieties should be interplanted and that honey bees be put among them during the blooming periods to insure good yields. The following are unusually good pollinizers and generally effect excel- lent sets of fruit on most other varie- ties: Ben Davis, Delicious, Fameuse, Golden Delicious, Grimes Golden, Jonathan, Mcintosh, Northern Spy, Rome Beauty, Wagener, Wealthy, Winter Banana, Yellow Transparent, and York Imperial. Others, including Arkansas, Bald- win, Gravenstein, Rhode Island Green- ing, Stark, Stayman Winesap, Thomp- kins King, and Winesap, produce mostly infertile pollen and conse- quently are ineffective as cross-polli- nizers. Most other commercial varieties grown in this country usually serve as satisfactory pollinizers. Interunfruitful combinations are rare except among closely related va- rieties and those having infertile pollen. Parent varieties are ineffective as cross- pollinizers for their color sports or bud mutations, and the mutations, in turn, are of no value as cross-pollinizers for the parent variety. For example, Deli- cious is interincompatible with any of its color sports, such as Redwin, Rich- ared, Starking, and Shotwell Delicious. An exception is Grimes Golden, which is ineffective as a pollinizer for Arkansas but an excellent pollinizer for other varieties. Presumably it is unre- lated to Arkansas. According to present evidence, based mainly on orchard observations, color mutations of apple varieties have the same pollination requirements and val- ue as cross-pollinizers as their parent varieties. The blooming periods of two vari- eties must overlap if cross-pollination is to be accomplished. According to W. H. Chandler and others, apple trees require more chilling before their buds will open evenly in the spring than most other fruits. Mild winters may Honey Bees as Agents of Pollination 101 widen the gap between the blooming dates. The relative order of blooming, however, is usually the same for any one locality. High spring temperatures following winters cold enough to meet the chilling requirements tend to short- en the blooming periods of all varie- ties. Under those conditions, all but the very early- and late-blooming varieties will overlap sufficiently for pollination. On the other hand, cold spring weath- er will tend to cause the blooming pe- riods of the early and late varieties to be much more widely separated. In most years, however, midseason varie- ties would overlap with the early- and late-blooming ones sufficiently to pro- vide an adequate pollen supply. The pollination requirements of plums have been studied at the California Agricultural Experiment Station for more than 40 years. Begin- ning in 1 9 16, A. H. Hendrickson made a series of reports showing self-unfruit- fulness of many varieties of European and Japanese plums. He also showed that bees must be provided in plum orchards for commercial crops even though the varieties are highly self- fruitful. Since then, work in England, New York, Michigan, and California has shown that the European plums may be classified as usually self-fruitful, partly self- fruitful, or self-unfruitful. The self-unfruitful varieties outnum- ber those that may be listed as partly or completely self-fruitful. Apparently no interincompatible pairs or groups of European plum varieties exist among those grown commercially in the United States. Because most vari- eties produce a high percentage of viable pollen, any variety should be effective in cross-pollinating another, provided their blossoming seasons overlap sufficiently. The blooming season of the Euro- pean plums in the East usually over- laps well enough to provide cross-pol- lination. In California the varieties are classed as either early or late blossom- ing. Most of the Japanese plums are self- unfruitful. A few varieties, such as Beauty, Climax, Methley, Red Rosa, and Santa Rosa, are partly self-fruit- ful, but, like the others, these five vari- eties will generally set much better when interplanted with other varieties for cross-pollination. Some of the earlier blossoming Japanese varieties are deficient in pollen production, and several varieties produce pollen that is low in viability. Other varieties, like Burbank, Duarte, Elephant Heart, Red Rosa, Redhart, Santa Rosa, and Wickson, are satisfactory pollinizers. The blossoming season of Tragedy, a European plum, coincides with several of the late Japanese varieties. Tragedy is also a moderately effective pollinizer for several Japanese plums, but it does not set fruit following cross-pollina- tion by them. Certain American plums are also effective pollinizers for several Japanese varieties. The necessity of finding a specific pollinizer is emphasized in the case of Elephant Heart. It is attractive and one of the largest of the Japanese plums. It has high quality and is a good shipper. It would undoubtedly be an important late-season plum except for its shy bearing habit. Between 1936 and 1948, workers at the California station tried 21 varieties as pollinizers for Elephant Heart. Finally in 1948, My- robalan 5Q, a selected Myrobalan seedling and one of 47 varieties of pol- len tested as pollinizers for Elephant Heart that year, gave a satisfactory fruit set. Extensive tests in 1949 proved that Elephant Heart can produce heavy crops with this source of cross- pollination. But the fruit of Myrobalan 5Q has no commercial value and the search for a suitable pollinizer was con- tinued. In 1950 it was discovered that a promising new Japanese variety, Redheart, developed in breeding work, will also bring about heavy fruit sets on Elephant Heart. Seed production is the object of pollination of legumes. The most fa- miliar large-seeded legumes such as peas and beans are generally self-pol- 102 linated, but many of the small-seeded ones require insect pollination. Even with the self-fertile legumes, cross-pol- lination is desirable because greater vigor results. The seedling plants in a legume field are of mixed heredity; that is ideal for true crossing and is dif- ferent from orchard trees, which are propagated asexually. Some of the self-fertile species of legumes require tripping — release of the staminal column — by insects be- fore they will set seed. In the process, cross-pollination is readily accom- plished. An orchard presents a rela- tively small number of blossoms, and only some of them are required to give a commercial crop. But a legume field has a tremendous number of blossoms, and it is desirable to set the greatest possible number of pods. The orchard- ist may accomplish his aim with one colony to the acre, but in some legume fields five or more colonies may be needed to set maximum crops. Alfalfa has become our leading legume hay crop. Adapted varieties are grown extensively even in the Middle West, where early attempts at produc- tion failed. The Intermountain States, particularly Utah, used to be the im- portant producers of alfalfa seed. Since 1925, however, their yields gradually declined from 8 to 10 bushels an acre to as low as 1 bushel. Injurious in- sects may have been a factor, but the reduction in the number of wild bees is considered to be one of the main causes of the lowered yields. Alfalfa blossoms require tripping (forcing the pistil out of the keel) and cross-pollination by insects for high- yields of seed. Native wild bees, notably leaf-cutting bees and alkali bees, took to alfalfa as a favorite source of pollen and nectar; where they are still abun- dant, tripping proceeds apace and high seed yields are maintained. As a rule, the pollen collectors are more efficient in tripping than the nec- tar collectors. The presence of more easily worked pollen sources within flight range, such as mustard, sweet- Yearbook of Agriculture 1952 clover, birdsfoot trefoil, and star- thistle, attracts the pollen-collecting bees away from alfalfa. For example, pollen trapped in the Cache Valley of Utah, which has other good pollen sources, contained little or no alfalfa pollen. A high proportion of alfalfa pollen was obtained in traps at Delta, Utah, where more attractive sources of pollen were limited. The use of honey bees to pollinate alfalfa blossoms has increased greatly. California, which has many bee colo- nies, has been making rapid strides in seed production. The State produced 15 million pounds in 1949; many yields ranged from 500 to 1,000 pounds or more of seed an acre. The 1950 yield was 33 million pounds. A survey by workers at the experiment station in- dicated that a production of 150 to 200 pounds of seed per acre per colony of bees was not uncommon. In one in- stance, a 132-acre field was supplied with bees during the flowering season at the rate of 5 or 6 colonies an acre. It was inspected daily for 2 months. Almost no wild bees were seen. Only limited amounts of pollen were col- lected by honey bees, but nectar-col- lecting bees were numerous. The field averaged 896 pounds of recleaned seed to the acre. Tripping was continuous but relatively slow in comparison to the rapid tripping by wild bees or pollen- collecting honey bees, as noted in some favored areas of other States. In earlier years bees were usually placed in large apiaries outside the field to be pollinated. The practice has been growing of scattering the colonies in small apiaries along drives crossing the field. Field experiments in 1949 and 1950 showed an increase in the rate of tripping around newly established groups of colonies — indicating the sup- periority of the newer arrangement. Until recently the honey bee has not been considered an effective polli- nator of red clover because measure- ments have shown that the length of the corolla tube of the floret exceeds the length of the honey bee's tongue. Honey Bees as Agents of Pollination The longer-tongued bumble bees have often been given credit for being the important pollinators of red clover be- cause they collect nectar and pollen from this plant in preference to many others. If bumble bees are numerous, no particular difficulty is experienced in setting seed, but they are becoming scarce in many areas. W. E. Dunham reported that of the insects responsible for red clover polli- nation in Ohio several years ago, 82 percent were honey bees, 15 percent were bumble bees, and 3 percent other insects. An acre of red clover is said to contain some 216 million individual florets. A bee takes about 30 minutes to visit 346 florets to get a load of pollen. R. G. Richmond showed that red clover caged with honey bees in Colo- rado produced 61.5 seeds per flower head and only 0.49 seeds per head when pollinating insects were excluded. He stated that first-cutting red clover set a good seed crop when conditions were inviting to honey bees. A 27-acre field of Kenland red clover in the Sacramento Valley pro- duced 616 pounds of clean seed to the acre in 1950. The field was planted that spring and no hay was cut. A large acreage of seed alfalfa was growing in the adjoining field. Honey bee colonies were in scattered groups in all fields. The grower stated that the bees worked the clover and the alfalfa about equally well. Pollination of red clover is accom- plished by the honey bee in this way: The bee approaches the floret over the keel and forces its head down directly between the keel and the standard petal. The fore and middle pairs of legs clutch and claw at the wings of the floret to spring them and the keel away from the standard, thus tripping the flower. Tripping exposes the stigma and anthers, which touch the bee on the underneath side of the head where it joins the thorax. Pollen is thus ac- cumulated and carried to the next blossom. White clover also needs pollina- 103 tion by insects for seed production. The several varieties, among them British, Dutch, and Ladino, are freely worked by honey bees. White clover is a leading source of honey in the North Central and Northeastern States, but Ladino clover (giant white) from Italy does not equal the Dutch clover (small white) in honey production. Because those two types readily cross, stands grown for seed must be far enough apart so insects cannot fly from one field to the other. In Oregon, from Ladino plants in cages where no insects could reach them, H. A. Scullen harvested 300 seeds from a sample of 100 heads; 100 heads just outside the cages yielded 14,900 seeds. At Thornton, Calif., where three colonies of bees per acre were placed for pollinating alfalfa and red clover, heads from stray Ladino plants averaged 276 seeds each, or a total of 27,600 seeds in 100 heads. The sweetclovers grown in the United States include many varieties, both yellow and white. Some are self- pollinating and self-fertile; others are self-fertile but require insect pollina- tion, and still others are self-sterile and require cross-pollination. As a source of both nectar and pollen, sweetclover is highly attractive to honey bees. Var- iation among plants is increased by cross-pollination effected by the large number of bees that visit the plants. Increased yields of sweetclover seed have been demonstrated many times by providing at least one colony of bees to the acre. Crimson clover, of the true clovers, is the most important winter annual cover crop in the United States. The different varieties are self-fertile, but their florets are not self-tripping, and insect visitation is required for heavy seed crops. Growers of crimson clover seed have become increasingly con- scious of the benefits from introducing colonies of honey bees into their fields. Observations in the lower Sacra- mento Valley disclosed that relatively few bees are required for pollination 104 of all Ladino clover florets, compared to the numbers required for pollina- tion of alfalfa. New flower buds do not open until the middle of the morning, and a concentration of two or three bees to the square yard can work all of them repeatedly in one afternoon. Following pollination, the florets turn down on the stem and close perma- nently at nightfall. Nectar is not secreted for 3 to 6 hours after a bud opens. In June and July the bees gen- erally work the blossoms for pollen. Alsike clover depends upon pollina- tion by honey bees and also to some extent on wild insects for seed produc- tion. Alsike produces an enormous number of individual florets, all of which must be cross-pollinated for heavy yields of seed. Field experiments in Ohio demonstrated that honey bees increased seed yields from a 10-year average of 1.6 bushels an acre to 8 bushels an acre. Yields as high as 20 bushels an acre were found to be possi- ble with maximum insect pollination. Alsike bloom produces a relatively poor supply of nectar in some years. If the concentration of bees is heavy enough to pollinate the crop ade- quately under such conditions, very little honey will be produced. Birdsfoot trefoil is becoming an important permanent pasture crop. Apparently it does well on many soils where clovers and alfalfa do poorly. As it is self-sterile, seed formation de- pends upon cross-pollination by in- sects. On caged insect-free plots in Ore- gon, H. A. Scullen obtained no seeds on either birdsfoot or big trefoil while similar plants exposed to bees seeded freely. The flowers of trefoil have great attraction for bees as a source of nec- tar and pollen. Bees were observed to leave a flowering alfalfa field near Davis, Calif., and fly 1 mile to gather trefoil pollen. Exceptionally heavy seed yields have been obtained where there were many bee colonies. Many vegetables do not require pollination to produce an edible crop, Yearbook of Agriculture 1952 but of these, carrots, radishes, turnips, cabbage, celery, and many others re- quire insect pollination for seed pro- duction. Both pollination and seed formation are essential in the produc- tion of the edible part of the pickling cucumber, cantaloup, and watermelon. Pollination of all sections of the com- pound ovary is evidently necessary for proper shape and quality of melons, as the deformed part of an incompletely pollinated cantaloup not only lacks seed but is also poor in sweetness and flavor. Honey bees are employed in the commercial production of the seed and fruit of several vegetable crops. Varietal crossing is generally unde- sirable in producing seeds for the prop- agation of vegetable crops. Hence the Department of Agriculture regulations require that in the production of seed of many vegetables the plots of the dif- ferent varieties must be at least one- fourth mile apart. An even greater dis- tance would be safer because pollen grains are always found on the honey bee and its flight range exceeds one- fourth mile. Cucumbers, muskmelons, watermel- ons, pumpkins, and squash have sim- ilar floral structures. The group — cucurbits — generally is characterized by having male and female blossoms on different parts of the same plant. Such an arrangement obviously re- quires insects for the transfer of pollen, as the plants are not wind-pollinated. Most varieties of cucurbits are self- and interfertile when pollinated by hand. Honey bees are widely used in green- house and field production of cucum- bers. Because the individual flowers are open for only a short time, a heavy con- centration of bees is advisable whether the cucumbers are grown for the fresh market, for pickling, or for seed. Watermelons and muskmelons often produce bisexual or complete flowers, instead of separate pistillate and stam- inate flowers. The complete flowers, however, do not fertilize themselves, and honey bees are as essential in their pollination as in the pistillate flowers. Honey Bees as Agents of Pollination Cabbage and the closely related cab- bagelike plants as cauliflower, broccoli, and brussels sprouts require cross-pol- lination by insects for good seed yields. Varieties of cabbage display various degrees of self-incompatibility. Cross- incompatibility is also common. Bees are effective agents in cross-pol- lination. Attempts to bring about self- pollination have had little success. Be- sides honey bees, cuckoo bees, leaf- cutting bees, mining bees, bumble bees, and bee flies are attracted to cabbage flowers. Some are said to work at lower temperatures than the honey bee. Be- cause cabbage for seed production is often grown in a cool location or dur- ing cool weather, some of these insects may be individually more effective than the honey bee in its pollination. The optimum temperature for pollen germination, however, is about 68° F., and bees are active in the field at tem- peratures as low as 6o°. English holly trees bear their pis- tillate and staminate flowers on sepa- rate plants. Although a small percent- age of the pistillate flowers on some trees develop parthenocarpic berries, facilities for cross-pollination are re- quired for commercial crops. The seeded berries resulting from cross-pol- lination are larger, less subject to pre- mature dropping, earlier maturing, and more resistant to withering after cutting. Bees are attracted to both staminate and pistillate holly flowers to such an extent that, when they are abundant, only one male tree is needed for pollinating 50 pistillate trees. The pollinizers should be selected for their foliage quality as well as their capacity to produce an abundance of viable pollen when the pistillate trees are blooming. Pollination by honey bees has thus become an essential factor in pro- ducing many crops, along with the factors that are taken for granted, such as the preparation of the soil, the sup- plying of moisture, and cultivation, pruning, and thinning. Because bee- 105 keeping is a specialty, just as fruit growing or the production of seeds are specialties, most growers will find it advantageous to rent bees rather than to keep their own. Cooperation be- tween grower and beekeeper thus be- comes important and is mutually advantageous. George H. Vansell, an apiculturist in the Department of Agriculture, is stationed at Davis, Calif. He has stud- ied in the University of Kansas, Har- vard and Stanford Universities, and the University of California. He has taught in the Universities of Kentucky and California. The activity of bees in collecting nectar and pollen, especially as agents of pollen distribution, has been his chief interest for many years. W. H. Griggs is assistant professor of pomology in the University of Cali- fornia at Davis. He has charge of investigations into the pollination of fruits and nuts. He received his train- ing in pomology in the University of Missouri and the University of Mary- land. Dr. Griggs was assistant professor of pomology in the University of Con- necticut in 1946 and 1947. For further reference: E. C. Auchter and H. B. Knapp: Orchard and Small Fruit Culture, John Wiley & Sons. 1937. A. D. Betts: The Constancy of the Pollen Collecting Bees, Bee World, volume 16, pages 111-113. 1935. G. Bonnier: Sur la Division du Travail chez les Abeilles, Academie des Science, Paris, Comptes Rendus, volume 143, pages 941-946. 1906. R. M. Bullock: Is Artificial Pollination Practical? American Fruit Grower, pages 14-15, May 1948; Handling and Applica- tion of Pollen to Fruit Trees, with F. L. Overley, Proceedings of the American So- ciety for Horticultural Science, volume 54, pages 125-132, 1949- C. G. Butler: The Behavior of Bees When Foraging, Journal of the Royal Society of Arts, volume 93, pages 501-51 1, 1945', The Behavior of a Population of Honey Bees on an Artificial and a Natural Crop, with E. P. Jeffree and H. Kalmus, Journal of Experi- mental Biology, volume 20, pages 65-73, '943- . . C. N. Buzzard: Bee Organization, Bee World, volume 17, pages 133-135, I93&; De reorganisation du Travail chez les io6 Abeilles, Societe d'Apiculteur des Alpes- Maritimes, Bulletin 15, pages 65-70, 1936. J. W. Carlson: Alfalfa-Seed Investiga- tions in Utah, Utah Agricultural Experi- ment Station Bulletin 258. 1935. D. B. Cast eel: The Behavior of the Honey Bee in Pollen Collecting, U. S. D. A. Bu- reau of Entomology Bulletin 121. 1912. W. H. Chandler, M. H. Kimball, G. L. Philp, W. P. Tufts, and George P. Weldon: Chilling Requirements for Opening of Buds on Deciduous Orchard Trees and Some Other Plants in California, California Agri- cultural Experiment Station Bulletin 611. "937- M. B. Crane and A. G. Brown: Incom- patibility and Sterility in the Gage and Des- sert Plums, Journal of Pomology and Horti- cultural Science, volume 17, pages 51—66. 1939- M . B, Cummings, E. W . Jenkins, and R. G. Dunning: Sterility in Pears, Vermont Agricultural Experiment Station Bulletin 408. 1936. Charles Robert Darwin: The Effects of Cross and Self Fertilisation in the Vegetable Kingdom, D. Appleton & Co. 1877. W. E. Dunham: Insect Pollination of Alsike Clover, Gleanings in Bee Culture, vol- ume 66, page 425, 1938; Insect Pollination of Red Clover in Western Ohio, Gleanings in Bee Culture, volume 67, pages 486-488, 1939- R. E. P. Dwyer and F. T. Bowman: Pol- lination of Williams (Bartlett) Pear in New South Wales. Part I. Investigations at Bat- hurst Experiment Farm, 1928-34, New South Wales Department of Agriculture Scientific Bulletin 62. 1938. Olav Einset: Experiments in Cherry Pol- lination, New York Agricultural Experiment Station {Geneva) Bulletin 617. 1932. S. W. Fletcher: Pollination of Bartlett and Kieffer Pears, Annual Report of Virginia Polytechnic Institute Agricultural Experi- ment Station for 1909, pages 213-224. 1910. K. von Frisch: Die Sprache der Bienen und ihre Nutzenwendung in der Landwirt- shaft, Experientia, volume 2, pages 397- 404. 1946. V. R. Gardner: A Preliminary Report on the Pollination of the Sweet Cherry, Oregon Agricultural Experiment Station Bulletin 116. 1913. J. H. Gourley and F. S. Howlett: Modern Fruit Production, The Macmillan Co. 1941. W. H. Griggs and George H. Vansell: The Use of Bee-Collected Pollen in Artifi- cial Pollination of Deciduous Fruits, Pro- ceedings of the American Society for Horti- cultural Science, volume 54, pages 1 18— 124, 1949; The Germinating Ability of Quick- Frozen Bee-Collected Apple Pollen Stored in a Dry Ice Container, with J. F. Rein- hardt, Journal of Economic Entomology, volume 43, page 54.9, 1950. Yearbook of Agriculture 1952 R. A. Grout: Pollination — An Agricul- tural Practice, Dadant & Sons, Inc. 1949. Q. A. Hare and George H. Vansell: Pol- len Collection by Honey Bees in the Delta, Utah, Alfalfa-Seed Producing Area, Journal of the American Society of Agronomy, vol- ume 38, pages 462—469. 1946. A. H. Hendrickson: The Common Honey Bee as an Agent in Prune Pollination, Cali- fornia Agricultural Experiment Station Bul- letin 291, 1918; Plum Pollination, Cali- fornia Agricultural Experiment Station Bul- letin 310, 1919; Further Experiments in Plum Pollination, California Agricultural Experiment Station Bulletin 352, 1922. George E. King and A. B. Burrell: An Improved Device to Facilitate Pollen Distri- bution by Bees, Proceedings of the American Society for Horticultural Science, volume 29, pages 156-159- 1933- Paul Knuth: Handbuch der Bliitenbiol- ogie, 5 volumes, Wilhelm Engelemann, Leip- zig, Germany, 1898. J. C. Kremer: Traps for the Collection and Distribution of Pollen in Orchards, Michigan Agricultural Experiment Station Quarterly Bulletin 31, No. 1, pages 12—21. 1948. C. I. Lewis and C. C. Vincent: Pollina- tion of the Apple, Oregon Agricultural Ex- periment Station Bulletin 104. 1909. L. H. MacDaniels: The Possibilities of Hand Pollination in the Orchard on a Com- mercial Scale, Proceedings of the American Society for Horticultural Science, volume 27> 37°~373> I93I> Pollination and Other Factors Affecting the Set of Fruit With Special Reference to the Apple, with A. J. Heinicke, Cornell Agricultural Experiment Station Bulletin 497, 1929. R. E. Marshall, S. Johnson, H. D. Hoot- man, and H. M. Wells: Pollination of Or- chard Fruits in Michigan, Michigan Agri- cultural Experiment Station Special Bulletin 188. 1929. W. J. Middlebrooke: Pollination of Fruit Trees, 1904-12, Journal of the Board of Agriculture, London, volume 22, pages 418- 443- 1915-16- A. Minderhoud: Untersuchungen Uber Das Betragen Der Honigbiene Als Bluten- bestauberin, Gartenvauwissenschaft, volume 4, pages 342-362. 1931. A. A. Moffett: Chromosome Number and Pollen Germination in Pears, Journal of Pomology and Horticultural Science, vol- ume 12, pages 321-326. 1934. F. L. Overley and R. M. Bullock: Pollen Diluents and the Application of Pollen to Tree Fruits, Proceedings of the American Society for Horticultural Science, volume 49, pages 163-169. 1947. O. H. Pearson: Observations on the Type of Sterility in Brassica Oleracea var. Capi- tata, Proceedings of the American Society for Horticultural Science, volume 26, pages 34—38, 1929; Breeding Plants of the Cab- bage Group, California Agricultural Experi- ment Station Bulletin 532, 1932. G. L. Philp: Cherry Culture in Cali- fornia, California Agricultural Extension Circular 46, revised. 1947. F. W. Rane: Fertilization of Muskmelon, Society for the Promotion of Agricultural Science, Report of the 19th Annual Meet- ing, pages 150-151. 1898. A. J. Pieters and E. A. Hollowell: Clover Improvement, Yearbook of Agriculture i937> pages 1 190-1214. C. R. Ribbands: The Foraging Method of Individual Honey Bees, Journal of Ani- mal Ecology, volume 18, pages 47—66. 1949. R. G. Richmond: Red Clover Pollination of Honey Bees in Colorado, Colorado Agri- cultural Experiment Station Bulletin 391. 1932. A. N. Roberts and C. A. Boiler: Pollina- tion Requirements of the English Holly, Ilex Aquifolium, Proceedings of the American Society for Horticultural Science, volume 52, pages 501-509. 1948. R. H. Roberts: Better Cherry Yields in Wisconsin, Wisconsin Agricultural Experi- ment Station Bulletin 344. 1922. J. T. Rosa: Fruiting Habit and Pollina- tion of Cantaloupe, Proceedings of the American Society for Horticultural Science, volume 21, pages 51-57, 1924; Pollination and Fruiting Habit of the Watermelon, Pro- ceedings of the American Society for Horti- cultural Science, volume 22, pages 331—333- 19*5- C. W. Schaefer and C. L. Farrar: The Use of Pollen Traps and Pollen Supple- ments in Developing Honey Bee Colonies, Bureau of Entomology and Plant Quaran- tine Circular E-531. 1941. J. S. Shoemaker: Cherry Pollination Studies, Ohio Agricultural Experiment Sta- tion Bulletin 422. 1928. C. E. Shuster: Pollination and Growing of the Cherry, Oregon Agricultural Experi- ment Station Bulletin 212. J 92 5. Sardar Singh: Behavior Studies of Honey Bees in Gathering Nectar and Pollen, New York Agricultural Experiment Station Mem. 288. 1950. F. W. L.Sladen: How Pollen Is Collected by the Social Bees, and the Part Played in the Process by the Auricle, British Bee Jour- nal, volume 39, pages 491—494. 1911. John C. Snyder: The Pollination of Tree Fruits and Nuts, Washington State College Extension Bulletin 342, reprint. 1947. F. E. Todd and O. Bretherick: Composi- tion of Pollens, Journal of Economic Ento- mology, volume 35, pages 312-317. 1942. W. P. Tufts: Almond Pollination, Cali- fornia Agricultural Experiment Station Bul- letin 306, 1 91 9; and California Agricultural Experiment Station Bulletins 346, Almond Pollination (1922), 373, Pear Pollination ('923), and 385, Pollination of the Sweet Cherry (1925), with G. L. Philp. Pollination by Native Insects George E. Bohart The earliest flowering plants in the fossil record were related to the mag- nolias, which to this day depend for pollination on the visits of beetles. Beetles, which comprise the order Coleoptera, were the most abundant and adaptable insects during the dawn period of flowering plants and thus, quite naturally, were the first pollina- tors. The flies and the sawflies and wasps were present but poor in variety and primitively developed. In the en- suing ages, however, their adaptation to the products of flowers became a dominant feature of their structure and habits. The moths and butterflies, which first appeared in the early days of flowers, soon adapted themselves completely to floral offerings. Now nearly all of them are highly developed for taking nectar from flowers. While the insects were thus becom- ing specialized to take advantage of flowers, plants were likewise becoming specialized to make more efficient use of insects. Certain flowers developed characteristics limiting them to polli- nation by certain types of insects, which in turn become highly adapted to these specialized flowers. Today we have many plants so constructed that only a few specially adapted insects can visit them successfully. Figs, orchids, Span- ish-bayonet, and monkshood are ex- amples. The so-called hawk-moth orchids (in the genera Habenaria, Angrae- cum, and others) exemplify the many intricate modifications possessed by orchids to insure pollination by specific kinds of insects. In these flowers the nectar, lying at the bottom of a long narrow tube, is accessible only to the long-tongued hawk moths. While 107 io8 probing for nectar, the moth brings each eye against a sticky disk to which a mass of pollen is attached, and flies away, carrying the masses on its eyes. The masses (called polfaiia) then bend forward on their stalks in such a way that, when the moth inserts its proboscis into the next flower, they fit perfectly against the stigma and adhere to it. From the presence in Africa of an orchid of this type, with a nectar tube 12 inches long, there is inferred the existence in that region of a hawk moth with a tongue equally long. In most acts of pollination the insect has no interest in the plant beyond its store of nectar or pollen, pollination on its part being an accident. It is the plant which, by its offering of nourish- ment and by the arrangement of floral parts, insures that such "accidents" will occur. The yucca moth, which is the sole pollinator of yucca ( Spanish-bayonet) , is a unique exception and provides a good example of symbiotic relation- ships between plants and animals. It is no mere nectar sipper. At first, oper- ating somewhat in the manner of the fig wasp, the female stabs the ovary of the yucca flower with her ovipositor and inserts an egg. That is common- place insect behavior, but her next acts, though instinctive, seem to display careful planning and an uncanny knowledge of botany. She mounts a stamen, scrapes together a wad of pol- len, carries it back to the pistil contain- ing her egg, and thrusts it into the funnel-shaped stigma. She takes neither nectar nor pollen for herself but performs the only act that will guarantee the proper food for her off- spring, the developing ovules of the plant. The yucca plant in its turn may lose a few seeds to the young worms — surely a small price to pay for such perfect pollination service. Some years ago, scientists argued hotly whether insects or flowers be- came specialized first or whether it was simultaneous. Voluminous papers at- tempted to explain why and how the Yearbook of Agriculture 1952 process of mutual adaptation devel- oped, but the subject finally became so controversial and unproductive that it was all but dropped. Recently, how- ever, technical advances in agriculture have demanded that progress on prob- lems of pollination keep pace. Knowl- edge gathered by the early workers in defense of their philosophical argu- ments is now being put to work in the applied field, but many of the old chal- lenging questions of insect-flower evo- lution remain unanswered. Granting the influence of pollinat- ing insects on biological history, what would happen if they should suddenly disappear? It would certainly not mean the end of the flowering plants, because many important plant types have secondarily become adapted to pollination by other agents than in- sects. The great family of the grasses depends upon cross-pollination by wind or automatic self-pollination within closed flowers. Most of the nut and acorn trees have become adapted to pollen transfer by wind. Even many species within the family of legumes, which is highly specialized for polli- nation by bees, have come secondarily to depend mainly upon automatic self- pollination within the young blossoms. Peas and beans are familiar examples. It is likely, therefore, that man could carry on without insects for pollina- tion. The grasses and self-pollinating legumes could form the basis of his agricultural economy. Many of the in- sect-pollinated plants could be main- tained by vegetative propagation, al- though most of them would be barren of fruit. Tomatoes and potatoes he would still have, but he would have difficulty finding substitutes for clover and alfalfa, and he would have to get along with reduced yields of a variety of crops ranging from cotton to onions. Perhaps the most drastic effects would be in uncultivated areas where a large share of the soil-holding and soil-en- riching plants would die out. Further- more, it would be a bleak springtime if no gay-colored flowers were to grow in the forest glens and open hillsides. Pollination by Native Insects So much for what did not happen and is not likely to happen. Let us ex- amine what has happened or may hap- pen in the future. Probably the insect pollinators will not disappear, and we can go right on eating apples and find- ing pieces of okra in our vegetable soup. When the first settlers arrived in America they found no honey bees but there were flowers, fruits, and vege- tables in the forests and fields. Further- more, they were able to produce na- tive American and introduced Euro- pean crops of many kinds for more than 50 years before honey bees were well established. Native insects were still abundant enough to pollinate the native and introduced insect-pollinated plants. Honey bees were colonized in North America before 1638, but for several decades they were probably more important as honey producers than as pollinators. So long as culti- vated areas were composed of small fields surrounded by wild land, native insects were able to handle the pollina- tion job without help from foreign la- bor. Inevitably, however, as the plow turned under large tracts of sod the native beneficial insects began to dis- appear. At the same time the available pollinators were spread more thinly over the ever-enlarging orchards and seed fields. Our native pollinators have suffered the same fate as other forms of wild- life. Certain species have been able to persist and even increase in cultivated areas by taking advantage of road cuts, outbuildings, eroded areas, and the like for nesting places. But most species have had to retreat into fence rows, stream gullies, wood lots, and waste fields to maintain themselves. In re- cent years, as clean cultivation and in- tensive land utilization have become the rule, such havens are fast disap- pearing within flight range of the crops that need insects for pollination. The logical question at this point is: How important or necessary are native pollinators to our agricultural set-up now that honey bees can be brought in 970134°— 52 0 109 large numbers to any field or orchard? There is no single answer to such a question. An estimated 80 percent of the insect pollination of our commer- cial crops is performed by honey bees — but that figure, which is only an esti- mate, does not tell us which crops are involved in the 20 percent pollinated by native insects and whether the whole 100 percent is adequate. Honey bees, unlike most of our na- tive pollinators, collect nectar or pollen or both from a wide assortment of plants. Consequently, there are few crops, whose blossoms are attractive to any insects, that do not hold some at- traction for honey bees. Besides, honey bees can be increased and moved about easily. Nevertheless, in the forests and ranges many herbs, shrubs, and trees will always have to depend on native insects for their reproduction. Like- wise, many forms of wildlife and range stock depend in whole or in part for food upon the plants or the seeds and fruit that the native pollinators make possible. Bee for bee, various native species are more efficient pollinators of certain crops, such as alfalfa, red clover, and sometimes even fruit, than honey bees. About that, more later; first let us look at the insects them- selves. Thousands of species of insects assist in the pollination of our entire fauna of insect-pollinated plants. They are distributed principally among the bees and wasps, the butterflies and moths, the flies and gnats, and the beetles. Even minute thrips may be important in the self-pollination of certain plants like carrots and some of the composites, which have tiny, closely aggregated flowers. The order Hymenoptera, even without the honey bee, is by far the most important order of insects in the pollination of commercial crops. Flies probably rank next in importance al- though the moths, which are very abundant, may do more pollinating .under cover of darkness than they are given credit for. However, the value of moths and butterflies as pollinators is more often than not offset by the dam- no age they do as larvae. Flies, likewise, are frequently harmful as larvae and many species are carriers of disease as adults. Among the Hymenoptera, bees, which comprise the superfamily Apoi- dea, are the most useful pollinators. Some other members of the order, such as the thread-waisted wasps, visit many flowers to partake of nectar, but theirs is a supplementary role on commercial crops and it is difficult to conceive of methods for making better use of them. The wild bees (after the honey bee) have rightfully received most of the attention accorded to our insect pollinators. At least 5,000 species of bees prob- ably exist in North America, many of them still undescribed by the taxono- mists. Most of the species are impor- tant only to wild plants, but at least several hundred take part in the polli- nation of cultivated crops. For exam- ple, more than 100 species have been reported as visitors to flowers of al- falfa alone. All but a few of our many species can be grouped in families thus: Col- letidae, obtuse-tongued bees; Halicti- dae, sweat bees and their allies; An- drenidae, mining bees; Megachilidac, thick-jawed bees; Anthophoridae, flower-loving bees; Xylocopidae, car- penter bees; Apidae, honey bees and bumble bees. The first three families are com- monly called short-tongued bees and the last four long-tongued bees, al- though that is not an invariable distinc- tion. Wild bees have great diversity in habits and habitats. Biological infor- mation has been published on fewer than 5 percent of the species. We do not even know where many of our more important pollinators nest. Rea- sonably complete biological studies have been made in this country for fewer than a score of species. By piec- ing knowledge of species in this coun- try with the more complete knowledge of their European relatives, however, Yearbook of Agriculture 1952 we have a ground work on which to build. Bees are characterized by the habit of providing a store of honey and pol- len for their offspring, although many species, cuckoolike, preempt the stores of their more industrious relatives. Most wild bees are solitary. Each female constructs, provisions, and lays eggs in her own nest without help from her neighbors. Each cell in the nest is sealed up as soon as it is provisioned and provided with an egg, and there is no further contact between parent and offspring. A number of species of the family Halictidae have advanced to the stage where the overwintered mother bee remains with her daugh- ters and assists them by guarding the communal nest entrance and laying fertilized eggs. Many of the so-called solitary bees are gregarious to a greater or lesser de- gree. Highly gregarious species may dig their burrows in the soil only an inch or two apart and cover acres with their bee towns. Populations in such sites are sometimes comparable to those of moderate-size apiaries of honey bee colonies. A nesting site of alkali bees in Utah was estimated to contain 200,- 000 nesting females. This site and an- other large one nearby provided good pollination for the alfalfa-seed fields within a radius of at least 2 miles. Most bee species are strictly solitary, showing no tendency toward neighbor- liness and often nesting in well-hidden places. In order to persist in effective numbers as pollinators, such species must have extensive areas suitable for their nesting. The recent experience of alfalfa-seed growers in Saskatchewan is a case in point. Their alfalfa is polli- nated principally by leaf-cutting bees, which nest in beetle burrows in the for- est timber. A few acres of seed sur- rounded by forest usually had plenty of leaf-cutting bees and good seed crops, but when the same area was given over to extensive cultivation, only a few seed fields next to the wild country were adequately pollinated. Social life is a striking but rather un- Pollination by Native Insects common attribute of bees. The true so- cial habit, involving division of labor and cooperation between parents and offspring, reaches its culmination in the complex society of the honey bee, but the glimmerings of social behavior are exhibited by several divergent stocks in various parts of the world. Among our native forms, bumble bees have the most complex society but their hive is a humble and untidy affair compared to that of the honey bee. The bumble bee, like the honey bee, belongs to the family Apidae, most of whose mem- bers are social. In the Tropics are many species of small stingless Apidae which, in some regards, are as highly devel- oped socially as the honey bee. They are the most abundant bees in many tropical areas and have been used by the Indians of South America for honey production, but attempts to col- onize them in this country have failed. Nesting places are nearly as varied as the bees themselves. The bumble bees choose well-protected cavities, which may be above ground or subter- ranean, depending upon the species. Carpenter bees (Xylocopa) and repre- sentatives of many genera of the Mega- chilidae nest in beetle burrows in wood or chisel their own tunnels. The small carpenter bees (Ceratina) and again many representatives of the Megachili- dae nest in the natural channels of hol- low or pithy-stemmed plants. Broad- tongued bees, sweat bees, mining bees, and flower-loving bees almost invari- ably construct burrows in the ground. Some of the less common environ- ments chosen by certain species include abandoned snail shells, small limb crotches, burrows of other bees, nests of mud-dauber wasps, and cavities in porous types of rock. Various mega- chilids are especially prone to develop tastes for unusual nesting places. Most bees nest in the soil. Depending upon the species, the soil may be moist or dry, loose or packed, or even solid rock. The surface may be bare or vege- tated, flat or vertical. Few species nest in rich organic soil or in densely shaded places. Most seem to like soil that packs III firmly, at least at the level of the brood cells. Nests of solitary bees in the soil are usually in the form of burrows with short or long branches containing brood cells. Some species make their tunnels only an inch or two long, but The large mountain carpenter bee and a series of brood cells tunneled in cedar. others drive the main shaft down for 2 or 3 feet. Some have vertical and others have horizontal cells. Some have several cells in a linear series, and others have only one. Some have cells in tight clusters like bunches of grapes, and others have them at the ends of short, horizontal branches along the main vertical shaft. Each genus of bees usually has a distinctive plan of archi- tecture, but plenty of leeway is left for one species to differ from another. Above the generic level, basic archi- tectural patterns are discernible in some cases (for example, in all halictid nests the entrance tunnel is wider than the branch tunnels) but in some fam- ilies, like Megachilidae, the diversity of nest types defies satisfactory classi- fication. Bees, like all insects that undergo complete metamorphosis, pass through four principal life-history stages, the egg, larva, pupa, and adult. The egg is always laid within a brood cell. Bumble bees and honey bees generally lay it in an empty cell, and the young larva is fed progressively by nurse bees in the hive. In this country all other bees lay their eggs on, within, or under 112 Yearbook of Agriculture 1952 Nest of Andrena subaustralis, exterior view (one cell cut open). Nest of Diadasia enevata, exterior view (one cell cut open). a mass of honey-moistened pollen, which becomes the sole nourishment of the growing larva. After laying the egg, the mother bee seals the cell. The eggs and developing larvae of most bees dry out readily. Consequently some sort of seal coat is applied to the inner walls of the cells by the mother bee. Mega- chilid larvae, which are less delicate, usually are not protected in such a fashion, although some of them are protected by cells lined with sections cut from leaves (leaf-cutting bees), with plant fibers (cotton bees) , or with pitch (resin bees). As I mentioned, the young of all bees are fed a combination of honey and pollen. Larvae of the honey bee are also fed a gland-secreted material called royal jelly. Queens particularly are fed large amounts of the material. Royal jelly or its equivalent may be added to the food of the solitary bees, but this has not been actually observed. The eggs of solitary bees, being rela- tively few in number, are much larger than those of honey bees and perhaps contain substances that are provided for honey bee larvae in royal jelly. The beebread, as the store of food is often called, is prepared in a variety of ways. Hylaeus, which is generally con- sidered one of the most primitive bees, does not collect pollen on her body but takes it into her honey stomach with the nectar. This distinctly liquid ma- terial is regurgitated into transparent waxen envelopes. The egg floats on the liquid in the envelope. The halictids fashion a flattened or egg-shaped ball of pollen, to which a small amount of nectar is added just before the egg is laid. This pellet of food material is measured exactly to serve the needs of the larva, and none is ever left over. The andrenids make a similar but more spherical ball. Sometimes differ- ent species within a genus may be dis- Pollination by Native Insects 113 Nest of Nomia melanderi, exterior view (one cell cut open). Nest of Halictus farinosus, exterior view (two cells cut open). tinguished by the shape of the pollen loaf. Size and shape of the egg and its method of placement also vary widely. The eggs generally hatch within 2 or 3 days, and the larvae attain full growth within from 1 to 3 weeks, molt- ing and usually eating their cast skins twice during the process. There follows a period of several days during which the great quantities of pollen in the digestive tract are absorbed and waste materials are discharged as fecal strips or pellets. Most bees, like the honey bee, maintain sanitary quarters during the feeding period. Among honey bees, at least, the midgut does not commu- nicate with the hindgut until the feed- ing period is over. In the large family of megachilids, however, the larvae may begin defecating when only one- third grown. Abandoned nests can often be identified by the type and manner of placement of the feces. Many of the megachilids use their pel- lets as building blocks in the construc- tion of their cocoons. Some bee larvae spin cocoons in which they pupate. Some do not. Honey bees and bumble bees, which are supposed to be at the top of the evolutionary scale, spin cocoons as do most of the megachilids, which are also thought of as advanced forms. Cocoon formation is scattered throughout the anthophorids, which are considered intermediate in the evolutionary scale. It is rare among the more lowly hal- ictids and andrenids, and absent among the most primitive of bees, the colletids. As bees are supposed to have developed from hunting wasps, most of which spin well-made cocoons, one would ex- pect the evolutionary trend, if any, to be away from rather than toward the cocoon-spinning habit. Mature bee larvae, after defecating and perhaps spinning a cocoon, become nearly motionless prepupae. The pre- "4 burrow Nest of Anthophora occidentalis, exterior view (two cells cut open). dirt plug transparent cells holding semiliquid food Nest of Colletes, interior view. Yearbook of Agriculture 1952 pupa is more resistant to cold, drying out, and disease than the growing larva and is for many bees the overwintering stage. Beginning with this stage, the major differences in the life cycles of the various genera become manifest. Andrena is the largest genus of bees. Most of its species have a simple life cycle, which may serve as a standard for comparison. Adults of both sexes emerge in the spring from their larval cells in the soil. After mating, the fe- males construct, provision, and lay eggs in nests of their own. The larvae grow rapidly and by the end of spring have pupated and become adults within their larval cells. They remain thus im- prisoned throughout the summer, fall, and winter; they escape the following spring when their host plants are in bloom. The alkali bee has a similar life his- tory except that the first activity of adults takes place in the summer, and overwintering is in the prepupal stage. In most localities, a second brood of adults appears in the late summer. It is composed predominantly of females. The scarcity of males at this time may account for the high percentage of males usually occurring in the over- wintering generation since males de- velop from unfertilized eggs. This life-history pattern, allowing for variation in time of adult activity and number of generations in the active season, is the predominant one for wild bees. It is safe to say that most genera of bees pass the winter in the prepupal stage. Halictus is a large and familiar genus of ground-nesting bees. Many of the species show a tendency toward social behavior. They are not related to bumble bees, but their life cycle is similar in several ways. Many Halictus have a life history somewhat as fol- lows: In the spring, overwintered females leave their hibernation bur- rows to construct and provision brood nests. Within a month or 6 weeks their progeny, all females, make their nests in the form of side burrows of the Pollination by Native Insects parental nest, or else dig new ones of their own. Their progeny, being un- fertilized, develop into males. The de- velopment of males from unfertilized eggs (parthenogenesis) is a general but not infallible rule among Hymenop- tera. The old, overwintered female continues to lay eggs, this time on pol- len balls of her daughters. These de- velop into females, which emerge in the summer and mate with the males, of which there is a large crop. The males soon die and the females dig themselves into hibernation burrows for the winter. Some species produce a third generation in the late summer composed of both males and females. Bumble bees carry this pattern to a higher social level. The mated, over- wintered females are large individuals known as queens. In the spring the queen leaves her hibernation quarters and spends considerable time feeding and searching for a nesting place. After finding one, she prepares a small bed of woolly material in which she constructs a ball of pollen and a waxen cup or two filled with honey. She then lays a group of eggs in a cavity in the pollen and feeds the young larvae honey, increas- ing the pollen supply as needed. This progressive feeding is a step forward in social development. Bumble bees have the birdlike habit of brooding on the eggs and young larvae. The queen's first brood gener- ally develops into four to eight small worker bees. The workers are females with small bodies and poorly developed ovaries, apparently resulting from a limited food supply in the larval stage. Shortly after emerging, the worker bees take over the field and hive duties. The queen then retires to a life of egg laying. Successive broods of workers tend to become larger as there are increasing numbers of bees to feed them in the larval stage. By the middle of the sum- mer a large share of the larvae are fed a maximum diet and develop into queens. At the same time the males (drones) begin to appear. Some of the males may come from unfertilized eggs "5 ^~—-^-^^-' fl,"ls fSs3g* — /'/"? "/ circular leaf pieces cell caps J of circular ■» leaf piece* fifi 3r B/Vl f'"' ''""' " '''' f V-VtT riljalja leaves Msfflu m*ci\w •V i \ "'"* l"'J t"eces Nest of Megachile dentitarsis, interior view. transparent cell made by mother bee Nests of Colletes, interior view. laid by the queen, but apparently most of them are the progeny of laying workers. The queens mate with males outside the nest, and after a few days or weeks of freedom they dig into sod or other material for hibernation. In the early fall, when no more female eggs are being laid by the exhausted old queen, the proportion of males in- creases and the colony gradually dies out. During the senescent period, scav- enging larvae of moths and beetles rap- idly destroy the nest. Many genera of bees have become specialized as social parasites — they live not on the tissues but on the food of their hosts. They are parasitic on other bees in all cases and, since they belong to various branches of the bee family tree, it is apparent that the para- n6 Yearbook of Agriculture 1952 Norma mclanden Norma triangulijera Andrena complexa Anlhophora ocadentatis Osmra hgnana H I Eggs and provisions of various solitary bees. Osrma montana sitic habit arose independently many times. A number of genera of parasitic bees are closely related to their hosts. Despite their diversity, parasitic bees all operate in much the same manner. The female spends most of her time searching for nests of her host. When she finds one, she waits for a propitious moment to slip in and place an egg on a completed pollen ball before the cell is sealed. Apparently the host bee then seals the cell without recognizing that one of the eggs is not her own. The parasite is well protected with heavy armor and a long sting in case the for- aging host returns and finds her in the nest. The young larva of the parasitic bee has long jaws adapted for pierc- ing the egg or young larva of the host. When this is accomplished, the in- truder develops on the stored food just as if it were the rightful progeny of the host. Let us now consider the usefulness of wild bees in the pollination of spe- cific crops. Although wild bees supple- ment the activities of honey bees in the pollination of many crops such as sweetclover and most of the fruits and cruciferous vegetables, honey bees are apparently at least as efficient and need only be supplied in reasonable numbers to handle the job alone. Red clover was recognized by Charles Darwin as a plant that re- quires bumble bees for satisfactory seed production. The flowers have a deep corolla tube and tend to produce little nectar. Consequently honey bees often find it difficult or unprofitable to take nectar from red clover. They can obtain pollen from it readily but more desirable sources of pollen in the vicinity may satisfy their needs. In New Zealand, where red clover is well adapted, seed production was almost nil until the end of the nineteenth cen- tury, when several species of bumble bees were successfully introduced and established. Bumble bees have longer tongues than honey bees, and most Pollination by Native Insects species regard red clover with special favor as a source of nectar and pollen. Apparently they have declined drasti- cally in numbers in the United States since 1900. In only a few districts are they adequate for the pollination of red clover; even there they are unre- liable because of yearly fluctuations in numbers. Fortunately in most regions honey bees, in sufficient numbers and properly managed, can be induced to pollinate red clover. Undeniably, how- ever, a general increase in bumble bees in the red clover seed areas would be a boon. Alfalfa presents a different problem. It is a favorite source of nectar for honey bees, especially in the West, but is not a preferred source of pollen. Honey bees pollinate most kinds of flowers equally well when gathering nectar or pollen, but in the case of al- falfa the nectar gatherer is able to "steal" nectar from the flower without tripping a special mechanism involved in the pollination process. Nectar gatherers accidentally trip a small per- centage of the flowers they visit, but for effective pollination they must be present in great numbers — greater, in fact, than the beekeeper is generally willing to supply when a honey crop is his primary goal. In some places the seed growers have largely overcome this difficulty by paying beekeepers to overstock the alfalfa fields with honey bees, but this does not seem to work equally well everywhere. It also in- volves difficult problems in financial arrangements between seed growers and beekeepers and in maintenance of colony strength. In some areas, where 5 to 25 percent of the honey bees visit- ing alfalfa collect pollen, the problem is much less acute, and overstocking is practiced only for exceptionally high yields. Many kinds of native bees visit al- falfa and most of them pollinate it efficiently, because they work it pri- marily for pollen, and trip the major- ity of the flowers they visit. Some species even seem to prefer alfalfa to neighboring pollen sources. Despite the 117 variety of bees that visit alfalfa, how- ever, there are not enough of them in most seed fields to provide adequate pollination — especially when the fields are large or an entire district is given over to seed production. The alkali bee is one of the few species that can build up sufficient numbers on small pieces of wasteland to pollinate extensive acreages of alfalfa. In a new seed dis- trict near Yakima, Wash., alkali bees are responsible for most of the pollina- tion on thousands of acres of high- yielding alfalfa-seed fields. This bee is an important pollinator in localized areas in most of the States west of the Great Plains. In Canada leaf-cutting bees (Megachilidae) are generally credited with most of the pollination of alfalfa, although they can do a good job only on small acreages surrounded by much wild land. An important fu- ture development in pollination by wild bees may be in the production of foundation and registered seed stocks which require isolation in order to maintain their purity. Fruit trees are pollinated principally by honey bees in this country. In most districts, however, various vernal species of wild bees have a supplemen- tary role. Even syrphid flies and blow flies are important in some localities, notably in pear orchards. Honey bees are generally satisfactory pollinators of fruit except in parts of New England and eastern Canada where weather unfavorable for honey bee activity is customary during the apple-blossom- ing season. When they are present there, bumble bees and a few other species active at cooler temperatures are more satisfactory. Tomatoes, peas, and string beans are examples of automatically self- pollinated crops. The principal exist- ing varieties are highly self-fertile and apparently receive no benefit from the cross-pollination accomplished by in- sects. Various wild bees are more at- tracted than honey bees to those crops. For example, bumble bees collect pol- len readily from tomatoes. Leaf-cut- ting bees are strongly attracted by cer- n8 tain varieties of peas. It is possible that wild bees could be important in the de- velopment of a hybrid-seed industry for several such vegetable crops. Several small species of sweat bees appear to be the only bees that visit the flowers of beets in Utah. Beets are generally considered to be wind-pol- linated, but insects are known to assist in the transfer of beet pollen. In Utah, where hybrid seed of sugar beets is being produced on experimental plots by planting alternate rows of male- sterile and pollen-parent varieties, the set of seed on the male-sterile lines is greatly enhanced by the presence of sweat bees. Such isolated experiences indicate that more seed crops are bene- fited by wild bees than is generally recognized. Any attempts to conserve wild bees must be based on a knowledge of their habits and on a knowledge of the natural and man-made factors that operate against them. Even in environments undisturbed by man, wild bees fall prey to an as- sortment of natural enemies. Philan- thinid wasps store them as food for their larvae. Robber flies pounce on them in the air and drain them of blood. Ambush bugs and crab spiders lie in wait on the flowers for a meal of bee blood. Back at the nests, conopid flies perch on spears of grass and seize passing bees for long enough to force an egg between their abdominal seg- ments, an egg that soon develops into a fat maggot occupying the entire body cavity of the host bee. Cuckoo bees lurk about the nesting sites and seize an opportunity when the mother bee is foraging to slip in and lay an egg in the cell being provisioned. Bee flies hover over the nest entrances and spray them with minute eggs. The eggs de- velop into hordes of spiny little mag- gots, which work their way into the bee cells before they are sealed and remain there until the bee larvae are full- grown. Only one maggot develops on a bee larva, but its persistent sucking gradually transfers the semiliquid con- Y ear book of Agriculture 1952 tents of the bee larva into its own grow- ing body and leaves only a dried-up husk. Toward the end of the nesting season, wingless velvet ants crawl over the ground in the. late afternoon, searching for any evidence of a nest. Once they find it, they force their way in, chew a hole through the host co- coon, and deposit an egg on the pre- pupa within. The invader then repairs the hole in the cocoon with salivary material and covers up the nest, leav- ing her offspring to fatten on its cell mate in security. In general, the gregarious species, more than the strictly solitary ones, are seriously harmed by parasites. Antho- phora occidentalis, a large western bee that nests gregariously in clay banks, is parasitized in Utah by a chalcid wasp, three meloid beetles, a clerid beetle, a velvet ant, two parasitic bees, and a bee fly. Total parasitism in some sites runs as high as 50 percent. The alkali bee, which nests by thousands in flat, alka- line ground, is parasitized in Utah by one parasitic bee, one meloid beetle, one conopid fly, and one bee fly. The first three are of minor importance, but the bee fly (Heterostylum robustum) nearly wiped out several large aggre- gations in Cache Valley in 1947; since then it has held them down, with para- sitism as high as 90 percent. Strangely enough, this same fly occurs in the large nesting areas of central Utah, but only a few maggots have been found in thousands of cells examined. Diseases are found among wild bees just as they are among honey bees. In- fections resembling the foul broods of honey bees have not been observed among the native species, but very likely they exist. Certainly, larvae in their cells in the ground are frequently seen to sicken and die. Probably the development of organisms on the stored food is more serious to the wild bees. Various types of mold attack the pollens and some invade the bodies of the bee larvae, although that may usually be secondary after the larva is weakened on account of the moldy food supply. On the wet soil used by Pollination by Native Insects the alkali bee the pollen balls may sud- denly liquefy, in which case the larva quickly dies. In some sites this has been observed in as many as one-quarter of the cells. Diseases of adults are not often seen but would usually be diffi- cult to observe or evaluate. In Califor- nia in the spring of 1934 a large popu- lation of Andrena corn pie xa gather- ing food from buttercups became in- fested with a fungus (probably Em- pusa sp.) and most of them died, still clinging to their host plants. The impression should not be gained that predators, parasites, and diseases are so serious that wild bees have no chance to increase. In central Utah the many kinds of insects and pathogens attacking brood of the alkali bee pre- vented only 30 percent from emerg- ing over a period of 3 years. During this period the known nesting sites in- creased in size and several new sites were founded. Predators and parasites of wild bees will probably prove difficult to control. The life history of many of them is so tied to that of their hosts that selective control measures may be impossible. Spoilage of the stores and molding of the larvae have been seen to in- crease following rain during the active nesting period of several species that nest in the soil. It is obvious that irri- gation and floodwater over the nests would be harmful then. Even during the dormant season, standing water would cause trouble, depending on the soil type and the species of the bees. The principal limiting factor in numbers of wild bees appears to be available forage. Particularly is that true in wild or thinly settled land. The close association between species of bees and particular genera of flowers was probably developed as a response to competition for forage; the less ag- gressive types had to specialize to survive. Competition has similarly forced many bees to restrict their season of activity to avoid periods of drought. In desert areas most bees can remain dormant for several years, if necessary, 119 until there is enough moisture for blos- soming of their host plants. Forage for bees is not generally abundant in densely timbered terri- tory, in deserts, or in open prairies. It is more often suitable for large popula- tions of bees in transitional zones at the edges of deserts or forests, in hilly country, or in abandoned agricultural areas that arc reverting to forest. For- age and bees are also usually abundant for limited periods in semiarid country where rain falling during a restricted season gives rise to short but intense periods of bloom. Some cultivated areas are highly productive of forage; a common condition is for flowers to be produced in greater quantity but lesser variety than before cultivation. For bees to build up sufficient num- bers of overwintering forms for a good emergence the following year, there must be a continuity of bloom during the season of foraging activity. The interrupted bloom common to most agricultural areas is thought to be largely responsible for the small exist- ing populations of wild bees. For ex- ample, it has been stated that wild bees will increase in alfalfa-seed-producing areas when the cutting schedule allows for a constant supply of bloom. Applied to wild bees in general, the statement is based on an oversimplified notion of their life histories. It would apply best to leaf-cutting bees, most of which, in Utah at least, have activity periods in- volving two to three generations, which last through the blossoming period of alfalfa. Good forage and weather con- ditions in the spring before alfalfa blooms are probably more important for bees like honey bees, bumble bees, and sweat bees, which have a long sea- son. Bees like Nomia, which do not appear until late summer, or Osmia, which disappear shortly after the first blooming of alfalfa, would be bene- fited more by a single cutting designed to achieve the maximum bloom at the proper time. The value of spring forage for bees with a long season is illustrated by events in an isolated alfalfa-seed dis- 120 trict near Fredonia, Ariz. Bumble bees were abundant in the summer of 1949 and provided excellent pollination for the alfalfa. An unprecedented drought in the area in 1950 prevented any spring bloom and, although queens were seen in the spring, there were no workers in the summer for pollination. It is likely in this instance that a few irrigated acres of an early-blooming crop like vetch would have allowed the bumble bees to increase as usual. More than half of our species of bees have a short season of activity. In most cases the timing of emergence of such bees with the first blooming of their natural host plants is remarkable. In the Sacramento Valley of California, where a nesting site of two species of Andrena was under observation, emer- gence of the bees and the first appear- ance of willow blossoms took place on the same day. Bad weather during the short period of activity of such bees is apt to be their most serious hazard. The presence of permanent and suit- able nesting sites may be as important as abundant forage for the mainte- nance of effective numbers of wild pollinators. The decline in populations of wild bees in agricultural areas has probably been brought about at least as much by destruction of nesting sites as by destruction of forage. In this con- nection it is interesting to speculate upon the probable history of popula- tions of the alkali bee in central Utah. In view of the fact that common, introduced plants like alfalfa, sweet- clover, and Russian-thistle are almost the sole forage plants for these bees in the area, it appears that they must have actually increased following the ap- pearance of white settlers. Many state- ments from the older farmers in the region attest to their abundance in the early days of alfalfa-seed growing. However, as cultivation increased, the nesting sites, although generally in poor soil, were plowed up and planted to alfalfa. Now only scattered areas are close enough to remaining nesting sites to be benefited. The best seed district in Utah from the standpoint of polli- Yearbook of Agriculture 1952 nation by alkali bees is adjacent to many acres of permanent saltgrass pas- ture that furnishes plenty of suitable land for nesting. Intensification of land utilization has played havoc with the nesting sites of wild bees. The old rail fences provided sites for many timber-inhabiting bees like leaf-cutting bees and Osmia and provided a network of areas of undis- turbed ground for nesting and of wild plants for forage. The clean cultivation now practiced to destroy weeds and soil-inhabiting insects is wiping out many of these last sanctuaries. It may soon become necessary to determine in each area how valuable the wild bees are for the pollination of crops and whether nesting sites can be reserved for them in a manner compatible with good agriculture. Destruction of harmful organisms to- gether with conservation of beneficial ones should be our aim. Too often the ravages of the destructive forms are so conspicuous that we lose sight of the value of the beneficial forms. This is clearly evident in the use of insecti- cides. The necessity for insecticidal control for many insect pests is unques- tioned. But it is becoming increasingly apparent that the simple question, "Will this application provide eco- nomic control of the pest concerned?" must be expanded to, "Will this appli- cation fit into a general program cal- culated to control all important pests without presenting a hazard to health or seriously affecting beneficial para- sites, predators, and pollinators?" Conservation programs for wild bees have never been tried or even formu- lated on an area-wide basis. Although it is encouraging to know that a few seed growers are taking steps to pro- tect known nesting sites, it is disheart- ening to know that most farmers do not appreciate the value of wild bees and are unlikely to take readily to conserva- tion measures involving setting aside pieces of land and complicating the cropping procedures. The following general measures should tend to conserve and even in- Pollination by Native Insects crease the numbers of many kinds of wild bees. Details for carrying them out would depend upon many local fac- tors; local conditions would probably call for certain additional measures. 1. Apply insecticides to blossoming plants only when there is no other way to control the harmful insects. Such applications should be made between 7 p. m. and 7 a. m. and should contain only toxaphene, methoxychlor, or other toxicants demonstrated to be rel- atively safe for bees when used at the proper strength. 2. Provide a continuous supply of bloom throughout the season. Forage crops such as vetch, clover, and alfalfa make a good series lasting from late spring through summer. Fruit trees, maples, hawthorns, elderberries, and other hedgerow plants generally pro- vide needed spring forage. Of course, each area would be best served by the plants suited to its own climate and agricultural needs. 3. Establish and maintain hedge- rows around agricultural fields and along roadways, ditch banks, and canals. Pithy-stemmed plants such as elderberry, sumac, and tree-of-Heaven should be encouraged in such hedge- rows. Light browsing would make them more suitable for nesting than if they were left undisturbed. 4. Hollow-stemmed plants such as milkthistle, wild parsnip, canebrake, and teasel should be broken over after the stalks are well developed. These will provide nesting places for leaf- cutting bees and harbor many hiber- nating species. 5. Establish and protect areas of bunch-type perennial grasses, espe- cially along the tops of banks. They will provide nesting places for bumble bees and tend to stabilize and shelter the banks. Banks so protected, espe- cially if nearly vertical, are ideal nest- ing places for many kinds of bees. 6. Preserve known nesting sites of gregarious bees from being cultivated, flooded, trampled, or encroached upon by dense vegetation. Expand the avail- able nesting ground if necessary, and 121 establish new areas with the same con- ditions as populated sites. In the past few years many nesting sites of the alkali bee have been discovered by al- falfa-seed growers. Once apprised of their value, the growers have usually been willing and even anxious to keep them in an unaltered state. Several and perhaps most of the gregarious Species of bees migrate in large groups to newly prepared areas. If other condi- tions for population increase are fav- orable, it should not take long for new areas to be populated. Another approach to the problem is through better utilization of available populations of native pollinators. The following principles should apply to many crops. 1 . Grow the crop in areas where na- tive pollinators are known to be abund- ant. In most cases such areas will be adjacent to or surrounded by untilled land. 2. Limit the acreage of the crop in bloom at one time to that which the native pollinators can handle. 3. Reduce competitive sources of pollen and nectar. 4. Time the blooming of the crop with the period of greatest natural abundance of the pollinator. (In gen- eral, only forage crops would be con- cerned here.) George E. Bohart, a member of the division of bee culture of the Bu- reau of Entomology and Plant Quar- antine, is in charge of the pollination studies in connection with the produc- tion of legume seed, conducted at tin- Legume Seed Research Laboratory in Logan, Utah. The attention of the reader is di- rected to the section of color drawings in which appears a drawing of an alkali bee (Nomia sp.) tripping an alfalfa blossom and the nesting sites and life stages of the bees. Opposite the draw- ing is a description of the life history and pollination activities of alkali bees. Breeding Bees Otto Mackensen, William C. Roberts Because honey bees produce honey and beeswax and help pollinate many plants, improving them through breed- ing benefits beekeepers and farmers. Man has kept bees for ages, but se- lective breeding of bees has lagged far behind that of other domesticated ani- mals and plants. The main reasons therefor stem from the social nature of honey bees, the mating of the queen and drone away from the hive, and a lethal mechanism that may kill a large percentage of eggs and brood. Honey bees will not mate, repro- duce, or survive in isolated pairs as nonsocial insects do. Each colony con- sists of a fertile queen and her many infertile daughters, the worker bees. All contribute to the performance of the colony. The colony rather than the individual is the unit upon which the selection of breeding individuals must be based. After a superior colony has been chosen, one can only use as breed- ing individuals the virgin queens that are sisters to the workers and the drones that are sons of the queen. The breeding quality of a colony can be obscured by environmental factors. A colony's large honey crop might be the result of its robbing activities rather than its industry in bringing in nectar from the field. A queen might have a high egg-production potential, but the actual number of eggs she lays each day depends on the size of the population of the colony, its food, and space. The drone, which develops from an unfertilized egg, is haploid — he carries only a single set of chromosomes and genes, the tiny elements of heredity. The sperms he produces are all genet- ically identical; they carry the same genes as the drone himself. The queens and workers, developed from fertilized eggs, are diploid; they carry a double set of chromosomes and genes. In the production of eggs, the genes segregate, so that each egg carries a sample half of the genes of the queen. A queen may mate with one or more drones, but after her mating period she does not mate again. The sperms are stored in the queen in a spherical structure called the spermatheca, and released a few at a time as the eggs are laid. If a queen is mated to one drone, all the workers of the hive receive identical genes from the drone and all the genetic variabil- ity comes from the queen. The same is true of queens reared from the colony. If the queen mates with two or more drones, there will be greater variability in the workers. The control of parentage obviously is essential to breed improvement. In this the honey bee presents a special problem because the queen leaves the hive to mate. She returns in about 15 ,or 20 minutes, bearing evidence of hav- ing copulated, but how and where mating takes place is still a debated point. Attempts to mate queens in con- finement have failed. Of the various methods employed to control mating, two have proved most practical: Isolation, by placing col- onies containing the breeding individ- uals (virgin queens and drones) in a location far away from other bees; and artificial insemination, by taking semen from one or more drones and injecting it into the queen by means of special instruments. Each method has advantages and disadvantages and a place in breeding programs. At a mating station several types of virgin queens can be mated at once and in large numbers. Only one type of drone can be allowed to fly there at one time, however. Because stray swarms may drift into the area unnoticed, one can never be certain that isolation is entirely effective, even when require- ments of distance have been met. In- dividual matings cannot be distin- 122 Breeding Bees guished because many queens mate more than once. With instrumental insemination, on the other hand, control of parentage can be absolute. Many types of drones can be used simultaneously in the same queen yard and individual- or multi- ple-drone matings can be made at will. Because the operation is time-consum- ing, however, only a limited number of inseminations can be made. The distances required for com- plete isolation at a mating station un- der various conditions have not been determined fully. In some experiments, virgin queens did not mate when the nearest source of drones was 6 miles away. A shorter distance may be all right when enough drones of the de- sired type are provided or when the location is geographically isolated, as by mountains or a body of water. For 2 years the division of bee cul- ture of the Bureau of Entomology and Plant Quarantine maintained a mating station at Grand Isle on the marshy coast of Louisiana. It was 20 miles from other bees and was adequately isolated. Queens and drones of a highly selected yellow strain that were mated there showed no evidence of having mated with strange drones. Kelleys Island, Ohio, 4.5 miles from the main- land in Lake Erie, has been used since 1948 as a mating station for the mass production of hybrid queens. It also is adequately isolated. Virgin queens did not mate during a complete lack of drones on the island although main- land colonies 5 or 6 miles away con- tained drones. Islands, marshy coasts, and desert areas probably offer the best locations for isolated mating stations. Instrumental insemination can be learned by anyone, but because ex- pensive equipment is needed and it takes so much time, few beekeepers have made use of it. The principal instruments needed are a stereoscopic microscope, a device for administering carbon dioxide as an anesthetic, and the insemination ap- 123 paratus itself (the manipulating stand, holding hooks, queen holder, and syringe). The procedure, as we prac- ticed it in 1952 and the results to be expected are given in the paragraphs that follow. The queen is allowed to back into the queen holder tube until the end of her abdomen projects. She is secured by means of a stopper, through which carbon dioxide is flowing, and placed in the manipulating stand. Then the chitinous plates at the tip of the ab- domen are separated with the holding hooks to expose the genital opening. Semen is taken into the syringe from the drone and injected into the genital opening of the queen. The operation takes about 5 minutes. The drone penis is relatively large and in copulation turns inside out, bringing the semen to the end of the everted penis. In artificial insemina- tion, partial eversion is brought about by exposure to chloroform fumes and completed by pressure. The semen ap- pears at the end of the everted penis as a cream-colored fluid accompanied by a white mucus. After a little prac- tice, the operator can easily take the semen into the syringe. A tonguelike structure, the valve fold, obstructs the opening to the ovi- duct. The fold must be pulled aside to permit the end of the syringe to pass. The semen must be deposited in the oviduct for successful insemination. The complete or partial failure of early investigators was probably due in large part to ignorance of this structure or disregard for it. From the oviducts, the sperms migrate into the spermatheca. During the insemination operation, carbon dioxide is allowed to flow through the queen holder. It acts as an anesthetic and it also stimulates early oviposition. Properly used, it reduces the average age at initial oviposition from about 40 days after emergence in untreated queens to 1 1 .5 days, which is about the age of initial oviposition after natural mating. Three exposures of 10 minutes' duration are given at 1 -day intervals. With or without in- 124 semination, this will stimulate egg pro- duction in queens. Other anesthetics and electric shock have the same effect. By using a counting chamber slide to count samples of sperm with a mi- croscope, we found that the sperma- thecae of naturally mated queens con- tained an average of 5.73 million sperms and those of queens insemi- nated with sperm from a single drone contained an average of 0.87 million. About 2.5 cubic millimeters can usually be taken from three drones. The aver- age number of sperms reaching the spermatheca when this amount was given one, two, three, and four times were 2.97, 4.1 1, 4.85, and 5.52 mil- lion, respectively. The number varied greatly in individual matings, but the variation was not so great when the amount of semen and number of injec- tions increased. When three and four inseminations were given, the variation was less than that of naturally mated queens. Two inseminations of about 3 to 4 cubic millimeters have given excel- lent results. Queens so inseminated have performed as well as naturally mated queens of the same parentage. Queens that lay unfertilized eggs in worker cells, where they intend to lay fertilized eggs, are called drone layers. With single-drone inseminations, a few queens will be partial or complete drone layers at the start and others will later become drone layers. Some have performed satisfactorily in small hives for a year or more, however. When larger inseminations are given, drone layers are rarely found at the beginning of egg laying. The percentage of good laying queens obtained varies with the stock and rearing conditions. The results we got in two lines during the 1950 season Yearbook of Agriculture 1952 give an idea of what might be ex- pected. Of 38 queens inseminated in one line, 36 (95 percent) started lay- ing; 33 (87 percent) were considered good queens. In another slightly in- bred line, 32 queens were inseminated; 25 (78 percent) started laying and 22 (69 percent) were considered good queens. All in both groups produced good worker brood. Several men have made noteworthy contributions. Lloyd R. Watson, of Al- fred, N. Y., developed a workable syringe and was the first to demon- strate (in 1926) a successful technique that could be used by others. W. J. Nolan, of the division of bee culture, developed the basic manipulating ap- paratus and queen holder. Harry H. Laidlaw, of the University of Cali- fornia at Davis, made a detailed an- atomical study of the reproductive or- gans of bees and pointed out the im- portance of the valve fold. We have been striving to improve the instrumental insemination appa- ratus and technique since 1935. We found that semen could be collected more quickly from the completely everted penis than from the seminal vesicles or dissected parts of the par- tially everted penis, as earlier workers had done. The greatest improvement in appa- ratus was that made in the syringe. A glass syringe tip was constructed that tapered at the end to an outside diam- eter under 0.3 millimeter (0.012 inch) so that it could be inserted into the ovi- duct, which usually has a diameter of 0.33 millimeter. The plunger barrel had an inside diameter of 0.41 milli- meter; that was large enough to give the syringe a capacity of about 3 cubic millimeters, or the amount of semen Plunger^ ==£ Plastic tip Diaphragm Sketch of section through syringe. Breeding Bees usually obtained from three to four drones. Later a less breakable tip of the same type was made of plastic. A syringe was finally designed that does away with the troublesome tight- fitting plunger. It employs a rubber diaphragm. The base of the tip fits against the diaphragm and has a cone- shaped depression into which the dia- phragm is pushed by a plunger acti- vated by a screw. In use, the tip is first filled with water, then some of the water is pushed out, and semen taken up in its place. This tip has a main barrel of 0.025 mcn (inside diameter) and then tapers 0.156 inch to an inside diameter of 0.006 inch. The outside diameter at the end is 0.0 10 inch (the average oviduct diameter is 0.013 inch). A lethal mechanism in bees com- plicates breeding. To clarify this mech- anism some preliminary explanations are necessary. We have explained the location of the hereditary determiners, the genes, in linear order on chromosomes and how they are inherited in bees. When genes are paired as in the diploid queen, and one member has a different action than the other, each gene is called an allele of the other. Individ- uals in the population may carry still other alleles at the same locus on the chromosomes, and there may be a large series, each having a slightly different action. Fertilization may bring to- gether various combinations of these alleles. Genes have various effects. Some have detrimental effects. In some in- stances this effect is so great that the gene kills the individual inheriting it. Such a gene is called a lethal. The lethals we are concerned with are members of a series of alleles which we have designated as a, b, c, d, et cetera. Females (queens and workers) are always heterozygous — that is, they contain two of these alleles that are different, as, for example, a and b. A queen of this composition produces eggs one-half of which are a and one- 125 half are b. Since the drones develop from unfertilized eggs, one-half of the sons are a and produce only a sperm, and the other half are b and produce only b sperm. When an egg such as a is fertilized by a sperm carrying a different lethal allele such as b, a queen or worker hav- ing the composition a/b results. If it is fertilized by a sperm carrying a simi- lar allele (a), this homozygous com- bination (a/ a) causes the individual to die before maturity, usually in the egg stage. When a queen (a/b) is mated to a single drone carrying a dif- ferent lethal allele such as c, then all the progeny resulting from fertilized eggs will have lethal alleles dissimilar (a/c, b/c) and will be viable — able to live to maturity. Efficiency in the brood nest will be high, and a populous col- ony will result. If, on the other hand, she is mated with a single drone having a similar lethal allele such as b, then one-half of her progeny resulting from fertilized eggs will be a/b and viable, and one-half will be homozygous (a/ a) and will die. Because most of the dying eggs are not removed until hatching time, 3 days after they are laid, effi- ciency is low in such a brood nest, and a weak colony will result. Failure of selection to eliminate the lethals indicates that a nonlethal gene does not exist at this locus. Inbreeding reduces the number of lethals in the population and increases the chances of similar lethals meeting to produce low viability. Outbreeding brings new lethals into the population and this in- creases the frequency of high viability. A similar series of lethals in a related insect Bracon hebetor, better known to geneticists as Habrobracon, has been studied by P. W. Whiting, of the Uni- versity of Pennsylvania. In that insect a definite association with sex has been established. Such an association has not been proved in the honey bee, where promotion of outbreeding may be jus- tification enough for the existence of such a wasteful lethal mechanism. As long as individual matings are made, the percentage of viable fertil- 070134° — 52- -10 126 ized eggs will either be near ioo per- cent or near 50 percent. Of course, there may be a small percentage of deaths from other causes. If mated naturally, many queens will mate twice and often intermediate viabilities will result, depending on the composition of queen and drone and the proportion of types of sperm reaching the sperma- theca. The same is true when several drones are used in artificial insemina- tion. By a series of individual matings something can be learned of the oppos- ing lethal alleles in a given cross. If all the progenies are highly viable, then the opposing lethal alleles are differ- ent ; if some of the progenies are poorly viable, however, then some of the op- posing alleles are similar. Lethal alleles can be identified most easily when we cross lines that contain only two alleles each. Crosses by indi- vidual matings between such lines will then fall into one of three classes: ( 1 ) All progenies of low viability, showing that the alleles are the same; (2) all progenies highly viable, show- ing the alleles to be different; and (3) one-half the progenies highly viable and one-half poorly viable, showing that the two lines have one allele in common. This procedure has been used to establish two-allele tester lines with definitely identified alleles for use in determining the alleles of any un- tested breeding stock. Two-allele lines can be readily estab- lished in one of two ways: (1) By making individual matings and breed- ing from a low-viability progeny or (2) by mating unfertilized queens to their own sons. The second way is done by inducing virgin queens to lay by exposure to carbon dioxide, rearing drones from them, and mating these drones back to their mothers. As the virgin queen can contain only two lethal alleles, there are only two alleles in the line established. Lethal alleles therefore are impor- tant in bee breeding. Matings that in- volve similar alleles cause low viability of the brood and lower colony popu- Yearbook of Agriculture 1952 lation. This in turn reduces produc- tivity of the colony. Selection for such qualities as honey production, which is profoundly influenced by colony popu- lation, is inefficient unless the lethal- allele conditions are comparable in all colonies. Because the lethals cannot be eliminated by selection, some form of controlled hybridization seems most promising. The early bee breeder raised vir- gin queens from his best colonies and thus controlled the female parent. He attempted to control male parentage by stimulating certain queens (col- onies) to produce great numbers of drones. Thus he increased the chances that his selected queens would mate with these selected drones. Some mass selection thus has been practiced since early times. Progress was made in se- lecting for body color, type, and tem- perament, but we doubt whether much improvement was made in less easily measured characteristics, such as honey production and vigor. In fact, continu- ous selection for color, type, and tem- perament has resulted in lower vigor and honey yield, as exemplified by the golden bees developed in the United States. They looked beautiful but were inferior in productivity. Seeing no real improvement through mass selection, the American bee breeder sought new stock from other beekeepers in this country or through the importation of races and strains from abroad. In mixing them with his own stock, he intentionally or unwit- tingly was hybridizing two races or strains. The superiority of the first few generations was inaccurately accred- ited to the new stock. Hybrids, of course, do not breed true, and it was impossible to maintain the superiority in later generations. As inbreeding pro- gressed, low viability due to the mating of similar lethal alleles became more and more frequent. The precepts of breed improvement successfully used by early plant and animal breeders included such ideas as like produces like or the likeness of Breeding Bees some ancestor, inbreeding produces prepotency or refinement, and breed the best to the best. The development of all breeds of livestock has included some inbreeding to produce uniformity within the breeds. If the beekeeper follows in the foot- steps of the animal breeder and tries to fix characteristics by inbreeding or line breeding, he immediately runs into difficulties. These systems of breeding will almost invariably in- crease the proportion of low-viability matings by reducing the number of lethal alleles in the line. What the beekeeper gains in uniformity and fixa- tion of desirable characteristics, there- fore, might be more than nullified by increase in mortality. In order to produce uniform colonies with high-viability brood, one has to cross races and strains that are likely to contain different lethal alleles or spe- cially selected lines of known lethal- allele composition. Hybrid breeding seems to be the bee breeders' best solution to their special problems. Plants and animals have fre- quently been improved by crossing. Hybrid plants are generally taller than their parents, larger in size, more vig- orous, longer lived, and more resistant to diseases. When it comes to heredity, animals behave as plants do. The ef- fects of hybridizing chickens, mice, guinea pigs, and rabbits are the same as in plants. The superiority of hybrid corn is attested by the fact that 8 1 per- cent of all corn planted in the United States in 1951 was hybrid seed. Hybrid bees therefore appear to offer the surest and fastest method of producing su- periority in production, egg viability, and performance. Inbreeding, followed by crossing, has been the successful method employed by plant and animal breeders. In- breeding is the mating of closely re- lated individuals such as parent-off- spring, brother-sister, or cousins. After several generations, each inbred line becomes constant and uniform within itself but distinctly different from other inbred lines. Inbred lines go through a 127 purification process such that only those individuals that possess much of the best that was in the original stocks in the beginning can survive. Although these inbred lines themselves will be inferior, they have possibilities as par- ents. By crossing inbred lines, one can gather together again the best qualities that have been distributed to the sev- eral inbred lines and create a new va- riety. Size, vigor, fertility, and viability can be fully restored in the hybrid with the advantage of real improvement through the elimination of undesirable characters. Crosses among certain inbred lines have shown a combination of desired characters that are definitely superior to those of the stocks from which the inbreds originated. This superiority could not have been reached as readily in the original stock by selection alone. To produce hybrid bees, the breeder may cross different races, strains, or in- bred lines of bees. Unless the races or strains are homozygous for the desired characters, the hybrids will be variable. Furthermore, the hybrids produced the following year or from other crosses of the same races or strains will differ from each other. The only sure method of having uniform hybrids is to cross strains or inbred lines that are homozy- gous for the desired characters. To produce inbred lines, the bee breeder must know which matings to make to obtain the desired inbreeding with the least expense of time and labor. Because of the mating habits of bees, it is an economic necessity that all inbreeding matings be made by artificial insemination. The first chart shows the percentage of inbreeding in successive generations by several sys- tems of inbreeding possible in bees. The percentage of inbreeding is the per- centage of heterozygous loci in the original selected individuals that be- come homozygous by inbreeding. In- breeding has no effect on genes already homozygous in the line so we are only concerned with those loci that are originally heterozygous. Since the bee breeder cannot know which genes were 128 PERCENT INBREEDING 100 80 60 Yearbook of Agriculture 1952 PERCENT HETEROZYGOSIS 0 [0 40 20 Backcros r to a Male ^^^ Mother-Son^}*' Rruther-Sister^ _. ' 'Aunt-Sejjiev^,^. Cousins ' Back cross to a Female--^ 20 30 40 50 0 10 12 14 16 GENERATION 1. The percentage of inbreeding and the percentage of heterozygosis (assuming the ini- tial value to be 50 percent) in successive generations of various systems in inbreeding in honey bees. originally heterozygous and what effect each gene has, he can only measure the relative purity of the stocks by the per- centage of inbreeding. The two systems of inbreeding that increase homozygosis fastest (backcross to a male and mother-son matings) are not advisable economically. Loss of breeding individuals and consequently loss of inbred lines is high when these systems are followed exclusively. The third most rapid method of increasing homozygosis is brother-sister matings. This is the most practical system. Be- cause drones mature more slowly than queens, backcrossing to a female for the first two generations produces in- breeding 37.5 percent faster, in time consumed per generation, than do the brother-sister matings, as illustrated in the first two generations in the second chart. Thus a combination of back- crossing to the original selected queen for the first two generations, followed by brother-sister matings in all future generations, is recommended for the production of inbred lines in the short- est period of time. Since all matings in this system can be multiple-drone mat- ings (all drones of each mating are sons of one queen), success in producing and maintaining inbred lines is insured. The bee breeder should know what inbreeding will do to his stocks. If he starts an inbred line by backcrossing for two generations and then makes brother-sister matings, he should ex- pect that each line will become more and more uniform as inbreeding pro- gresses. Most noticeable, however, for the first few generations will be the quality of the brood. If queen B, a daughter of A, is mated to several drones (sons of A), the brood viability of queen B will average 75 percent. A daughter queen C is then mated to sons of queen A and will have brood viability that will average either 75 or 50 percent. If it is 50 percent, the line has been reduced to two lethal alleles and the brood of queens D, E, and F will also be 50 percent if mated as shown in the diagram. If brood of queen C is 75 percent viable, then that of D may also be 75 percent but some- where not far from E or F in the dia- gram the viability will probably drop to 50 percent and all future genera- tions will remain at that level. How- ever, by selection it is possible to keep viability at 75 percent but the breeder would be reducing the effectiveness of inbreeding slightly by selection for heterozygosity of lethal alleles and Breeding Bees other genes linked to these. It is prob- ably advisable to select the matings that produce 50 percent viable brood in the C or D generation and thus quickly reduce all inbred lines to two lethal alleles and consequently have 50-percent viability in all inbred lines. If this is done, an analysis of the lethal alleles in all inbred lines is more readily accomplished. By test crossing to identify the lethal alleles in each line, the breeder can then predict which crosses will give high brood viability in hybrids and which crosses will give intermediate or low viability. In one season of inbreeding it is possible to get as far as producing a number of sister queens of the D gen- eration and get these mated to their brothers (drones produced by their mother queen C). These queens will be wintered, and the following year the breeder can make test crosses while continuing to inbreed the lines by brother-sister matings. It is advisable to test the inbred lines at the E genera- tion of queens (50-percent inbred). One generation of brother-sister mat- ing should be made each year after the first season. This insures continuing the inbred lines until they are selected in hybrid combinations. The bee breeder can accomplish very little by selection while inbreed- ing. He can surely select queens and drones in each generation for color and general appearance. In a sense he can progeny-test each generation by meas- uring such qualities in the workers as tongue length, wing length, color, or temper — but the economic value of bees is measured by the total produc- tivity of the entire colony. Thus very little selection can be made by testing inbreds as inbreds mated to drones of the same line. Since viability of brood in two-allele lines is only 50 percent, colonies headed by inbred queens lacking in vigor do not develop sufficient populations to ac- curately evaluate such economically important characteristics as honey pro- duction, swarming tendencies, and wintering qualities. This selection in 129 inbreds as inbreds is supplemental rather than substitutable for selection between inbred lines when in crosses with other inbred lines. 2. Arrow diagram of a recommended sys- tem of mating for inbreeding in bees. After two generations of back-crossing to a selected queen the line is continued by brother-sister matings. The division of bee culture under- took bee breeding in earnest in 1937 when a program for development of disease-resistant strains was initiated. Earlier work had been limited largely to control of mating, introduction of races, and studies of the characteristics of races. The program was begun in coopera- tion with the State experiment stations of Iowa, Texas, Wisconsin, and Wyo- ming. Work in Iowa had shown that some stocks were more resistant than others. Each year colonies headed by daughters of highly resistant queens of the previous year were tested. The queens were mated at isolated mating stations. In the three principal lines carried until 1945, one showed definite increase in resistance, another less in- crease in resistance, and a third line ap- parently little or no increase. The lines became stabilized at characteristic levels considerably below complete re- sistance. Selection of negative colonies J30 (showing no disease after inoculation) was more effective than selection of re- covery colonies ( recovered from disease produced in inoculation) . In the resistance work, artificial in- seminations were first used in 1943 on a small scale and were so successful that natural matings were discontinued in 1946. Resistance increased immedi- ately, indicating that the slow progress with use of natural matings might have been due to mismating. Soon entire test groups developed no disease. Hy- brids produced by crossing the resistant lines were resistant and also more pro- ductive than the inbreds. These encouraging results with dis- ease-resistant hybrids stimulated ex- pansion of this method of breeding to include other economically important characteristics. Inbred lines of bees were developed from a number of queens that produced outstanding col- onies of bees. These queens were se- lected because their colonies not only produced large crops of honey but pos- sessed other desired qualities such as gentleness, vigor, high egg production, or nonswarming tendencies. After the lines were inbred, they were crossed and tested in various hybrid combina- tions. Artificially inseminated queens were tested for honey production for the first time in 1943. As expected, dif- ferences between various hybrid com- binations were apparent early in the breeding and testing program. It soon became obvious that testing of hybrids under a wide range of cli- matic and environmental conditions was desirable. The Department of Ag- riculture, division of bee culture, there- fore entered into an agreement with the nonprofit Honey Bee Improvement Cooperative Association. Through this agency a large number of hybrid queens of various types were distrib- uted and tested by beekeepers through- out the United States. The queens are produced on Kelleys Island in Lake Erie. As the island is isolated, one can control matings. All hybrid queens are allowed to mate naturally to the drones produced by other unrelated hybrid Yearbook of Agriculture 1952 queens. Thus the test colonies are headed by single-hybrid queens and the workers in these colonies are double hybrids. Having only one such isolated mating station, the various types of single-hybrid queens must all be mated to drones of a single type of hybrid. Some of the queens are tested at the various bee culture laboratories of the Department of Agriculture. The De- partment also is continuing to test other hybrids that are artificially in- seminated. Beekeepers who have obtained test queens have been favorably impressed by the superiority of certain double hy- brids, which have produced as much as 50 percent more honey than com- parable commercial lines. The hybrids also have shown greater uniformity, more brood per colony, and brood of higher viability than the commercial lines. We have seen that the problems involved in breeding bees are too great for the individual breeder to go far in improving his own stock. He can do little more than avoid the mating of closely related individuals, select for high brood quality, and outcross to unrelated strains whenever low brood viability becomes too frequent. Breeding and testing have shown that hybridization can produce supe- rior bees. The best solution is thus a hybrid-breeding program such as only State or Federal research organiza- tions, widely supported cooperative or- ganizations, or large commercial firms can conduct. The ultimate objective of such programs is a number of four- way hybrids adapted to different re- gions or systems of management. The research agencies can then supply the foundation stock for the production of large numbers of these hybrids. Otto Mackensen is in charge of the bee breeding work of the division of bee culture, Bureau of Entomology and Plant Quarantine. He has special- ized in bee breeding and artificial in- semination research since 1935, when he joined the staff of the Southern States Bee Culture Laboratory, which is maintained in cooperation with the Louisiana State University at Baton Rouge. Dr. Mackensen, a native of Texas, holds degrees from Texas Agri- cultural and Mechanical College and Texas University. William C. Roberts, an apicultur- ist in the Bureau of Entomology and Plant Quarantine, has been associated with the North Central States Bee Cul- ture Laboratory and the University of Wisconsin since IQ43- Dr. Roberts, a native of Louisiana, is a graduate of the Universities of Louisiana and Wis- consin. Between ig35 and 1943 he worked at the Southern States Bee Culture Laboratory at Baton Rouge, and was an instructor in apiculture at Louisiana State University. His pres- ent work in bee breeding includes supervision of the Kelleys Island, Ohio, hybrid queen production project. Suggested for further reading: Harry H. Laidlaw, Jr.: Artificial Insemi- nation of the Queen Bee {Apis mellifera L. ), Morphological Basis and Results, Jour- nal of Morphology, volume 74, pages 429- 465, 1944; Development of Precision In- struments for Artificial Insemination of Queen Bees, Journal of Economic Entomol- ogy, volume 42, pages 234—261, 1949. Otto Mackensen: Effect of Carbon Diox- ide on Initial Oviposition of Artificially In- seminated and Virgin Queen Bees, Journal of Economic Entomology, volume 40, pages 344-349, 1947; A New Syringe for the Arti- ficial Insemination of Queen Bees, Ameri- can Bee Journal, volume 88, page 412, 1948; Viability and Sex Determination in the Honey Bee (Apis mellifera L-)> Genet- ics, volume 36, pages 500—509, 1951; A Manual for the Artificial Insemination of Queen Bees, with W. C. Roberts, Bureau of Entomology and Plant Quarantine, ET— 250, 1948. William C. Roberts: The Performance of the Queen Bee, American Bee Journal, vol- ume 86, pages 185-186, 211, 1946; Breed- ing Improved Honey Bees, with Otto Mack- ensen, American Bee Journal, volume 91, pages 292-294, 328-330, 382-384, 418- 42i, 473-475, i95i- P. W. Whiting: Multiple Alleles in Sex Determination of Habrobracon, Journal of Morphology, volume 66, pages 323-355, 1940; Multiple Alleles in Complementary Sex Determination of Habrobracon, Gene- tics, volume 28, pages 365-382, 1943. Insecticides and Bees Frank E. Todd, S. E. McGregor Bees are so important in agriculture and so important to so many of us that we cannot afford to destroy them along with the harmful insects. Two-thirds of the 5,600,000 colonies of bees in the United States are east of the Mississippi River. About one- half of them are in the Southern States. Of the 1,946,000 colonies west of the Mississippi, the Plains States have about 30 percent, California 24 percent, and the Intermountain and the Southwestern States 20 percent each. Six percent are in the Pacific Northwest. Although 500,000 persons keep bees in the United States, 80 per- cent of the colonies belong to about 50,000 beekeepers, about 1,000 of whom depend on bees for their liveli- hood. The latter group controls about two-fifths of the colonies. But the extensive use of insecticides has driven beekeeping out of many lo- calities. In apple-growing areas, for example, growers have to pay rental fees to entice beekeepers into the areas during blossom time to insure pollina- tion, and growers of legume seed are beginning to follow this practice. In many cotton-growing areas, spreading arsenical dusts by airplane has nearly wiped out the bee industry. About three-fourths of the annual honey crop comes from cultivated al- falfa, buckwheat, clovers, cotton, and oranges. The honey crop is the bee- keeper's source of livelihood, and un- less it covers his expenses he cannot stay in business. A widespread aban- donment of beekeeping in turn would reduce the supply of pollinators for agricultural crops. Although bees visit most of the flowering plants to obtain food for colony maintenance, few 131 132 species contribute enough nectar to make a honey crop. Apple blossoms, for example, contribute pollen and nectar for colony maintenance, but apple honey is unknown on the market. Dandelion, mustard, goldenrod, and gum weeds are examples of important sources of food for maintenance only; their elimination by weed spraying may limit the amount of beekeeping an area can support. Native bees depend on weeds even more than honey bees do. Their foods must be obtained locally. Often their survival requires a conti- nuity of sources, usually weeds. Spray- ing grain fields in the Sacramento Val- ley of California to remove weeds has caused the near disappearance of star thistle honey from the market. Herbi- cides are also being applied along road- sides for sweetclover and mesquite, both important sources of the honey crop. Bees are not killed by weed sprays, but food sources may be seri- ously reduced by their widespread use. Controlling harmful insects on agri- cultural crops is often beneficial to beekeeping. Almost always can insec- ticides be applied so as not to harm bees— but control programs that dis- regard bees usually are followed by an acute bee poison problem. The problem began in the early 1870's. A strange malady appeared then among colonies of honey bees. In the spring dead bees piled up around the hives, colonies failed later to re- cover strength, and many died out- right. The malady and the use of paris green to control codling moth on ap- ples and pears appeared at the same time; the use of paris green spread rapidly and so did the sickness among bees. Beekeepers soon learned the source of trouble: The trees were be- ing sprayed while they were in bloom. C. M. Brose, of the Colorado Agri- cultural Experiment Station, in 1888 reported finding arsenic in dead bees fed london purple and paris green in sirup. He found no arsenic in the stored honey. A. J. Cook, of the Michigan Agricultural Experiment Station, re- ported that bees died soon after they Yearbook of Agriculture 1952 fed on sirup or water containing lon- don purple at the strengths used in spraying. He strongly advocated laws to prohibit the spraying of fruit trees in blossom. Beekeepers made vigorous com- plaints. In 1 89 1 the Association of Economic Entomologists appointed a committee to find an answer to the question: "Will arsenical sprays ap- plied to flowering fruit trees kill honey bees?" The chairman was F. M. Web- ster, of the Ohio Agricultural Experi- ment Station. In 1892 Webster made the commit- tee's first report. He sprayed a flower- ing plum tree with paris green and then caged it, enclosing a colony of bees. An analysis of the dead bees showed arsenic, before and after they were washed to remove external con- tamination. Experiments on apple trees in the open were less convincing. His second report in 1895 covered more detailed work. He found arsenic in dead bees taken from sprayed apple trees and in bees taken from a colony that had died shortly after the apple orchard in which it was located had been sprayed. That evidence con- vinced everybody. Research gave proof also that bees do not injure fruit (al- though they may suck juices from over- ripe fruit after it is punctured by birds and yellow-jackets) and that honey bees are an economic necessity as pol- linators. About 1920 two new factors arose — the development of the cheaper cal- cium arsenate dust and the use of the airplane to apply insecticides. The boll weevil had become established in the South, but repeated applications of calcium arsenate re- duced its damage. The land of cotton covered more territory than the fruit areas; calcium arsenate was just as toxic as the sprays used in orchards; consequently losses of bees were more extensive. Not uncommonly did bee- keepers lose 500 colonies in a season. The choice was to move the bees or go out of business. Moving often meant Insecticides and Bees transporting several truck loads of hives ioo miles or more to strange and less profitable honey locations ; besides, the beekeeper often would not know whether insecticides were being ap- plied in the locality until he saw a dusting plane or found an apiary al- ready poisoned. Beekeeping in cotton areas declined. Most beekeepers quit, some moved to distant areas, and a few developed specialties, such as rear- ing queens away from the cotton areas. Arsenicals applied on crops else- where caused corresponding losses of bees, even though the crops (such as tomatoes, potatoes, or lettuce) were unattractive to bees. The reason was that the dust was drifting onto plants attractive to bees along the borders or outside the treated fields. Analyses dis- closed that, regardless of place or method of application, all arsenicals were highly toxic to bees — about one- third of a part per million of the bee's body weight was enough to cause death. Furthermore, any arsenical car- ried into the hive with pollen on the bee's legs and stored for future food remained poisonous for months. One would expect that some of the enormous quantities of insecticides ap- plied on cultivated crops would show up in honey. That is not the case. Nec- tar is carried in the honey sac, a spe- cialized part of the alimentary tract. When the nectar contains poison, the carrier is quickly affected. Instead of returning to the hive, the bee attempts to throw off the effect of the poison and becomes lost or dies in the field. Should the bee return with a load of poisoned nectar, there is a second safety factor. Every drop of nectar is rehandled by the hive bees, which are exposed to poison longer than the field bees. Hive bees tend to leave the colony when poisoned, carrying with them the poisoned nectar. It is therefore unlikely that poisons would ever be stored with honey. Chemical analysis of honey stored in the brood nests of colonies affected with arsenical poisoning has failed to reveal any trace of arsenic. Calcium arsenate was used in in- 133 creasing amounts until 1946. Other materials were below it in volume or toxicity. The arsenicals are some 50 times more toxic to bees than cryolite. Large amounts of sulfur were used, but as applied to field crops it is safe for bees. Several plant derivatives — nico- tine, pyrethrum, sabadilla — were ap- plied, but losses were minor as they are safe for bees within a few hours after application. Since 1946 the synthetic insecticides have brought new problems. They dif- fer in relative killing powers and affect colonies differently. Some, like the ar- senicals, cause a large number of bees to die near the hive entrance. Others, like chlordane, cause the bees to die in the field away from the hive. Benzene hexachloride causes bees from affected colonies to be furiously mean. DDT causes slight stupefaction. Lethal ef- fects from exposure to dieldrin may continue for a week and from para- thion (less toxic than dieldrin but very dangerous) for 2 to 4 days. Losses from applications of toxaphene may be neg- ligible. Toxaphene, the least dangerous to honey bees, gives good control of a number of harmful insects. Applied to such crops as alfalfa, the protected crop produces more flowers — actually a ben- efit to the honey bees. That considera- tion led to the establishment by the Department of Agriculture of a labo- ratory at Tucson, Ariz., in 1949 to study the effects of insecticides on bees. Insecticides that kill colonies quickly or remain in their stores of food for long periods, such as the arsenicals, are most damaging to beekeeping. Several of the synthetic insecticides kill only the field force that comes in con- tact with the material, and the affected colony recovers its strength in time. In areas with such concentrated plantings as cotton or alfalfa grown for seed, however, repeated exposure may stead- ily reduce the field force so that the honey crop fails and the colony may die from a weakened condition or lack of food. Up to now no synthetic in- 134 secticide used on a commercial scale has caused as much damage to bee- keeping as the arsenicals have. Many organic insecticides have been tested against bees in the laboratory and in the field. In the laboratory most of them have been found to be toxic as stomach poisons, by contact, or both. Some have been classified as to their relative effect on bees when applied to flowering plants, as follows: May be used with safety — toxaphene, methoxy- chlor, sulfur. Safety questionable, ex- perience variable — DDT, chlordane. Unsafe — BHC, lindane, aldrin. Very destructive — parathion, dieldrin, ar- senicals. The organic insecticides tested gen- erally can be used safely on plants not in flower. With one exception, fields treated while they are in flower are safe for bees within 48 hours after an application. Fields treated with diel- drin are not safe for bees for a whole week. Insecticides applied during the hours bees are visiting the field are much more damaging than those ap- plied at night when no bees are pres- ent. Insecticides that kill more than 10 percent of bee visitors to the field are considered unsafe for use on plants while they are in flower. Beekeepers have experimented with various means of resolving the problem of bee poisoning. When it was confined to the fruit-producing areas, legislation was sponsored to prohibit the spraying of fruit trees in bloom. Such laws were passed by Ontario in 1892, Vermont in 1896, New York in 1898, Michigan in 1905, Nebraska and Colorado in 19 13, Kentucky in 1915, Utah in 1919, and Prince Edward Island, Canada, in 1920. The laws provided no special enforcement agency and low penalties and soon proved they were not the solution. Losses because of insecticides spread by airplane brought some law- suits, several of which resulted in judgments in favor of the beekeeper. The right to own bees as personal prop- erty has been recognized. The law will Yearbook of Agriculture 1952 protect bees as it will any other form of property. The principle has been sustained that a person may not use his property in such a manner that dam- age to his neighbor is a foreseeable re- sult. The principle has been recognized that the dusting of growing crops to prevent the inroads of insects is fre- quently necessary and a legitimate op- eration, but it must be conducted at a time and manner so as not to endanger other legitimate industries, such as apiculture. In California, where airplane dust- ing affects several industries and the county agricultural commissioners are organized for thorough enforcement, county ordinances are used. The ordi- nances require the pest-control opera- tors to obtain permits and to operate under strict conditions set up by the commissioners. Although burdensome to operator and commissioner, the method has reduced the poisoning of bees, but only because of the close su- pervision made possible by the Cali- fornia system of enforcing agricultural statutes. A community approach was made in Arizona. In 1945 a survey revealed that an estimated 10,000 colonies were killed as a result of dusting programs. In 1946 leaders of the insecticide trade and the operators of airplane dusters reached an agreement whereby the sale and use of an arsenical as an insecticide was practically discon- tinued and DDT or other materials were substituted. As a result, in the fol- lowing 5 years, severe losses from bee poisoning were largely eliminated, re- lations improved, and better crops were produced. In Millard County, Utah, beekeep- ers suffered honey-crop failures in 1946 because alfalfa-seed crops were dusted with DDT while they were in full bloom. Research workers had devel- oped a program of bud-stage dusting, which adequately controlled lygus bugs. Growers had difficulty estimating lygus bug populations on their fields, and so they were also dusting alfalfa in the bloom stages, with consequent dan- ger to honey bees. To improve the situ- ation, the county supervisors and the growers agreed to finance jointly the services of an entomologist. His duties were to make surveys of insect popula- tions and recommend control measures to the growers, giving due considera- tion to the protection of bees. The sav- ings in insecticides his advice made pos- sible exceeded his salary and expenses, and damage to beekeeping was prac- tically eliminated. As long as insecticides are used, bee poisoning probably will continue to be a problem. Although the bee industry is still absorbing greater losses from in- secticide poisoning than it should, there is a growing spirit of cooperation be- tween growers and beekeepers. In that lies the best chance of solution. Frank E. Todd is apiculturist in charge of the Southwestern States Bee Culture Laboratory of the Department of Agriculture in Tucson, Ariz. S. E. McGregor, apiculturist with the division of bee culture of the Bu- reau of Entomology and Plant Quaran' tine, has been in bee work since ig2§ in Texas, Arkansas, Louisiana, New York, Wisconsin, and Arizona. The drone fly closely mimics the honey bee in color, size, and actions. Insects To Control a Weed James K. Holloway, C. B. Huffaker Over many square miles of western range lands millions of pea-sized, bright, metallic-colored beetles are de- stroying a common weed that for years has caused huge economic losses. The weed, Hypericum perforatum, has a number of common names. In California it is called Klamath weed because it was first reported, about 1900, in northern California in the vicinity of the Klamath River. In many of the Western States it is sometimes referred to as goat weed. The recog- nized common name in Europe, orig- inal home of the weed, is St. Johnswort because, according to legend, it blooms on June 24, the day of St. John the Baptist. It has invaded extensive temperate regions throughout the world. It is con- sidered a noxious weed in the range lands of Australia, New Zealand, Can- ada, and the United States. The in- fested areas in California are esti- mated at 400,000 acres. Oregon, Wash- ington, Idaho, Nevada, and Montana also have many thousands of infested acres. Klamath weed causes losses by dis- placing desirable range plants. It is poisonous to livestock, but death as a result is rare. Animals that eat much of it become scabby, sore-mouthed, and unthrifty. It causes the white parts of the skin to become photosensitive and, when exposed to sunlight, blisters form on the unpigmented skin areas. Cattle are more sensitive to it than sheep. In some localities in California the grasses dry rapidly in the spring, and the most abundant remaining green plant is Klamath weed. In those local- ities cattle are usually moved from the 135 136 ranges before the condition arises. But on occasion unavoidable delays do oc- cur, and the animals may then con- sume damaging quantities of the weed. The ingested plant causes cattle to become irritable so that they are diffi- cult to corral; sometimes it is almost impossible to load them into trucks and it may be necessary to confine them and give them other feed for a day or two, until the effects of feeding on the weed wear off. Many attempts have been made to control the weed, a perennial, with chemicals — borax, 2,4-D, and others. But the materials are expensive and the land to be treated is mostly extensive and inaccessible. Control of the weed by insects has been under consideration for several •years. The general method was success- ful in other countries, notably Aus- tralia, but its use in this country is a recent development. The Commonwealth of Australia first began a search for insect enemies of St. Johnswort in 1920 in England. Early in 1935, after the insects im- ported from Britain proved apparently unsuccessful in Australia, the search was transferred to southern France. The early work in Europe comprised tests by starvation and breeding of many insects on 42 species of economic plants, representing 19 botanical fami- lies, to determine whether the insects could feed and breed on them. At the satisfactory conclusion of the tests in Europe, the species that had shown neither feeding nor reproduc- tion upon the test plants in Europe were shipped to Australia. Before they could be liberated, however, additional tests had to be made on plants that had not been tested in Europe. About 8 years after two species of the leaf -feeding beetles, Chrysolina, were released in Australia, encouraging results were reported. Subsequently men at the University of California who had watched the experiments with great interest were authorized by the Department of Agriculture to import Chrysolina hyperici, C. gemellata, and Yearbook of Agriculture 1952 a root borer, Agrilus hyperici. The stipulation was made that feeding tests be made on sugar beet, flax, hemp, swcetpotato, tobacco, and cotton. A project for the importation, test- ing, and colonization of the three species was then set up by the Bureau of Entomology and Plant Quarantine and the University of California. The war made it impossible to collect the insects in Europe. It was learned, though, that abundant material was available in Australia and would be transported to California by the United States Army Air Transport Command. The Australian Council for Scientific and Industrial Research offered to col- lect and ship the material. Leaders in those activities were Frank Wilson and Harry S. Smith. Wil- son was sent to southern Europe in 1935 when the first shipments of the natural enemies of Hypericum perfora- tum from England to Australia failed to progress satisfactorily. Wilson had been associated with the work in Eng- land and he continued it in France un- til 1940. Professor Smith, who was head of the division of biological con- trol in the University of California un- til his retirement in 1951, is regarded as one of the world's foremost propo- nents of biological control. Ever since the biological control of Klamath weed was advocated by Dr. R. J. Tillyard of Australia in 1926 he followed the de- velopments. In correspondence with Dr. A. J. Nicholson of Australia in 1944, Smith found that the biological control was beginning to make prog- ress. He then took the steps that led to the project between the University of California and the Department of Agriculture. When the importations began, James K. Holloway was put in charge of the investigations. The first importations were made in October 1944. The initial problem was to adjust the life cycles so that they would be in phase with the seasons of the Northern Hemisphere. The specimens of the root borer (Agri- lus hyperici) were received as mature Insects To Control a Weed larvae in roots. Some of them were re- tarded in cold storage, but others were forced to emerge upon arrival. Neither method proved satisfactory, and fur- ther importations were curtailed until the work in Europe could be resumed. The two species of Chrysolina were occasionally shipped as mature larvae, which would emerge as adults upon ar- rival, feed, and enter summer dorman- cy about 3 weeks later. Most of the shipments, however, consisted of sum- mer-dormant adults. Either way, the problem was to bring the adults out of aestivation into the egg-laying phase. By subjecting the adults to fine sprays of water each day, a state similar to normal winter moisture conditions was reproduced in the laboratory, and the beetles came out of aestivation, mated, and began producing fertile eggs with- in 2 to 3 weeks. During the first year of importations, enough C. hyperici were received to conduct the feeding tests. The tests were completed in May 1945. No feed- ing had taken place on any of the test plants, and four colonies were released late in the season. In January 1946 the feeding tests with C. gemellata were completed. Permission was obtained to release 13,650 adults that were being retained in quarantine. They were divided into two colonies of 5,000 each, one colony of 2,000, and one of 1,650. The experimental releases of both species were made in the Coastal Range, northern Sacramento River Valley, and the Sierra foothills — local- ities considered representative of the grazing areas in which the weed occurs in California. A total of 330,000 adults of C. hy- perici, shipped from* Australia, was re- leased in 1947 at 66 sites in 15 counties of California. Two experimental col- onies of 5,000 each were released in Oregon through the cooperation of the Oregon Agricultural Experiment Sta- tion. Both species of Chrysolina were well established by 1948, and we did not need to import more. Two of the orig- 137 inal releases of C. gemellata had shown a remarkable increase, and from them we collected 212,000 adult beetles, which we placed in 52 new locations in 16 counties of California. Three other releases were made in Oregon and two in Idaho. Initial releases of C. hyperici were made in Washington, Idaho, and Montana in 1948 through the cooper- ation of the State experiment stations. The Forest Service joined the project in 1949 in Oregon, Washington, Idaho, and Montana. In May of that year 140,000 adult C. hyperici were col- lected and shipped to those areas in units of 5,000 each, making 28 new locations in the Northwest. C. gemellata was so numerous and widespread in California by 1950 that redistribution became a local problem. Perhaps 3 million adult beetles were collected and redistributed in May I95°- • • 1 , The success of C. hyperici has been limited. It has become established in other localities, but an increase com- parable to that of C. gemellata has been restricted mainly to the coastal mountains in California. The effectiveness of the leaf-feeding beetles in controlling Klamath weed is associated with their life cycles and their weed host. The balance be- tween the propagative ability of the weed and that of its insect enemy is determined by factors of soil condition, climate, and the influences of inter- related plants and animals. The two species of weed-feeding beetles differ slightly in their environ- mental requirements. Yet that small difference means that one species re- produces abundantly and the other's reproduction is curtailed under Cali- fornia conditions. C. gemellata starts reproducing quickly when the fall rains come. Con- sequently its progeny have enough time to make the necessary growth before the dry season arrives in late spring and early summer — conditions that are haz- ardous to pupal development. Also, the egg-laying period is longer, and greater numbers of eggs are deposited. i38 But C. hyperici reacts slowly to moist conditions in the fall and under aver- age California conditions deposits most of its eggs so late that there is insuffi- cient time to complete the necessary phases of development before dry weather sets in. The life history of the more success- ful species is attuned to the phases of weed growth and to local climatic con- ditions. The adult beetles issue from their pupal cells just beneath the sur- face of the soil in April and early May. They feed voraciously during May and June on the foliage of the plants, which then are flowering. By late June and early July the beetles have completed preparation, by feeding and sunning, for their summer sleep. This inactive, dry-season stage is spent beneath de- bris, under small stones, and in crevices of the soil. The beetles spend 4 to 6 months in this inactive condition without food or water. During this period the weed, too, enters a relatively dormant phase. It develops and ripens its seed crop but drops most of its leaves and becomes hard and woody. The larvae feed actively in warm periods in winter and spring. Their in- tensive feeding keeps the plant stripped of leaves over a long period when its food reserves are at a low ebb. Thus the root system and the plant die of starvation. Adult feeding, voracious as it is, does not last long enough to pro- duce wholesale death of the plants without the previous feeding by the larvae. The rains in fall and early winter re- activate weed and beetle. The weed sends out vigorous, prostrate, leafy shoots in rosettes at the base of the flowering stalks. The beetles mate, and many eggs are placed on the leafy growth. The larvae from the eggs and the host weed grow during the winter in relation to the temperatures. All stages of the beetles can survive heavy snows and cold. By midwinter and early spring in favorable locations the larvae reach a half-grown to nearly mature grub stage. Yearbook of Agriculture 1952 From then on the weed suffers pro- gressive destruction of its foliage by the larvae. The fully mature larvae enter the soil to pupate at about the time the plant begins to develop the shoots that become the flower-bearing stalks. The appearance of the adult beetles com- pletes a single cycle, which covers a year. The beetles can move in effective numbers into new areas, but enough of both sexes must be present to assure fertility of the eggs. About 3 years gen- erally are required to give local control in a remote area where only a few thou- sand beetles have been released. In the third reproductive year the numbers reach a level at which they can exert controlling pressure on the weed. Local dispersion is normally accom- plished by the crawling adult beetles, which often are seen moving in great numbers across roads or ravines from centers of overpopulation where the weed has been cleared. Dispersion by flight is less commonly observed but oc- curs under conditions of high beetle density, complete depletion of the food supply, and hot, sunny weather. Col- onies have appeared which apparently are the results of single or repeated flights from production centers as far as 3 miles distant. Through its natural powers of dis- persion, plus a supplemental influence from establishment of secondary initial colonies in the area, C. gemellata has now spread and effected general con- trol of the weed over hundreds of square miles in southern Humboldt County, California. The same species has cleared an open range area in Placer County and moved from that limited infestation through small, isolated patches of weed to points up to 3 miles away in various directions. No additional releases were made anywhere in the area. A second and younger colony in Placer County was located in an area notably unfa- vorable as to climate, yet it cleared that field of the weed within 3 years. C. hyperici, though poorly adapted to the California conditions generally, Insects To Control a Weed brought under control 4 to 5 square miles in an upland area of Humboldt County, where retention of soil mois- ture late in the season has operated to it? advantage. Ranchers and farm workers attest to the complete destruction of the weed by the beetles. They have seen how the hungry insects have removed the weed from a large area near Blocksburg, Calif. The beetles also can locate and de- stroy small, isolated stands of weeds that were missed in previous years. The appearance of the seedling weeds in cleared fields is common, but so far enough beetles are present in the area to find them. These scattered reinfes- tation spots and plants in the edges of heavily wooded borders (less preferred •by the beetles) maintain the general distribution of the beetles in an area after the weeds cease to be a range problem. That fact may assure the re- turn of the beetles in effective numbers quickly enough to take care of rein- festations before they can reach seri- ous proportions. The control of a weed by the bi- ological method involves several as- pects of ecology. Klamath weed is pri- marily a pest in range areas where soil moisture is ample from winter to early summer but deficient later in the year. Overgrazing fosters its spread. Under such conditions, its deep root system enables it to overcome even the stur- diest grass competitors, particularly when grazing has been so heavy that seed production by the more vigorous perennial grasses is curtailed. An insect may control weeds by more subtle means than direct destruction. If its action is such as to remove the competitive advantage of the weed host over desirable plant species, the weed may then be overcome by plants that cannot alone compete with it. That does not explain the control of Klamath weed, but the pressure of the beetles on the weed at a time when vigorous competing range plants oc- cupy the area may be enough to pre- 139 vent the return of the weed in such fields under proper grazing manage- ment. Several investigators believe that three-fourths of the land south of Mount Shasta and from the coast to the Sierra foothills in California was originally covered with perennial bunchgrasses. Annual plants now make up most of the forage there. Their re- placements by perennials would be im- possible (and not necessarily desirable) everywhere in the region. It therefore seems probable that with the destruction of Klamath weed the predominant annual-plant cover characteristic of the region may re- gain the land under normal conditions. That has happened in the areas where the weed has been cleared for three successive seasons. In Placer County, annual grasses, dominated by soft chess (Bromus hordeaceus) , legumes such as birdsfoot trefoil, clovers, and lupines, and desirable forbs such as filaree, have returned as thickly as they are in neigh- boring range lands that have remained free of Klamath weed. The success of the beetles in Hum- boldt County is attended by circum- stances favorable to return of a forage cover of maximum value. In the areas most heavily infested with the weed (indicative of a favorable soil and site), the main perennial bunchgrass of earlier years (Danthonia calif or- nica) has managed to survive along animal trails and about the edges of seepage areas, which were too wet for Klamath weed in winter. The destruc- tion of the dense stands of the aggres- sive weed has permitted a gradual re- turn of this fine range plant. Although its distribution in the original beetle- release remained spotty for some years, it soon began to develop a vigorous cover that spread slowly over new ground. At the end of a 50-acre field that had been cleared of weed by the beetles for 3 years, a rather complete stand of the hardy bunchgrass developed. Over the whole field — most of which had been weed-free only 2 years — Dan- 140 thonia increased from 9.2 percent of the total plant cover in 1947 to 23.4 percent in 1950. By considering soft chess (Brom.us hordeaceus) , Dantho- nia, and desirable legumes together, one gets a general picture of the total forage improvement due to the beetle action. The three desirable types in- creased from 14.8 percent in 1947 to 43.4 percent in 1950. Klamath weed was reduced from predominance (57.6 percent) to complete absence. Thus the position of the weed and the posi- tion of the three desirable plants was practically reversed. The Chrysolina beetles have become a permanent part of the natural fauna. Their future success will depend mostly on how closely the life processes of the beetles and their host coincide with changes in weather, for on that syn- chronization depends how fast the beetles multiply and how intensive is their action on uninvaded weed stands and reinfested fields. Indications are that the beetles can duplicate throughout the Northwest the success they have had in California. It would be an economical, self-per- petuating way to combat a serious pest, now that the first intensive research and exhaustive explorations are completed. James K. Holloway is an entomol- ogist in the Bureau of Entomology and Plant Quarantine, the division of for- eign parasite introduction, and special- ist in biological control in the Univer- sity of California College of Agricul- ture. He has been engaged in research in biological control since 192J and has had responsibility for carrying forward the project on the biological control of Klamath weed in this country since its inception in 1944. He studied at Missis- sippi State College and Ohio State University. C. B. Huffaker, an entomologist and ecologist, has been engaged in eco- logical research since 1940. In 1946 he was appointed assistant entomolo- gist in the division of biological control of the University of California, where he has been particularly interested in Yearbook of Agriculture 1952 the population complexes of insects and the weeds attacked by them as com- ponents of a natural range environ- ment. Dr. Huffaker holds degrees from the University of Tennessee and Ohio State University. The authors suggest for further reading Bio-Ecology by Frederic E. Clements and Victor E. Shelf ord, published by John Wiley & Sons, Inc., in 1939, and the following publications: Australian Council for Scientific and In- dustrial Research — Bulletin 169, The En- tomological Control of St. John's Wort (Hy- pericum perforatum L.) . . ., by Frank Wil- son, 1943; and Pamphlet 29, The Possibility of the Entomological Control of St. John's Wort in Australia — Progress Report, by G. A. Currie and S. Garthside, 1932. California Agricultural Experiment Sta- tion Bulletins — 615, The Chemical Control of St. Johnswort, by R. N. Raynor, 1937; and 503, St. Johnswort on Range Lands of California, by Arthur W. Sampson and Ken- neth W. Parker, 1930. California Forest and Range Experiment Station Technical Note 21, Standards for Judging the Degree of Forage Utilization on California Annual-Type Ranges, by A. L. Hormay and A. Fausett, 1942. In Ecology, The Return of Native Per- ennial Bunchgrass Following the Removal of Klamath Weed [Hypericum perforatum) L.) by Imported Beetles, by C. B. Huffaker, volume 32, pages 443-458, 1951. In the Journal of Economic Entomology, The Role of Chrysolina gemellata in the Biological Control of Klamath Weed, by J. K. Holloway and C. B. Huffaker, vol- ume 44, pages 244-247, 1951. In the Proceedings of the Entomological Society of Washington, Biological Control of Weeds in the United States, by H. S. Smith, volume 49, number 6, pages 169- no, 1947- Insects as Destroyers Losses Caused by Insects G. J. Haeussler Every minute of the day and night billions of insects are chewing, sucking, biting, and boring away at our crops, livestock, timber, gardens, homes, mills, warehouses, and ourselves. How much damage they do is hard to say. Many variables and complicat- ing factors are involved. The damage by one kind of insect to a crop differs from year to year and from one area to another. Pests cause losses in un- counted ways. The infestations reduce the yield of crops, lower the quality, increase the cost of production and harvesting, and require outlays for materials and equipment to apply control measures. The products must be screened or washed to remove insects or insect frag- ments; washed, brushed, trimmed, or otherwise treated to remove insecticide residues; and graded to eliminate or otherwise allow for injury. Livestock pests lower the production of meat and milk and the value of hides. Mosquitoes, house flies, ticks, and fleas exact a toll in human diseases and in efficiency and money — time lost from work, the cost of screens on homes, interference with the cultiva- tion or harvesting of crops, the loss of business at resort places. Insects cause direct losses to timber production. They also cause indirect 970134' 1 1 ones: The fire hazards of insect-killed trees in the forest, the effect on conser- vation, spoiling of beauty in parks and other scenic areas and on streets and properties in towns and cities. Food and homes suffer. Insects at- tack grains while they are in farm storage and in transit and while they are stored in elevators. Others infest dried fruits during and after the drying process. Clothes moths, carpet beetles, pantry pests, and termites invade homes, infest food, ruin clothing, dam- age the timbers of houses. A compilation of estimated losses due to 6o-odd insects in the United States was made in 1938 by J. A. Hy- slop, of the Department of Agriculture. He set the total damage, including the cost of control measures, at $1,601,- 527,000 annually. His estimates were based on prices far lower than those of today and did not take into account all injurious insects. We can be more specific now about losses to certain crops and commod- ities. The European corn borer, one of our insect immigrants, has been one of the farmers' worst enemies. Surveys to determine its distribution, abun- dance, and damage show how the losses increased as the corn borer spread throughout the Corn Belt. In 1949, when conditions were especially favor- able, the damage reached an all-time high. Fortunately weather conditions unfavorable to the borer and other natural factors sometimes check its ravages, as they did in 1950. Practical methods are now available for control- ling it in field corn, sweet corn, and seed corn, but by no means has it been eliminated. 141 142 Yearbook of Agriculture 1952 Estimated VaJue of Crops Destroyed by Grasshoppers and Saved by Control Measures Tear '925- 1926. 1927. 1928 . I929- >930- J931- J932- '933- 1934- '935- 1936. r937- '938- '939- 1940. J941- 1942. '943- '944- :945- 1946. '947- 1948. '949- !95°- „, , Estimated value of crops States - J , Destroyed by grasshoppers Saved by control measures Number Dollars Dollars 20 I O, 484, 904 2 1 9' 757^51 21 IO, 506, 901 20 12, 818, 951 21 15, 688, 194 22 20, 516, 174 23 34. 073. 351 23 4i.968-578 23 58> 403, 961 23 35. 765- 862 18 14,753,080 5,540,803 14 102,029,061 25,817,848 20 65,836,215 102,288,178 24 83,841,727 176,442,672 24 48,811,430 128,483,225 22 24,087,117 44,568,833 23 23,822,713 35, 583, 136 20 14,016,475 29,307,683 19 13,217, 884 6, 89 1 , 46 1 21 13,486,060 22,712,485 23 12,671,604 29,708,832 20 22,743,328 41,150,436 20 22, 342, 835 50, 368, 599 19 36,826,624 67,586,232 20 27, 376, 479 72, 077, 868 18 19,333,402 25,327,876 Losses Caused by Vegetable and Truck Crop Insects Insect Mexican bean beetle . Beet leafhopper . Cabbage caterpillars . Onion thrips Pea aphid Aphids Sweetpotato weevil. . Tobacco hornworm . . Celery leaf tier Crops affected Beans /"Sugar beets, toma- toes, beans, can- taloups. Beets, sugar and table. ,Bcans, dry Cabbage and caul- iflower. Onions Peas, canning and market. Potato Sweetpotato . Tobacco. . . . Celery Area involved New Mexico, Ari- zona, Colorado, and eastern United States ex- cept Michigan. Western United States. Intermountain re- gion. Idaho, Oregon . . . f United States. . . . Southern States and California. United States .... United States .... Northern States, except Idaho. Gulf Coast States . Southern States. . . California and Florida. Estimated an- nual losses Period Dollars '944 5,502,000 1930's 2, 430, OOO 1944 3, 676, OOO 1944 2, 446, OOO 1928-32... 5,433,000 1944 7, 663, OOO 1944 14, 500, OOO 1944 3. 969'°°° 1 944 66, 467, 000 1944 5,031,000 1944 84, 073, 000 Outbreak i, ooo, ooo years. Losses Caused by Insects Grasshoppers damage a variety of crops and range plants. More than 75 years ago C. V. Riley estimated that the grasshoppers caused crop losses amounting to 200 million dollars in a number of Western States from 1874 to 1877. Hyslop recorded that entomolo- gists in 23 Western and Midwestern States estimated the average annual value of crops destroyed by grasshop- pers from 1925 to 1934 at about 25 million dollars. The losses remain high, especially in outbreak years, but con- trol campaigns and better control methods have meant great savings of crops. Now that practical, effective materials are available to the individ- ual farmer, grasshoppers should never again be allowed to cause such losses as those in the i93o's. Cereal and forage crops are attacked by many other pests. Among them are the corn earworm, hessian fly, chinch bug, velvetbean caterpillar, lygus bugs, and greenbug. Among the many kinds of insects that attack vegetable and truck crops are aphids, leafhoppers, sucking bugs, beetles and weevils, caterpillars, thrips, spider mites, cutworms, wireworms, and mole crickets. Many cause direct injury. Certain aphids and leafhoppers and some others cause indirect damage by transmitting diseases to potatoes, sugar beets, and similar plants. No at- tempt has ever been made to bring to- gether estimates of all these losses, but one of the tables gives some idea of them. As for the fruit insects: Yearly losses in our apple crop because of the codling moth from 1940 to 1944 were set at about 15 percent of the crop value, or $25,245,000. That did not in- clude the cost of measures to combat the pest, which cost an estimated 25 million dollars more. DDT has been used extensively and effectively against the codling moth, so that the average annual losses from codling moth from 1944 to 1948 were about 4 percent of the crop value, or $9,176,000. The citrus crops of California are said to have suffered losses of about 10 143 million dollars in 1 943-1 944 because of the California red scale. In 1943 and 1944 losses to peach growers east of the Rocky Mountains because of the plum curculio and the cost of applying control measures have been estimated at nearly 8 million dollars a year. The peach tree borer is another serious pest over much of the eastern two-thirds of the country. If not controlled, infesta- tions of the borers weaken and often kill peach trees. The extent of the dam- age is difficult to determine, but the annual cost of applying control meas- ures alone was estimated at $3,200,000 in 1943 and 1944. The boll weevil takes a big bite out of our cotton crop each year. The cut in production from 1909 to 1949 in the 13 States in which the boll weevil oc- curs meant an estimated average loss of cotton and cottonseed of more than 203 million dollars annually. The loss was more than 500 million dollars in each of 5 years, between 400 million and 500 million dollars 1 year, and be- tween 300 million and 400 million dol- lars in each of 3 years. It was 200 mil- lion to 300 million dollars in each of 6 years. The estimated loss was below 100 million dollars in only 16 of the 41 years. The value of the cotton was computed at the seasonal average price received by farmers and does not con- sider what they might have received had the yield not been reduced by in- sects. To those losses must be added the damage caused by other insect pests. One of them, the bollworm, is esti- mated to have destroyed cotton in Texas alone to the extent of 85 million dollars in some years. Robert C. Jackson, of the National Cotton Council, cited estimates of the Bureau of Agricultural Economics that insects destroyed 15.1 percent of the 1946 cotton crop and drew these con- clusions: Besides the lint, there was lost 6 1 3,000 tons of cottonseed, or, based on the season's average price, more than 44 million dollars worth. The 613,000 tons of cottonseed that was destroyed would have produced 179 million pounds of refined cottonseed oil, which i44 Yearbook of Agriculture 1952 Estimated Losses to Livestock, 1940-44 Pest Animals affected Cattle grubs Cattle Horn flies, stable flies, deer flies Cattle, horses, mules Screw-worms and blow flies Cattle, horses, hogs, sheep, goats . Lice Cattle, goats, hogs, sheep Ticks Cattle, sheep, horses Lice, mites, fleas, ticks Poultry and eggs. Average annual loss Dollars 160, ooo, ooo ioo, ooo, ooo 15, 000, 000 30, 000, 000 6, 500, 000 85, 000, 000 Estimated Savings from Control of Livestock Pests, 1949 Animals and pests Cattle treated for grubs Cattle treated for flies Cattle treated for lice Sheep treated for ticks and related pests . Hogs treated for mites and related pests . Poultry treated for lice and related pests. States re- porting Number 29 28 28 27 29 25 Animals treated Number 3, 889, 344 13, 769,846 6, 469, 493 3, 540, 922 4, 538, 256 43,482,013 Estimated savings Dollars 14,643, 708 47, 245, 628 14, 083, 966 3, 568, '73 5,781,720 7, °56, 9*5 Estimates of Losses to Corn Caused by the European Corn Borer Tear 1939 1940 J94! 1942 '943 '944 J945 1946 1947 1948 1949 '95° Areas known to be infested Areas included in estimate Estimated value of crop loss States Number 20 20 20 22 23 26 26 28 28 29 29 36 TT~, ^ rr~ Com harvested for c * Counties Slates Counties ■ J Sweet corn grain Number Number Number Dollars Dollars 455 479 556 661 791 883 9r3 959 '.053 1, 169 1,314 1,405 16 19 18 18 20 22 22 22 22 25 26 26 285 258 258 308 337 400 398 446 806 892 1, 001 1, 001 1,846,335 4, 140,479 4, 260, 248 15,211,895 27, 800, 740 20, 185, 153 32, 846, 459 26, 679, 552 93, 532, 296 99, 107, 000 349, 635, °°° 84, 911, 000 2, 130, 791 2, 539, 348 675, 742 1,817, 181 5, 562, 778 2,528, 77o 3, 918, 106 2, 061, 237 3, 238, 495 4, 129, 000 Total loss Dollars 3,977, 126 6, 679, 827 4> 935, 99° 17, 029, 076 33, 363, 51 8 22,713,923 36, 764, 565 28, 740, 789 96, 770, 791 103, 236. 000 349, 635, °°° 84, 911, 000 Losses Caused by Cereal and Forage Crop Pests Insect Corn earworm . Hessian fly ... . Chinch bug . Crops affected Corn Wheat [Corn Velvetbean caterpil- lar. Pea aphid Lygus bugs Vetch weevil Wheat, barley, rye, [ oats. Peanuts and soy- beans. Alfalfa Alfalfa seed Hairy vetch seed . . . (Oats, and wheat .... Area involved United States . . . United States . . . United States . . . United States . . . Period '945 ■944 '945 '934 '934 Green bug. Wheat. Oats. . { Barley. Southeastern States. 1946 United States !944 United States !944 United States x944 Kansas, Oklahoma, 1907 Texas. Oklahoma x950 Oklahoma '95° Oklahoma x950 Estimated annual losses $140, 000, 000 47, 400, 000 37, 000, 000 27, 500, 000 28, 000, 000 5, 000, 000 30, 580, 000 15, 800, 000 2, 290, 000 1 50, 000, 000 1 22, 000, 000 1 2, coo, 000 1 800, 000 1 Bushels. Losses Caused by Insects might have provided the total mini- mum edible fat requirements for more than 8 million persons. The oil would have made 200 million pounds of mar- garine, which is more than one-third the amount consumed in the United States in 1946. The 613,000 tons of cottonseed would have turned out 276,- 000 tons of high-protein meal and 152,000 tons of cottonseed hulls. The meal would have provided enough protein to produce 178 million addi- tional pounds of beef, or to take care of enough cows to produce 690 million gallons of milk — enough to provide every individual in this country with 19 quarts of milk. Estimates prepared by the National Cotton Council placed the value of cotton lint and seed destroyed by in- sects in the United States in 1950 at $907,884,000, the highest in history. As in the case of insect pests of other crops, losses to cotton caused by insects vary greatly from area to area and even from field to field. Moreover, the loss in quality of lint and seed is sometimes serious even though no great reduction in yield occurs. Applications of insecti- cides to control sucking bugs, especially in the Southwestern States, often pay dividends because of the resulting im- provement in the grade or quality of the lint. Livestock pests each year cost this country about 500 million dollars, mostly in wasted feed, lower produc- tion of meat and milk, and damaged hides. Cattle lose energy and weight when they have to fight off attacks of horn flies, stable flies, and horse flies, which also rob cattle of blood. The combined loss of energy and blood rep- resents a great waste of food and for- age. Animals protected from horn flies may gain one-half pound in weight a day more than unprotected animals. Horn flies can cut milk flow as much as 10 to 20 percent. Heel flies cause such annoyance when they are laying their eggs that milk flow suffers and beef animals fail to put on finish nor- mally. The total annual loss in cattle hides and calfskins due to injury by 145 cattle grubs has been estimated at more than 2 million dollars. A few examples of estimated losses to livestock, taken from published rec- ords, are given in a table. They were considered conservative at the time they were made ; later increases in the values of livestock and livestock prod- ucts have made the losses even more alarming. Some estimates of the amount of savings resulting from control of in- sects and related pests of livestock in 1949 were assembled and summarized by the Extension Service. They are given in an accompanying table. They pertain to fewer than 30 States, but they give an indication of the savings ranchers and stockmen make when they control insect pests. In a report in 1950, Lyle F. Watts, Chief of the Forest Service, wrote : "In- sects and diseases rank with fire as de- stroyers of forests. Ordinarily the dam- age caused by these pests is less con- spicuous. But they are at work every year, and no forest area is entirely free from them. Their total effect probably exceeds that of fire." The actual amount of loss insects cause to forests is hard to measure, but a few estimates have been made. An outbreak of the spruce budworm from 1910 to 1920 in balsam, fir, and spruce forests of Minnesota and Maine killed about 70 to 90 percent of the mature stand. The loss of timber was estimated at about 4.5 million dollars annually during that period. The Engelmann spruce beetle, in an outbreak in 1940 to 1946, destroyed about 20 percent of the Engelmann spruce timber in Colo- rado. The average yearly loss amounted to about 500 million board feet, valued at about 1 million dollars. The out- break continued in 1951. An outbreak of the mountain pine beetle caused an estimated annual loss of 60 million board feet of lodgcpole pine in Wy- oming in 1946 and 1947. Some 15 million feet of ponderosa pine was destroyed by the Black Hills beetle in South Dakota in 1947. 146 An outbreak of the Douglas-fir tus- sock moth on more than 400,000 acres of forest near Moscow, Idaho, was brought under control in 1947 by DDT sprays applied from airplanes. The in- sect had defoliated the stands in 1946 and had killed the timber on about 16,000 acres. An estimated additional 1,518,856,000 board feet of timber, valued at $84,328,000, might have been killed had no steps been taken to pre- vent further defoliation. Insects cause an average annual loss of at least 5 percent of the rice, corn, wheat, barley, oats, grain sor- ghums, and similar crops after they are harvested and while they are in storage on the farm, in elevators, or in warehouses. Much of this loss comes right on the farm and is more severe in the southern parts of the country where in the warmer temperatures the weevils, beetles, and moths breed and feed through most of the year. The actual amount of grain lost annually because of these pests has been esti- mated at 300 million bushels, worth more than 500 million dollars at 1 951 prices. In the fall of 1947, entomolo- gists estimated from samples of wheat taken from untreated bins in a Mid- western State that the farmers there and then were giving 380 billion in- sects free board and lodging in their grain bins. Processed foods and packaged goods of various kinds get their share of in- sect damage, although such contami- nation is far less today than it was in our grandparents' time. One recalls the barrels of flour and cornmeal and the open boxes of dried prunes common in the local store not many years ago. How often, in buying these products, did one carry home meal infested with weevils and prunes covered with the excrement of the worms that infested them? Today if the housewife finds a sign of an insect in a package it goes back to the dealer. Our food and drug laws now insist that our food be free from insect contamination. Despite the advances, the meal and flour moths and the flour, grain, rice, and cigarette Yearbook of Agriculture 1952 beetles still cause great damage to some processed foods and packaged goods. The annual loss in this country from those pests was estimated at 150 mil- lion dollars between 1940 and 1944. The estimate includes the destruction caused by the pests in processing plants, warehouses, retail stores, and homes. Every now and then a housekeeper has to discard a partly used package of cereal, meal, nuts, dried fruit, or other food which, forgotten on the pantry shelf, has become infested by moths, worms, or weevils — the pantry pests. Suppose each family in the United States discarded only 50 cents worth of infested products a year: The loss would be about 20 million dollars. The losses to clothing, rugs, furni- ture, and other furnishings by clothes moths, carpet beetles, and similar pests are estimated by entomologists to be from 200 million to 500 million dollars annually. Such figures should give us pause. They are figures for fewer than 1 00 of the 600 or more injurious species of in- sects of primary importance that are known to occur in North America. They emphasize that everyone is af- fected in many ways by many insects, even though he might go for months without even seeing or noticing an in- sect or any signs of insect damage. Losses caused by all insects in the United States add up to a staggering amount, whether we regard it in terms of dollars, lost food and fiber, or time and materials used in combatting them. That amount, in the opinion of en- tomologists, is at least 4 billion dollars for an average year — 4 billion dollars. G. J. Haeussler is head of the divi- sion of truck crop and garden insect investigations in the Bureau of Ento- mology and Plant Quarantine. From 1944 to 795/ he was in charge of the division of insect survey and informa- tion. A graduate of the University of Massachusetts, he joined the Depart- ment in 1925. He was engaged for 16 years in investigations on the biologi- cal control of fruit insects. Carriers of Human Diseases F. C. Bishop p, Cornelius B. Philip Through the centuries people have been plagued by insects and have died by the millions from diseases carried by them. Man is gradually gaining mastery over them, but the battle is long and expensive, the burden is too heavy for the poor in many parts of the world, and we still have much to learn about these agents of death. Probably 10,000 kinds of mites, ticks, and insects infect man directly or indi- rectly with disease. Most of them are only occasional and accidental carriers. Many spread diseases among livestock and wildlife and carry them from the animal reservoirs of infections back to persons. Insects transmit disease in many complex ways. First, their mere presence or attack, without the transfer of germs, may produce a disease or harmful condition. Itch mites and screw-worms that in- vade the tissues are of this type. Some insects cause accidental injury to sense organs. Others produce intense itch- ing and allergies, such as are caused by body lice, bee stings, and bites of chig- gers and ticks. Some persons have idio- syncrasies that intensify their reaction to such attacks. A fly or other insect that walks over and feeds on filth and then deposits the germ-laden contaminants on food by crawling over it, vomiting on it, or de- fecating on it is spoken of as a mechan- ical carrier. An insect, such as a horse fly, is also a mechanical carrier when it picks up germs by biting a diseased animal or person and then carries the germ on its beak until it bites a healthy individual. More complex is the relationship among insect, disease, and man when the disease germ multiplies in the insect but does not change greatly in form. That occurs in fleas when they ingest plague organisms with the blood of a plague-stricken rat. The most complex relationship is illustrated by anopheline mosquitoes in transmitting malaria. The malaria organisms in the blood of man at times produce male and female cells. The mosquito ingests the cells when it bites. The cells mate in the mosquito's stom- ach and develop into active ookinetes, which penetrate the stomach wall of the mosquito and thereon form cysts. Cell division takes place in the cysts, and hundreds of small, spindle-shaped sporozoites are formed. The greatly enlarged oocyst then bursts open within the insect's body cavity. The active sporozoites swarm out, soon reach and penetrate the sali- vary glands, and are ready to pass into the blood stream of the next person the mosquito bites. This cycle, which takes 7 to 10 days, is called the essen- tial or sexual cycle. Upon entering the blood stream, the minute malaria or- ganisms— the sporozoites — enter such organs as the liver. In a few days they attack the red blood cells, in which they go through another cycle of growth and multiplication. Some ulti- mately become sexually mature, ready for other mosquitoes to ingest, and so repeat the sexual cycle in the insect. Many variations occur in this method of disease transmission, which is called obligatory or cyclic because the disease organism is dependent on an insect for its continued natural transmission. Insects carry disease organisms of many types, among them micro- scopic viruses, bacteria, and protozoa and the larger roundworms and tape- worms. Ways by which disease organ- isms are kept alive in higher animals and insects and are passed from one generation to another frequently are very complex. Unraveling them has often required great scientific imagina- 147 148 tion and patient skill. We give some ex- amples later. Sometimes disease organ- isms are carried from one stage of an insect host to another, with the inter- mediate stage or stages not transmit- ting infection or even living as para- sites. In many instances the disease agents pass through the egg from one host generation to the next. The disease cycle can be broken by destroying the insect vector, by using drugs to kill or suppress the disease organisms in the human host, or by immunization. Most successful usu- ally is a combination of the three, plus isolation of infected persons (to pre- vent the insect vector from acquiring the disease organism) and such sani- tary measures as screening to protect healthy persons. Does the disease make the insect sick or kill it? Sometimes the infected insect is not injured in any way — ap- parently it has become tolerant. Some- times its life span may be shortened. Occasionally it may be killed — when that happens, that particular kind of insect is not a usual or well-adapted carrier of those particular disease germs. Insects of the order diptera, or two-winged flies, perhaps are respon- sible for more human illness and death than any other group. They may rank with the world's top killers of man. Mosquitoes inhabit practically all parts of the earth except the polar regions. They alone carry malaria, yellow fever, dengue, and bancroftian and malayan filariasis. They also carry certain types of encephalitis and may be involved occasionally in the mechanical transfer of tularemia and anthrax. Malaria, the great disabler, prevails throughout the Tropics and much of the temperate regions. Outbreaks have occurred in Canada and as far north as Archangel in Soviet Russia. Species of dapple- winged Anopheles mosqui- toes are the carriers of human malaria. Large areas of the United States once were malarious, but as the swamps were drained and the land tilled and Yearbook of Agriculture 1952 people got into screened houses, the malady was pushed southward. There the mild climate and abundant water areas gave opportunity for mosquitoes to breed in numbers during the long summer. The malaria parasites also developed in the mosquitoes and peo- ple were more exposed, because they spent more time outside during the warm evenings. Poorly built and un- screened houses sometimes permitted infection to occur even indoors. Since 1943 the disease has been further re- duced by the use of DDT. There is little malaria now in the United States. About a dozen species of Anopheles mosquitoes occur in the United States, but only one in the Eastern and South- ern States has been important in trans- mitting malaria. Likewise in the Pacific States a single species, but a different one, is the natural carrier. Scores of different kinds of Anopheles exist in various parts of the world. Their vary- ing breeding and biting habits deter- mine which control measures are instituted in any area. Some carry malaria. Others have no part in infect- ing man. Yellow fever, or yellow jack, periodi- cally put terror in the hearts of our people, especially in the South in the early days. When Reed, Carroll, La- zear, and Agramonte proved in 1901 that a semidomestic mosquito, now known as the yellow-fever mosquito, was the vector, some of the terror dis- appeared. But although we know how to control or eradicate the mosquito, and although a protective vaccine has been developed, the disease is still re- garded as a serious threat to this coun- try and to many other warmer parts of the world. A deadly virus disease, it still lurks in the jungles of South Amer- ica and Africa. To start serious trouble, the virus needs only to be transferred by jungle species of mosquitoes from an infected monkey to a man, who in turn may infect the yellow-fever mos- quito in a populous area. Indeed, this insidious disease has suddenly flared up since 1950 in the jungles of Panama and Costa Rica, where it was thought Carriers of Human Diseases to be stamped out, resulting in some- what hysteric, unfounded reports even in Mexico. The yellow-fever mosquito lives close to humans. It breeds in water in old tin cans, flower vases, and dis- carded tires. It is seldom found more than a quarter of a mile from a house. Only female mosquitoes bite. Females of the yellow-fever species slip out of hiding places at twilight, find exposed ankles or arms, and dart away at the slightest motion. The Nobel Prize in Medicine for 1951 was awarded Max Theiler for de- velopment of a vaccine of living, atten- uated virus, which has not only pro- tected thousands of exposed civilians and troops but has undoubtedly been instrumental in keeping this dread dis- ease out of the Far East, despite the increase in travel by air. Dengue, or breakbone fever, also carried by the yellow-fever mosquito, is a painful and debilitating but not fatal virus disease that strikes occa- sionally. In an epidemic in 1922, Texas had more than a half million cases. For short periods it incapacitated large numbers of our troops on Guam and other Pacific islands during the Sec- ond World War. Encephalitis, caused by several kinds of viruses that attack the central nervous systems of vertebrates, is trans- mitted by several species of mosquitoes. One species may be a vector of one virus strain and not of another. An outbreak of the so-called St. Louis type of encephalitis in 1933 is thought to have been carried by the northern house mosquito. A strain of the disease that has caused several hundred cases of human encephalitis each year is carried primarily by Culex tarsalis. Sev- eral species of mosquitoes can transmit the serious "Japanese B" encephalitis, which has caused serious epidemics in Japan and adjacent areas. Two types of equine sleeping sickness that have killed thousands of horses in the United States also cause illness in man and are probably transferred by a number 149 of our common species of mosquitoes. Some infected parasitic bugs and bird mites have been found in the wild. Elephantiasis, a disfiguring malady of people in the Tropics and subtrop- ics, is carried by mosquitoes. The ex- tremities and genitals often become greatly swollen because of small round- worms that establish themselves in the lymph glands. Into the blood stream the worms discharge eggs, which, after developing to active embryos known as microfilariae, are picked up by mos- quitoes when biting. Some strains of the young worms swarm in the blood near the surface of the body at the time of the day or night when the favored species of mosquito is likely to bite. Upon reaching the stomach of the mosquito, the young worms wiggle out. of their saclike sheaths in an hour or so. They work through the stomach wall and into the thoracic muscles. There they grow for 2 or 3 weeks. Then they migrate to the beak of the mos- quito, curl up, and await a chance to gain entrance to the skin of a person when the mosquito again bites. The worms, about one-twentieth inch long, burrow into the skin, reach the capil- laries, and are carried in the blood stream to a lymph gland, where they develop to maturity. The female worms are 3 to 4 inches long. The males are about half that size. The cycle is complete when mating takes place and production of microfilariae begins. Elephantiasis does not necessarily follow infestation from an infected mosquito bite, but skin irritation and fever are often manifest. Infection by these little worms is called filariasis. The malady occurred a number of years in the vicinity of Charleston, S. C, but it appears to have died out. No other endemic foci are known in the United States, although the carrier, the southern house mosquito, is widely distributed in the South. The development and use of various ways to control mosquitoes are dis- cussed on page 476. 150 Indians gave the name no-see-ums to the tiny mottled winged gnats that can readily crawl through a fine screen. They are also called punkies or sand flies (although they are not the same as Phlebotomus, discussed later) , and are known scientifically as Culi- coides. Their bites can be extremely irritating. Often they produce delayed reactions. The 20 species in North America differ greatly in breeding habits, but all develop in water or moist places. The larvae of the most trouble- some kinds develop in the mud on salt marshes and in rot holes in trees where decaying leaves and water are held. The insects, however annoying, are not known to carry human disease in this country. They were mistakenly accused of causing "sand-fly fever" in Ameri- can troops in New Guinea. They are an intermediate host of certain round- worms (nematodes) in Africa and elsewhere in the Tropics. These round- worms in the blood of man apparently do not cause illness. Installing dikes and tide gates to pro- tect salt-marsh areas, clearing and deepening the margins of ponds and streams, and filling tree holes are steps that reduce breeding. Insecticidal sprays and fogs protect communities against the adults. Painting screens with 5 percent DDT in kerosene, the use of close-woven bed nets, and the application of repellents to exposed parts of the body give some relief. Black flies, of the family Simuliidae, are annoying pests to lumbermen, campers, fishermen, and others in the north woods. These rather small, hump-backed gnats are not confined to the north country, however. Some 75 species exist in the United States. Many others occur in other countries. All breed in flowing water. Some kinds live only in fast mountain streams. They lay their eggs on sticks and rocks projecting from the water. The larvae cling to objects in the water, from which they gather food with a set of motile brushes around the mouth. They spin weblike pockets under the water and pupate in them. Yearbook of Agriculture 1952 As carriers of human diseases, black flies are not serious in this country, al- though many persons get severe der- matitis or allergic reactions from the bites. In Mexico, Central America, South America, and Africa, some species are hosts for early stages of a roundworm, which they transfer from one person to another. The worms form nodules under the skin, principally on the head and upper part of the body which cause so-called onchoceriasis. Some get into the eyes and may pro- duce blindness. The larvae of black flies can be killed by adding small amounts of DDT to the infested stream. Dosages required to control larvae will not injure fish, but care should be taken not to apply excessive amounts, which will kill fish. DDT fogs applied from the air or ground help to destroy these gnats. Damming streams to eliminate rapids has some merit. Repellents are not entirely satisfactory. Sand flies are annoying blood- suckers and carriers of at least two serious diseases, although none of the half dozen species of this group (Phle- botomus) that occur uncommonly in this country is a disease carrier. The dangerous verruga or Oroya fever, which occurs in Peru, Ecuador, Bolivia, and other South American countries, is carried by sand flies. They also trans- mit pappataci, sand-fly, or 8-day fever of the Mediterranean region, Near East, southern China, Ceylon, and In- dia. It is a mild febrile disease of man. Kala azar, a leishmaniasis endemic in the Mediterranean area, Iraq, south- ern Russia, India, and China, is carried by sand flies, as is a repulsive skin dis- ease, Oriental sore, in that general area. The insects breed in damp animal and vegetable wastes and in crevices in rocks and walls. A spray of DDT in kerosene in corners of sleeping quar- ters, around the base of houses, and their other breeding and resting places controls the sand flies and stops in- fections. Carriers of Human Diseases Close relatives of sand flies are other moth flics that have no thirst for blood. Some of them breed in sewage filters. Often they emerge from the sewer beds in large numbers and invade near- by houses. They are not attracted to food, but occasionally get on it or on dishes and utensils, which they un- doubtedly contaminate. The larvae on the stones in the filter beds require much oxygen and may be killed simply by flooding the beds with a few inches of quiet water or sewage. Insecticides (such as DDT emulsions) are also ef- fective but may destroy other organ- isms that aid in keeping the filters open. Horse flies and deer flies are serious pests of livestock but usually are less troublesome to man. These aggres- sive bloodsuckers will also attack man. Their bites are painful. Bathers and picnickers on beaches near salt marshes along the Atlantic seaboard are often driven away by attacks of "greenhead" horse flies. Outdoorsmen in the north woods are familiar with swarms of the flies. Deer flies often attack man. In the summer of 1935, 1 70 young men of the Civilian Conservation Corps were pre- paring a game refuge on salt marshes near Bear Lake, Utah. The deer flies were very annoying; 30 men contracted tularemia, or rabbit fever, in 2 weeks, and the camp had to be closed. The flies carry tularemia on their beaks. Occasionally anthrax germs also are carried in that way by horse flies and deer flies between diseased and healthy animals and sometimes to man. In tropical West Africa, deer flies of two or more species are hosts of the filarial parasite {Loa loa) of man. The parasite lodges in the connective tissues under the skin and often invades the eyes. One can combat horse flies and deer flies in several ways. Drainage of marshy breeding areas is frequently impractical. Spraying in early sum- mer with DDT solution of marshes and swamps where the flies breed has some value. Because of the danger of 151 injuring wildlife, the use of insecti- cides should be under the direction of an experienced person. Repellents to protect livestock have so far not proved very practical. Tsetse flies cany African sleep- ing sickness, a deadly disease caused by Tsetse fly. minute, single-cell organisms, the tryp- anosomes. One form, Gambian sleep- ing sickness, is carried mainly by a fly, Glossina palpalis, which resembles the stable fly. Another kind, which brings death more rapidly, is Rhodesian sleep- ing sickness. It is carried principally by G. morsitans. Besides these diseases of humans, tsetse flies carry several re- lated diseases of livestock and wildlife. The disease organisms are taken up by the fly in the blood meal, pass through developmental stages, and multiply in the digestive tract. They invade the salivary glands when mature and at a subsequent feeding of the fly they gain entrance to another animal host. These dangerous flies are confined to tropical and subtropical Africa, where they hinder settlement and development. 152 Tsetse flies live 3 to 6 months. They can travel considerable distances, al- though G. palpalis stays close to the banks of lakes or the timber along streams. The insects differ from most flies in that the eggs hatch and the larvae develop in a uterine pouch with- in the body of the mother fly. One larva develops at a time. When it is full-grown, it is dropped in the shade and near water. There it burrows into the soil and in 3 weeks to 2 months changes into a fly. The house fly has shared man's food and developed in his wastes and those of his domestic animals since the world was young. The house fly may lay 2 1 batches of eggs, live 5 months, and complete a generation every 2 weeks. House flies breed in fermenting veg- etable and animal matter and other filth, without which they cannot exist, despite a high reproductive capacity. Its ability to travel at least 13 miles, its filthy habits, and its greedy appe- tite make the house fly a formidable germ carrier, but many of the germs it carries to our food do not cause dis- ease. When disease organisms are in the wastes, however, the house fly car- ries them. In earlier days, many cases of typhoid were clearly chargeable to it and some still are. Dysentery, diar- rhea, and other digestive troubles are often due to contamination of foods and utensils by flies. House flies are believed to have a part in spreading the germs of cholera, yaws, trachoma, and tuberculosis. They also transport certain parasitic worms. After the Second World War, DDT — applied as a residual or long- lasting spray to walls and ceilings of buildings — made the house fly almost a rarity for a time. But in line with na- ture's defenses to perpetuate a species regardless of man's wishes, strains of flies resistant to the effects of DDT be- gan to appear in 1947 in various parts of the world; those strains became more resistant and widespread in the next years. Yearbook of Agriculture 1952 Other insecticides, somewhat like DDT chemically, were found to have the same residual killing effect, but were less persistent. Among them were benzene hexachloride, lindane, meth- oxychlor, TDE, chlordane, toxaphene and dieldrin. No doubt others will be discovered, but the house fly has dem- onstrated its ability to develop resist- ance to each of the materials after a number of generations have been ex- posed to them. To help meet the situation, the old and safe pyrethrum has been brought back more fully into use, alone and in mixtures. Unlike DDT, it is a quick killer. Screens and other means of exclud- ing flies from buildings and food are of great value and will undoubtedly con- tinue to be necessary for protection against the house fly, mosquitoes, and other troublesome and dangerous in- sects. Sticky fly paper, traps, and elec- trocuting devices are also useful in destroying flies that breed despite rigid sanitation. Blow flies, often called green bottle flies and blue bottle flies, are of many kinds. They have life cycles and habits somewhat like those of the house fly, but they breed mainly in carcasses of dead animals and in meat in gar- bage. They are seldom so numerous as house flies but carry many of the same disease-producing organisms. Laboratory studies have incrimi- nated field-collected blow flies in the conveyance of poliomyelitis virus, al- though their role in causing human infection is still a moot question. The habits of blow flies would seem to give opportunity for them to transmit that disease and many others. The larvae of blow flies also develop in wounds or natural openings of the body. Such attacks are called myiasis. Some species, true parasites, develop in the tissues of living animals. Other species, when accidentally ingested as eggs or young maggots, may continue to grow in the digestive tract and pro- duce severe irritation, nausea, vomit- ing, and diarrhea. Carriers of Human Diseases The true parasites may suck blood, as the human-infesting Congo floor maggot does, or they may invade wounds or inflamed nasal passages, as the screw-worm does. Screw-worms, if not promptly killed and removed, may destroy enough tissue to produce disfigurement or death of the victim. Some other insects that customarily live as parasites in livestock occasion- ally attack man. The sheep bot fly sometimes darts at the eye of a person and deposits in it a droplet containing a number of minute, active, spiny lar- vae. The larvae crawl over the eyeball and cause inflammation. Shepherds in North Africa are said to be blinded by repeated attacks. Horse bot flies and cattle grubs in the first stage of their development occasionally get into the skin of people working around live- stock. The horse bots burrow about in the skin, producing what is likened to creeping eruption. The cattle grubs penetrate deeper and usually work up- ward as they do in cattle. Often they come to the surface on the neck or head and produce a boil-like swelling, from which they can be removed. They sometimes cause severe illness. Dermatobia is a fly that produces serious losses to livestock in the tropical Americas. Often it infests man and has therefore been called the human bot fly. The larvae develop in pockets be- neath the skin and maintain an open- ing through which to get air. To get its larvae to a suitable host, the fly catches a mosquito or other bloodsuck- ing insect, attaches eggs to its body, and then releases it. When the mos- quito bites a warm-blooded animal, the little maggots pop out of the eggs and burrow into the skin. Fleas, like the biting flies, are among the higher insects that have complete metamorphosis. They have developed highly specialized parasitic habits in the adult stage only. The wingless adults have laterally com- pressed bodies and strong, spiny legs, which help them move rapidly among the hairs or feathers of their hosts. 153 Their mouth parts are fitted for pierc- ing and sucking. All species, as far as we know, are parasites of higher verte- brates. Fleas have astonishing strength in proportion to their small size. The human flea can jump 13 inches. Some persons attract fleas more than do others under the same exposure. In one person, an area of inflammation immediately surrounds the bite; in others, a delayed irritation occurs. Pulex irritans is thus an appropriate name for the human flea, which has adapted itself to residence in folds of man's clothing as a substitute for the fur of the lower animals. Their eggs are dropped promiscuously and are not fastened to clothing or hair as are those of lice. The maggotlike larvae live on organic waste about the premises. Other species that may become an- noying in human abodes are the rat, cat, and dog fleas, which do not have so restricted a host preference as do some of their cousins on various ro- dents in the field. The chigoe is an especially irritating kind of flea to man and animals in the Tropics. The females bury themselves in the skin, particularly of the feet, and cause persistent, ulcerlike craters, from which the fleas have to be removed be- fore the wound can heal. This flea is not a known disease carrier. Of greater concern are the species that carry the serious and widely occur- ring rodent infections, bubonic plague and murine typhus, to man. They live everywhere in warm climates. One of the authors during the Second World War watched the pests jump in all di- rections from the wrinkles in the panta- loons of Arabs while he was studying the effects of DDT in Egypt; boil up into his clothes from the straw in aban- doned pillboxes and from cave floors occupied by refugees in Sicily, where he was investigating mosquitoes and sand flies; and emerge by thousands from the ground litter of a small, aban- doned native village along a mountain stream in the Philippines. Regardless of locality, race and color, they were after human blood. 154 Bubonic plague is by all odds the most serious of the human diseases at- tributed to the flea. Think of the rav- ages of the Black Death in the Middle Ages, particularly among the popula- tions of port cities. Plague still stalks the earth. In military operations in the Tropics we may have unavoidably spread the disease to new areas through beachhead or landing operations when it was not possible to use safeguards, like collared anchor cables and inspec- tion, which are observed in peacetime to restrict the emigration of rats and their rat fleas into new ports and settled areas. One of us lived in 1930 in a West African port city where people re- garded the annual human death rate of about 600 as not unusual. The disease waxes and wanes in the Tropics, but the antibiotics developed since 1940 give promise of relief if they are available. No one now need die of this once dreaded disease if diagnosis is made early enough and suitable drugs are available. Experimental data indicate that a combination of strepto- mycin and aureomycin is the treatment of choice. The ecology of the so-called "sylvatic plague" in the western half of the United States has been quite obscure. There the infection continues to wipe out whole populations of field rodents locally with only an occasional human case. The disease has not affected rats in cities to any great extent. The special fleas of the affected ground squirrels, prairie dogs, and rabbits are less prone to bite human beings than are the oriental and the northern rat fleas. Murine typhus, or endemic typhus, is much like the louse-borne type, which in numbers of cases, but not in virulence, outweighs plague as a world- wide human disease. The spread of murine typhus from man to man by lice, after establishment from fleas, has been reported in Mexico and Man- churia. New laboratory techniques and careful diagnosis are required to verify such reports. Murine typhus occurs widely in trop- Yearbook of Agriculture 1952 ical and temperate climates. Treatment with antibiotics has been effective, but preventive measures are still the most important. Constant vigilance is re- quired against the spread of rat fleas by domestic rats from foreign ports. In endemic localities DDT or other toxi- cants are used in rat runways to reduce existing flea populations on rats. An- other weapon is rat poisons, which have been spectacularly improved in recent years; one of them is the comparatively safe warfarin. Almost everyone has been stung by bees, wasps, and ants. Some have suffered the more painful sting of a velvet ant, the "cow-killer." All these insects use their stings merely in self- defense, but the pain is none the less severe. The effects generally do not last long, but some persons who are allergic to the poison that the insect injects may be seriously affected or even killed, par- ticularly when they get many stings. The material that causes the pain and the stinging mechanism vary among the different insects. The venom is usually a complex protein material. Among ants, formic acid is partly re- sponsible for the pain of the bite or sting. Most insects can sting more than once, but a honey bee loses its life when it stings. The barbed sting holds fast in the flesh, and the tip of the abdomen and the two poison glands are torn off. The muscles that operate the stinging organ keep contracting for a few minutes, force the sting deeper into the skin, and pump the venom into the wound. The sting therefore should be removed quickly. That is best done by scraping the sting off with a knife or the fingernail; pulling it out with the fingers might squeeze more venom in. Caterpillars do not carry human diseases, but often they cause painful injuries. The hairs or spines on the bodies of the larvae are mainly re- sponsible. The hollow spines, connected at their bases with poison glands, con- tain poisonous materials. They are Carriers of Human Diseases broken off in the skin of man when a sensitive part of the body comes in con- tact with the caterpillar or its shed skin. The spines help protect the caterpil- lar against its predatory enemies, but they do not prevent its destruction by parasitic flies and wasps, which kill a high percentage of them. Several species in seven or eight fam- ilies of moths can sting in that way. Some of the worst, such as the puss caterpillar, look quite innocent, but many ugly and dangerous-appearing caterpillars, such as the hickory horned devil, are harmless. Some urticating caterpillars are crop or forest pests, such as the brown-tail moth in New England, the flannel moth of the Northern States, the io moth, the saddleback caterpillar, and the puss caterpillar. The last named occasionally strips the leaves from elms, hackberries, and other shade trees in the South. The hairs of the brown-tail moth re- tain their poison for a long time and, when the insects are numerous, may irritate the skin and eyes of many peo- ple. The sting of most of the species, although painful, does not last long. The puss caterpillar can have a more lasting, severe sting, which can give persons the symptoms of paralysis. Before they reach full growth, the caterpillars can be controlled by spray- ing the infested shade trees or shrub- bery with arsenate of lead or DDT. The likelihood of a person coming in contact with them is increased when the caterpillars are crawling around seeking a place to pupate. There is no specific remedy for the sting, although packs of bicarbonate of soda and cool- ing lotions are advised. Beetles of the family Meloidae have in their body fluids a poisonous substance, cantharidin, which blisters the skin. The beetles are collected and dried and the cantharidin is extracted and sold as a drug. Of the many spe- cies of blister beetles, one is widely known in this country as the old-fash- ioned potato beetle. 155 Some of the rove beetles (family Staphylinidae) also can*cause blisters, which are often slow to heal. The so- called toddy disease among natives in the Marshall Islands is supposedly caused by rove beetles that get into palm pulp, which is fermented to make an alcoholic drink. The poison in the beetle is extracted by the alcohol and causes symptoms when the liquid is imbibed. To the entomologist the term "bug" refers specifically to an order of insects, the Hemiptera, which includes the wingless bed bugs of various spe- cies, and the winged, biting, or blood- sucking insects known as "kissing bugs," assassin bugs, conenoses, and their relatives. Few insects have been more often maligned and less confirmed as car- riers of disease than the bed bug or its close relative in the Tropics and sub- tropics, the Indian bed bug. Some other species of the bed bug family that are customary parasites of poul- try, swallows, or bats may invade houses on occasion but cause little com- plaint among human inhabitants. An infestation of bed bugs is considered a sign of filth and uncleanliness in houses and hotels, but the bugs are seen sometimes in public conveyances so that any person might take one home. A few persons are sensitive to their bites and may develop a temporary rash, local swelling, or irritation fol- lowing such attacks. The bugs are noc- turnal. In daytime they retreat into mattresses, joints of wooden bedsteads, cracks, and other hideouts in bed- rooms. Their flat bodies can edge into tight crevices. They have a distinctive, pungent odor. All active stages are parasitic from the time they hatch from eggs, which are laid in their hiding places. Adults can live a year without a meal of blood but will usually migrate during such a starvation period. No one so far has proved that bed bugs are the actual, natural vectors of any important human disease. The causative agents of several diseases, 156 including those of kala azar of Asia, Chagas' disease of South America, re- lapsing fever, infectious jaundice, lym- phocytic choriomeningitis, tularemia, and also plague, have been shown ex- perimentally to persist some times in the bodies of the bugs. They also have been reported as vectors of Rocky Mountain spotted fever in Brazil. From earlier experimental work in Poland similar conclusions were drawn, but such experimental results have not been found by other investigators. Certain of the conenoses, sometimes called kissing bugs, of the family Tri- atomidae have been shown to be the natural carriers of trypanosomiasis in the American Tropics and even in tem- perate parts of South America. Chagas' disease, named for a Brazilian investi- gator, causes high mortality among children in some localities. The disease is caused by minute organisms, tryp- anosomes, which multiply in human or- gans and pass through a definite devel- opmental cycle in bugs that have fed on persons having them in their blood. Some 40 species of the triatomids have been found naturally infected, but fewer than 12 are of any importance in human transmission. Several species habitually pass excrement while biting or soon thereafter. Transmission prob- ably occurs through fecal contamina- tion of the bite wounds and other abrasions or of the mucous membranes of the victim, rather than through di- rect inoculation by the mouth parts of the feeding bug. A number of native and domestic animals are reported to be reservoirs. Several species of the bugs occur in various parts of the Southern States, where they infest rodent nests and other animal habitations. They may invade human dwellings or camps and attack man himself. Natural infection of trypanosomes in wood rats, and in their parasites, Triatoma protracta or T. uhleri, has been reported in a few localities. Human infection in this country has been suspected only on serological evidence. Triatomids were found naturally infected with western Yearbook of Agriculture 1952 equine encephalomyelitis virus in Kan- sas, but it is doubtful whether they are of importance in either human or equine transmission. Assassin bugs prey on other insects but are not parasitic on animals. If annoyed or accidentally touched, some species can inflict severe and painful '"stings" with their beaks. Bed bugs are controlled with prepar- ations of DDT. The conenoses are sus- ceptible to the new insecticides, but the delayed action of DDT and their abil- ity to fly and quickly reinfest premises complicate the problem of control. DDT can be applied as a 10-percent dust or a 5-percent solution in deodor- ized kerosene to the hiding places of the bugs. Cockroaches are the only mem- bers of the grasshopper group (order Orthoptera) that are involved in the contamination of food. Crickets often enter houses and occasionally eat holes in clothing but are not attracted to food. Of the many species of cockroaches, only five are common house inhabitors. They are the large American, oriental, and Australian cockroaches and the smaller German cockroach, or "water bug," and the brown-banded roach. Their habits are similar. They prefer secluded, warm, damp places, as be- hind sinks, around drain pipes, and in furnace rooms. They lay their eggs in- side a pod. The German cockroach carries the egg capsule attached to the tip of the abdomen until a day or two before the eggs hatch. The young in- sects resemble the adults except in size and lack of wings. Cockroaches grow slowly. The American cockroach usu- ally takes a full year to develop from egg to adult. Then and later the filthy pest at night or on dark days busily runs about, sampling filth and foods and imparting to infested areas his fetid, roachy smell. People have long assumed that cock- roaches must be carriers of various enteric diseases and tuberculosis. Now we know that certain disease organisms Carriers of Human Diseases {Salmonella) were still alive when passed 10 to 20 days after being fed to three common kinds of household roaches. The germs remained infective in the fecal pellets for more than 199 days afterward. These insects furnish one of the important sources for spread of intercurrent salmonellosis in labo- ratory animals. Cockroaches can be controlled by thorough sanitation, elimination of breeding and hiding places, and in- secticides. Sodium fluoride dust (at least 50 percent) has been used in this way. Chlordane, applied as a 2-percent spray or a 5-percent powder to re- stricted areas where the insects hide has given good results. A 5 percent DDT spray or 1 o percent DDT powder is also recommended but is less effective than chlordane. Pyrethrum sprays are recommended in situations where foods might be contaminated with chlordane or DDT. Pyrethrum must be reapplied frequently because it loses its effective- ness in a few days. Of the two kinds of lice, we are concerned here with the sucking lice (Anoplura). The biting lice (Mallo- phaga) are chiefly parasites of birds. Three kinds of sucking lice com- monly infest man, the head, body, and crab lice. Crab lice, also called pubic lice, pre- fer hairy parts of the human body in the pubic region and armpits. In severe infestations they may be found in the eyebrows and lashes. They may cause intense itching but have no known effect on the health of the host. The other lice have greatly in- fluenced human history. They have af- fected the outcome of many military campaigns because they spread epi- demic typhus. This disease, the "red death" of the Middle Ages, has been the scourge of soldiers and displaced peoples during times of their greatest misery, when they were least able to exercise customary habits of sanitation. No insect has shown greater adapta- tion to the habits of its human host than has the body louse. It alone of all 157 the members of its order has forsaken the fur of its host and found refuge in man's inner garments, on which its eggs are fastened and to which it clings except while feeding. An adult may feed as often as six times a day. The longer the garments are worn next to the skin, the better the body louse thrives. The head louse is related to the body louse, but it is slightly smaller and darker in color and is found only among hairs of the head. The more luxuriant the head adornment of na- tives, the greater opportunity is pro- vided for lice to develop. Adult lice may survive up to 5 days without a meal of blood. The younger stages must feed oftener than once a day to stay healthy. A person's first ex- posure means little or no discomfort, but a sensitivity occurs after a week or 10 days, when the average person de- velops an intense itching from the feed- ing of the lice. A tolerance is later de- veloped by constantly infested peoples, who take little notice of their presence. Infestations of head lice are not un- common in the United States; occa- sionally they occur among school children. Louse-borne typhus, like plague, has been one of the historic scourges of vermin-infested mankind. Like ma- laria, on the other hand, it is one of the few serious insect-transmitted dis- eases in which man himself serves as the so-called animal reservoir; there is no known cycle in some lower animal. Epidemics, heretofore, have followed in the wake of war as certainly as death and taxes, simply because habitual sanitation and segregation, which pre- vent the spread of lice, could not be constantly maintained among the sol- diers or civilians. At the beginning of the First World War as many as 2,500 new cases a day were hospitalized with typhus in the Serbian Army; among civilians the number was said to be three times greater. The outbreak tem- porarily checked the impending inva- sion of the Austrian Army at the Ser- bian borders more effectively than any 970134' --2 i58 military strategy. Napoleon's retreat from Moscow is believed to have been due more to typhus than to cold weather. Trench fever, another disease of sol- diers, is also louse-borne. It occurred in the First World War and also in the Second World War on the Russian- German front. The causative agent is thought to be related to typhus fever. It does not kill, but it can be a debilitat- ing epidemic disease among louse- infested troops. Relapsing fever, also transmitted by the body louse, is caused by a spiro- chete, an organism entirely different from the one that causes typhus. The disease is most prevalent in parts of North Africa and Asia. Here again the louse-man-louse cycle is all that is needed to maintain the infection. How louse-borne relapsing fever and ty- phus are maintained between epidem- ics is a mystery, because transmission through the egg from one generation of lice to the next seldom occurs, if ever. Lice can be controlled easily by 10 percent DDT powder. In 1944 a simple method was devised to apply DDT powder by blowing it into the openings of the clothing while on the person. Thousands of individuals living under refugee conditions in Naples were so treated and a threatened outbreak of typhus was averted. However, the find- ing of DDT-resistant lice during the Korean conflict has made it necessary to use pyrethrum or lindane instead of DDT. Ticks and mites comprise the Ac- arina — a class of arthropods separate from the Insecta because they have four instead of three pairs of legs in the nymphal and adult stages and their bodies lack the separate thoracic region of true insects. Mites generally are smaller than ticks and diverse in habits. Only a few at- tack man and animals. Ticks require blood for their development and repro- duction, but probably fewer than half of the species feed upon man. Yearbook of Agriculture 1952 Scabies of man is a condition caused by the itch mite, which burrows in the skin, where it lays its eggs. The mite causes intense itching and irritation. In aggravated cases, an extensive crust- ing and scabbing results, particularly over the arms and hands. The offenders are never seen except by careful dis- section under a good lens. Observa- tions in England during the war dem- onstrated that the major source of new cases was provided by actual body con- tact rather than through towels, bed clothing, and wearing apparel used by infested people. Grocer's itch and harvester's rash — transient but often annoying — are the result of exposure to mites that ordi- narily infest grain and stored-food products. The tropical rat mite and a less com- mon but also widely spread house- mouse-infesting mite, Allodermanyssus sanguineus, occasionally cause com- plaints through infestation of houses invaded by their rat or mouse hosts. The first species has been accused of acting as a vector of murine typhus and of plague, but experimental evidence has been conflicting, and its importance in this regard remains doubtful. But it can transmit rickettsialpox, the most recently discovered of the typhuslike diseases, which has occurred in some mouse-infested premises in New York and Boston. A. sanguineus is the nat- ural transmitter of rickettsialpox in New York suburban settlements. Some of these mites have also been reported as transmitting tularemia in Russia. Chicken mites and related species oc- casionally annoy man by their blood- sucking habit but do not remain on him long. Chiggers belong to another group of mites of the family Trombiculidae. Only the first or larval stage is parasitic on vertebrates and must have a blood meal for further development. Chig- gers are so minute that they are seldom seen by man even though the numerous sites of attachment such as the belt line are evident. In eastern and south- ern areas of the United States where Carriers of Human Diseases chiggers are most prevalent, they are not known to carry any disease, but their attack produces severe itching, which may result in secondary infec- tions. This type of attack in the south- west Pacific area is known as scrub itch. Scrub typhus, a serious malady in the Far East, is called tsutsugamushi disease in Japan, where it was first rec- ognized. It is carried by certain species of the chiggers. The disease agent, related to the typhus group, is passed from one generation of mites to the next through the egg. The chiggers usually parasitize rats and other rodents, but certain species will attack man. The mortality from scrub typhus can be extremely high — more than 60 percent of cases in some parts of Japan, or as low as 0.6 percent among Americans in an epidemic in the Schouten Islands, where 1,469 cases occurred in 6 months. Even with this low death rate, the situation is serious when incapacitation averages 2 months or more. Chloromycetin and aureomycin, however, are markedly reducing hospitalization and will aid in progress against this dread disease. More people have an acquaintance with ticks than with mites. Yet many do not know that there is one family, the Argasidae, or soft ticks, which feed on man rather rapidly like bed bugs and do not remain attached. The notorious relapsing-fever tick of tropical Africa has become almost com- pletely domesticated, and natives sleep- ing on the floors of their huts pay little attention to it. A number of re- lated species in various parts of the world, including the United States, have since been found to cause human cases of relapsing fever. These are mostly species that rodents bring into living quarters, mountain cabins, na- tive huts, and the like. Several similar forms of relapsing fever are carried by different species of these soft ticks. Another group of ticks, the hard ticks, of the family Ixodidae, custom- arily require several days to complete engorgement after attachment. In tem- 159 perate climates where cold seasons in- tervene, some species may require 2 years to complete a generation. The Rocky Mountain wood tick is an ex- ample. The adults have been known to survive three winters when kept out- side. Many ticks have simple eyes with which they can discern the passing shadow of a potential victim. Others have no eyes. Special sense organs en- able them to detect animals 25 feet or more away, so that an unwitting camper may attract ticks from a con- siderable area. Many ticks are thus able to select favorable sites- — game trails, for instance — for seeking their host. Ticks have numerous progeny and few enemies, are not greatly affected by weather conditions, can feed upon vari- ous kinds of animals, and permit the passage of disease organisms from stage to stage as well as from one generation to another through the egg. Instances of secondary infection at the point of tick attack occur often. "Spring-summer" encephalitis in So- viet Russia and Siberia is the most important of the filterable viruses car- ried by several Russian species. Another tick-borne virus is the rather mild, nonfatal Colorado tick fever of our Rocky Mountain region. Tick-borne typhuslike diseases are assuming increasing importance in va- rious parts of the world. These include American or Rocky Mountain spotted fever in various countries of the New World, and a group of usually less severe diseases in Europe, Africa, Asia, and probably Australia, related to bou- tonneuse fever, which was first recog- nized in the Mediterranean region. The latter group of diseases includes South African tick-bite fever, Kenya typhus, and Siberian, Indian, and probably Queensland tick typhuses. In the United States, the Rocky Moun- tain wood tick in the West, the Amer- ican dog tick in the East and South, and probably the lone star tick in the South are the chief criminals in human infection with spotted fever. Q fever is a peculiar, recently recog- nized disease due to a typhuslike agent i6o which is being discovered in many parts of the world. Ticks have been found naturally infected in North America, Australia, Spain, and parts of North Africa, but only in Austra- lia have they shown any importance in relation to human infection. Tularemia, an important bacterial disease affecting man, occasionally is transmitted by ticks. Tick paralysis of man and animals is due to a presumed toxin secreted in the saliva of ticks. Though more fre- quently observed in tick-infested ani- mals, a number of human cases have been reported in the United States, Canada, and Australia. The ascend- ing paralysis is caused by a rapidly en- gorging female tick attached to the base of the head, where the hair may hide its presence for longer periods. Complete recovery has followed within 24 to 48 hours of removal of the of- fending tick. Death occurs if the as- cending paralysis reaches the respira- tory centers of the human or animal victim before the tick has dropped off or has been removed. The Acarina are more difficult than insects to control because they are more resistant to insecticides. Benzyl benzoate ointment is one of the best materials for the control of scabies. This acaricide in combination with di- butyl phthalate is an excellent clothing impregnant against chiggers and is more durable than many chemicals previously tried. Chiggers can be con- trolled in infested areas by applying i to 2 pounds of chlordane or toxa- phene or one-fourth pound of lindane per acre, employing dusts or sprays. Such mixtures are being constantly improved. Several substances effective against ticks have been discovered. The con- trol of livestock and wild animals upon which ticks feed reduces the number of these pests. Many of the ticks in in- fested ground areas can be destroyed by spraying with a 5 percent DDT emulsion. Certain ticks congregate near roads and trails and an insecticide should be applied especially on those Yearbook of Agriculture 1952 areas. Clearing of brush and close cut- ting of grass is a material aid in tick control. The best way to remove ticks that are attached to persons is to pull them out. Some ticks have long beaks and their removal may require a needle or knife. F. C. Bishopp has general direction of the research in the Bureau of En- tomology and Plant Quarantine on in- jurious and beneficial insects. In igog he began investigating ticks and var- ious insects as annoy ers of man and carriers of disease. In ig26 he was put in charge of the Bureau's division of in- sects affecting man and animals and in ig^.1 became assistant chief of the Bureau. Cornelius B. Philip is principal m.edical entomologist and assistant di- rector of the Rocky Mountain Labora- tory, United States Public Health Serv- ice, Hamilton, Mont., and a member of the National Defense Virus and Rickettsial Disease Commission. He has studied insect-borne diseases in many parts of the world, in part as a colonel in the S-anitary Corps of the Army duritig the Second World War, and is an authority on medical ento- mology. Pacific Coast tick. Carriers of Animal Diseases Gerard Dikmans, A. O. Foster C. D. Stein, L. T. Giltner Flies, ticks, and other arthropods spread and perpetuate many livestock diseases. Most are of comparatively minor concern as direct causes of in- jury or annoyance but, like the fever ticks of cattle, are important as reser- voirs and vectors of disease-causing or- ganisms. Some, like the tsetse flies in Africa, are of no consequence as pests, yet are a limiting factor in the produc- tion of livestock. The remarkable ways of insects and their allies in transmitting diseases are as varied and spectacular as the dis- eases themselves and the vectors that transmit them. Common house flies, however benign and unspectacular they may seem, often carry on their feet and mouth parts or in their bodies the contaminating germs of everyday skin and generalized infections and some- times even the dreaded bacilli of an- thrax or the fatal toxin of botulism. Other insects, mites, and bugs of vari- ous kinds transmit the infective stages of numerous parasites, final infection usually resulting from accidental swal- lowing of the infected vector by a sus- ceptible animal. Disease caused by tropical warble flies, which are next in importance to the cattle fever tick among the external parasites of live- stock in Latin America and analogous to the better known warble flies of Temperate Zones, is essentially insect- borne — the flies attach their eggs to captured mosquitoes (Psorophora) , which in turn transport the infection to cattle and other animals. The afore-mentioned examples are illustrative only of the ways in which insects carry diseases. With the notable exceptions of tick-borne fever of cattle and tsetse fly disease, they are scarcely typical of the principal arthropod- borne diseases of livestock. As might be suspected, the chief vectors are pre- dominantly bloodsucking species, and the diseases transmitted by them are essentially blood infections. Those vec- tors ordinarily spread and propagate disease in two ways. One, mechanical transmission, is the direct transfer (or its equivalent) of infective blood from diseased to healthy animals. The other, biological transmission, represents a specialized and complex relationship among vector, organism, and host, which is characterized by reproduction and structural change of the disease- causing organism within the body of the vector. Some biting flies function naturally in both ways. For a short period, probably not more than 2 hours, after feeding on the blood of a diseased animal, the dangerous organ- isms may be carried to healthy, sus- ceptible animals on which the fly may chance to feed. For a longer period thereafter, from a few days to several weeks, the fly is incapable of trans- mitting the infection. Then it may again become infective in consequence of a biological reconstitution of the or- ganism, culminating in the production of new infective stages in its salivary glands or other tissues. The arthropod-borne diseases of domestic animals are of two main kinds: Those caused by plant micro- organisms and those caused by animal micro-organisms. The former comprise bacteria, spirochetes (Borrelia) , Rick- ettsiae (Coxiella) , and viruses. The latter, in part, are pathogenic protozoa, including piroplasms (Babesia) , Thei- leriae, the trypanosomes, Leishmaniae, Leucocytozoa, and a species of Haemo- proteus. Animals that recover from dis- ease caused by some of these organisms may remain carriers, or apparently healthy animals that are dangerous seedbeds of infection, for long periods or for life. The bacterial diseases that are sometimes carried by arthropods are 161 1 62 anthrax, tularemia, swine erysipelas, and botulism (limber neck of birds). All are spread by other means, and the role of arthropods is accidental and mechanical. Anthrax, an acute disease caused by Bacillus anthracis, affects all classes of mammals, including man. Infections in livestock are generally acquired dur- ing grazing. Incidence is especially high during the fly season, and out- breaks in cattle have been ascribed to fly transmission. The vectors are the black horse fly and other horse flies, the stable fly, mosquitoes (Psorophora sayi and Aedes sylvestris) , and several nonbiting species, including the house fly and blow flies (Calliphora) . The ear tick and even ants also have been suspected. Tularemia, caused by Bacterium tularense, is primarily an infection of small wild animals, such as rabbits, squirrels, rats, mice, woodchucks, opos- sums, and grouse, but it can be trans- mitted to man, sheep, swine, dogs, and cats. The disease is transmitted com- monly by contact, sometimes by the ingestion of contaminated food and water, and occasionally by the bites of ticks, flies, lice, and bed bugs. Swine erysipelas is a prevalent in- fectious disease that biting flies may spread from pig to pig. We do not know the extent to which it is insect-borne, but such transmission has been demon- strated experimentally with the stable fly. The infectious organism, Erysipelo- thrix rhusiopathiae, is an invader of the blood, joint membranes, and other tissues. Death is uncommon, but con- dition and marketability are seriously affected. Botulism, or limber neck of chick- ens, is a fatal condition induced by the potent toxin of Clostridium botulinum. Ordinarily it follows ingestion of canned vegetables that have become contaminated with the organism. At times chickens become ill and die from ingesting blow fly maggots that have developed in contaminated meat. Spirochetes affect all animals and Yearbook of Agriculture 1952 get from host to host in many ways. They are minute, spiral organisms, which show some affinities to the pro- tozoa but are commonly regarded as bacteria. We mention two examples, both tick-borne. Borrelia theileri is responsible in South Africa for a benign, febrile dis- ease of cattle, sheep, and horses. It oc- curs in the blood stream and is trans- mitted biologically by one- and two- host ticks, Boophilus decoloratus and Rhipicephalus evertsi, and possibly by others. Borrelia anserina causes relapsing fever, or spirochetosis, in chickens, tur- keys, ducks, and geese. It occurs in Asia, Africa, South America, and else- where. It has been found in a few epi- zootics of turkeys in the United States. It causes a rapidly fatal blood infec- tion. The fowl tick and probably the chicken mite are vectors. Mosquitoes also are suspected. Rickettsiae, which are intermedi- ate between bacteria and viruses, cause many serious diseases of man as well as animals. One of these, Q fever, caused by Coxiella burnetii, is a disease of man, but cattle probably are the source of most human infections. The disease is recognized now in many parts of the world, including the United States. Ticks carry the organisms, and natural infections have been found in numer- ous species (Rocky Mountain wood tick, Pacific Coast tick, lone star tick, brown dog tick, and others) . A few other rickettsial infections of livestock, such as heart water fever of ruminants, occur outside the United States. Ticks, as far as we know, are the only vectors. Virus diseases are numerous, and many are mechanically transmitted, wholly or in part, by arthropods, par- ticularly biting flies. The ones transmitted mainly by these agents are equine infectious anemia, infectious ecjuine encephalomyelitis, African horse sickness, Japanese B en- cephalitis, louping ill of sheep, Nairobi Carriers of Animal Diseases disease of sheep, blue tongue of sheep, and rift valley fever. At times arthro- pods are presumably instrumental also in the transmission of fowl pox, swine pox, myxomatosis of rabbits, and in- fectious enteritis of cats. Equine infectious anemia, or swamp fever, occurs throughout the world. It destroys the working efficiency of thou- sands of horses, mules, and donkeys. Its natural spread is imperfectly under- stood, but the disease is readily pro- duced experimentally in susceptible animals by injection of infectious ma- terial, such as blood or other tissue fluids from infected animals. Under ex- perimental conditions, the virus has been transmitted by horse flies (Taba- nus septentrionalis and T. sulcifrons) , stable flies, mosquitoes (Psorophora columbiae) , and biting lice (Bovicola pilosa?) . The probability that direct mechanical transmission by biting flies commonly occurs is emphasized both by the summer intensity of the disease and the persistence of the virus in the blood of infected hosts. Infectious equine encephalomyelitis is caused by so-called Eastern, Western, and Venezuelan types of virus in North and South America and neighboring islands. Man is susceptible to all types. Natural reservoirs, particularly birds, are a probable source of infection to mosquitoes, which are the common vectors of the virus. The virus types have been recovered from or experi- mentally transmitted by a large number of arthropod species, among them the yellow-fever mosquito, salt-marsh mos- quitoes (Aedes), other mosquitoes (Culex, Culiseta, and Mansonia) , the bloodsucking conenose, the Rocky Mountain wood tick, the chicken mite, and chicken lice {Menopon pallidum and Eumenacanthus stramineus) . In the tick, the virus is present at all stages of development, but scientists do not know yet whether the virus passes through the egg. African horse sickness, an acute and virulent infection of equine species in central and southern Africa, is pre- sumed to be transmitted by arthropods, 163 mainly because of apparently con- vincing evidence that the disease does not pass directly from animal to ani- mal. Mosquitoes, horse flies, midges, and other insects have been suspected as vectors. Japanese B encephalitis, a fatal virus infection of man, is not a disease of domestic animals, but domestic ani- mals, especially the horse, are danger- ous reservoirs of the virus. It occurs in the Far East, where it is transmitted biologically by hibernating culicine mosquitoes (the southern house mos- quito and others) . Several virus infections of sheep, all of which affect other animals in some degree and most of which are trans- missible to man, are biologically and exclusively spread by arthropods. They are serious diseases in several parts of the world. Louping ill, transmitted by the castor bean tick {Ixodes ricinus) , is prevalent in the British Isles. Rift valley fever, transmitted by mosquitoes (Eretmopodites) , and Nairobi disease, carried by ticks (Rhipicephalus ap- pendiculatus) , occur in Kenya, British East Africa. Blue tongue, a more wide- spread disease, is carried by midges (Culicoides) . Fowl pox, a widespread and serious disease of chickens, turkeys, and pheas- ants, is often transmitted by the north- ern house mosquito and the yellow- fever mosquito, possibly, in some in- stances, by a biological mechanism. Ordinarily, however, it is passed di- rectly from bird to bird. Swine pox, another virus disease, does not appear to be directly infectious but is probably transmitted mainly by the sucking lice of hogs {Haematopinus suis) . Feline infectious enteritis is a common, fatal disease of kittens. The cat flea is pre- sumed to be an important vector, al- though the disease is more commonly spread by direct contact with diseased animals or contaminated quarters. Myxomatosis, which occurs in Cali- fornia, is a fast-spreading, fatal disease of rabbits, that is carried both by con- tact and by mosquitoes {Culex annuli- rostris and Aedes theoboldi?) . 164 Tick paralysis of cattle is actually caused, rather than carried, by ticks, yet it is a specific clinical entity char- acterized by complete paralysis in se- vere cases. Outbreaks in the Rocky Mountain States and British Columbia have resulted from infestation with the Rocky Mountain wood tick. Tick re- moval usually affords prompt relief. Other ticks (the American dog tick in the East and species of Ixodes in Australia and elsewhere) cause the condition. The symptoms are appar- ently due to the injection of toxin by female ticks at a particular stage in their sexual development. Arthropod-borne, protozoan dis- eases of livestock, in contrast to most of the diseases already discussed, are distinct from those affecting man. The only exceptions, known technically as leishmaniasis and Chagas' disease (Trypanosoma cruzi infection), are predominantly human afflictions, al- though they also occur naturally in dogs and some other animals. A disease of extreme importance in man and of even greater importance in livestock is sometimes referred to as African trypanosomiasis but the specific organ- isms that cause disease in animals are not infective to man. On the other hand, the species affecting man ( Tryp- anosoma gambiense and T. rhodesi- ense) , although experimentally trans- missible, are diagnosable in animals only as T. brucel, which is a most seri- ous disease-causing species. Poultry (chickens, turkeys, pigeons, ducks, and geese) are subject to two insect-borne diseases caused by related protozoan parasites, namely, Haemo- proteus columbae and Leucocytozoan smithi. Both occur in the United States. The former is a parasite of pigeons, although this or related species also occur in other birds. Commonly re- ferred to as pigeon malaria, the disease is carried by pigeon flies (Lynchia maura and Pseudolynchia canari- ensis) . The latter, possibly comprising more than one species, affects other classes of poultry and is transmitted by Yearbook of Agriculture 1952 black flies (Simulium occidentale, S. venustum, S. nigroparum, S. slos- sonae, and S. jenningsi) and perhaps by mosquitoes. Anaplasmosis, caused by Anaplasma marginale, an organism of indefinite classification, clinically resembles cattle fever and was not recognized as a wide- spread disease in the United States until fever ticks were eradicated from a large part of the cattle fever area. Because the disease is coextensive with cattle fever, vectors of the latter are presumed to be transmitters of ana- plasmosis. This incriminates the prin- cipal species of fever ticks (Boophilus annulatus, B. micro plus, and B. decolo- ratus) . However, the persistence of the disease, which is not contagious, in areas without fever ticks indicated clearly that there were other vectors. Much experimental work has revealed that other ticks (American dog tick, Rocky Mountain wood tick, Pacific Coast tick, brown dog tick, and possi- bly others) can transmit the disease, although we do not know their natural capacity to do so. Several species of horse flies and some mosquitoes have also been demonstrated to be potential mechanical vectors. Aegyptianellosis, caused by Aegypti- anella pullorum, another organism of uncertain zoological classification, af- fects chickens, ducks, and geese. It is transmitted by the fowl tick and occurs in Africa and in parts of Europe and Asia. Piroplasmosis, one of the most dev- astating groups of diseases of domestic animals, affects cattle, horses, sheep, swine, and dogs. The causative organisms, all bio- logically transmitted by ticks, are: In cattle, Babesia bigemina, B. argen- tina, and B. bovis; in horses, B. equi and B. caballi; in sheep, B. motasi and B. ovis; in swine, B. trautmanni and B. perroncitoi; and in dogs, B. canis and B. gibsoni. These are microscopic, sin- gle-celled parasites that enter and de- stroy the red blood cells. When the disease is acute, or fulminating, the parasites multiply rapidly and cause Carriers of Animal Diseases death in most cases. The symptoms are those associated with destruction of red blood cells, namely, fever, anemia, jaundice, thick bile, enlargement of liver and spleen, and emaciation. Ticks ingest the organisms during their en- gorgement on infected animals. Car- riers, rather than acutely ill animals, are commonly the chief source of tick infection. In ticks, the parasites mul- tiply and invade all tissues, including the salivary glands. The cattle tick, all stages of which live on one animal, transmits the parasites through its eggs to the next generation of larvae, or seed ticks, which carry the disease to a new host. Cattle tick fever, or bovine piro- plasmosis, is prevalent throughout the world. It causes incalculable losses. The history of the disease in the United States is a remarkable chapter of medi- cal science. First came the demonstra- tion in 1893 of tick transmission by Theobald Smith and F. L. Kilborne, of the Department of Agriculture — a discovery of immense benefit because it pointed the way to the solution of other disease problems. Then came the remarkable campaign, begun in 1906, to eradicate the cattle tick, and with it. cattle fever. The painstaking studies that laid the groundwork for success included researches into the habits and distribution of cattle ticks, determina- tions of their capacity to transmit cattle fever, establishment of the northern limits of the disease, the promulgation and enforcement of quarantines gov- erning the shipment of cattle, and the critical evaluation of arsenical dips against the cattle tick. When the pro- gram of tick eradication was finally de- vised, cattle fever was causing losses exceeding 40 million dollars annually in the South and Southwest. As of 1952, and for more than a decade, the dis- ease and fever ticks have been all but eradicated from the country. The fight against this disease is the most exten- sive campaign ever waged against par- asitic disease in livestock. The total cost amounted to scarcely more than the 165 toll that was formerly taken by the dis- ease in a single year. In the areas of their respective dis- tributions, many ticks transmit the three afore-mentioned species of Ba- besia among cattle. Chief among them are Boophilus annulatus, B. micro plus, B. decoloratus, Rhipice phalus appen- diculatus, R. evertsi, R. bursa, Ixodes ricinus, I. persulcatus, and Haemaphy- salis cinnabarina. Tick fever of horses does not occur in the United States but is a common and serious disease throughout tropical and temperate zones. The two species of Babesia utilize tick vectors belonging principally to the genera Rhipicepha- lus, Hyalomma, and Dermacentor. The story of the piroplasmoses af- fecting other animals is similar, al- though Babesia infections of sheep, swine, and dogs are not of comparable economic importance to those affect- ing cattle and horses. Canine piroplas- mosis occurs in the United States. It is the only Babesia infection of domestic animals that has been found in this country since 1939. The brown dog tick and probably other ticks (Derma- centor reticulatus, D. andersoni, Hae- maphysalis leachi, and H. bispinosa) are vectors of the species of Babesia affecting dogs. Theileriasis principally affects cat- tle in Africa, but it occurs in sheep, goats, and dogs and on other conti- nents. It is similar to piroplasmosis but does not commonly cause jaundice, hemoglobinuria (hemoglobin in the urine), or anemia. Theileriae do not multiply in and destroy red cells but enter them only after multiplication in the so-called endothelial tissues. The causative organisms are: In cattle, Theileria parva, T. annulata, and T. mutans; in sheep and goats, T. ovis and T. recondita; and in dogs, Ran- gelia vitali. T. mutans is an essentially harmless species. The species of Thei- leria are transmitted by two- and three- host ticks in contrast to the usual trans- mission of Babesia of cattle fever by one-host ticks. Associated with this dif- i66 ference in vectors are differences in the biology of transmission. Theileriae de- velop in successive stages of ticks and are not transmitted hereditarily, or through the eggs, to successive genera- tions as in the case of Babesia. Ticks usually acquire the organisms during their larval stages and carry them to new hosts during succeeding nymphal stages. East coast fever,, caused by Thei- leria parva, is a fatal disease of cattle in South Africa. The chief vectors are species of Rhipicephalus (R. appendic- ulatus, R. capensis, R. evertsi, and R. simus) . A milder form of bovine thei- leriasis, caused by T. annulata, is trans- mitted by Hyalomma mauritanicum in North Africa and by H. dromedarii asiaticum in Central Asia. In sheep, goats, and dogs, theileri- asis, although a serious infection, is not of comparable importance to the dis- ease in cattle. Of interest is the fact that a soft, or argasid, tick (Ornitho- dorus lahorensis) is presumed on ex- perimental grounds to be a vector of the disease among sheep and goats. All other ticks that transmit protozoan in- fections are hard, or ixodid, ticks. Trypanosomiasis is a group name for several related diseases, each of which is caused by a specific trypano- some. It includes some of the worst illnesses of domestic animals and man. It is the only disease that by itself has denied vast areas of land to all domes- tic animals other than poultry. The areas of complete denial are all in Africa. One-fourth of Africa is con- trolled by tsetse flies (Glossina) , which are principal vectors of trypanosomes. The disease is, however, a major live- stock scourge in every continent except Australia. Moreover, the exclusive oc- currence of tsetses in Africa makes it evident that other vectors and mecha- nisms are responsible for the spread of the disease outside of Africa and, in- deed, that they are operative within the tsetse fly area. Nine species of trypanosomes pro- Yearbook of Agriculture 1952 duce disease of livestock. Four {Tryp- anosoma congolense, T. brucei, T. simiac, and T. unijorme) are found only in the tsetse fly areas. Two others (T. vivax and T. theileri) exist therein but are also established in other areas, where they are mechanically trans- mitted by horse flies (Tabanus) and other biting flies, exclusive of tsetses. These are .the half-dozen species re- sponsible for nagana, the African animal trypanosomiasis. The three re- maining species (T. evansi, T. equin- ium, and T. equiperdum) cause severe diseases in horses (surra, mal de ca- deras, and dourine). Mal de caderas occurs in South America. Surra and dourine occur worldwide. Both have occurred in the United States, but con- stant vigilance and prompt eradicative measures have kept them from becom- ing established. Surra and dourine also employ particularly interesting meth- ods of transmission. The former is usu- ally transmitted mechanically by horse flies and other biting insects. However, it is probably identical with murrina, an affliction of horses in Panama that has heretofore been ascribed to Tryp- anosoma hippicum rather than T. evansi. This infection (murrina) in- criminates the vampire bat (Desmodus rotundus murinus) , this being the only instance of mammalian transmission of a protozoan disease. Dourine, on the other hand, is ordinarily transmitted during coitus, and is therefore fre- quently referred to as horse syphilis. It is consequently confined to horses and donkeys, occurring chiefly among breeding stock. Eight (all but T. equinium) of the nine disease-causing trypanosomes of livestock are encountered in Africa; namely, the four afore-mentioned, in- digenous species, two of cosmopolitan distribution that affect horses, and two extra-African species that also cause nagana. The latter, T. vivax and T. theileri, have outgrown their depend- ence on tsetse flies. In Africa, however, T. vivax is associated closely with Glos- sina and is as dependent as T. congo- lense. T. vivax is a cause of cattle tryp- Carriers of Animal Diseases anosomiasis in South and Central America, where biting flies presumably transmit it. T. theileri, similarly spread, is of cosmopolitan distribution in cattle but only occasionally causes severe dis- ease. It occurs in North America, where it is apparently noninjurious. Nagana, or African trypanosomiasis of animals, caused by some six species, already named, affects all mammals. Economically it is the most important protozoan disease of livestock. In cattle the species responsible for the disease, in order of importance, are T. congo- lense, T. vivax, and T. uniforme. The first two account for most of the cases. T. congolense occurs throughout the tsetse fly areas and is the most virulent trypanosome affecting animals. The organisms are found only in the blood. In the case of T. vivax, organisms are not readily found in the blood stream but may generally be demonstrated in a gland smear. A study of cattle losses in Nigeria re- vealed that 30 of every 100 deaths were due to nagana. In horses, the principal species caus- ing nagana are T. brucei and T. con- golense. T. vivax sometimes infects horses but rarely causes symptoms. T. brucei, like T. vivax, is more readily found in glandular tissue than in blood. Horses infected with either T. brucei or T. congolense almost always die un- less they are adequately treated. In sheep and goats, nagana is caused by the same species that cause it in horses. T. congolense infections, in con- trast to those associated with other trypanosomes, is characterized by a sameness of grave disease in cattle, horses, sheep, and goats. In swine, the chief pathogen is T. simiae, which has been called the light- ning destroyer of pigs. Swine are sus- ceptible to infection with T. brucei and T. congolense, but these species rarely produce symptoms. T. simiae infection of pigs is extremely acute. Animals in apparently good health are taken ill overnight and die the next day. Camels and dogs are notably sus- ceptible to trypanosomiases; in them, 167 T. evansi, the cause of surra in horses, produces the same disease. Some au- thorities regard surra as a predomi- nantly camel disease. Camels and dogs are also victims of severe and fatal dis- ease caused by T. congolense and T. brucei. Camels, but not dogs, are sub- ject to the same hyperacute disease caused by T. simiae that occurs in pigs. This species, as suggested by its name, also causes fatal illness in monkeys. The transmission of nagana is both biological and mechanical. Tsetse flies are the only biological vectors, but they and other biting flies transmit the in- fections mechanically. The cyclical de- velopment of trypanosomes in tsetse flies is exceedingly complicated, since it varies with different species of tryp- anosomes and even with the same species in different tsetse fly species. In general, T. congolense, for example, initiates its development in the ali- mentary tract of the fly. Then elon- gated organisms move to the hypo- pharynx, where attached intermedi- ate forms and free trypanosomes suc- cessively develop. In the case of T. vivax, all development takes place in the mouth parts of the fly. Usually from 2 to 4 weeks are required for multipli- cation and metamorphosis in the fly. All forms of nagana are also spread by the interrupted feeding of biting flies, including tsetses, and this may be the normal method of transmission when outbreaks occur. Throughout the tsetse fly region of equatorial Africa, there exist numerous horse flies and other biting flies, notably Chrysops and Haematopota. With reference to these genera, probably all species act as me- chanical vectors. Small flies are poorer vectors than large ones. Extensive studies of nagana seem to warrant the general deduction that mechanical vectors have a large part in the spread of African trypanosomiases but that cyclical development in tsetse flies is essential to the perpetuation of the diseases. Eradication of tsetse flies from any area has always eliminated nagana completely. Tsetse flies, found only in Africa, i68 owe their importance entirely to the fact that they are vectors of trypano- somes. The principal species, about 20, vary considerably in size, abundance, distribution, habits, susceptibility to ad- verse environment, and economic im- portance. They are about the size of house flies. Low mean temperatures generally are unfavorable to them. They cannot endure dry heat or tem- perature above 1060 F., even in areas of high humidity. Vegetation must be ample for the support of reservoir and other host animals, since blood is the sole food of tsetses, but treeless grass- land, deciduous bushland, and wood- lands with a thick underbrush are unfavorable. Rainfall or fresh-water streams must be abundant where the flies and their mammalian hosts reach maximum populations. Some tsetses in East Africa, however, are well adapted to comparatively arid districts. Because of this delicate environmental adjust- ment, tree clearance, burning of grass and brush, establishment of zones of vegetation-clearance, and like meas- ures have been useful in controlling the flies. Seasonal changes and other natural factors cause expansion and contraction of fly belts. Unlike mosquitoes, which are the only insects of greater medical impor- tance than the tsetses, males as well as females are bloodsuckers. They feed mainly on large game and domestic animals. Native game, however, are comparatively resistant but serve as reservoirs of trypanosomes. Probably no trypanosome is pathogenic to its normal host. In any event, notwith- standing a complete dependence on large mammals, neither tsetse flies nor trypanosomes are especially host-spe- cific. Some authorities also believe that any species of tsetse fly probably can transmit any species of pathogenic trypanosome with which it comes in common contact. In addition to peculiar feeding habits and the comparative immunity of native game reservoirs, the method of reproduction of tsetse flies increases the difficulties of control. Females do Yearbook of Agriculture 1952 not lay eggs like most insects. They give birth to live young and deposit the larvae in haunts that are peculiar to the individual species. Larvicides there- fore are of no avail, and breeding places cannot be eradicated. Adult tsetse flies probably do not live longer than 8 or 10 months. Their cycle of development is comparatively simple and direct. Females produce their first larvae about 3 or 4 weeks after mating. One large larva is pro- duced at a time, but a new larva begins its development as soon as one is born. Successive larvae are produced every 9 to 14 days. The larvae pupate promptly in warm, loose soil of pro- tected, shady areas. Pupation lasts 2 weeks to 4 months, and the adults rarely emerge unless the temperature is above 700 and below 870. The control of nagana — tsetse fly disease* or African animal trypanoso- miasis— is much more than an ento- mological or veterinary problem. It is acutely beset with economic and so- ciological obstacles and with the basic agricultural problems of land usage and soil erosion. But such considera- tions do not lower the value of con- tinued effort to achieve better control through therapy, prophylaxis, and im- munization directed against the tryp- anosomes, through eradicative and limiting measures directed against tsetse flies, and through modifications of the environment to make it unfa- vorable for the continuance of trypano- somal diseases. Increased utilization of disease-resistant breeds of livestock, such as the West African Shorthorn cattle, for example, may also be a measure of great potential value. The outlook for better control of insect-borne diseases is bright. The dis- coveries of new insecticides and the devising of effective formulations and methods of application have in large measure provided the means for a con- certed attack upon the insect vectors. New chemicals for treatment of these diseases and methods of immunization against them are also available. Finally, an ever-increasing knowledge of all aspects of insect-borne diseases has pro- vided the foundations essential to the success of applied control measures. Seemingly the major limiting factor in the achievement of unprecedented, constructive victories is the modest eco- nomic burden that would be tempo- rarily imposed. Gerard Dikmans, a graduate veter- inarian from Michigan State College and holder of a master's degree from Minnesota and a doctor's degree from Georgetown University, has been a parasitologist in the Bureau of Animal Industry since ig26. For several years he has been in charge of investigations of ruminant parasites. A. O. Foster, a parasitologist in the Bureau of Animal Industry, is in charge of anthelmintic investigations. He was trained at the Johns Hopkins Uni- versity School of Hygiene and Public Health and served for 5 years on the staff of the Gorgas Memorial Labora- tory of Tropical and Preventive Medi- cine in Panama. C. D. Stein is a veterinarian in the Bureau of Animal Industry. For many years he has contributed important re- searches on anthrax, equine infectious anemia, and other diseases of large and small animals. L. T. Giltner, a veterinarian, is pathology consultant in the Bureau of Animal Industry. He was assistant chief of the pathological division for many years and has pursued or directed investigations on nearly all aspects of animal diseases. Camponotus castaneus, a common ant. Insects and Helminths Everett E. Wehr, John T. Lucker Many species of the helminths, or parasitic worms, of livestock and poul- try can pass through certain of their early stages only within the body of an insect. These species are transmitted, in the true sense, by insects. Beetle mites or grass mites similarly transmit others. One species is transmitted by a tick as well as by insects. These par- ticular species of worms are obligatory parasites of insects, or their allies, just as truly as they are obligatory parasites of farm animals or birds. For their con- tinued existence and propagation, for their survival as species, they depend equally upon insect and upon avian or mammalian hosts. The life cycle of a helminth of one of these species, like the life cycle of all other parasitic helminths, is initiated by the eggs or microscopic larvae pro- duced by the mature female or her- maphroditic individual. But depending on its specific identity, its eggs or larvae are infectious only to an insect or a mite or perhaps a tick. If ingested by a suitable insect, for example, each egg or larva gives rise to a more advanced developmental stage of the parasite, which takes up its abode in some part of the insect's body. There, however, the development of the worm stops at a stage far short of reproductive ma- turity. Unless this arrested-develop- mental stage gains access to the body of a suitable vertebrate animal, the life cycle of the parasite cannot be com- pleted. Obviously, therefore, any step that can be taken to destroy infected insects will aid in preventing the in- fection of livestock and poultry with worms that have this type of life cycle. The world-wide extermination of the insect vectors, were this possible, would 169 170 result automatically in the extermina- tion of a goodly proportion of the spe- cies of worms that now afflict man and his domestic animals and many others that live in wild animals and birds. Worms that are obliged to undergo development in two or more hosts are called heteroxenous parasites. The hosts in which they can reach repro- ductive maturity are called final or definitive hosts. Hosts in which their larval stages must develop before the parasites can take up life in a final host are called intermediate hosts. Some instances of the transmission of parasitic worms by insects were dis- covered before it was learned that in- sects are also vectors of some of the most devastating protozoal and infec- tious diseases known to medical and veterinary science. Some years before the transmission of malaria or yellow fever by mosquitoes or of southern cattle fever by ticks was discovered, it had been demonstrated that the larvae of a nematode worm, Wuchereria bancrofti, which causes human filiari- asis, could be sucked up by a feeding mosquito and would undergo develop- mental transformation in its body. Very few kinds of parasitic worms can multiply — that is, reproduce through successive generations — en- tirely within the body of the animal in which they mature. The eggs or larvae of nearly all species must leave the host's body to perpetuate the parasite. The eggs or larvae of worms that live in the digestive tract or in an organ or system (such as the liver or respiratory system) that communicates with the digestive tract or in the uri- nary system ordinarily pass from the host with its feces or urine. The pres- ence of the progeny of the worms in those substances, which in natural cir- cumstances are deposited by livestock and poultry on the ground, leads to their ingestion by various kinds of in- vertebrate and vertebrate animals. Although the insects and their close relatives are perhaps the most ubiq- uitous of the invertebrates and are of outstanding importance as vectors of Yearbook of Agriculture 1952 parasitic worms, the eggs and larvae of some of the heteroxenous worms of farm animals and birds are not infec- tious to them. Other arthropods, snails, slugs, earthworms, or other animals serve as intermediate hosts in those instances. Some of the insect vectors of worms that produce eggs, which leave the definitive host's body in the manner described, habitually feed upon the excrement of higher animals. In the process they ingest the worm eggs and thus become infected. Others are not susceptible as adults to infection or at least do not become infected. They habitually deposit their eggs in excre- ment or in materials contaminated by it. The larvae that hatch from their eggs ingest the worm eggs and are sus- ceptible to infection by them. In other instances the insects involved cannot be classified as coprophagous — dung eating — nor do they customarily or preferentially breed in manure. But natural forces continually scatter worm eggs into their habitats. They ingest quite incidentally the worm eggs that contaminate their normal food supply. Some of the vectors are themselves ectoparasites of farm animals. They normally feed upon the cellular debris or detritus on the skin of their hosts. They take in worm eggs when the skin is contaminated with fecal matter or crushed parts of worms. Some of the heteroxenous worms live in situations, such as the circulatory system or subcutaneous tissues, that have no connection with the external body openings of the host. They include several species of viviparous round- worms, or Nematoda, which eject the larvae they produce into their host's blood, or lymph, or dermal skin layers. There the larvae remain, ultimately to perish unless they are ingested by a biting or bloodsucking insect. Not only do the habits, habitats, and structural modifications of the various insects and certain of their close relatives lead these arthropods to ingest the microscopic progeny of parasitic worms of many kinds. The insects like- Insects and Helminths wise afford an almost ideal means of transport of the infectious stages of the worms back to the definitive hosts, live- stock or poultry. Many of them form part of the normal diet of birds. In grazing, swine, sheep, cattle, and horses cannot avoid taking in beetles, mites, and similar insects along with the herb- age they consume. A dog or cat suffer- ing from infestation by fleas or lice, bites and licks at the noxious creatures and swallows some of them. A female mosquito must have a blood meal be- fore it can lay fertile eggs and a further blood meal between every two batches of eggs it lays. If, between meals, in- fectious worm larvae have developed in its body, it injects these into the blood of the next animal it bites. Many adaptations exist among in- sects, parasitic worms, and the defin- itive hosts of the worms. Farmers can take advantage of some of these adaptations to protect livestock and poultry against the inroads of insect- borne worm infections. Insects frequent fecal and re- lated waste materials because those substances are essential for their growth and development or because they con- tain something that attracts insects — a bright or moving object, for example. Segments of tapeworms, because of their bright color or ability to move, readily attract insects and mites and often are eaten by them. Insect-borne worms of livestock and poultry include representatives of all four of the major groups of helminths: Roundworms (Nematoda) , tapeworms (Cestoda), thorny-headed worms (Acanthocephala) , and flukes (Tre- matoda). Those that are transmitted by habitual or accidental dung feeders inhabit the digestive tract of the defin- itive host or organs that communicate with this tract; as has been noted, the eggs or larvae of worms living in these situations occur in the host's feces. Various species of tumble bugs and dung beetles are intermediate hosts for worms occurring in swine, sheep, cat- tle, poultry, cats, and dogs. Two stom- 171 ach worms, Ascarops strongylina and Physocephalus sexalatus, of swine, and the gullet worm, Gongylonema pul- chrum, which occurs in swine, sheep and cattle, utilize such coprophagous beetles as Copris, Aphodius, Passa- lurus, Onthophagus, Scarabaeus, Gym- nopleurus, Ataenius, Canthon, Phan- aeus, and Geotrupes as intermediate hosts. The German cockroach also serves as an intermediate host of the gullet worm. The eggs ingested by the insects contain well-developed em- bryos at the time of oviposition. On hatching in the insect's gut, the larvae first enter the abdominal cavity of the intermediate host and finally come to rest in the walls of the Malpighian tu- bules or musculature, where they be- come encysted. Completely formed cysts are usually found free in the ab- dominal part of the body cavity. The larvae become infective in the inter- mediate host in a month or so. The larvae of the esophageal worm, Spirocerca lupi, of the dog develop to the infective stage in the beetle, Scara- baeus sacer, and other beetles. The in- fective larvae become encysted in these insects, chiefly on the tracheal tubes. If such beetles are swallowed by an unsuitable host, such as a frog, snake, bird, or a small mammal, the larval worms become encysted again in the esophagus, mesentery, or other organs of these animals. This phenomenon is also known to occur in the case of the swine stomach worm, Physocephalus sexalatus, the larvae of which have been found naturally reencysted in the wall of the digestive tract of such birds as the loggerhead shrike, screech owl, and red-tailed hawk in southern Geor- gia and northern Florida. Reencyst- ment of the larvae was found in experi- ments to occur in many different ani- mals, including birds, mammals, and reptiles, to which beetles containing in- fective larvae were fed. One of the commonest species of tapeworms, Hymenolepis carioca, found in the domestic fowl, is trans- mitted by beetles {Aphodius, Choerid- ium, Hister, and maybe Anisotarsus) . 172 Another species of tapeworm, Hy- menolepis cantaniana, found in chick- ens, turkeys, and quail of the Eastern States, develops in the beetles Ataenius and Choeridium. Its development in its intermediate host is unusual. The larva elongates to form a somewhat branched myceliumlike structure; buds along the branches develop into the cysticercoids, or small larval forms, which contain the tapeworm heads. Tapeworms belonging to the genera J oyeuxiella and Diplopylidium, which are closely related to Dipylidium, occur in cats and apparently develop in dung beetles and related insects. It takes about 3 weeks to 2 months, depending on temperature, for the cysticercoids to develop within the insect host. Com- pletely developed cysts are found in its body cavity. The tapeworm Metrolias- thes lucida, commonly found in the small intestine of the domestic and wild turkey, is reported to have the grass- hoppers Melanoplus sp., Chorthippus longicornis, and Paroxya clavuliger as intermediate hosts. Guinea fowls are also susceptible to infection with this tapeworm. Dermestid beetles, darkling beetles, fungus beetles, and other groups of beetles and several species of grass- hoppers have been infected experi- mentally, or found to be infected nat- urally with the larvae of the gizzard worm of poultry, Cheilospirura hamu- losa. Small numbers of these worms in the gizzard do not produce any serious re- sults. In heavy infections, the lining of the gizzard may show ulcerations, which may also involve the muscula- ture. Soft nodules enclosing the para- sites are often found in the muscular portions, especially in the thinner parts of the gizzard. In the intermediate host, the infective larva of the gizzard worm is found encysted in the musculature of the body wall, where it is found to be tightly coiled. The infective stage is reached in about 19 days. Darkling beetles {Alphitobius, Gon- ocephalum, and Ammophorus), the ring-legged earwig, and the hide beetle Yearbook of Agriculture 1952 have been reported as being infected with the third-stage larvae of Subu- lura brumpti, the cecal worm of poul- try. The final host becomes infected through the ingestion of the infected intermediate host and the larvae pass to the cecum, the blind gut. Many species of darkling beetles and ground beetles have been incriminated as in- termediate hosts of Raillietina cesti- cillus, the broad-headed tapeworm of poultry and of another poultry tape- worm, Choanotaenia infundibulam. The latter also develops in the red- legged grasshopper and in the house fly. The chief effect of this tapeworm, even in heavy infestations, is to retard the growth rate of its host. The Surinam roach, and possibly other species of cockroaches, is an in- termediate host for three nematodes of poultry, namely, the eyeworms, Oxy- spirura mansoni and O. parvovum, and the proventricular worm, Seurocyrnea colini, of the turkey and bobwhite quail. Infections with the eyeworms result in a marked irritation, which interferes seriously with vision. It often is accompanied by continual winking as if to dislodge a foreign body. The nictitating membrane of the eye be- comes inflamed and appears as a puffy elevation. Heavy infestations may cause blindness. The German cock- roach has been shown in experiments to serve as an intermediate host for Seurocyrnea colini. This cockroach, the red-legged grasshopper, and the dif- ferential grasshopper have been re- ported to be suitable intermediate hosts for the globular stomach worm, Tet- rameres americana, of chickens, bob- white quail, and turkeys. After the eggs are ingested by the intermediate hosts, the larvae of this stomach worm pass into the body cavity and become quite active for the first 10 days after in- fection. They then penetrate the mus- cles and become loosely encysted. In about 42 days, or possibly sooner, the infective larvae have completed their development. The vitality of grass- hoppers is greatly reduced by infections with this parasite. Some die and some Insects and Helminths become inactive and an easy prey to birds. The infection is transmitted to the bird through the ingestion of the infected intermediate host. Serious in- fections with these species of stomach worms have not been noted in do- mestic birds in the United States. Several species of ants, of the genera Tetramorium and Pheidole, are natu- rally infected with cysticercoids of two closely related poultry tapeworms, Raillietina tetragona and R. echino- bothrida. Experimental attempts to in- fect ants of these and other genera by feeding to them eggs of these tape- worms resulted in failure, but naturally infected ants were fed to chickens and infections resulted. Quail are said to be seriously parasit- ized by R. tetragona and death losses have been attributed to this tapeworm, but its pathogenicity has not been ex- perimentally verified. However, R. echinobothrida is defi- nitely known to be one of the most in- jurious tapeworm parasites of poultry. It causes the formation of tuberclelike nodules on the intestinal wall, which closely resemble the nodules of tuber- culosis. The absence of the nodules in the liver, spleen, and other internal organs and the presence of tapeworms in the small intestine warrant the diag- nosis of this infection and exclude tuberculosis. Biting lice are minor vectors of worm parasites. The only known instance in- volves the dog biting louse, which is reported to be an intermediate host of the double-pored tapeworm, Dipy- lidium caninum, of the dog and cat. Because this louse normally feeds on particles of dried skin of its host, it can hardly be classified as a coprophagous insect. It is presumed that the skin of the dog and cat, especially in the peri- anal region, becomes contaminated with eggs of the tapeworm, which are more or less incidentally eaten by the louse. Beetle mites, also known as oriba- tid or galumnid mites, serve as vectors of the broad tapeworm, Moniezia ex- pansa, of cattle, sheep, and goats. 970134°— 52 13 After being expelled with the host's feces, the tapeworm eggs must become fairly dry and well anchored before the mites can ingest them. The mites usually do not eat the entire egg. They make a hole in its shell and ingest its contents. This tapeworm adversely affects the growth of infected lambs. Several investigators have reported that M. expansa produced scouring in range lambs. In experiments, how- ever, infected lambs have not shown scouring. The life history of M. expansa, which had defied investigators for many years, was solved in 1937. Since then it has been shown that oribatid mites also are the vectors of several other anoplocephalid tapeworms of domestic animals. They transmit Cit- totaenia ctenoides and C. denticulata of rabbits; Anoplocephala perfoliate, A. magna, and Par anoplocephala mamillana of horses; and Moniezia benedini and Thysaniezia giardi of ruminants. Beetle mites are most apt to be abundant in moist, shady places. They are found in pastures both winter and summer, but they increase markedly in numbers with the new growth in spring. The mites are generally dis- tributed throughout the world. The number of groups of insects in which infection takes place in the lar- val or immature stage is small, com- pared to those that acquire the infec- tion in the adult stage. In some instances the mouth parts of the adult insect are of the sucking type so that solid materials cannot be ingested, or its feeding habits are such that it does not come in contact with materials containing the worm eggs and larvae. The larval insects, however, hatch out in such material, and their mouth parts are adapted for its ingestion. The house fly and the stable fly breed abundantly in horse manure. Their maggots migrate extensively through- out manure piles and feed promiscu- ously on the materials found therein. The maggots of the house fly are the i74 intermediate hosts for two nematodes (roundworms) commonly found in the stomachs of horses — Habronema mus- cae and Drashia megastoma. Those of the stable fly are suitable hosts for the development of a third horse-stomach worm, Habronema majus. The nematode larvae undergo sev- eral molts within the body of the fly maggot and reach the infective stage about the time the fly hatches. The adult fly harbors the infective larvae free in the body cavity, but some of the larvae may migrate into the mouth parts of the fly. The horse presumably becomes infected when the flies in feeding deposit worm larvae on its lips or by ingesting flies which get into its food or water. The thorny-headed worm is a rather common parasite of swine, particularly in the South. Characteristic nodules form at the sites of attachment of the worms to the wall of the small intes- tine. Sometimes they change their places of attachment, thus leaving the previous sites of attachment to become ulcerative. Perforation of the intestinal wall occasionally may occur. The para- site makes the intestines worthless for sausage casings. White grubs, the lar- vae of May and June beetles, serve as its intermediate hosts. White grubs are found abundantly just below the sur- face of the soil, particularly in grass- lands, and are relished by hogs, which uncover them as they root up the ground. The eggs of the thorny-headed worm, which are expelled in the feces of the swine, hatch when they are in- gested by the white grubs. In the grubs, the larvae hatching from the eggs are released in the midgut; then they mi- grate to the body cavity and there develop into the infective stage within 2 to 3 months in summer. Since the infective larvae persist when the pupal and beetle stages of the insect develop, pigs become infected by ingesting in- fected grubs, pupae, or adults. The larvae of the dog and cat fleas are vectors of the double-pored tape- worm, Dipylidium caninum. Because Yearbook of Agriculture 1952 the adult flea has sucking mouth parts, infection in this stage is impossible. Flea larvae ingest the eggs of the tape- worm. In the larva, the tapeworm grows but slightly. It grows more in the pupal stage and transforms into the infective stage in the adult flea. The White grub. cysticercoid lies free in the body cavity of the flea. The cat or dog becomes infected by ingesting fleas or lice. More than one kind of intermedi- ate host is required in the development of some of the heteroxenous worms. One of them may be an insect — as in the case of the oviduct fluke of poultry, Prosthogonimus macrorchis, which uti- lizes a snail, Amnicola limosa porata, as its first intermediate host and drag- onflies as its second intermediate hosts. Species of several genera of dragonflies, Leucorrhinia, Tetragoneuria, Epicor- dulia, and Mesothemis, may serve in the capacity of secondary intermediate hosts for this trematode. In the United States, the oviduct fluke is found naturally in ducks, Can- ada geese, and chickens, chiefly in the Great Lakes region. Here the snail that is the intermediate host is found in abundance on the under sides of boards and sticks and may be found traveling along the lake bottom in water i to 2 feet deep. The snail becomes infected by ingest- ing the eggs of the fluke. After going through several stages of development in the snail, the young flukes (cer- cariae) escape from its body and swim freely about in the water. The free- swimming organisms are drawn into the anal openings of aquatic naiads, or immature dragonflies, with the water that is alternately taken in and Insects and Helminths forced out by the organs of respiration located at the posterior end of the ali- mentary canal. After entrance into the body of the naiad, the young flukes encyst in the muscles and in most in- stances are found in the ventral por- tions of the posterior part of the body. Infection of the secondary intermedi- ate host usually takes place during the late spring and early summer, so that the young flukes sometimes remain in the insect host for i or 2 years before they are ingested by the definitive host. Infection of the bird host occurs usually at the end of May or beginning of June, when the dragonfly naiads are transforming into adults. Infection may also occur by the ingestion of in- fected mature dragonflies, which can be easily captured by birds in the early morning. The immature worms pass posteriorly to the diverticulum of the cloaca or to the cloaca itself, where they develop to maturity. Some of the worms develop to maturity in the ovi- duct and have been found in eggs laid by infected hens. The presence of this fluke in laying hens may result in a sharp drop in egg production, the lay- ing of soft-shelled eggs, and, in ad- vanced, cases, peritonitis. Prosthogonimus macr orchis is prob- ably the most important fluke parasite of poultry in the United States. How- ever, it is localized around certain sec- tions of the Great Lakes region. That is cause for not too great alarm, be- cause the sections are not important poultry centers. Many adult insects depend on blood for food. Among those that have been reported to be vectors for worms of livestock are mosquitoes, midges, fleas, sucking lice, and ticks. They in- gest worm larvae as they feed on the blood or lymph of infested animals. All the worms they transmit are round- worms of the group Filarioidea. Mosquitoes of the genera Anopheles, Aedes, and Culex, the dog flea, and the cat flea are suitable intermediate hosts of the dog and cat heartworm. This large worm, 5 to 12 inches long, oc- 175 curs mainly in the right ventricle of the heart and the pulmonary artery of the dog, cat, fox, and wolf. Many studies indicate that it occurs princi- pally in the Southern States. It may occur throughout most of the United States, although not endemically. Hunting dogs are more seriously af- fected than other breeds. The infected animal tires easily, gasps for breath, and may collapse. Severe complica- tions, such as inflammation of the kid- ney and urinary bladder, may arise. In severe cases, the animal becomes poor, and the hair and skin are dry. Abnormal heart sounds are infre- quently noted, but moist rales are oc- casionally present. About 24 to 36 hours after the mos- quito or flea has sucked blood of an infected animal, the larvae or micro- filariae may be found within the tissue cells of the Malpighian tubules. There they develop to the infective stage within 5 to 1 o days, when they migrate to the mouth parts of the intermediate hosts and are ready to be transferred to a final host during the act of biting. Mosquitoes of the genera Aedes and Anopheles are vectors for Dirofilaria repens, a rather small worm occurring in the subcutaneous tissue of dogs in southern Europe, Asia, and South America. The worms may cause pru- ritis without skin lesions. The dog sucking louse, the dog flea, and the brown dog tick have been re- ported as vectors of Dipetalonema re- conditum, originally reported from the perirenal tissue of the dog in Europe. The worm also occurs in other organs and tissues, including the vascular sys- tem, lungs, and liver. The brown dog tick also transmits D. grassi, which occurs in the subcutaneous tissue and body cavity of the dog in Italy. Twelve days after microfilariae of Dirofilaria scapiceps, which lives under the skin in the loins and in the sub- cutaneous tissues of the fore and hind legs of wild rabbits in the United States, had been ingested by Aedes mosquitoes, infective larvae were seen actively moving in the proboscis of the 176 insects. Microfilariae also were ob- served in the gut contents of an uniden- tified engorged tick. Attempts to infect rabbits experimentally with this round- worm by allowing infected mosquitoes to feed on them have failed. The rabbit tick may also be a suitable intermediate host of this worm, although it has not been incriminated. Species of biting midges, or sand flies, and black flies have been incrimi- nated as intermediate hosts of species of the genus Onchocerca. O. reticulata occurs in various countries in the large tendon supporting the neck of the horse and mule and has been reported as a possible causative agent of poll evil and fistulous withers. This worm sup- posedly is transmitted by Culicoides nubeculosus. Simulium ornatum is the vector of Onchocerca gutterosa, which occurs in the neck tendon and other parts of the body of cattle. Onchocerca gibsoni, which lives in the subcutane- ous connective tissue of cattle, often giving rise on the brisket and the ex- ternal surfaces of the hind limbs to nodules, in which the worms lie coiled up, is reported to develop in Culicoides pungens and also in black flies. The microfilariae, infrequently found in the nodules or worm nests, are more often found in the walls of the blood vessels and along the lymph spaces. Infected animals show no symptoms except the nodular swellings under the skin, but their carcasses are condemned as un- suitable for sale on most markets. The stable fly is a reported vector for Setaria cervi, which occurs free in the body cavity of cattle and various species of antelope and deer. This worm has been found in the eyes of horses and the udder of a cow. Other reports indicate that in Asia this parasite is transmitted by three species of mosquitoes (Anopheles hyr- cans sinensis, Armigeres obturbans, and Aedes togoi) . The last is also a vector for S. equina of horses. Larvae of both species are said to invade the central nervous system of horses, caus- ing lumbar paralysis. Skin lesions due to the presence in Yearbook of Agriculture 1952 the lesions of both adults and micro- filariae of Stephano filar ia stilesi, S. de- doesi, S. kaeli, and S. assamensis have been reported from the abdomen and legs of cattle in North America, Java, Malay Peninsula, and India, respec- tively. Presumably insects transmit them. The invasion of the skin of sheep by the microfilariae of Elaeophora schnei- deri, which lives in its host's carotid and iliac arteries, produces a derma- titis primarily in the back part of the head but tending to spread over the face to the nostrils. Similar lesions are sometimes noted on the hind foot used to scratch the head. The presence of the larvae in the tissues results in in- tense itching, which causes the animal to scratch itself. The scratching causes destruction of tissue. The condition has been confined to summer mountain ranges in New Mexico, Arizona, Colo- rado, and possibly Utah. The life his- tory is unknown, but it is suspected that bloodsucking insects serve as in- termediate hosts of the parasite. In the solution of the problem of the control of insect-borne worm infec- tions of livestock and poultry, six gen- eral avenues of approach are available : The use of drugs therapeutically; the use of drugs prophylactically; physical and chemical sterilization of stable and poultry manure and sanitary disposal of excrements; elimination of the breeding places of insects; destruction of insects and their larvae chemically and mechanically; and mechanical prevention of the access of insects to farm animals. Although the primary purpose of therapeutic treatments directed against the worm infections is to improve the health and efficiency of the sick animal, they have some value in control. After treatment, which eliminates worms from the animal's body or kills them in the body, there is, until reinfection occurs, a reduction in the number of eggs or larvae voided by the animal or the number of larvae entering its tissues. Drugs may be used prophy- Insects and Helminths tactically to combat certain of the in- sect-borne worm infections. Hetrazan administered orally to persons having filariasis causes a rapid and marked re- duction in the number of microfilariae in the blood even though the adult worms are not killed. Fouadin, one of the standard drugs in the treatment of heartworm infections in dogs, has a similar effect when injected into these animals. It also inhibits the reproduc- tive capacity of the adult female worms. It is. likely that other drugs may be found to operate similarly against the microfilariae of other worms of domesticated animals. When the economic value of the ani- mal to be protected warrants, stable manure or poultry manure may be promptly collected and stored so as to exclude flies and perhaps other insects from it. Horse manure may be stored in piles so that some of the worm eggs and larvae in it will be destroyed by the heat of its decomposition. This effect may be heightened by storing it and cow manure in covered insulated wooden manure boxes. Evidently it has not been determined specifically that the eggs and larvae of heteroxe- nous worms are killed by these proce- dures; however, in all cases investigated it has been found that the eggs and larvae of worm parasites generally are killed by approximately the same de- gree of heat (about 1400 F.). Several chemical agents will kill the eggs and larvae of monoxenous worm parasites in stable manure. None, to our knowledge, has been specifically demonstrated to be effective against the eggs and larvae of the heteroxenous worms. Some of the agents do kill as- carid eggs, which are thick-shelled, and the means for killing chemically all types of worm eggs and larvae in ma- nure probably are at hand. Investiga- tion to prove this is needed, however. Stable or poultry manure which has not been processed in some manner ought not be used on the farm for fertilizer. The destruction of breeding places and direct attacks against insects and 177 their larvae are weapons that can be applied generally to control these vec- tors. Usually both lines of attack should be employed, but the habits and life histories of insects are so diverse that the weapon of choice — habitat de- struction or larvicide or destruction of the adult — may differ with the insect to be fought. Attacks against the house fly can be directed most feasibly and easily against the larvae. DDT has been re- ported to be effective against the mag- gots of this fly when- used in a water emulsion. Such an emulsion was found to be effective also against certain other species of flies breeding in poul- try manure. DDT, methoxychlor, chlordane, lindane, and other insecti- cides are recommended as residual sprays directed toward the control of the adults. Important supplemental measures include disposal of manure, chemical treatment of manure, the use of properly baited fly traps, and the use of pyrethrum fly sprays. The stable fly likewise is most vul- nerable to attack in its larval stage. A principal measure for its control — applicable also in the case of the house fly — is the destruction of its breeding places. When it is impossible to locate and eliminate all of these, insecticides as recommended for controlling the housefly are distinctly useful against the adult stable flies. Mosquitoes, biting midges, and black flies breed in water, and the elimina- tion of their breeding places is not always feasible or desirable. Ponds, small pools, and useless swampy areas may often be filled in or drained. Since the maintenance of large ponds and streams is desirable, treating the water with oils to kill the larvae me- chanically and with such larvicides as paris green long has been one of the approaches to the problem of mosquito control. DDT when incorporated into an oily vehicle for application to the water surface is effective for the destruction of the mosquito larvae. This insecticide also is of value in kill- ing the adults of mosquitoes, biting 178 midges, and black flies. Tests have in- dicated that the larvae of black flies are susceptible to DDT, TDE, and other new chlorinated insecticides. Cat and dog fleas in and around buildings may be controlled effectively by the use of DDT sprays or dusts. One to two gallons of 5 percent DDT in oil sprayed lightly over areas of 1,000 to 2,000 square feet has been found effective in the complete eradi- cation of adult fleas. Five percent DDT powder, applied with a dust can, is recommended for the destruction of fleas on dogs. The application of the dust to the building will destroy the larvae and adults as they emerge. Methods are available for the con- trol of grasshoppers, earwigs, and cock- roaches. Beetles frequenting poultry manure likewise may be controlled chemically. The use of insecticides against these beetles probably would not be practical in seeking to control worms in poultry flocks having access to large areas, but the confinement of birds, as presently widely practiced, favors the feasibility of measures for beetle destruction in accumulated manure. In theory, beetles frequenting ma- nure on pastures no doubt also may be dealt with by means of insecticidal dusts or sprays, but we know of no work demonstrating that this is prac- tical. Since it has been demonstrated that the feeding of small amounts of drugs, such as phenothiazine, to cattle prevents the development of horn flies in their dung, it would seem advisable to investigate the possibility that beetles might be controlled as worm vectors by the routine incorporation of suitable insecticidal materials into the diet of farm animals and birds. Manure de- posits on pastures may be broken up and spread to reduce the attractiveness of the manure to insects. The maxi- mum adverse effects of dryness and sunlight on worm eggs and larvae may also be had by this step. The chemical destruction of beetle mites on pastures and grazing land apparently has not been investigated, but even were it pos- Yearbook of Agriculture 1952 sible, its practicability seems doubtful. It seems probable that other means will have to be sought for the pre- vention of tapeworm infections trans- mitted by these mites. Everett E. Wehr is a parasitologist doing research in parasitology in the zoological division, Bureau of Animal Industry, at Beltsville, Md. He has been associated with the division since ig28 and has been in charge of its investi- gations on the parasitic diseases of poultry since 1936. He is the author of numerous papers on the worms and other parasites of livestock and poultry. He has particularly investigated the nematodes of birds. Dr. Wehr is a grad- uate of the University of Idaho, University of California, and George Washington University. John T. Lucker is a parasitologist and has been associated with the zoo- logical division since ig^o. He also is stationed at Beltsville. The identifica- tion of nematodes has been one of his chief assignments since IQ40. He is a graduate of the University of Wash- ington and George Washington Uni- versity. For further reference: Joseph E. Alicata: Early Developmental Stages of Nematodes Occurring in Swine, U. S. D. A. Technical Bulletin 489, 1935; The Life History of the Gizzard Worm (Cheilospirura hamulosa) and Its Mode of Transmission to Chickens With Special Ref- erence to Hawaiian Conditions, in Livro Jubilar do Professor Lauro Travassos, 1938. Eloise B. Cram: Developmental Stages of Some Nematodes of the Spiruroidea Parasi- tic in Poultry and Game Birds, U. S. D. A. Technical Bulletin 22J. 1931. Ashton C. Cuckler and Joseph E. Alicata: The Life History of Subulura brumpti, a Cecal Nematode of Poultry in Hawaii, Transactions of the American Microscopical Society, volume 63, pages 345-357. 1944- G. Dikmans: Skin Lesions of Domestic Animals in the United States Due to Nema- tode Infestation, The Cornell Veterinarian, volume 38, pages 3-23. 1948. R. I. Hewitt, E. White, D. B. Hewitt, S. M. Hardy, W. S. Wallace, and R. Anduze: The First Year's Results of a Mass Treat- ment Program With Hetrazan for the Con- trol of Bancroftian Filariasis on St. Croix, American Virgin Islands, American Journal of Tropical Medicine, volume 30, pages 443-452- 1950. Paul R. Highby: Development of the Mi- crofilaria of Dirofilaria scapiceps (Leidy, 1886) in Mosquitoes of Minnesota, Journal of Parasitology (Supplement) , volume 24, page 36. 1938. Myrna F. Jones: Life History of Metro- liasthes lucida, a Tapeworm of the Turkey, Journal of Parasitology, volume 17, page 53> I93° {part of Proceedings of the Hel- minthological Society of Washington) ; Metroliasthes lucida, a Cestode of Galliform Birds, in Arthropod and Avian Hosts, Pro- ceedings of the Helminthological Society of Washington, volume 3, pages 26—30, 1936; Development and Morphology of the Ces- tode Hymenolepis cantaniana, in Coleop- teran and Avian Hosts, with J. E. Alicata, Journal of the Washington Academy of Sciences, volume 25, pages 237-247, 1935. Kenneth C. Kates: Development of the Swine Thorn-Headed Worm, Macracan- thorhynchus hirundinaceus, in its Interme- diate Host, American Journal of Veterinary Research, volume 4, pages 173-181 , 1943; Observations on Oribatid Mite Vectors of Moniezia expansa on Pastures, with a Re- port of Several New Vectors from the United States, with C. E. Runkel, Proceed- ings of the Helminthological Society of Washington, vol. 15, pp. 10, 19-33, J94^- H. E. Kemper: Filarial Dermatosis of Sheep, North American Veterinarian, vol- ume 19, No. 9, pages 36-41. 1938. Wendell H. Krull: Observations on the Distribution and Ecology of the Oribatid Mites, Journal of the Washington Academy of Science, volume 29, pages 519-528. 1939. George W. Luttermoser: Meal Beetle Larvae as Intermediate Hosts of the Poultry Tapeworm Raillietina cesticillus, Poultry Science, volume 19, pages 177-179. 1940. Ralph W. Macy: Studies on the Taxon- omy, Morphology, and Biology of Prostho- gonimus macrorchis Macy, A Common Ovi- duct Fluke of Domestic Fowls in North America, Minnesota Agricultural Experi- ment Station Technical Bulletin 98. 1934. Horace W. Stunkard: The Life Cycle of Anoplocephaline Cestodes, Journal of Para- sitology, volume 23, page 569, 1937; The Development of Moniezia expansa in the Intermediate Host, Parasitology, volume 30, pages 491-501, 1939. William A. Summers: Fleas as Accept- able Intermediate Hosts of the Dog Heart- worm, Dirofilaria immitis, Proceedings of the Society of Experimental Biology and Medicine, volume 43, pages 448-450. 1940. Y. Tanada, F. G. Holdaway, and J. H. Quisenberry: DDT to Control Flies Breed- ing in Poultry Manure, Journal of Economic Entomology, volume 43, pages 30-36. 1950. Willard H. Wright and^Paul C. Under- wood: Fouadin in the Treatment of Infes- tations With the Dog Heartworm, Dirofi- laria immitis, Veterinary Medicine, volume ?9> Pages 234-246. 1934. Insects and the Plant Viruses L. D. Christenson, Floyd F. Smith The Russian scientist D. Iwanowski demonstrated in 1892 that sap from tobacco plants with a mosaic disease is infectious after passing through a bacteria-proof filter. It was the first dis- covery of an amazing group of agents that cannot be seen with ordinary microscopes and that now are called viruses. Many of our most serious and difficult plant-disease problems have been shown to be the results of infec- tions of plants by these minute entities, which are smaller than bacteria. A few of the many different kinds of viruses are even smaller than the largest mole- cules known to chemists. Tulip mosaic, peach yellows, aster yellows, sugar-beet curly top, phloem necrosis of elm, tobacco mosaic, rasp- berry mosaic, blueberry stunt disease, potato leaf roll, pea mosaic, tomato spotted wilt, and sugarcane mosaic are examples of plant diseases caused by viruses. Virus agents also cause serious diseases of man and animals — small- pox, measles, mumps, the common cold, rabies, distemper, and foot-and- mouth disease. Others, like the sac- brood virus of honey bees, infect in- vertebrate animals. For a long time we knew little about the nature of viruses. Now, as a result of the studies of W. M. Stanley, F. C. Bawden, N. W. Pirie, and others, they are believed to consist of complex nucleoproteins that have some of the attributes of living organisms. Like liv- ing organisms, the individual virus particles can reproduce or multiply. They also can change or mutate dur- ing the multiplication process. They do not seem able to grow or multiply, however, except within the living cells of their hosts, and, unlike living or- 179 i8o ganisms, they cannot carry on the com- plicated processes of respiration, diges- tion, and other metabolic functions. Most of the plant viruses have been discovered since 1900, but they are not of recent origin. Old Dutch masters recorded in their paintings the varie- gations in the petals of tulips caused by a virus now known as tulip mosaic. Dutch bulb growers knew as early as 1637 how to graft healthy bulbs with variegated bulbs to get the coveted many-colored flowers even though they did not know what caused them. Potato viruses had become so abundant in Europe by 1775 that the production of potatoes had to be abandoned in many areas because of what was then termed the "running-out" of potatoes. In the United States, the virus disease now known as peach yellows was described as early as 1 79 1 . We have evidence that it was doing damage in peach orchards as early as 1750. Only a few viruses kill the plants they infect. Plants affected by most of them never recover, but they do not die as a result of the infection. Their growth and productivity may be seri- ously affected. Some species and vari- eties of plants apparently are not attacked by viruses. Others may be tolerant of them or only mildly affected when their tissues are invaded by the virus particles. Trees, shrubs, other plants in uncultivated areas, and weeds on farms may be infected by viruses that also attack cultivated plants. When that is so, the wild plants serve as important sources of danger to the cultivated plants. Otherwise the viruses in the uncultivated plants are not eco- nomically important. No viruses are yet .known that attack coniferous trees, such as pine and spruce. Our cultivated crops annually suffer heavy losses because of virus diseases. Phony peach has plagued peach growers in the Southeastern States for at least half a century, making it neces- sary for them to take out more than 2,600,000 peach trees. Years ago in the Northeastern States, peach yellows de- stroyed the productiveness of hundreds Yearbook of Agriculture 1952 of thousands of trees. Sometimes it was necessary to destroy entire orchards. Tobacco mosaic has been estimated to cause an annual loss of millions of pounds of tobacco. Viruses have seri- ously affected the production of pota- toes each year. To reduce their losses, the growers here and in England and other countries have to expend large sums to get healthy seed potatoes grown in areas where potato viruses are not serious. Production of head lettuce in the East has not been profit- able because of infection by the virus known as aster yellows. Losses caused by the curly-top virus in sugar beets have been so severe in the Western States that some sugar factories have had to be abandoned. The same virus has caused crop failures in tomato fields. Similar heavy tolls may be levied by the viruses that attack many of our ornamental plants and flowers. Plant diseases caused by viruses spread in several ways. Some are so in- fectious that contact between the leaves of normal and diseased plants is all that is necessary. Highly conta- gious diseases such as these may be spread by mechanical means. A few instances of spread through seeds are known. A serious method of spread is through the use of parts of infected plants to start new plantings. For ex- ample, viruses that persist from year to year in potato tubers, in bulbs, and in rhizomes infect plants growing from them. Viruses may also be spread through cuttings or suckers from in- fected plants and through budding and grafting procedures employed in nurseries. Insects are the worst spreaders. A Japanese scientist in 1901 found that a leafhopper could transmit stunt disease of rice from diseased rice plants to healthy plants. The first insect to gain prominence in North America as a carrier of a plant virus was the beet leafhopper. It was found to be spreading curly-top disease in sugar-beet fields in Utah and other Western States only a few years after the discovery of the insect Insects and the Plant Viruses 181 carrier of stunt disease of rice. We now know that many of our plant virus disease outbreaks are the result of in- sect-carrier activity, and it is suspected that insects are involved in the spread of many other plant virus diseases. Insect carriers of plant viruses are Six-spotted leafhopper. known to occur in only six of the major orders of insects — the Homoptera ( aphids, leafhoppers, whiteflies, mealy- bugs, scales), Thysanoptera (thrips), Heteroptera (plant bugs, lace bugs), Coleoptera (the beetles), Orthoptera (grasshoppers), and Dermaptera (ear- wigs). Most of the carriers have suck- ing mouth parts, and among them the aphids and leafhoppers seem to be the most proficient. A few insects with chewing mouth parts, such as grass- hoppers and leaf-feeding beetles, also spread certain virus diseases. To accomplish transmission, the vec- tor has to get the virus from a diseased plant, which it does while feeding, and then move to a healthy plant, which it infects during the feeding process. With the sucking insects, the virus par- ticles apparently are injected into plants with the saliva. The relationships between plant viruses and their vectors have com- manded the attention of many ento- mologists, plant pathologists, and other biologists. Striking advances have been made, and we now know a great deal about many insects that transmit vi- ruses, something about what happens to the virus during its period in the insect body, and something about the factors involved in the transmission process. There is still much to be explained, however: We do not know why cer- tain species can transmit viruses while other similar insects cannot, or why certain insects can transmit so many different kinds of plant viruses but not others. The many other vectors await- ing discovery also remain a challenge. Plant viruses are considered as be- longing to two general groups. In the group called the nonpersist- ent viruses, the insect carrier can trans- mit the virus soon after feeding on a diseased plant. This ability to cause new infections is quickly lost, however, after the insects feed on healthy or im- mune plants. A starvation period be- fore feeding on infected plants usually increases the transmission efficiency of the vectors of the viruses, which usually can be transmitted by mechanical means, as by wiping the sap of an in- fected plant over the leaves of a healthy plant. The insect carriers some- times include many different kinds of insects. Many viruses transmitted by aphids and chewing insects belong to this group. Perhaps some of the non- persistent viruses are transmitted through contamination of the mouth parts of the insect carriers with virus particles, but for many others the transmission process does not seem to be that simple. The other group includes the per- sistent viruses. When they are taken in with the food of their vectors, an in- terval (the incubation, or latent, period) is necessary before the insects can infect healthy plants with them. Once having the ability, insect carriers of persistent viruses usually can trans- mit them to healthy plants for an ex- l82 tended period, often for life. In two instances involving leafhoppers, per- sistent viruses are transmitted to the succeeding generation through the eggs. Some of these viruses are trans- mitted by only one or a few closely re- lated insects. Most of the viruses that leafhoppers transmit are persistent viruses. A few aphids or other insects also transmit persistent viruses. The incubation period of the per- sistent viruses in insects sometimes lasts only a few hours or less. It may last as long as 5 days in some aphids or several weeks in some leafhoppers. The incubation period of the virus that causes western X-disease of peach is usually longer than 30 days in the geminate leafhopper. Incubation pe- riods as long as 40 days have been reported for some leafhoppers that transmit aster yellows, although in most of them the period is about 2 weeks. One of the four leafhopper car- riers of phony peach in the Southeast- ern States has transmitted the disease to healthy peach trees 14 days after first feeding on a phony tree, but in another leafhopper the shortest incu- bation period observed thus far has been 19 days. Temperature may influ- ence the length of an incubation period of a virus in an insect. The meaning of the incubation period of the viruses is moot. Some in- vestigators believe it is a true incuba- tion period, during which the virus goes through some kind of necessary developmental or reproductive stage. Others consider it merely the time necessary for the virus particles to make their way through the intestinal walls of the insect into the blood stream and thence into the salivary glands, where they can be introduced with saliva into healthy plants during feeding. Both points of view can be justified, depending on the virus involved. Some leafhoppers transmit persistent viruses throughout their lives once they be- come infective. Others may lose the ability after a period. In some indi- viduals the ability to transmit a virus may become much less pronounced as Yearbook of Agriculture 1952 they near the end of their life span — perhaps the original supplies of virus taken in have become exhausted dur- ing the intervening feeding periods on healthy plants and there has been no multiplication of the virus particles within the insect, or at least not suffi- cient reproduction to maintain an in- fective charge of the virus. There is no evidence that viruses undergo bio- logical changes in insects, but one scientist has reported that clover club- leaf virus reproduces in its leafhopper carriers. The leafhoppers remain in- fective through successive generations long after there would be any chance for the original quantity of virus to be involved. There also seems to be con- vincing proof that the virus causing aster yellows and the one that causes stunt disease of rice in the Orient mul- tiply in their insect carriers. Some insects that transmit viruses can become infective after a feeding period of only 1 minute or after only a single feeding on a diseased plant. Different species vary with respect to their effi- ciency in transmitting virus diseases, and there are instances where the nymphs or immature stages seem to be less efficient than the adult insects. Some vectors can pick up viruses while they are in immature stages but can- not transmit them until the adult stage is reached. The suggested explanation, in the case of a leafhopper carrier of aster yellows, is that the incubation period is not completed before the nymphs reach the adult stage. But that is not the explanation in the case of thrips, which transmit spotted wilt virus, because adults become infective only after picking up the virus while in the larval stage. A plant virus may have a single species of insect serving as its vector, or there may be several kinds able to transmit the same virus. Sometimes the latter are entirely unrelated species. Single insects can infect plants with virus diseases, but even an infective in- dividual cannot cause an infection every time it feeds on a healthy plant. In some instances this seems to be be- Insects and the Plant Viruses Macrosiphum ambrosiae, aphids. cause the virus must be introduced into certain types of plant tissue which the vector does not always reach with its mouth parts ; in other cases the reasons are not apparent. Viruses do not seem to affect their insect carriers in any way, even though they cause serious diseases of plants. The aphids, or plant-lice, have de- veloped the ability to serve as carriers of plant viruses to the greatest degree. These minute, soft-bodied insects feed by sucking sap through their beaks, which they insert into plant tissues. They attack practically all kinds of plants. Most species produce both winged and wingless individuals. The former are chiefly responsible for the spread of virus diseases in fields. The green peach aphid is outstand- ing among aphid carriers of plant virus diseases. It is known to transmit more than 50 kinds, mostly of the nonper- sistent type. The green peach aphid occurs al- most everywhere and feeds on many kinds of plants. It is a serious pest of potatoes because it can transmit leaf roll and other viruses. In potato-grow- ing areas where the winters are mild, the green peach aphid spends the win- ter on weeds and such vegetables as spinach and kale. Winged individuals 183 produced on the winter host plants mi- grate into the potato fields when the plants are small. As they move from plant to plant, the winged migrants leave a few young aphids here and there and spread potato viruses from diseased plants to healthy plants. The young aphids left behind start new aphid colonies throughout the potato field. When the colonies become over- crowded, enormous numbers of winged aphids may be produced. They swarm over the field and cause another wave of infection. Individual potato farmers are helpless in their efforts to protect their crops when tremendous numbers of migrating aphids are present. In northern Maine and other po- tato-growing areas where winters are cold, the green peach aphid over- winters in the egg stage. The eggs are laid on twigs of peach and plum trees by female aphids, which are produced in the late summer or early fall. Rela- tively few winged aphids are produced in colonies developing from these eggs, and consequently infestations in po- tato fields are extremely light early in the spring. Although large numbers of winged aphids may be present later in the summer, there is usually not so much spread of virus diseases in north- ern potato-growing areas as there is in areas with warmer winters. The amount of potato leaf roll in the fol- lowing year's crop may be predicted rather accurately from the abundance of the winged forms of peach aphid during the summer. The green peach aphid and other aphids that develop on potatoes and other plants may migrate across a glad- iolus field and pick up yellow bean mosaic virus. The virus causes only mild symptoms in gladiolus, but when the aphids transmit it to beans, a de- structive disease results. Celery in Flor- ida is infected with cucumber mosaic by aphids which pick it up as they feed on commelina, a weed that grows along ditchbanks. Lilies in fields containing a few plants infected with the nonpersistent coarse mottle and cucumber mosaic vi- 184 Yearbook of Agriculture 1952 Examples of Plant Viruses and Some of Their Insect Vectors Virus Potato spindle tuber. Strawberry yellow edge Strawberry crinkle .... Onion yellow dwarf. Cucumber mosaic. Raspberry mosaics . Pea mosaic Potato leaf roll. Sugarcane mosaic. . . Citrus quick decline . Potato yellow dwarf. Sugar-beet curly top. Pierce's disease of grapevines. . < Phloem necrosis of elm. Peach yellows Phony peach . Western X-disease of peach . Papaya bunchy-top Cranberry false-blossom Blueberry stunt disease. Tomato spotted wilt. . . Tobacco mosaic Latent potato virus (potato virus X). Tobacco ringspot Cotton leaf curl (in Africa). . . Vector Common name 'Melanoplus spp grasshoppers. Epitrix cucumeris potato flea beetle. Systena taeniata flea beetle. bisonycha triangularis leaf beetle. Leptinotarsa decemlineata Colorado potato beetle. Lygus oblineatus tarnished plant bug. JAyzus persicae green peach aphid. Pentatrichopus fragar^ae aphid. Pentatrichopus fragariae aphid. {Aphis gossypii melon aphid. Myzus persicae green peach aphid. Brevicoryne brassicae cabbage aphid. Aphis maidis corn leaf aphid. Other Aphidae At least 50 species of aphids transmit this virus. {Aphis gossypii melon aphid. Myzus persicae green peach aphid. Myzus circumflexus crescent-marked lily aphid. Myzus solani foxglove aphid. (Amphorophora rubi aphid. [Amphorophora sensoriata aphid. {Macrosiphum pisi pea aphid. {Myzus persicae green peach aphid. {Myzus persicae green peach aphid. Myzus circumflexus crescent-marked lily aphid. Myzus solani foxglove aphid. Macrosiphum solanifolii potato aphid. I Aphis maidis corn leaf aphid. \Hysteroneura setariae rusty plum aphid. Aphis gossypii melon aphid. {Aceratagallia sanguinolenta . . . clover leafhopper. Aceratagallia curvata leafhopper. Aceratagallia longula leafhopper. Aceratagallia obscura leafhopper. Circulifer tenellus beet leafhopper. ' Draeculacephala minerva leafhopper. Helochara delta leafhopper. Carneocephala fulgida leafhopper. Other Cicadellidae At least 14 species can trans- mit this virus. Aphrophora annulata spittlebug. Aphrophora permutata spittlebug. Clastoptera brunnea spittlebug. ^Philaenus leucophthalmus meadow spittlebug. Scaphoideus luteolus leafhopper. Macropsis trimaculata plum leafhopper. {Homalodisca trique.tr a leafhopper. Oncometopia undata leafhopper. Graphocephala versuta leafhopper. Cuerna costalis leafhopper. Colladonus geminatus geminate leafhopper. Empoasca papayae leafhopper. Scleroracus vaccinii blunt-nosed cranberry leaf- hopper. Scaphytopius sp leafhopper. Thysanoptera thrips. {Aphidae A few aphids have been re- ported to transmit this virus. Melanoplus differentialis differential grasshopper. Melanoplus differentialis differentia] grasshopper. Melanoplus differentialis differential grasshopper. Bemisia gossypiperda whitefly. Insects and the Plant Viruses ruses soon become almost completely diseased when the fields are planted near potatoes or other plants where the aphid carriers of these diseases develop. Lily rosette, a persistent virus, is trans- mitted by the melon aphid after an in- cubation period of the virus in the aphid lasting 3 or 4 days. This aphid develops on young lily plants; both the wingless aphids (which crawl to adja- cent plants) or winged migrants (which fly to plants farther away) may spread lily rosette. The melon aphid also transmits a virus that causes a condition known as lily symptomless disease. The disease has spread slowly throughout most commercial stocks of lilies. In itself it is not serious, but when the same plants get cucumber mosaic the double infec- tion termed necrotic fleck makes them worthless. Necrotic fleck was chiefly re- sponsible for the failure of Easter lily bulb production in the United States. To meet our needs, as many as 25 mil- lion Easter lily bulbs have been im- ported in a year. The strawberry aphid in England transmits three viruses of strawberries, which cause the "running out" of de- sirable varieties. This aphid and two related species occur in the United States and live throughout the year on strawberry plants. Similar diseases and possibly others are devastating straw- berries in the United States. These three strawberry aphids have been shown to be vectors of strawberry vi- ruses in America and are believed to be chiefly responsible for their dispersal under field conditions. The Depart- ment of Agriculture has helped the strawberry industry by locating virus- free strawberry plants of the more val- uable varieties and furnishing founda- tion stocks to cooperating nurseries for mass propagation and replacement of infected plants. Winged aphids from overwintering pea aphid colonies on alfalfa transmit a serious virus disease of peas, which kills the tips and interferes with the productivity of the plants. Aphids may also spread viruses that 185 affect trees. An example is the citrus quick decline disease, which in a few years has caused the loss of many thou- sands of orange trees in California. The vector of quick decline is the melon aphid. Another aphid, which does not occur in the United States, is the vec- tor of a similar virus disease of citrus in South America. The leafhoppers are our second most important carriers of plant vi- ruses. They are small, slender, variously colored insects, which have sucking beaks similar to those of the aphids. They are active jumpers. The adults fly freely and some of them can cover long distances in migratory flights. A characteristic habit of young and adults is that of walking sideways. All leafhoppers are plant feeders. Certain kinds are called sharpshooters, and other names such as whitefly and green- fly have been used for some of them. Leafhoppers transmit at least three serious virus diseases to peach trees. The oldest is peach yellows. Its vector was a mystery until the 1930's, when it was discovered that the plum leaf- hopper is the carrier. The plum leafhopper feeds on the twigs and is seldom seen on the leaves. Plum is its favored host. Rarely is it found on peach. The leafhopper may obtain the virus, which it transmits to peach trees, from peach and plum trees. The latter are symptomless car- riers of the yellows virus. In orchards adjacent to woodlands, correlations be- tween the numbers of the leafhoppers, the abundance of wild plum, and the amount of yellows disease in peach have been noted. No other vectors of peach yellows have been discovered. The plum leafhopper is present in all areas where peach yellows occurs. Although peach yellows is no longer a serious problem, new cases each year give warning that the vector is still active and that peach growers in the Northeastern States cannot afford to relax their vigilance with respect to the disease. Phony peach disease poses a problem for peach growers in Southeastern States, particularly in parts of Georgia and Alabama, where it is difficult to control with the usual method of in- specting orchards and removing all dis- eased trees. An intensive 1 2-year search for vectors ended in 1 949, when it was Leafhopper. announced that four leafhoppers can spread the disease. They are all general feeders. Two are believed to be main vectors in spreading the disease in orchards. They spend the winter as adults and occasionally as nymphs under trash and debris in woodlands and possibly along ditchbanks. In spring they become active, leave the woods, and move to a variety of plants, including peach trees, where they feed on the twigs. When preferred herbaceous plants later become available, they leave the peach trees and go to them. Very few are found on peach trees in summer but they reappear on this host early in the fall. They continue to feed on the twigs of peach trees even after the trees become fully dormant until they are forced into hibernation by cold weather. Presumably phony peach spreads mostly during the periods in spring and fall when the leafhoppers are on peach trees. After sucking sap from infected Yearbook of Agriculture 1952 trees, they cannot cause phony infec- tions until after an incubation period of 14 to 40 days. Infective leafhoppers can transmit the phony peach virus for a long time, possibly for life, but we do not know whether the virus persists in them through the long periods of hi- bernation. When feeding, the hoppers insert their beaks into the woody tissue of peach twigs where the phony peach virus seems to be localized. They can obtain the virus from both diseased peach and wild plum trees, but not all peach trees seem to be equally good sources of the virus. Fruit growers in the Pacific North- west are plagued by several devastating peach and cherry virus diseases. One, the western X-disease of peach, is transmitted by the geminate leafhop- per, which might also spread a little- cherry condition caused by the same virus. The incubation period of the virus in this leafhopper is usually longer than 30 days. Single leafhoppers have transmitted western X-disease; some have retained the ability to cause in- fections for at least 80 days. The leaf- hopper prefers legumes and grasses, but it feeds on many other plants. Nymphs seldom occur on peach trees but the adults frequently visit them and other stone fruits. The leafhopper also occurs on chokecherry, a wild host of the western X-disease. At least 14 species of leafhoppers transmit the virus that causes Pierce's disease of grapevines. The same virus also infects alfalfa, causing a disease called alfalfa dwarf. A remarkable thing about the leafhopper carriers of Pierce's disease is that they are all closely related and belong to the same subfamily, which includes all known vectors of phony peach disease. The leafhoppers vary greatly in their effi- ciency and importance as vectors of Pierce's disease. Infective leafhoppers have been found far from vineyards or alfalfa fields — perhaps there are still other plant hosts of the virus. Aster yellows virus affects many vegetables, flowers, and other herba- ceous plants. In the Eastern States only Insects and the Plant Viruses one strain of the virus and only one vector, the six-spotted leafhopper, are known. A number of leafhoppers can transmit the western strains of the virus. The strain affecting celery, for example, is carried by at least 22 kinds of leafhoppers. The geminate and mountain leafhoppers transmit the cel- ery strain of the virus, but they cannot transmit a related strain that causes yellows disease in asters. Both of these western virus strains, however, are transmitted by the six-spotted leafhop- per. This curious relationship, and a similar situation found among the leaf- hoppers which transmit potato yellow dwarf virus, suggest that some virus strains may have developed in rela- tion to their insect vectors rather than their plant hosts. The vector of aster yellows in the East cannot cause infec- tions when exposed to high tempera- tures, but it regains the ability when the temperature goes down. The wide variety of leafhoppers that transmit aster yellows, Pierce's disease of grapevines, and phony peach has brought up the point that the ability to transmit virus diseases may be deter- mined somewhat by the ability and in- clination of leafhoppers to feed on a definite part of the plant host. Of course that would be true only of the species that meet all biological require- ments necessary for them to serve as vectors. Curly-top virus causes serious dis- eases of sugar beets, tomatoes, and beans in Western States. The only vec- tor known in this country is the beet leafhopper, apparently an introduced species with no close relatives in the New World. The curly-top problem and its leafhopper vector are the sub- jects of another article, on page 544. Other kinds of sucking insects spread plant virus diseases. Besides many leafhoppers, four species of frog- hoppers, or spittlebugs, transmit Pierce's disease of grapevines. A lace bug is a vector of a virus disease of sugar beets. Mealybugs and whiteflies transmit serious diseases of cacao, cas- sava, or cotton in other countries. A 187 scale insect may be involved in the spread of sudden death of clove trees. Thrips are tiny insects that feed by macerating the. surface layers of plant cells and then sucking up the juices. Certain thrips are notorious as vectors of the spotted wilt virus of tomatoes and pineapples. The virus, or strains of it, occurs in many parts of the world and affects many kinds of plants. It causes one of the major diseases of pineapples in the Hawaiian Islands. Adult thrips cannot acquire the virus by feeding on infected plants. How- ever, the adults that develop from nymphs that have fed on diseased plants become infective; the virus sur- vives the pupal, or resting, stage which the insects undergo. In spotted wilt of tomatoes, the incubation period of the virus in the insects is 5 to 7 days and the ability to cause infections is re- tained for several weeks. Thrips de- velop on a wide variety of host plants and can cause severe damage even when not transmitting viruses. Insects with biting and chewing mouth parts are involved in the trans- mission of a few plant viruses. Grass- hoppers and leaf-feeding beetles are vectors of the highly infectious disease of potatoes called spindle tuber. Cu- cumber beetles transmit a mosaic disease of cucumber. The role of these insects — of spindle tuber at least — seems to be that of a mechanical car- rier. The disease is also spread by many kinds of insects. The differential grasshopper appar- ently can transmit tobacco mosaic, latent potato virus, and tobacco ring- spot virus to healthy tobacco plants. Aphid vectors had been reported for tobacco mosaic, but repeated trials have failed to implicate insects in the transmission of latent potato virus or tobacco ringspot virus. The differential grasshopper apparently can infect to- bacco plants immediately after feeding on diseased plants; infection results after only one or two feedings on a healthy plant. The transmission proc- ess is believed to be a simple mechan- ical transfer of virus particles on the mouth parts of the grasshopper. It is likely that virus particles on the feet of the grasshoppers may also start in- fections. Some of the reports of insect transmission of plant viruses upon fur- ther investigation may be found to be a result of direct-feeding injuries that resemble symptoms of virus diseases. The foxglove aphid on lilies and sev- eral vegetable crops and an aphid on carnation cause spotting and distortion of leaves that look like viruses in the same hosts. Tarnished plant bugs cause stunting, distortion, and dead areas much like virus infection in some plants. Alfalfa yellows and a condition in potatoes known as hopperburn were suspected of being virus diseases until investigations showed that they re- sulted from direct feeding by the potato leafhopper. Feeding by the broad mite causes mottling, distortion, and stunt- ing that have been mistaken for virus diseases. Infections by leaf-infesting nematodes result in yellowing, mot- tling, and dead areas like virus symp- toms. Such direct injuries appear to be due to toxic principles in the saliva injected while feeding or to mutilation of cells in very young tissue that later develops abnormally or declines pre- maturely. Symptoms left by the potato leafhopper result from injury to vas- cular tissue in the plants, which inter- feres with translocation of food. In a few diseases, the viruses move into new shoots less rapidly than growth occurs. When that happens (for example, when dahlia roots are in- fected with spotted wilt) healthy plants can be obtained from shoots that grow from the crowns if cuttings are removed before they are invaded by the virus. The use of healthy planting ma- terial is an obvious first precaution for reducing losses caused by many virus diseases. Rotation of crops sometimes elimi- nates virus sources in volunteer plants that would infect the crop if it were Yearbook of Agriculture 1952 grown in the same field the following year. Rogueing, the removal of infected plants as soon as symptoms of diseases appear, maintains or even improves the health of potato, raspberry, strawberry, and other crops. The procedure, to- gether with nursery practices to make sure that young trees used for new plantings are not infected, has been the principal method for controlling serious virus diseases of stone fruits such as peach yellows, phony peach, and peach mosaic. In isolated areas, where the vectors apparently are not very active, it has even been possible to achieve eradication of these diseases by this method. When they are available, the use of resistant or immune varieties is an ef- fective way to prevent losses caused by virus diseases. Losses can also be avoided by growing crops in areas where serious virus diseases are not present or where vector activity is at a low ebb. Means have been sought for curing plants affected by virus diseases with heat treatments or chemicals adminis- tered internally. Often viruses may be killed by exposures to high tempera- tures that are tolerated by the infected plant tissue. Heat cures are of practical value for eliminating the viruses of sereh and chlorotic streak diseases in sugarcane seed pieces. For stone fruits a heat treatment has been suggested for yellows and X-diseases of peach but has not been used practically as yet. Its value is primarily in providing disease- free planting material. A practical chemical treatment for inactivating viruses in plants, usable under field conditions, would be a boon to agriculturists everywhere. The spread of plant viruses may also be prevented or retarded by methods that eliminate or reduce the insect car- riers below critical transmission levels. The problem is not simple. Treatments must be exceptionally effective, even more than when the direct injury caused by the insects is the only con- cern. A light population of the insect Insects and the Plant Viruses carriers may be able to infect many ad- ditional plants when abundant sources of virus are available, or start a new outbreak of disease. The presence of numerous widely distributed carriers with different seasonal histories fur- ther complicates the problem. Because insects may move in constantly from untreated areas, some of them al- ready infective, continuous protection throughout the growing season may be required when insecticides are used. Despite such difficulties, some progress can be reported. Some benefits have been obtained with methods for reducing the num- bers of insect carriers or for preventing or avoiding their activity without us- ing insecticides. The elimination of host plants of insect carriers is often beneficial. Cloth of a special coarse weave, supported by posts and wire, effectively excludes the leafhoppers that transmit yellows infection to China asters. Potato virus diseases are largely con- trolled by using seed potatoes grown in isolated areas or in places where the aphid vectors are scarce. Relatively few potatoes are infected under those conditions, and the seed pieces pro- duce a high proportion of healthy plants. Northern locations or high alti- tudes with cool temperatures and al- most constant winds are best for grow- ing seed potatoes, because those con- ditions are unfavorable for aphid de- velopment or flight. Frequent rogue- ing and applications of insecticides help to maintain the healthy seed stock. Similar procedures are used for devel- oping and maintaining healthy source stocks of strawberries in England, and they may be practical for lilies, gladi- olus, and other economic plants in the United States. Many experiments have been made to determine the usefulness of insecti- cides in controlling the carriers of plant virus diseases. The materials available before 1940 were seldom ef- fective enough. The situation has im- proved with the development of new insecticides, such as DDT. 189 Applications of insecticides to culti- vated crops can be expected to control virus diseases best if the diseases are spread solely within the crop by the in- sect carriers that develop on the crop. Residual insecticides may be of value in reducing the amount of disease caused by carriers coming in from outside sources. To be effective, the insecticide must kill rapidly enough to destroy the insects before they can do much feeding. They must also remain toxic to later invaders for several days or until the next application of insec- ticide is made. The application of in- secticides to vector breeding areas to destroy the insects before they reach cultivated plantings may have merit in certain situations. DDT has been the most useful of the new insecticides for controlling insect carriers of plant virus diseases. It is effective against nearly all leafhoppers and it destroys some of the important aphid vectors. It is now almost uni- versally applied to potato fields to eliminate aphids. The applications greatly reduce the number of wingless aphids and winged summer migrants which develop, and the spread of po- tato leaf roll is now much less than in former years. Aster yellows has been reduced by about 90 percent in lettuce fields in New York and Maryland by DDT applications, which destroy the six-spotted leafhopper, the most im- portant carrier of the disease. The DDT residues are also effective against additional leafhoppers that move into lettuce fields each day. Good results with DDT for controlling aster yellows in carrots have also been reported. DDT has been studied in Western States to determine its usefulness in preventing curly-top virus infections in sugar beets, tomatoes, and beans. The DDT reduces the number of beet leaf- hoppers and has good residual toxicity, but it does not prevent the feeding of the leafhoppers that reinfest the fields. The incidence of the disease in toma- toes therefore may not be appreciably reduced by DDT if reinfestation oc- curs. In fields where reinfestation does 9701^.4' -52- -14 190 not occur, single applications may give good results. Insecticidal control of leafhoppers on weed hosts growing on idle and waste land, which contribute large populations to cultivated areas, has been used in California to combat a serious curly-top problem. Experi- ments with the method have also been made in Idaho. When control of the leafhoppers in their breeding grounds is undertaken, it is desirable to elimi- nate the host plants of the insects as fast as possible, and replace them with plants, such as grasses, on which the beet leafhoppers do not breed. First results of experiments suggest that DDT may have an appreciable effect on the insect carriers of phony peach disease and that it may be pos- sible to retard its spread with DDT, but much remains to be done on the prob- lem before practical suggestions for the use of DDT for the purpose can be made. Systemic insecticides, which invade entire plants after being taken in through the roots or leaves, are toxic to aphids that feed on the treated plants. The spread of yellows in beets and other virus diseases in strawberries, all aphid-transmitted, is said to have been greatly reduced through the use of systemic insecticides on farms in England. Studies in the United States indicate that the method has possi- bilities for aphid-transmitted viruses, which attack ornamental plants, such as lilies, tulips, narcissus, and other plants propagated in nurseries. The method may also be feasible for treat- ing food crops if it is found that the insecticide or its decomposition prod- ucts in the plant are not harmful. In greenhouses, the spread of viruses is easily prevented by maintaining strict control over all insects. Fumiga- tion with various materials or the use of aerosols containing one of the new organic phosphate insecticides are effective. The new advances in insecticidal control of plant virus diseases probably will lead to others. With such an array of new insecticides for evaluation and Yearbook of Agriculture 1952 with the new equipment for applying them rapidly and effectively, the en- tomologists may make even greater contributions to the control of plant virus diseases than has been possible in the past. But it is too much to expect that the problem will be solved entirely even then: Still needed will be cooperation among growers in control programs, constant emphasis on preventive meas- ures, and the enforcement of quaran- tines to prevent the spread of viruses into new localities and to prevent the introduction of additional virus dis- eases into the United States. L. D. Christen son is entomologist in charge, oriental fruit fly investiga- tions, Bureau of Entomology and Plant Quarantine, in Hawaii. He attended the Utah State Agricultural College, the University of Minnesota, and the University of California. From 1929 to 1942 his principal assignments were concerned with studies of the relation- ships of insects to sugarcane, cotton, and stone fruit diseases in Cuba and in Southern and Western States. From 1946 to 1951 he was assistant to the chief of fruit insect investigations, Bureau of Entomology and Plant Quarantine. Floyd F. Smith, a senior entomol- ogist in the Bureau of Entomology and Plant Quarantine, has devoted 28 years to the study of insects affecting greenhouse and ornamental plants. He has published many articles on the bi- ology and control of those pests and on insects as vectors of plant diseases. In recognition of his research on the aero- sol method of applying insecticides in greenhouses, the Society of American Florists gave him an award for the most important contribution to floriculture in 1947. He is a graduate of Ohio State University. Seed-corn maggot. Insects, Bacteria, and Fungi /. G. Leach Some insects do great damage by aiding in the spread and development of plant diseases. The insect first proved by experi- ments to be a vector of a plant disease was the honey bee, which everyone considered completely beneficial; no one had thought of suspecting one of man's best friends. The experiments that indicted the honey bee were a landmark in agricul- tural science. For many years the dam- age done to plants by insects had been measured only in terms of direct injury from their feeding and breeding. M. B. Waite, an employee of the De- partment of Agriculture, discovered in 1 89 1 and proved experimentally that the honey bee, while visiting apple and pear blossoms in search of nectar, be- came contaminated with the bacteria causing fire blight and transmitted the disease from blossom to blossom and from tree to tree. That was a new idea, one that plant pathologists and entomologists were slow to accept. The recognized import- ance of the honey bee in pollinating flowering plants and producing honey made many reluctant to believe that it could be guilty of transmitting a disease. All this was discouraging to Waite, but his work was confirmed by J. C. Arthur, working at the New York Agri- cultural Experiment Station at Ge- neva. Soon plant pathologists and en- tomologists began to suspect other in- sects of transmitting plant diseases. A few years later Erwin S. Smith, an- other pioneer worker of the Depart- ment of Agriculture, and his associates reported that a destructive bacterial wilt of cucumber and muskmelons was transmitted by two species of cucumber beetles. Further work has demonstrated that the bacteria causing the disease survive the winter within the bodies of the insects and that, in nature, the dis- ease depends completely on the insects both for survival over winter and for spread from plant to plant in summer. A similar relationship exists between the bacterial wilt of sweet corn and two species of flea beetles. Ergot of rye and related cereals and grasses was perhaps the earliest fungus disease to be recognized as transmitted by insects. The fungus affects the young flowers and replaces the normal seed with a hard, black mass called a sclerotium. In early stages of blossom infection, the fungus secretes a sugary fluid in which masses of spores are pro- duced. The fluid has a foul odor that attracts flies. When the flies feed on the sugary solution, they become contami- nated internally and externally with the ergot spores. Some of the flies also feed on the pollen grains of the healthy flowers. On them they deposit the ergot spores and thus spread the disease from plant to plant. In this instance, a mutu- ally beneficial relationship exists be- tween the fungus and its insect vector. The flies derive nourishment from the sugary fluid. In return for the food, the flies transmit the spores of the fungus from flower to flower and en- able the fungus to survive. An associa- tion of this type is called mutualistic symbiosis. Similar mutualistic symbiosis occurs in other instances of insect transmis- sion. The seed-corn maggot and other dipterous insects carry the soft rot bac- teria which affect many vegetable crops. The flies lay their eggs in the soil near vegetable tissue or directly on it When the eggs hatch, the young maggots bore into the plant tissues, tak- ing the soft rot bacteria with them. The maggots will not grow and de- velop normally in sterile plant tissue but grow rapidly when the tissues are decayed by the bacteria. Thus the bac- teria are essential for the normal de- velopment of the insect. The bacteria 191 192 may also provide essential vitamins for the insect and aid in the digestion of plant tissues. The soft rot bacteria are wound parasites and cannot penetrate uninjured plant tissues. The insects make the necessary wounds. The bac- teria in return provide the necessary vitamins and aid the insects in deriv- ing nourishment from the plant. Both the insect and the bacteria thus benefit from the association. Because the young maggot would be helpless without the bacteria, which it may or may not obtain from the soil, the insect insures their presence when needed by harboring the bacteria within its body. The bacteria survive within the intestinal tract of the insect in all stages of metamorphosis. Freshly deposited eggs are usually contami- nated. The insect carries with it at all times a culture of the bacteria that are essential for the nourishment of the young maggots. It is evident that the transmission of plant diseases by in- sects often is not a simple matter of chance but is a complicated association that has evolved over a long period. Fire blight is a bacterial disease of orchard fruits, principally pears and apples. It chiefly affects blossoms and young tender shoots. It may also form destructive cankers on the trunk and larger branches. It is caused by bac- teria that overwinter in the bark sur- rounding the cankers. Sap oozes in spring from the edges of infected cank- ers. A microscopic examination of the sap shows it to be teeming with the fire blight bacteria. Insects, principally ants and flies, feed on the ooze and then visit blossoms in search of nectar. Thus the bacteria are introduced into the nectar, from which they spread into the blossoms, causing the blossom- blight stage of the disease. Bees, wasps, and other insects that visit the flowers in search of nectar or pollen spread the bacteria from blossom to blossom and from one tree to another. Shortly after the blossoms have been blighted, the young and tender shoots become infected, turn black or brown, and wither. Heavily infected trees look Yearbook of Agriculture 1952 as if they had been scorched by fire, hence the name fire blight. The young shoots are inoculated with the bacteria by sucking insects, including several species of aphids and leafhoppers. These insects become contaminated by feeding upon or crawling over infected tissue. Later, when the contaminated insects pierce healthy twigs with their needlelike mouth parts, the bacteria are carried deep into the tissues and the twig is inoculated. The bacteria of fire blight may be disseminated also by wind-blown rain and by pruning tools. Whatever the relative importance of the various methods of spread, it is agreed that if all dissemination by insects could be eliminated the disease would be much less serious. Bacterial wilt of cucurbits damages cucumbers, muskmelons, and squashes in the North and East. The bacteria causing the disease are found in the water-conducting vessels of the plants, in which they grow in white, sticky masses and interfere with normal movement of water from root to leaves. Affected plants wilt as if suffering from drought and usually die before any fruits mature. The bacteria gain entrance into the plant only through the feeding wounds made by two species of cucumber beetles, the striped cucumber beetle and the spotted cucumber beetle. The bacteria survive the winter within the bodies of the beetles and are introduced into the wounds from their mouth parts. Not all beetles are contaminated with the bacteria, but any beetle that feeds upon a diseased plant is likely to become contaminated. The beetles hi- bernate in the adult stage and in some years a relatively high percentage of overwintering beetles harbor the bac- teria. Such beetles may transmit the disease to any susceptible plant on which they feed in the spring. No other method of infection or sur- vival over winter is known to occur in nature. The only way to control the disease is to prevent the beetles from feeding on the plants. The only satis- Insects, Bacteria, and Fungi factory way to protect the plants used to be to grow them under insectproof cages. Some organic insecticides, such as medioxychlor, have given promise Striped cucumber beetle. against the beetles and may be a more practical means of controlling bacte- rial wilt. Bacterial soft rot of vegetables is caused by several related strains of bac- teria. It affects a variety of plants, in- cluding most plants with succulent tissue that is not too acid in reaction. The bacteria are strictly wound para- sites and generally do not penetrate uninjured tissues. A wounded plant normally attempts to heal the wound by laying down a layer of cork cells, which will prevent infection. If con- ditions do not favor cork formation, the bacteria may infect and cause a rot before the healing action is completed. The soft rot bacteria can be found in most agricultural soils. Any wound in susceptible tissue thus is a potential point of infection for soft rot. If the wound heals quickly enough, infection may not take place, but if something interferes with wound-cork formation, the disease is likely to occur. It is com- mon practice in some potato-growing regions therefore to store cut seed 193 pieces under conditions that permit rapid healing or suberization of the cut surfaces. Wounds made by insects on the roots of plants or on stems or leaves near the ground are common points of infec- tion. Among the most effective insects in making the wounds are the dipter- ous insects, such as the seed-corn mag- got, the cabbage maggot, and the onion maggot, which live in mutualistic sym- biosis with bacteria. They harbor the bacteria within their bodies. When the maggots burrow into the plant tissues, they usually introduce the bacteria into the plant. Moreover, the maggots, by continually burrowing into the tissues, prevent the wound from healing or puncture each new layer of cork as it is formed. Because the insects live in decaying plant tissue, they used to be considered harmless scavengers, coming in only after the plant tissues had already de- cayed. Actually, however, the insects, by transmitting the soft rot bacteria and making the necessary wound, inoc- ulate the plant, and thereby produce their own rotted tissue. The bacterial wilt of sweet corn for a long time was a highly destructive disease, but new wilt-resistant hybrids have reduced its importance. It may kill susceptible varieties in any stage of their development. The bacteria are found chiefly in the vascular bundles, through which they may spread to all parts of the plant. Sometimes, when the plant is not killed until the ears have formed, the bacteria may reach the young kernels and penetrate be- neath the seed coat. People therefore once believed that the disease was transmitted through the seeds. We now know that even though the bacteria may be present under the seed coat the disease will not develop unless wounds are made on the young plant. Such wounds, through which the bac- teria may infect the plant, are made chiefly by the larvae of the spotted cucumber beetle, commonly called the southern corn rootworm when it is found on corn. 194 A more common means of transmis- sion of the disease is provided by two species of small, black flea beetles, the toothed flea beetle and the corn flea beetle. They feed on the corn leaves and so cause wounds into which they introduce the bacteria. The beetles usually pick up the bacteria when they feed on leaves of diseased plants. The bacteria may live over winter within the bodies of adult beetles. Such beetles are responsible for the primary infec- tion each year as well as for secondary spread throughout the summer. Among the fungus diseases trans- mitted by insects are the Dutch elm disease and the blue stain of conifers, which are transmitted almost entirely by bark beetles. They are discussed on page 688. Another group of insect-transmitted fungus diseases are the fruit-spoilage diseases of figs. Most common are en- dosepsis, smut, and souring. Endosepsis, or internal rot, of the caprified fig is caused by a fungus and is transmitted by the fig wasp. The wasp develops only in figs. It is neces- sary for the pollination and normal development of the fig. It overwinters in the fruit of the mammae, or late summer, crop. In early spring the male wasps fertilize the females while they are still within the fruit. The females then leave the old mammae fruit and enter the young spring, or profichi, crop. There they lay eggs in the ovules of the young flowers that are within the young fig fruit. The eggs hatch into a new brood of wasps, whose fe- males emerge and enter the fruits of the summer, or mammoni, crop to ovi- posit. When they leave the profichi fruit they rub against staminate flowers that surround the "eye" and become covered with pollen. The pollen is car- ried into the mammoni fruit and ferti- lizes the developing florets. The in- sects cannot oviposit in mammoni fruit because the styles of its flowers are too long. Thus, although the mammoni fruit is effectively pollinated and de- velops normally, the insects do not de- Yearbook of Agriculture 1952 velop. The fruits of the mammoni crop constitute the edible fig of commerce. The female insects that transport the pollen from the profichi flowers to the mammoni flowers may also transport the spores of the fungus that causes the internal rot. The fungus forms numer- ous spores in the infected mammae and profichi fruits. The spores adhere to the body of the fig wasp as readily as do the pollen grains. Without the aid of the wasp, few or no spores would find their way into the fruit through the small "eye" through which the wasp enters. Some success in the control of the disease has been obtained by collecting the mammae fruits and disinfecting them internally to destroy the fungus spores before the insects emerge. When the insects emerge from the disinfected fruits, they are caught in glass tubes and later liberated in the orchards where they enter the profichi fruit free of fungus spores. This insures a healthy profichi crop so that the wasps leaving the profichi fruits and entering the mammoni fruits will not be contami- nated with fungus spores. Souring of figs begins as a fermenta- tion of the sugary sap of the ripe fruit by several species of yeast. The fruit is further decomposed by secondary fungi and bacteria. Both the common fig (which does not require pollination by insects) and the caprified fig are affected, but the disease is more prev- alent on the common fig. The yeasts are introduced into the fig fruit through the "eye" by two insects, the dried- fruit beetle and the pomace or vinegar flies, which enter in search of food. The yeasts seem constantly to be asso- ciated internally and externally with the insects. Because the yeasts grow also on many other kinds of spoiled fruits and the insects breed in them, the control of souring depends largely on destruction of the waste fruits in which the insects and the fungi breed. Smut of figs is not a true smut but a mold. It is caused by a strain of the common black mold that grows on all kinds of spoiled fruits. The fungus pro- duces spores in a black smutlike mass. Insects, Bacteria, and Fungi Some of the spores are introduced into the healthy fig fruits through the "eyes" by the same insects that trans- port the yeasts that cause souring. The control measures for smut, like those for souring, are based on sanitation and control of the insects by destroying the waste fruit in which they breed. Stigmatomycosis applies to a type of injury to plants long known to be associated with the feeding punctures of several kinds of true bugs. For many years the injury was attributed to the supposed toxic effect of the salivary secretions of the insects. It is really caused by fungi introduced into the plant tissues by the insects while feed- ing. The role of fungi in stigmatomycosis was discovered in a study of the bugs known as cotton stainers (Pyrrhocori- dae), which feed on cotton bolls. The bugs pierce the cotton bolls with their needlelike mouth parts. The cotton fibers beneath each puncture become stained and matted in a hard clump of worthless fibers. The staining of the fibers is caused by several species of a yeastlike fungus that are introduced on the mouth parts of the bugs when they feed on the cotton bolls. The staining occurs only on bolls that have been punctured by the bugs, and all evi- dence indicates that the disease de- pends entirely on the bugs for its en- trance into the bolls. The fungi are not constantly associated with the bugs but are picked up by them while they are feeding on infected material. When noncontaminated bugs feed on cotton bolls, the fiber is not stained and little injury is caused. Once a bug has been contaminated, however, it apparently remains contaminated for the rest of its life and introduces the fungus with each feeding puncture. Like relationships exist between other bugs and similar fungi on other crops. For example, the green stink bug transmits the fungus that causes the destructive yeast spot on lima beans. The kernel spot of pecan, long consid- ered to be caused by the mechanical 195 injury and toxic substances associated with the feeding of a stink bug, is now known to be caused by a fungus trans- mitted by the bug. In most of my examples of insect transmission, the insects make the wounds through which the fungi or bacteria penetrate the plant and also transport the micro-organisms from plant to plant. In some instances the insect may not be so important in trans- porting the spores of a fungus but may provide wounds through which wind- blown spores may enter. That appears to be the case in the association between the brown rot of peaches and plums and the plum cur- culio. A relationship between the cur- culio and brown rot of peaches and plums has been observed for a long time, but the importance of the re- lationship was not fully realized until organic insecticides like benzene hexa- chloride and parathion became avail- able and effective control of the cur- culio became possible. In orchards where the curculio is effectively controlled, much less brown rot occurs than where the curculio is not controlled. There is no evidence that the curculio is a major factor in disseminating the spores of brown rot, which are readily wind-blown. But the insects influence the development of brown rot by making wounds in imma- ture plums and peaches through which wind-blown spores are able to infect. The fungus has difficulty in infecting immature fruits if the skin is uninjured, but the fungus grows readily in the punctures made by the curculio. Spores formed on the injured green fruits provide an abundant source of infec- tion for the ripening fruit later in the season. The logical method of control for insect-borne diseases is to control the insect vectors. But that has not always worked, because our best methods for controlling the insects were not good enough. Many insect-control measures have reduced losses from direct-feeding injuries but have permitted enough in- 196 sects to survive to transmit the disease effectively. DDT, lindane, parathion, methoxychlor, and other new organic insecticides have given more complete control — good enough to give us the idea that the possible control of all in- sect-transmitted diseases should be re- considered from the standpoint of better control of the insect vector. Moreover, when it has not been pos- sible to control an insect effectively, it has been difficult to determine accu- rately to what extent the insect is re- sponsible for transmitting a disease. By using the more effective insecticides to get more nearly complete control of known or suspected insect vectors, a more accurate measure of the impor- tance of insect transmission of many plant diseases can be had. A relationship also exists between insects and the rust fungi. Many of the rust fungi, such as the destructive black stem rust of cereals, reproduce sexually and produce structures that have func- tions comparable to the male and fe- male organs of the flowering plants. To complete the life cycle, a male cell must enter the female organ so ferti- lization can take place. The rust fungi depend largely on insects for this proc- ess of "pollination." In the black stem rust of wheat the process occurs on the leaves of the bar- berry bush (Berberis vulgaris, B. cana- densis, and B. fendleri) , which is the alternate host of the fungus. The spores come from the grass host and infect the barberry leaf. They are of two sexes usually designated as + and — , because there are no morphological differences that would identify them as male and female. On the barberry leaf each spore produces a spot in which are formed numerous flask-shaped structures, the pyenia. Each pyenium produces thou- sands of small spores (pyeniospores) and numerous short hyphae. If the pyenium originated from a + spore, the nuclei in the pyeniospores and hyphae are of the + sex. Those arising from — spores have nuclei of the — sex. The spores function as gametes com- Yearbook of Agriculture 1952 parable to the pollen of higher plants. The fungus hypha correspond in func- tion to the stigmata and are called re- ceptive hyphae. If a pyeniospore comes in contact with a receptive hypha of the opposite sex, it germinates and fuses with a cell of the receptive hypha. The nucleus of the spore passes into the cell of the receptive hypha and be- comes associated with the nucleus of the opposite sex and eventually fuses with it, thus effecting fertilization. The pyenia are self-sterile — the spores pro- duced in a + pyenium will not fuse with the receptive hyphae of the same pyenium or of other pyenia of the same sex. They must be transported to a re- ceptive hypha of the opposite sex if fertilization is to take place. The pyenia are produced on the upper side of the barberry leaf in a bright yellow spot, and the spores and receptive hyphae are covered with a drop of sugary, fragrant solution. Flies and other insects are attracted to in- fected barberry leaves by the bright color of the spots and the solution, on which they feed. In feeding on the so- lution and moving from one spot to an- other, the insects transfer spores from + to — pyenia and vice versa, thus insuring "pollination" of the fungus. Sexual reproduction often results in hybridization between different races of rust and results in the production of new races, some of which will attack the new varieties of wheat that have been bred for rust resistance. Thus the insects, with the aid of the barberry bushes, are breeding new varieties of rust almost as fast as the plant breeders can breed new varieties of wheat. J. G. Leach has been head of the department of plant pathology and bacteriology in West Virginia Univer- sity since 1938. Before that he was pro- fessor of plant pathology in the Uni- versity of Minnesota. He has done ex- tensive research with insects in relation to plant diseases. He is author of a book, Insect Transmission of Plant Dis- eases. He is a former president of the American Phytopathological Society. The Nature of Insecticides Can Insects Be Eradicated? Clay Lyle We know that insects have survived for 250 million years and that they are endowed with marvelous mechanisms by which they should be able to survive for many more years. We know also that no species of insect has disap- peared from the earth because of man's activities, as have the dodo, the pas- senger pigeon, and some other animals. Yet I give an unqualified yes to the question, Can insects be eradicated? It is possible to wipe out destructive insects and it is desirable to do so. When insects first migrate to a new locality they should be destroyed, while their numbers are still small, even at great expense, lest they continue to spread and cause losses to farmers that year and every succeeding year. Several insects have been eradicated from such large areas that the complete extermination of their species through- out the world could probably be ac- complished. It is true, though, that climate, natural enemies, food supply, and some other factors that affect any one species vary so greatly the world over that eradication might be practi- cable in one country and unimportant or impossible in another. Three insects and one snail which had become well established in the United States have been eradicated and were not known to occur within our continental limits in the year 1952. The Mediterranean fruit fly was ex- terminated from parts of 20 counties in Florida in about a year — an out- standing example of eradication. The parlatoria date scale was de- stroyed in several places in Arizona, California, and Texas. The citrus blackfly was expelled from Key West, Fla., although fears of a reinfestation of the United States, from Mexico have been expressed. The white garden snail has been eradicated from several counties in southern California. Several other pests have been ex- terminated within definite areas, al- though they are still present in other sections of the United States or even in the same areas after reinfestation from outside sources. Among them are : Pink bollworm, from northern Flor- ida, Georgia, and large areas in Texas, New Mexico, Arizona, Oklahoma, and Louisiana, some of which have been re- infested from Mexico. Sweetpotato weevil, from areas in several Southern States, which have since become reinfested. Gypsy moth, entirely from Pennsyl- vania and New Jersey and greatly re- duced in some other Eastern States. Argentine ant, from several towns in Mississippi. Citrus whitefly, from 16 counties in California; new infestations occurred after 1942 in 2 counties in California, but were eradicated by 1 950. Obscure scale, from Los Angeles and San Diego Counties, Calif. Cattle tick, practically eradicated from the United States after a fight of more than 50 years. Effective methods of eradication were known before 1900 197 i98 but could not be used successfully until the farmers of the South realized the importance of livestock production. The tick carries the protozoan organ- ism that causes Texas fever. Efforts to eradicate insects in other countries have also been successful in several instances. The Colorado potato beetle first ap- peared in Europe in Germany in 1877. It was found again in 1887, 1914, and 1 934. Each time it was promptly eradi- cated, but Germany, Belgium, the Netherlands, and Switzerland have since become infested through its spread from France, where it was first found in 1922. Infestations in England in 1 90 1 and 1933 were quickly stopped. Rcinfestations still occur in England as the beetle spreads over continental Europe. The dangerous African mosquito {Anopheles gambiae) , which caused 20,000 deaths in Brazil in 1938 and 130,000 in Egypt in 1943 by transmit- ting malaria, was apparently eradi- cated from Brazil by 1940 and from Egypt by the end of 1945. The Rocke- feller Foundation assisted the govern- ments of the two countries in the work. Large areas of Brazil have also been freed of the yellow-fever mosquito. Sleeping sickness in sections of Africa is being reduced through the eradica- tion of several species of tsetse flies (Glossina spp.) by chemical and cul- tural methods. The brown-tail moth has been eradi- cated from Nova Scotia and New Brunswick ; the gypsy moth from south- ern Quebec and New Brunswick; the codling moth from western Australia; and two species of cattle grubs (north- ern cattle grub and common cattle grub) from Clare Island, Ireland. Most of the foregoing examples of eradication occurred before the devel- opment of the new insecticides and equipment. Several other pests could doubtless have been wiped out except that the necessary measures would have been considered as interference with an individual. For example, hu- man lice could easily be eradicated in Yearbook of Agriculture 1952 Sheep bot fly. this country but a Nation-wide com- pulsory physical examination would be necessary to find the few infested per- sons. The boll weevil, which has caused millions of dollars of damage to cotton every year, could be eradi- cated quickly by establishing a series of zones across the Cotton States in which no cotton could be grown for a while. The farm adjustments and loss of income for even a year to ginners, oil millers, and others would keep any State from adopting the necessary leg- islation, however. If we examine again the insect prob- lems of the United States and take into consideration the value of the new chemicals and machines, very likely we would agree on the practicability of a full-scale onslaught against other pests, especially those that attack livestock. One of the first would be the two species of cattle grubs, which cause an estimated annual loss in the United States of 100 million to 300 million dollars. The eradication of both from Clare Island by the slow painful method of squeezing the grubs out of the animals by hand shows that the present convenient and inexpensive chemical treatment could be effectively used for eradication if the public de- manded it. Any community that undertakes to eradicate cattle grubs might well in- Can Insects Be Eradicated? Human bot fly. elude cattle lice in the program; the cost of eliminating the two groups of pests would be little more than the cost for one alone. Cattle lice were almost unknown in parts of the South during the compulsory tick-eradication pro- gram, and it seems certain that present methods of controlling lice would re- sult in quick eradication in any areas that undertake to do so. The screw-worm does not ordinarily overwinter north of Florida and the extreme southern part of Texas, but it occurs much farther north in mild win- ters. From those areas it spreads north each season. If it would be stamped out in Florida during a cold winter, all the Southeastern States would be freed from attacks. Eradication seems quite possible technically, especially with the improved treatments that kill the adult flies and the larvae, but the presence of wild hogs and other wild animals in remote areas would make eradication more difficult. The effective control of the sheep bot fly with a Lysol nose drench makes it possible to get rid of the pest, because it overwinters only in the nasal pas- sages of sheep. Associations of sheep raisers might well consider a combined program to eradicate the sheep-tick and the sheep bot fly at one time. The reduction in the number of horses on farms in the United States 199 has made the eradication of the horse bot fly, nose bot fly, and throat bot fly only a question of whether there is enough interest to justify such an un- dertaking. Controls are effective and areas could be cleaned up quickly in a vigorous campaign. The eradication in any State of the several livestock pests I have men- tioned probably would not be too diffi- cult technically, but much of the value would be lost unless the programs were undertaken on a national or continen- tal basis, for some of the pests would quickly spread back from other areas. Required, therefore, would be the con- certed, simultaneous effort of the States to bring about the desired results. Pests more or less restricted to the bodies of their hosts, such as cattle lice, the sheep bot fly, and the sheep-tick, might be eradicated within limited areas and their reintroduction prevented by strict enforcement of quarantine measures, but I think it would be more desirable, even so, to have a Nation-wide pro- gram. That is a challenge to entomologists and farmers. No eradication project can succeed, no matter how effective the controls devised by the entomolo- gist, without the full cooperation of farmers in initiating and supporting the necessary enforcement laws and regulations and in carrying out the recommendations. Clay Lyle is director of the Missis- sippi Agricultural Experiment Station and Agricultural Extension Service and dean of the School of Agriculture of Mississippi State College. Before he as- sumed those positions on July 1 , 1951, he was entomologist for the Mississippi Agricultural Experiment Station, en- tomologist and executive officer of the State Plant Board of Mississippi, pro- fessor and head of the Department of Zoology and Entomology, and dean of the School of Science of Mississippi State College. He began his entomolog- ical work in Mississippi in IQ20. He has degrees from Mississippi State College and Iowa State College. How Insecticides Are Developed R. C. Roark New insecticides are developed in two ways. The first is by determining the struc- ture of the active principles of plants recognized as toxic to insects. Then the principles or other compounds closely related to them are synthesized — put together again to make the whole. The second is by testing compounds of known structure and unknown tox- icity upon several species of insects and selecting the ones that are effective. The first method starts with a mate- rial of known toxicity but unknown structure. The second starts with a compound of known structure but un- known toxic value. The insecticidal principles — parts or elements* — of plants have a compli- cated make-up. Even after their for- mulas are known it may be impossible to reconstruct them: The structural formulas of rotenone and deguelin have been known since 1932, but the chemist does not know how to attempt their synthesis. An example of the first method is the synthesis of anabasine. For that, nico- tine, a compound of known structure isolated from a plant, was used as a model. The chief insecticidal principle of tobacco, the liquid alkaloid nicotine, i-methyl-2-(3-pyridyl ) pyrrolidine, kills many kinds of insects. When a systematic search for new synthetic in- secticides was undertaken in the De- part of Agriculture in 1922 by C. R. Smith, he naturally turned to nicotine as the first model of compounds to be synthesized. Its structure was deter- mined a half century ago, but no com- mercially feasible process of making it synthetically on a large scale is known. Many derivatives of pyridine and pyrrolidine, the two rings of carbon and nitrogen atoms that are found in the nicotine molecule, then were prepared. The derivatives were tested on the bean aphid, a species highly susceptible to nicotine, but none approached nicotine in insecticidal effectiveness. Finally in 1928 Smith, by the action of sodium on pyridine, prepared 2- (3- pyridyl) piperidine, a compound con- taining the same number of carbon, hydrogen, and nitrogen atoms as nico- tine but arranged differently. This isomer of nicotine proved even more effective than nicotine for killing aphids. Because of its resemblance to nicotine, Smith named the new com- pound neonicotine. Shortly after its synthesis was announced, Russian chemists found the alkaloid in Anabasis aphylla, a weed belonging to the goose- foot family, and named it anabasine. Another example is the synthesis of allethrin. For nearly 30 years chemists sought to learn the nature of the insec- ticidal constituents of the pyrethrum flowers. Two Swiss chemists, H. Stau- dinger and L. Ruzicka, in 1924 an- nounced that two compounds contain- ing carbon, hydrogen, and oxygen in pyrethrum flowers were responsible for the insecticidal value of the flowers. They explained the structure of the compounds, called pyrethrin I and pyrethrin II, and described their un- successful efforts to synthesize them. F. B. LaForge and associates in the De- partment of Agriculture reexamined pyrethrum in 1934 and discovered two additional insecticidal esters in the flowers. They named them cinerin I and cinerin II. Of the four active principles in pyre- thrum flowers, cinerin I has the sim- plest structure. It was taken as the pat- tern when synthetic work was under- taken. Compounds closely related to cinerin I were made. One of them, called the allyl homolog of cinerin I, was found to ecjual the natural com- pound in killing house flies. About a dozen steps are required in synthesizing it. The large-scale manufacture of the 200 How Insecticides Are Developed ester has been accomplished, and 10,- ooo pounds of it were used in 1951 in liquefied-gas aerosol bombs. To avoid the cumbersome "allyl homolog of cin- erin I," the name allethrin was coined for the compound. Allethrin is a light-yellow, viscous liquid. It has a slight but pleasant odor. It is insoluble in water but readily sol- uble in the solvents used in fly sprays and in Freons 1 1 and 12, used in aero- sol bombs. It is more stable than pyre- thrum extract and is free from the Freon-insoluble material present in the natural product. Allethrin has been tested by pharmacologists and pro- nounced to be as safe as pyrethrum to man; pyrethrum is regarded as the least objectional of all insecticides in toxicity to people. Its many desirable properties should mean a wide use of allethrin in aerosol bombs, fly sprays, and agricultural insecticides. The de- velopment of allethrin is a vindication of the thesis that it is possible to de- velop synthetic insecticides that rival the constituents of insecticidal plants, but this achievement is possible only when the structure of the plant insecti- cide is known. Another insecticide of plant origin is scabrin, a constituent of the root of Heliopsis scabra. An. account of the work leading to the discovery of this toxicant illustrates the method of de- veloping new insecticidal chemicals through research on insecticidal plants. In 1943 the division of insecticide investigations of the Bureau of Entom- ology and Plant Quarantine received from Mexico City the roots of a plant reported to be used by Mexicans as an insecticide. The plant was incorrectly labeled Erigeron affinis, but Depart- ment botanists later identified it as Heliopsis longipes. The active principle was isolated and was identified as n- isobutyl-2,6,8-decatrienamide. Three other species of the genus Heliopsis were collected in several parts of the United States and tested for in- secticidal value. Laboratory tests dis- closed that all the species, particularly their roots, were toxic to house flies. 201 From the most toxic of the species, Heliopsis scabra, there was isolated an amide C22H35NO, called scabrin, which proved to be nearly three times as toxic as the pyrethrins to house flies. The first synthetic organic com- pounds used to kill insects were em- ployed as fumigants. Carbon disulfide, made by the direct combination of car- bon and sulfur, may be regarded as one of the simplest organic compounds. It was first used as an insecticide nearly 100 years ago in France. Paradichloro- benzene, originally a byproduct in the manufacture of chlorobenzene, was used as a substitute for naphthalene in combatting clothes moths in Germany in 191 1. Chloropicrin emulsified in water was proposed as an insecticide in Austria in 1907 and was tested as a fumigant in the United States about I9I7- In 1922 a systematic search for new fumigants was undertaken by Depart- ment chemists and entomologists with the object of finding substitutes for the dangerously inflammable carbon disul- fide widely used for fumigating weevily grain. The search resulted in the dis- covery of several new fumigants, all synthetic organic compounds. Among those that have come into commercial use are ethylene dichloride; propylene dichloride; dichloroethyl ether; ethyl- ene dibromide; the methyl, ethyl, and isopropyl esters of formic acid; and ethylene oxide. About 10 years later methyl bromide was first used as an insecticidal fumigant in France. D-D mixture (containing 1,3-dichloropro- pene, 1,2-dichloropropane, and other chlorides) has come into use in Cali- fornia and the Hawaiian Islands as a soil fumigant. As I mentioned, the early synthetic work in the Department of Agriculture to develop new contact and stomach poisons for insects was based on nico- tine as a model. Later the empirical method of approach was used — syn- thetic organic compounds were tested irrespective of their structure. The work led to the development of pheno- thiazine as a pesticide. First it was tested against mosquito larvae and found to be highly toxic to them. It was then tested against a variety of agricultural pests and found to be amazingly effective in controlling codling moth on apple. More than 3 million pounds were used as an intes- tinal worm remedy in 1951 . The modern synthetic chlorinated organic insecticides DDT and benzene hexachloride were discovered in the same way that phenothiazine was dis- covered— that is, by screening thou- sands of compounds of known struc- ture but unknown toxic value. As yet too little is known of the relationship between the chemical structure of com- pounds and their insecticidal value to serve as a guide to the synthesis of new insecticides. Every candidate insecti- cide must be tested against the insect it is designed to control. Often compounds closely related chemically differ widely in insecticidal value. As more compounds are tested, the chemist should be able to find a relationship between the chemical structure and insecticidal value of or- ganic compounds. Eventually he will be able to synthesize a compound for the control of a specific insect pest. Meanwhile the study of the constitu- ents of insecticidal plants will help en- large our knowledge of how chemical structure affects toxicity. R. C. Roark is in charge of the di- vision of insecticide investigations, Bu- reau of Entomology and Plant Quar- antine. He has been engaged in re- search on insecticides since igio. He is a native of Kentucky and holds de- grees from the University of Cincin- nati, University of Illinois, and George Washington University, hi 1948 his di- vision received an award for Distin- guished Service from the Secretary of Agriculture for chemical research that discovered new insecticidal chemicals, new means of increasing the usefulness of insecticides, new methods of chem- ical analysis, and new ways of applying insecticides. How Insecticides Are Mixed H. L. Haller Insecticidal chemicals have to be mixed properly before they can be used as insecticides. To use them in pure or undiluted form would often be too costly, and their physical properties — usually they are coarse or sticky solids or viscous liquids — make them un- suited for direct application. Dust dil- uents, solvents, and wetting, emulsify- ing, spreading, penetrating, sticking, and stabilizing agents are added to them. Proper mixing of the accessory com- ponents has to take into account sev- eral factors : Whether the preparation will be applied to plants, animals, or humans ; whether it will come into con- tact with foods or feeds; the insect to be controlled; the cost of the treat- ment; the ease of application; and the effect of the accessory materials on the toxicity. For example : An oil solution would be chosen to protect a wooden post against termites because oil helps the insecticide to penetrate the post and give more than surface protection. But oils harm plants and on them water emulsions or dusts are used. Five types of insecticide formula- tions are employed to control insect pests: Dusts, wettable powders, emul- sions, solutions, and aerosols. Dusts are applied in hand dusters or power- driven devices; wettable powders, emulsions, and solutions are applied as sprays. Aerosols are of the liquefied gas type or smokes or mechanically gen- erated oil clouds. Before proceeding to consider them, let me define some terms that apply here and to other chapters in this book. A pesticide is a substance or mixture of substances that may be used to de- 202 How Insecticides Are Mixed stroy or otherwise control any un- wanted form of plant or animal life. (The ending -cide means killer.) Among the many types of pesticides are: An insecticide is used against insects and their near relatives. Insecticides of more specific use are often designated by such terms as larvicide, aphicide, or miticide, which kill larvae, aphids, and mites, respectively. A fungicide is used against fungi, particularly those causing diseases of plants. Some fungicides also act as in- secticides. A herbicide is used against plants growing as weeds. It often is called a weed killer. A rodenticide is used against ro- dents, especially rats and mice. Some of the terms pertaining to in- secticides are: Wettable powders are insecticidal materials manufactured into powders that can be readily mixed with water. They often contain wetting and condi- tioning agents. Suspension sprays are mixtures in which the finely divided particles of powdered insecticide are dispersed in a liquid. Emulsion concentrates are insecti- cidal materials manufactured into liquid concentrates so formulated that they will form an emulsion when mixed with water or another liquid. Emulsion sprays are mixtures made with emulsion concentrates and a liquid, usually water. Conventional or dilute sprays con- tain a relatively small amount of insec- ticide in a relatively large amount of water, such as 4 pounds per 100 gallons. Concentrated sprays contain large amounts of insecticides in small amounts of liquid, such as 1 pound in 1 to 5 gallons. Terms like stomach insecticide and contact insecticide indicate the way the insecticide enters the body of the in- sect. A stomach insecticide is eaten and swallowed. A contact insecticide enters through the skin. The terms have no 203 significance as to how or where the ma- terials exert their effect. Some sub- stances can enter in only one way. Others reach the vital organs in both ways. Insecticidal dusts usually are made with talcs, clays, and diatoma- ceous earth. Sometimes finely ground plant material, such as walnut-shell flour, is used. Diluents of dusts are classed accord- ing to whether they have low or high bulk density. By this is meant the weight of the dust occupying a definite volume. The low bulk density, or light, type is illustrated by silica gel, hy- drated alumina, calcium silicate, and diatomaceous earth. Examples of the high bulk density, or heavy, type are pyrophyllite, talc, calcite, and clays. Mixtures of both types often are used to prepare products that have practical bulk-density values and will also resist caking on storage at high tempera- tures. The use of the heavy diluents alone may yield products that become packed or lumpy on storage. Dusts also may be prepared by mix- ing a solution of the insecticidal chem- ical in a volatile organic solvent, such as acetone or benzene, with the dust diluent; the solvent is then allowed to evaporate, and the mixture is ground. Or a solution of the insecticide may be sprayed into the dust diluent during the mixing and grinding process. Sometimes the chemical is dissolved in a nonvolatile solvent and mixed with the diluent. When this is done, care must be taken so that the amount of the solvent used is not so great as to im- pair the dusting qualities of the fin- ished dust. The concentration of the active ingredient of dusts ranges from 1 to 20 percent, depending on the in- secticide and its use. Wettable powders, which can be dis- persed or suspended in water for use as sprays or dips, are made by adding wetting agents to dusts. With some kao- lin types of clay as the diluent the addition of a wetting agent is unneces- sary. The wetting agent may be ad- 204 versely affected by the type of diluent or kind of water, such as extremely hard or highly alkaline. The amount of wetting agent must be carefully ad- justed to avoid excessive run-off when the spray is applied to plants. Emulsions are obtained by adding water to an emulsifiable — or emul- sion— concentrate. Such concentrates are made by dissolving the insecticidal chemical and an emulsifying agent in an organic solvent. Usually the solvent is substantially insoluble in water as water-miscible solvents have not in general proved satisfactory. Two general types of solvents have been used : ( i ) Solvents, such as tolu- ene or xylene, which evaporate after spraying or dipping to leave a deposit of the toxicant; and (2) nonvolatile solvents, such as alkylated naphtha- lenes or a petroleum oil, which leave the treated surface coated with a solu- tion of the toxicant in oil after the water has evaporated. Solvents such as toluene and xylene under certain con- ditions may constitute a fire hazard. The use of high-boiling aromatic sol- vents, such as the alkylated naphtha- lenes, may be dangerous when the emulsion is applied to animals. Three classes of emulsifiers are gen- erally recognized — anion-active, cat- ion-active, and nonionic. Soap is typi- cal of an anion-active agent. Lauryl pyridinium chloride is an example of a cation-active emulsifier. Nonionic emulsifiers, as the name implies, do not ionize. An example of a nonionic emul- sifier is the reaction product obtained from 10 to 12 moles of ethylene oxide to 1 mole of dodecyl alcohol. Several hundred emulsifiers are commercially available under various trade names. No one class of emulsifiers may be said to be superior to another. The type best suited will depend on the insecticide and can only be determined by ex- perimentation. When extremely small particle size or permanence of the emulsion is not essential, or if agitation can be maintained after the concen- trate has been diluted with water, the Yearbook of Agriculture 1952 proportion of emulsifying agent in the concentrate may be reduced consid- erably. Oil solutions of insecticidal chemi- cals are usually made with crude or refined kerosene and other petroleum oils. The selection of a solvent depends on its ability to hold the chemical in solution at ordinary temperatures, whether it is toxic to plants, and whether it constitutes a fire hazard. Sometimes more than one solvent is used, particularly when a preferred solvent does not dissolve sufficient of the insecticidal chemical to provide a solution of the desired concentration. An example is DDT in refined kero- sene. This solvent does not dissolve enough DDT to permit the preparation of a 5-percent solution. One has to add an auxiliary solvent. Auxiliary solvents dissolve larger quantities of the chemi- cal. Cost, toxicity, and fire hazard keep them from being used as the only sol- vent. When a solution is made up of two solvents, one of which is a poor one and the other a good one for a particular chemical, the solubility in the mixed solvent may not equal the sum of the solubilities in the individual components. The quantity of a chemi- cal that a solvent will dissolve varies widely with the temperature. All in all, so many factors are in- volved in the formulation of insecti- cides that it usually behooves a person to buy the ready-to-use insecticides rather than to try to mix the chemicals himself. H. L. Haller is assistant chief of the Bureau of Entomology and Plant Quarantine. His duties cover the vari- ous chemical aspects of the Bureau's problems and involve the development and use of products for the control of insect pests. He has been engaged since iQ2g in studies on insecticides in the Department, which he joined in igig following service in the First World War. From ig2^ to ig2g he was on the staff of the Rockefeller Institute for Medical Research as an associate in chemistry. How Insecticides Poison Insects John J. Pratt, Jr., Frank H. Babers Somebody has said that because in- sects are small an insecticide kills them all over. Our knowledge of the subject is incomplete, but it is enough to belie the statement. Poisons affect the normal functions of specific cells and tissues of insects just as they are known to do in humans and other higher animals. Basically some chemical process in the animal is affected so as to bring about changes in its functions. Those changes are sec- ondary to the original process that was affected and are frequently mistaken for the initial action of the poison. A complete knowledge of the way a chemical poisons an insect would have great value in the formulation of in- secticides. While preparing an insecti- cidal mixture, for example, we could add a substance that would help the poison reach the target — the organ or tissue it acts upon. Chemicals could be added to weaken or destroy the mecha- nisms that protect the insect against the poison in question. If we know how one poison acts, we could select or synthesize other chemicals of similar action. Research is giving us that knowledge so that before too long such ideals should become realities. Insecticides have been classified ac- cording to the way they get into the insect's body cavity: Stomach poisons are eaten, contact poisons enter through the skin, and fumigants enter through the breathing tubes or the skin as gases. Some insecticides may enter by all three routes. But often such a classification is used wrongly to refer to the mode of action of an insecti- cide— an entirely different term, which means the way in which a chemical acts on an animal's system. 970134° — 52 15 In studying the mode of action of an insecticide, we often rely for clues on what we know of the action of the poi- son on man or other higher animals. Sometimes the mode of action may be similar in vertebrates and in insects, but without experimental evidence it is unwise to assume that such a similarity exists. The poisonous properties of the in- organic arsenic compounds (paris green, calcium and lead arsenate, sodium arsenite) are due to the forma- tion of the water-soluble compounds, arsenious or arsenic acid, in the diges- tive tract. Arsenic is considered a general pro- toplasmic poison; that is, it poisons the contents of all types of cells. Most tis- sues and organs therefore are affected in arsenic poisoning. One well-known effect of arsenic on vertebrate animals is the abrasion and destruction of the lining of the intestine. A similar de- struction occurs in the mid-intestine of insects. Often it is said that such de- struction is the primary reason that arsenic insecticides kill insects. If that were true, it still would not explain what biochemical process is disturbed in order to bring about destruction of the intestinal cells. Investigations with vertebrate animals have shown that arsenic poisons unidentified enzymes, which function in the metabolism of carbohydrates by cells. Probably arse- nic acts on the insect system in the same manner. Nicotine first stimulates and then de- presses the nervous system of animals. Paralysis follows rapidly and results in the failure of organs to function. In insects, as in higher animals, the poi- soning action of nicotine occurs in the nerve ganglia, which are clumps of nerve tissue at various places in the nervous system. Nicotine seems to have practically no effect on nerve fibers or on the junctions of nerves with mus- cles. The chemical process of nicotine poisoning in insects is not known. Pyrethrum powder, the ground flow- ers of certain species of the chrysan- themum, contains the chemicals, py- 205 206 rethrin I and II and cinerin I and II, which are the main toxic principles. The rapid paralyzing action of pyre- thrum is evident to anybody who has sprayed a room with a household fly spray and watched the flies drop almost immediately to the floor. The insects recover from the paralysis, however, unless a lethal amount of the poison gets on them. Pyrethrin acts directly on the central nervous system of insects. The paralysis is a result of the block- ing of transmission of nerve impulses. We know that destructive changes oc- cur in the nervous tissue of insects poi- soned with pyrethrin, but the reason for the changes is obscure. Rotenone causes paralysis of the breathing mechanism in mammals, possibly by acting on bronchial tissues. All we know now about the method by which rotenone kills insects is that it slows the rate of heart action and breathing. The symptoms may indicate disturbances in the functions of prac- tically any tissues so they really tell us little of the fundamental basis for rotenone poisoning. Several theories have been advanced to explain how oils kill insects: Oils penetrate the insect's breathing tubes, thus causing suffocation; or they pene- trate the tissues and poison them; or certain poisonous, volatile substances in the oils kill by penetrating the tissues as gases. None of the theories has been proved. Maybe each may have some merit, depending on the oil in ques- tion. Nonvolatile oils (such as mineral oil) that contain no poisonous com- pounds might kill an insect through suffocation. For oils (such as kerosene) that contain volatile, poisonous con- stituents, the second and third theories might account for the killing action. In vertebrates, such volatile petro- leums as gasoline act first as stimulants then as depressants of the central nerv- ous system. Death is due to respiratory failure if the animal is exposed to the oil for a long time. Work done by George D. Shafer many years ago at the Michigan Agricultural Experiment Yearbook of Agriculture 1952 Station indicates that a similar action occurs in insects. E. H. Smith and G. W. Pearce of the New York State Agri- cultural Experiment Station demon- strated that oil does not kill eggs of the oriental fruit moth by depriving them of oxygen (suffocation) . They ob- tained some evidence that the oil pre- vented unknown poisonous substances formed by the egg from passing out- ward through the eggshell. The dinitrophenols are used in sev- eral phases of insect control — most commonly the sodium, calcium, and dicyclohexylamine salts of 2,4,dinitro- 6-cyclohexylphenol and the sodium and calcium salts of 4,6,dinitro-o-cresol. Dinitrophenol increases the meta- bolic rate of warm-blooded animals. Perhaps the poison acts directly on cells, causing them to increase the rate at which they use oxygen. Fat metab- olism is involved because the excess oxygen is used only for burning this body food. Dinitrophenol and dinitro- cresol act in the same manner on in- sects and raise the oxygen require- ments by as much as three times the normal amount. The mechanism by which the dinitrophenols cause cells to use abnormally high amounts of oxy- gen has not been determined. The characteristic tremors of DDT poisoning are symptoms of a disturb- ance of the nervous system. The sensory nerves — which carry impulses to the central nervous sys- tem— are the most sensitive to DDT poisoning, the nerve ganglia the least sensitive. When DDT gets on an in- sect's body, it affects hundreds of sen- sory nerve endings. The nerves then produce impulses faster and stronger than normal. These cause the nerves responsible for moving muscles to pro- duce the tremors typical of DDT poi- soning. The capacity of the central nervous system to coordinate sensory impulses is also disrupted — as seen in the stumbling gait and general in- stability of the insect. We do not know why DDT poisons nervous tissue. It has been suspected that DDT poisons the enzymes cholin- How Insecticides Poison Insects esterase, which is important in the proper functioning of nerves, but con- siderable research has failed to show that DDT affects the enzyme. Perhaps another enzyme system in nervous tissue is involved. One theory is that DDT causes a depletion of calcium in nervous tissue, which in turn causes spontaneous activity of the nerve. Promising leads are emerging from research on house flies that are resistant to DDT. Flies can change DDT in their bodies to a nonpoisonous sub- stance and DDT-resistant flies can do this faster than susceptible flies can. The chemical processes involved in this breakdown of DDT are being elu- cidated and should tell us much about the mode of action of DDT. Other effects of DDT on the physi- ology of insects include an increase in the consumption of oxygen and a de- crease in the amount of stored food substances in the body. Those are probably secondary effects of DDT poisoning. Benzene hexachloride occurs in sev- eral forms, or isomers, each of which has a slightly different molecular shape. Of the 1 6 possible isomers, 5 are known — the alpha, beta, gamma, delta, and epsilon. The gamma isomer, com- monly called lindane, is several hun- dred times more toxic to insects than the others are. In vertebrate animals, gamma ben- zene hexachloride causes stimulation of the central nervous system, but the beta and delta isomers cause depres- sion. The external symptoms of poison- ing in insects resemble those of DDT, except that they usually appear more rapidly. As in DDT poisoning, the tremors suggest an effect upon the nervous system, but whether the mech- anism of poisoning is the same as that of DDT remains for future research to explain. Shortly after the insecticidal prop- erties of benzene hexachloride were discovered, it was suggested that (be- cause of possible similarity in molecular shape) the poison might act as an anti- metabolite to myoinositol, one of the 207 B vitamins — that is, it might compete with and replace myoinositol in some vital physiological process. Myoinosi- tol will alleviate somewhat the poison- ing of certain yeasts by gamma benzene hexachloride, but several attempts to demonstrate a similar process in insects have failed. Chemical investigations, which now indicate that myoinositol and gamma benzene hexachloride do not have similar molecular shapes, may explain the failure to prove the hy- pothesis. The organic phosphates — hexaethyl tetraphosphate (HETP), tetraethyl pyrophosphate (TEPP), and diethyl jfr-nitrophenyl thiophosphate (para- thion) — are highly toxic to animals. In insects and in warm-blooded animals, they poison the cholinesterase. A chemical called acetylcholine is formed in certain nerves and aids in the transmission of nerve impulses. If it is not destroyed immediately after it has served its purpose, it will continue to cause impulses to move along the nerve. The enzyme cholin- esterase is always at hand to destroy the acetylcholine. The organic phosphate insecticides poison the enzyme, thus allowing the acetylcholine to accu- mulate, and cause uncoordinated nerv- ous activity through the whole animal. The results are tremors, convulsions, muscle paralysis, and finally death. It is possible that the organic phosphates poison insects in other ways, but the action we described is the major one now known. Another organic phosphorus com- pound that shows much promise for control of some insects and mites is schradan (octamethyl pyrophosphora- mide) . Many plants absorb it from the soil. Insects and mites that feed on the plant sap are poisoned. Schradan seems to have little effect on the cho- linesterase system of insects; it is not particularly toxic when it is sprayed on them. But the fact that the sap of plants that have taken it up is highly poisonous to cholinesterase indicates that the mode of action is the same as that of the other phosphates — only, 208 however, after it has been changed in some manner by plant tissue. Animal liver cells also increase the anticholin- esterase activity of schradan. Of the cyanides used in controlling insects, hydrocyanic acid, or prussic acid, is a liquid that evaporates rap- idly; calcium cyanide is a solid that gives off hydrogen cyanide gas more slowly. Both are classed as fumigants because the killing action is due to gas- eous hydrogen cyanide. Hydrogen cyanide is extremely toxic and acts quickly on all animals. In warm-blooded animals it poisons the enzymes that enable cells to use the oxygen supplied to them. As all living cells require a constant supply of oxygen, the failure of the supply results in the rapid and widespread poisoning of tissues that is characteristic of cyanide. The poisoning action of cya- nide on insects is probably the same, for the enzymes involved are common to practically all living cells. Methyl bromide, also used as a fumigant, is less toxic than hydrogen cyanide, and its poisoning action is much slower. The mode of action of methyl bro- mide on insects has not been studied. Research with vertebrates has yielded two opposing theories. One states that methyl bromide is changed in the animal to methyl alcohol and a harm- less bromine salt. The methyl alcohol then poisons the animal. Another the- ory proposes that the methyl bromide is not changed in the animal but poi- sons as methyl bromide. Whatever the mode of action may be in vertebrates, it will probably be similar in insects, for the effects of methyl bromide seem to be common to all animals. Ten years ago we had a dozen or so insecticides and knew little about their modes of action. Today we have sev- eral dozen new ones and know nothing of how they act. Entomologists are gradually turning from trial-and-error ways of discovering new insecticides, however. These are being replaced by research on the fundamental aspects of poisoning action. Eventually we will Yearbook of Agriculture 1952 be able to predict whether a chemical will be poisonous and to what insects. Then we can make insecticides to suit our needs. John J. Pratt, Jr., is an entomol- ogist in the Bureau of Entomology and Plant Quarantine. He has degrees from the University of Massachusetts, North Carolina State College, and Cornell University. During the war he served with the Army and the United States Public Health Service, and joined the Bureau of Entomology and Plant Quarantine in 1^48. Dr. Pratt conducts research on the physiology of insects. Frank H'. Babers, a biochemist in the Bureau of Entomology and Plant Quarantine, has charge of research on insect physiology and the mode of action of insecticides. For further reading: Dietrich Bodenstein: Investigation on the Locus of Action of DDT in Flies (Droso- phila), Biological Bulletin, volume go, pages 148-157. 1946. G. J. Goble and R. L. Patton: The Mode of Toxic Action of Dinitro Compounds on the Honeybee, Journal of Economic En- tomology, volume 39, pages 1 77-180. 1946. Louis Goodman and Alfred Gilman: The Pharmacological Basis of Therapeutics, The Macmillan Co., New York. 1941. Harold T. Gordon and John H. Welsh: The Role of Ions in Axon Surface Reac- tions to Toxic Organic Compounds, Journal of Cellular and Comparative Physiology, volume 31, pages 395—420. 1948. W. M. Hoskins: Recent Contributions of Insect Physiology to Insect Toxicology and Control, Hilgardia, volume 13, pages 307— 386. 1940. D. D. Irish, E. M. Adams, H. C. Spencer, and V. K. Rowe: Chemical Changes of Methyl Bromide in the Animal Body in Re- lation to Its Physiological Effects, Journal of Industrial Hygiene and Toxicology, volume 22, pages 408-41 1. 1941. S. Kirkwood and Paul H. Phillips: The Antiinositol Effect of y-Hexachlorocyclo- hexane, Journal of Biological Chemistry, volume 163, pages 251-254. 1946. Bernard P. McNamara and Stephen Krop: Observations on the Pharmacology of the Isomers of Hexachlorocyclohexane, Journal of Pharmacology and Experimental Therapeutics, volume 92, pages 140—146. 1948. Robert L. Metcalf: The Mode of Action of Organic Insecticides, National Research Council, Washington, 1948; Studies of the Mode of Action of Parathion and Its Deriva- tives and Their Toxicity to Insects, with Ralph B. March, Journal of Economic En- tomology, volume 42, pages 721-728, 1949. W. E. Ripper, R. M. Greenslade, and L. A. Lickerish: Combined Chemical and Bio- logical Control of Insects by Means of a Systemic Insecticide, Nature (London), volume 163, pages 787-/89. 1949. Kenneth D. Roeder and Elizabeth A. Weiant: The Site of Action of DDT in the Cockroach, Science, volume 103, pages 304— 307, 1946; The Effect of DDT on Sensory and Motor Structures in the Cockroach Leg, Journal of Cellular and Comparative Physi- ology, volume 32, pages 175—186, 1948. George D. Shafer: How Contact Insecti- cides Kill. I and II, Michigan Agricultural College Technical Bulletin 11, 191 1; How Contact Insecticides Kill. Ill, Technical Bulletin 21 , 1915. E. H. Smith and G. W. Pearce: The Mode of Action of Petroleum Oils as Ovicides, Journal of Economic Entomology, volume 41, pages 173-180. 1948. J. M. Tobias and J. J. Kollros: Loci of Action of DDT in the Cockroach (Peri- planeta americana), Biological Bulletin, volume 91, pages 247-255. 1946. G. W. van Vloten, Ch. A. Kruissink, B. Strijk, and J. M. Bijvoet: Crystal Structure of "Gammexane," Nature (London) , vol- ume 162, page 771. 1948. J. Franklin Y eager and Sam C. Munson: Physiological Evidence of a Site of Action of DDT in an Insect, Science, volume 102, pages 305-307- *945- The Organic Insecticides C.V.Bowen, S.A.Hall The best known of the synthetic or- ganic insecticides is DDT, but it was not the first. Some of them have been in use for decades. Carbon disulfide, /j-dichlorobenzene, and naphthalene stand out as old-timers. Ethylene di- chloride, ethylene dibromide, methyl bromide, and thiocyanates have been used for the past quarter century. Thousands of similar compounds — man-made materials whose basis is carbon — have been investigated as to insecticidal value. The Department of Agriculture in 1922 or so began a study of their use as repellents and fumigants and began later the synthesis of mate- rials for testing as poisons for insects. Phenothiazine, thiodiphenylamine, introduced as an insecticide in 1935, may be considered one of the early members of the newer synthetic age. It is used now to only a limited extent as a codling moth insecticide, but it is used extensively for the internal medi- cation of livestock for the control and removal of injurious nematodes that infest cattle, horses, sheep, and goats. H H CSC / \ / \ / \ HC C C CH I CH Fiea beetle. HC C C \ / \ / \ / C N C H H H Phenothiazine Azobenzene, an orange crystalline material, was found in 1 943 to be effec- tive as a fumigant for the control of mites in greenhouses. Because azoben- zene sublimes readily, a solution con- taining it may be applied to steam pipes and allowed to vaporize. The de- 209 210 velopment of the organic phosphorus compounds, however, has greatly less- ened its use. H H C C / \ / X HC C— N=N— C CH HC \ CH HC C II c II / CH Azobenzene A group of dinitro derivatives of phenol and eresol came into use before the Second World War as dormant sprays in apple orchards. The simplest of these, 4,6-dinitro-o-cresol, DNOC, formerly known as 3,5-dinitro-o-cresol, is a solid melting at 85.8 ° C. and not very soluble in water. The sodium de- rivative (called a salt) often is used in the dormant sprays because of its greater solubility in water. Analogs — similar substances — in which the methyl group of the eresol has been replaced by cyclohexyl or by some other group are also used. Salts other than sodium, such as the dicyclohexylamine or triethanolamine salt, are also in use. OH i / \ 02NC CCH3 I II HC CH \ / C N02 4,6-Dinitro-o-cresol OH 02NC HC / C \ CCeHn c N02 2-Cyclohexyl- 4,6-dinitrophenol (DNOCHP) Aliphatic, alicyclic, and aromatic esters of thiocyanic acid were used be- Yearbook of Agriculture 1952 fore the Second World War in house- hold and horticultural sprays. The so- called lauryl thiocyanate is a mixture of compounds containing alkyl groups derived from the natural fatty acids of coconut oil in which the lauryl, or 12-carbon, chain predominates. Other thiocyanates used as insecticides are the 2-(2-butoxyethoxy)ethyl ester of thiocyanic acid, diethylene glycol di- ester of thiocyanic acid, /?-thiocyano- ethyl esters of aliphatic fatty acids averaging from 1 o to 18 carbon atoms, and isobornyl thiocyanoacetate. Be- cause some of them may injure grow- ing plants, care should be exercised in using them. In recent years a great deal has been said about chlorinated hydrocar- bons as insecticides. The use of this type of compound is not new, for car- bon tetrachloride and p-dichloroben- zene, which have been used for years, are chlorinated hydrocarbons. DDT is a chlorinated hydrocarbon insecticide. The raw materials for its manufacture are chlorine, benzene, and alcohol. DDT was first described in 1874 W a German chemist, Othmar Zeidler, but its insecticidal value was not discovered until about 1939 by Paul Miiller, in Switzerland. It was first introduced into the United States in August 1942 when the dye firm of J. R. Geigy shipped from Switzerland to New York two formulations — a dust and a wettable powder — for testing by American entomologists. Later undi- luted DDT was imported, and in June 1943 the manufacture of DDT was be- gun in the United States for use by the Armed Forces. When the end of the Second World War made DDT avail- able for civilian use, it came into large- scale use as an insecticide. The symbol DDT combines the first letters in the name dichloro-diphenyl- trichloroethane. The precise chemical name for the principal toxic ingredient of technical DDT is 1,1,1-trichloro- 2, 2-bis(/?-chlorophenyl) ethane. A dis- cussion of its chemistry was presented in Science in Farming, the Yearbook The Organic Insecticides of Agriculture for 1943- 1947. DDT is unstable in the presence of alkalies and consequently is not compatible with alkaline agricultural chemicals. It is also decomposed by iron and some iron salts. Benzene hexachloride, or 1,2,3,4,5,6- hexachlorocyclohexane (i. e., BHC, HCH, or HCCH) , is a chlorinated hy- drocarbon made by reacting chlorine with benzene in the presence of ultra- violet light to produce a compound with the molecular formula which the English designated as 666. The mate- rial was first made by Michael Faraday in 1825, but its insecticidal action was not known until many years later. Harry Bender, an American chemist, in a United States patent application for a method of chlorinating hydrocar- bons, mentioned in 1933 that the ben- zene hexachlorides appeared to be good insecticides, but apparently no use was made of the idea. A. P. W. Dupire in France in 1941 applied for a French patent on the use of benzene hexachloride as an insecticide based on entomological tests conducted in 1940. In 1942 a sample of benzene hexa- chloride made by F. D. Leicester in England was found to be insecticidal, and the compound came into use in that country in place of derris in flea beetle powder. Technical benzene hexachloride is made up of a mixture of isomers, com- pounds that are identical in chemical structure except for a difference in the orientation in space of some of the con- stituent atoms. F. J. D. Thomas in England in 1943 found that the insec- ticidal principle of technical benzene hexachloride was the gamma isomer. The isomers had been named alpha, beta, gamma, and delta in the order in which they had been isolated, alpha and beta by F. E. Matthews in 1891 and gamma and delta by T. von der Linden, a German chemist, in 19 12. In 1949, the common name lindane was selected for the gamma isomer of benzene hexachloride of not less than 99 percent purity after von der Linden. The gamma isomer comprises about 211 12.5 percent of crude benzene hexa- chloride. Because of its odor and the off-flavor it imparts to certain food products, technical benzene hexa- chloride is limited in use. Lindane, however, is practically odorless. Inves- tigations were started in 1947 to check its effect on the flavor of fruits, vegeta- bles, and meats. It is a white crystalline solid, soluble in most of the common organic solvents but insoluble in water. It has some fumigant properties and is a contact and stomach poison. In the early study of DDT, analyses of the technical material revealed 4 percent of an impurity that has insecti- cidal properties. The impurity was identified as i,i-dichloro-2,2-bis(/?- chlorophenyl) ethane, a byproduct of the reaction used in making DDT. The compound has been referred to as DDE) and as TDE, from its generic names dichloro - diphenyl - dichloro- ethane and tetrachloro - diphenyl- ethane. It is closely related to DDT in chemical structure and properties. It will react with alkalies and conse- quently should not be formulated with alkaline materials. H H H H C=C C— C / \ H / \ C1C C— C— C CC1 \ / I \ / C— C HCCI2 c=c H H H H TDE Another of the analogs of DDT has been given the common name meth- oxychlor because it has a formula in which two of the chlorine atoms of DDT have been replaced by the meth- oxy group (CHsO-) . Like DDT, tech- nical methoxychlor (which contains about 80 percent of i,i,i-trichloro-2-2- bis ( p-methoxyphenyl ) ethane ) also is a white solid, soluble in the common organic solvents and insoluble in water. It is less effective than DDT against most insects but is less toxic to warm- blooded animals. Alkaline materials promote decomposition of methoxy- chlor and consequently must not be used in its formulations. 212 Analogs of DDT containing bromine and fluorine have been tested for insec- ticidal action. 2,2-Bis(/?-bromophe- nyl ) - 1 , i , i -trichloroethane sometimes has been referred to as Colorado 9. 1,1,1 - Trichloro - 2,2-bis(p-fluorophe- nyl) ethane is a constituent of an insec- ticide which the Germans called Gix. Although insecticidal in action, neither material has come into commercial use in the United States, probably because their cost is greater than that of DDT. H H H H C— C XCCI2 C=C S \ I / \ R— C C— C— C C— R \ / I \ -/ C=C H C— C H H H H R=C1 X=C1 DDT =CHsO- =C1 Methoxychlor =Br =C1 Colorado 9 =F =C1 Gix or DFDT =C1 =H TDE or DDD Toxaphene, C10H10C1S, is the com- mon name for a product obtained by reacting chlorine with camphene. It is more complex than benzene hexachlo- ride. Its structure is not completely known. The technical material consists of a mixture of compounds, which con- tain 67-69 percent chlorine. It was originally known as Hercules 3956, but a more descriptive name is chlorinated camphene. It is a cream-colored solid of waxy consistency. It melts over a range of 65 ° to 90 ° C. Toxaphene is readily soluble in the common organic solvents. Toxaphene will dehydro- chlorinate in the presence of alkalies. Like DDT, it slowly splits off hydro- chloric acid on heating and in the pres- ence of materials, such as iron com- pounds, that may act as catalysts. Chlordane, 1 ,2,4,5,6,7,8,8-octachlo- ro-2,3,3a,4,7,7a-hexahydro-4,7 - meth- anoindene, formerly known as Velsicol 1068, is a chlorinated hydrocarbon ob- tained by subjecting two compounds called hexachlorocyclopentadiene and cyclopentadiene to a reaction of a type developed by two German chemists, Otto Diels and Kurt Alder, and treat- ing the resulting product with chlorine. Yearbook of Agriculture 1952 Diels and Alder received the Nobel prize in chemistry in 1950 for their work on this type of diene synthesis. Chlordane is a nearly odorless, viscous, amber-colored liquid that can be dis- tilled only under high vacuum and is soluble in the common organic sol- vents. Chlordane is also a good solvent for DDT. It decomposes in the pres- ence of alkalies with a resulting loss of insecticidal toxicity; consequently it cannot b*e formulated with alkaline materials. CI I C C1C H 'C CH2 CC12 C1C \ \ C II H C CI ? / \ I H CI CI Chlordane TWO OF THE NEWER SYNTHETIC chlorinated hydrocarbons, which were known during their experimental test- ing period as Julius Hyman and Com- pany Compounds 118 and 497, have been given the common names, aldrin and dieldrin, honoring Alder and Diels. Aldrin has been defined as con- taining not less than 95 percent of 1 , 2,3,4, l °j l o-hexachloro- 1 ,4,4a,5,8,8a- hexahydro- 1 ,4,5,8-dimethanonaphtha- lene. Dieldrin (pronounced deel-dr'm) has been defined as containing not less than 85 percent of 1,2,3,4, 10,1 o-hexa- chloro-6,7-epoxy- 1, 4,43,5,6, 7,8,8a - oc- tahydro - 1,4,5,8 - dimethanonaphtha- lene. Aldrin is a white solid with a melt- ing point of io4°-io4.5° C. It is prac- tically odorless at room temperature, but it has a pinelike odor when warm. Dieldrin melts at i75°-i76° C. and is odorless. Aldrin is soluble in the common organic solvents. Dieldrin is moderately soluble in the same solvents. Neither is soluble in water. Aldrin is stable in the presence of organic and The Organic Insecticides inorganic alkalies and hydrated metal- lic chlorides and therefore is compati- ble with most agricultural chemicals. Unlike DDT, the DDT analogs, toxaphene, and chlordane, dieldrin is unaffected by alkalies. Insecticidal ef- fectiveness is not lost in the presence of alkaline and acid materials that would occur in formulation; thus it is com- patible with most agricultural chem- icals. It does react chemically with strong acids. Aldrin, with a vapor pres- sure approximating that of lindane, is about 20 times more volatile than dieldrin. Because aldrin and dieldrin are highly toxic, technical products and insecticidal formulations containing them must be handled with extreme care. CI II CI / \ c c c H /l\ \ / I \ C CH CI c / \ CC12 CHS H \|/ C / CH CI H Aldrin CI \ ecu If c / II CI / C / H \ ji Dieldrin C |\ CH2 O 1/ c c I H Bis(p-chlorophenoxy) methane, for- merly called K-1875, is a soud that melts at 68°-68.5° C. It is rather sol- uble in acetone, benzene, and ethyl ether. It is not appreciably soluble in 213 ethanol and the aliphatic hydrocar- bons. It is insoluble in water. It belongs to the class of compounds known as acetals and is stable to alkalies, but on boiling with dilute aqueous acids it is hydrolyzed. It is used to control mites in fruit orchards. H H H H C=C H C— C / \ 1 y \ C1C C-0-C-O-C CC1 \ /- I \ / C— C H C=C H H H H Bis (/>-chlorophenoxy) methane One of the new insecticides that came out of the Second World War the Germans called "Lauseto neu." It is chloromethyl p-chlorophenyl sulfone and is not a chlorinated hydrocarbon. It is a good insecticide but is less effec- tive than DDT against certain strains of lice, flies, and mosquitoes. H H 0 C = =C II / C1CH2- -fi- -C ll \ 0 c- -C H H Lauseto neu CC1 An interesting accomplishment in the preparation of synthetic organic insecticides was the synthesis of com- pounds that resemble the pyrethrins and cinerins, the toxic materials in pyrethrum flowers. In 1948, after 15 years of investigation of the structure of these naturally occurring insecti- cides, chemists of the Bureau of Ento- mology and Plant Quarantine pre- pared pyrethrin-type esters similar to cinerin I. One of these synthetic esters, thed/-2-allyl-4-hydroxy-3-methyl-2-cy- clopenten- 1 -one ester of a mixture of cis and trans ^/-chrysanthemum car- boxylic acid, has been produced com- mercially and has been given the com- mon name of allethrin by the Inter- departmental Committee on Pest Con- trol. Allethrin is a light, yellow-colored oil and possesses solubilities similiar to those of the natural products so that it 214 may be used in the same manner in fly sprays and aerosols. CHs CH3 HO H C c c — c — o — c CH HC— CH=C(CH3)2 H2C CCHjCHiCHj C = 0 Allethrin Diphenylamine, (CfiH5)2NH, is an- other type of organic compound that has been used successfully for screw- worm control. i,i-Bis(/?-chlorophen- yl)ethanol, (Cl2C6H4)2COHCH3, (also called DMC from the generic name dichlorophenyl methylcarbinol) is used against mites. Pentachlorophe- nol, C6Cl5OH, is used to control termites. A new field of organic phosphorus insecticides was opened up during the Second World War by Gerhard Schrader, a German chemist who was engaged primarily in the search for more powerful agents of chemical war- fare. Schrader discovered a new series of highly toxic organic phosphorus compounds. From them, through ex- tensive tests, came several effective in- secticides. The list includes parathion, tetraethyl pyrophosphate (including the so-called hexaethyl tetraphos- phate) , and octamethyl pyrophosphor- amide. Parathion, a remarkably effective in- secticide, has been put to use in many countries to control many kinds of in- sects infesting various crops. Tetraethyl pyrophosphate and octa- methyl pyrophosphoramide are used chiefly against aphids and some mites. Tetraethyl pyrophosphate kills in- sects rapidly, almost as soon as the ma- terial is applied, and then, having per- formed its task, the toxic insecticide soon decomposes by hydrolysis into nontoxic and water-soluble products. Thus, there is no spray-residue prob- lem connected with its use. Octamethyl pyrophosphoramide has been manufactured in the United States only on a relatively small scale. Yearbook of Agriculture 1952 British investigators have called it a systemic insecticide because, when it is applied either to the leaf or root system of a living plant, it is absorbed into the sap stream and translocated, rendering the plant insecticidal to certain insect species for several weeks. Its general use on food or fodder crops is not recommended. Experiments have indi- cated that it may prove useful on orna- mental plants and cotton plants. Cot- tonseed has been soaked in a very di- lute water solution of octamethyl pyro- phosphoramide and then planted. The cotton seedlings that emerged were found to be insecticidal to aphids and mites for about a month. It may also be useful on sugar beets to kill aphids that carry virus yellows disease. Because of their extreme toxicity to warm-blooded animals, these potent insecticides may not be used to control insects affecting man and animals, such as household pests and cattle and sheep pests. Because they are effective at ex- tremely low dosages against a wide range of insect species, and when prop- erly applied leave a negligible spray residue on an agricultural crop, their potential usefulness is great. H H C2H50 S C— C Ml /■ X P— O— C C-NO2 / \ / C2H50 C=C H H Parathion C2H5O 0 O OC2H6 Ml II / p— o— P / \ C2H5O 0C2H5 Tetraethyl pyrophosphate (CH3)2N O O N(CH3)2 Ml II / P— O— P / \ (CH3)2N N(CH3)2 Octamethyl pyrophosphoramide Synthetic organic chemicals have been used as fumigants for nearly a century. They are low-boiling com- The Organic Insecticides pounds of rather simple structure. They include hydrocarbon derivatives that contain sulfur, oxygen, chlorine, bromine, and nitrogen. Carbon disul- fide (CS2), prepared now from sulfur and coke by heating in an electric fur- nace, was the pioneer. Ethylene oxide, (CH2) :0, is a gas at ordinary temper- ature and was proposed as a fumigant in 1928. One part is used with 10 parts of carbon dioxide to reduce the fire hazards. Among the chlorinated hydrocar- bons we find carbon tetrachloride, CC14, used with ethylene dichloride, C2H4CL, (1:3) since 1927; propylene dichloride, C3Hr,Cl2; ethylene dichlor- ide alone ; and a mixture of 1 ,2-dichlor- opropane, C3Hf,Cl2, and 1,3-dichloro- propylene, C3H4C12, known as D-D. These materials and certain bromine compounds — methyl bromide, CH3Br, and ethylene dibromide, C2H4Br2 — are of value against wireworms and nematodes. Hydrocyanic acid, or hydrogen cya- nide (HCN) , is a highly poisonous gas used in fumigation of citrus trees as well as a space fumigant for ware- houses and other enclosed places. Some compounds of higher boiling point and more complex structure are also used as fumigants because of their high vapor pressure. Chloropicrin, CLCNO., boils at 112.40 C. and is used as a fumigant for grain and soil. It is most effective in a mixture of 1 pound to 1 gallon of carbon tetrachlor- ide. Dichloroethyl ether— (C2H4C1) 20, bis(2-chloroethyl) ether — with a boil- ing point of 1 78. 50 C. produces vapors much heavier than air and is of value as a soil fumigant. Naphthalene, C10H8, is one of the older organic insecticides not obtained from plants or oil. It is a hydrocarbon obtained by the destructive distillation of coal. This flaky white solid has been used for a half century to protect woolen cloth against clothes moths. It has a fumigating action, but its objec- tionable odor is not easily removed from the fabric. The newer moth- 215 proofing materials do not have that disadvantage. p-Dichlorobenzene, Cr,H4Cl2, a white, odorous solid, which melts at about 530 C, has wide use for control of peach tree borers and clothes moths. It is synthesized by reacting chlorine with benzene in the presence of the proper catalysts. It is one of the best known fumigants because of its long and wide usage. H H C C / \ / \ HC C CH H C C C II \ / \ / c c H H Naphthalene CI 1 1 C HC CH HC CH V 1 1 CI p-Dichlorobenzene A desire to find materials that would increase the toxicity and thus extend the supply of scarce insecticides, such as pyrethrum, has encouraged investi- gation in this field. Such materials are known as synergists. N-isobutylundecy- lenamide, N-isobutylhendecenamide, the first synergist developed for pyre- thrum, was introduced in 1938. It may be considered a synthetic material, al- though castor oil is the basic material for its preparation. The value of sesame oil as a synergist for pyrethrum was discovered about the same time. Its effectiveness was shown to be due to the presence of sesamin. Knowledge of the structure of sesamin led to the syn- thesis of related compounds, including piperonyl cyclonene and piperonyl butoxide. Piperonyl butoxide, also known as (butyl carbitol) (6-propylpiperonyl) 2l6 CH2 O I I c I / \ O — C CH I II HC CCH2OC2H4OC2H4OC4H9 Yearbook of Agriculture 1952 HC / / H C \ c C3H7 Piperonyl butoxide CH2 O I C / \ O — C CH I II HC CH V CH / \ CH2 CHCOOC2H5 Alkyl— C C = 0 \ / CH CH2 O C / \ O— C CH HC CH \ / C CH / \ C H2 C H9 Alkyl— C C=0 \ / CH Piperonyl cyclonene \ H O \ / // c — c CH, HC \ NC8Hx7 C C \ \ H O HC HC H MGK 264 H C C— COOR II C— COOR C II R=CH3 Dimethyl phthalate R=C4Hy Dibutyl phthalate CH3 H H H OH CH2 H H— C— C— C— C C C— OH H H H H H H Rutgers 612 O II C / \ H2C CH CH3 \ \ C C— COOC4H9 CH3 O Indalone H COOC3H7 H \ / C C H / V / \ / O— C C C— COOC3H7 CH: CH, c o— c c \ / \ / \ C C H H HH n-Propyl isome H HC II HC / ./ CH: H \:— C00CH3 C — COOCH3 H Dimethyl carbate The Organic Insecticides ether, is a thick, viscous liquid that con- tains as its principal active constituent a-[2 - (2 - butoxyethoxy)ethoxy] - 4,5- methylenedioxy- 2-propyltoluene. Pip- eronyl cyclonene, formerly known as piperonyl cyclohexenone, is the com- mon name for a mixture comprised of 3-alkyl - 6 - carbethoxy - 5 - (3,4-meth- ylenedioxyphenyl)-2-cyclohexen-i-one and 3 - alkyl - 5- (3,4 - methylenedioxy- phenyl)-2-cyclohexen-i-one, in which the "alkyl" refers to aliphatic radicals that may be varied. It is a thick, viscous liquid. Another synergist for pyrethrum is n-propyl isome, the dipropyl ester of i,2,3,4-tetrahydro-3-methyl-6,7-meth- ylenedioxy-1,2 - naphthalenedicarbox- ylic acid. Like sesamin, the three mate- rials all contain the methylenedioxy- phenyl group. One of the later organic insecticides, N-octylbicyclo[2.2.i]-5-heptene-2,3-di- carboximide, MGK 264, was intro- duced as a synergist for pyrethrins but has also been found to be effective as an ovicide. It is an amber-colored and rather viscous liquid. It is slightly heavier than water. It is readily solu- ble in the usual organic solvents and is itself a good solvent for quite a few of the other newly discovered insecti- cides. Oil of citronella, a plant product, was the standard repellent for mosqui- toes before the Second World War. During the war, however, the need for repellents for chiggers, mosquitoes, and fleas instigated the testing of many synthetic organic compounds, Benzil, C6H5COCOCf)H5, and benzyl benzo- ate, C6H5COOCH2C6H5, were found to be repellent to chiggers; dimethyl phthalate to mosquitoes and mites; Rutgers 612 (2-ethyl-i,3-hexanediol), Indalone (often called n -butyl mesityl oxide oxalate but more properly the butyl ester of 3,4-dihydro-2,2-dimethyl- 4-0x0- 2H-pyran-6-carboxylic acid) and dimethyl carbate ( the dimethyl es- ter of cw-bicyclo [2.2.i]-5-heptene-2,3- dicarboxylic acid) to mosquitoes, chig- gers, and fleas. A mixture of Indalone, dimethyl phthalate, and Rutgers 612 is used as an all-purpose insect repellent. 217 Only a few synthetic organic com- pounds have been used to attract in- sects. Metaldehyde, (C2H40)4, a poly- mer (condensation product) of acetal- dehyde, is used in baits for the control of garden snails and slugs. Isoamyl sali- cylate, HOCsHiCOOCgHn, an ester, is used to attract tobacco hornworm moths into traps. Methyl eugenol has proved attractive to the male oriental fruit fly in tests in Hawaii. The paucity of synthetic materials used as attract- ants would indicate that this might be a good subject for more intensive investigation. Besides the synthetic organic insecti- cides we have discussed, others of less importance are in use. Still others are in the experimental and develop- mental stages. The wide variety of compounds that we have considered here gives evidence that the chemistry of synthetic organic insecticides covers the entire field of organic chemistry. C. V. Bowen, head chemist at the Orlando, Fla., laboratory of the divi- sion of insects affecting man and ani- mals, Bureau of Entomology and Plant Quarantine, entered Government em- ploy in 1923 in the Insecticide and Fungicide Board. He taught chemistry at Washington and Jefferson College from 1925 until 1937, when he re- turned to the division of insecticide in- vestigations. His principal interest has been in research on the preparation, analysis, and formulation of synthetic organic insecticides. S. A. Hall, a chemist, began analyti- cal work with the Treasury Depart- ment in 1934 and in 1939 transferred to the Bureau of Agricultural and In- dustrial Chemistry to do research on naval stores. In 1943 he joined the di- vision of insecticide investigations, Bu- reau of Entomology and Plant Quar- antine, at Beltsville, Md., where he first worked on DDT and the development of insect repellents. For further reading on organic insecti- • cides the authors recommend: Bureau of Entomology and Plant Quar- antine publications: E-J33, Results of Screening Tests with Materials Evaluated as Insecticides, Miticides, and Repellents at the Orlando, Fla., Laboratory, April 1942 to April 1947, 1947; E-802, A Digest of In- formation on Toxaphene, by R. C. Roark. 1950. In Advances in Chemistry, volume 1: Organic Phosphorus Insecticides, by S. A. Hall, pages 150-159; Alkali-Stable Poly- chloro Organic Insect Toxicants, Aldrin and Dieldrin, by R. E. Lidov, H. Bluestone, S. B. Soloway, and C. W. Kearns, pages 175-183. 1950. In Chemistry and Industry: The Gamma- Isomer of Hexachlorocyclohexane (Gam- mexane), by R. E. Slade, volume 40, pages 314-319- "950. In Science in Farming, Yearbook of Agriculture 1943-1947: The Chemistry of DDT, by H. L. Holler and Ruth L. Busbey, pages 616-622; Pests That Attack Man, by E. F. Knipling, pages 632-642. 1947. N. Y. State Flower Growers Bulletin 7, Revised Recommendations for Azobenzene, by W. E. Blauvelt, pages 15-16. 1946. United States Patents: 2,291,193, Insecti- cide, patented by Lloyd E. Smith, July 28, 1942 (U. S. Patent Office Official Gazette, volume 540, page 827); 2,010,841, Chlori- nation, patented by Harry Bender, August J3> 1935 {volume 457, page 302). The Inorganic Insecticides This cylindrical fungus-feeding beetle is admirably suited for living in round tun- nels which it bores into forest trees for the propagation of its food. 2l8 R. H. Carter Inorganic insecticides are of mineral origin, mainly compounds of antimony, arsenic, barium, boron, copper, fluo- rine, mercury, selenium, sulfur, thal- lium, and zinc,, and elemental phos- phorus and sulfur. Antimonyl potassium tartrate, tar- tar emetic, K(SbO)C4H406.i/2H,0, is a white powder soluble in water. It is sometimes used as the toxic agent in ant poisons and for the control of thrips. Arsenical compounds are the most widely used inorganic insecticides. Recommendations for their use date from 1 68 1. They were probably used before that. The poisonous properties of arsenic trioxide were well known during the Middle Ages and it was a favorite instrument of murder as prac- ticed by the Borgias. This knowledge of the poisonous properties of arsenic compounds probably led to their use as insecticides. Arsenic trioxide, As203, also called arsenious oxide, is a white crystalline material sometimes referred to as white or gray arsenic. It is the starting material in the manufacture of arseni- cal compounds used as plant insecti- cides and it is sometimes used in weed killers. It is obtained from the flue dust from copper smelters. Our supply comes from domestic and foreign sources. It is sometimes used as the toxic agent in baits to control grass- hoppers, cutworms, and other insects. The calcium arsenate that is sold commercially as an insecticide is not a single chemical compound but a com- plex mixture of several calcium arsen- ates and an excess of calcium hydrox- ide. The material is made from arsenic trioxide by first oxidizing it to arsenic The Inorganic Insecticides pentoxide with nitric acid and then reacting the solution of arsenic pen- toxide or arsenic acid with a slurry of calcium hydroxide. The conditions of temperature, concentration, and dura- tion of reaction are important because of their influence on the physical na- ture of the product. Commercial cal- cium arsenate generally is colored pink and is alkaline in reaction. It is a finely divided powder. It has been used ex- tensively against certain insects affect- ing field crops, especially cotton. It can- not be used safely on apples, peaches, beans, and some other crops because of its burning effect on the foliage and fruit. Calcium arsenate-calcium arsenite mixture is sold under the name london purple. It has some use for poisoning insects on cotton. Among the insecticidal materials containing copper and arsenic, copper- aceto-arsenite (or paris green), 3 Cu- (As02)2.Cu(C2H302)2, is by far the most important. It has been used as an insecticide since about 1870. For many years it was the most widely used insec- ticide in the United States for control of Colorado potato beetles. It has largely been supplanted by some of the newer materials, but approximately 4 million pounds are used annually by farmers and gardeners. Copper arsenite, Cu3(As03)2.xH20; copper meta arsenite, Cu(As03)2- H20; and basic copper arsenate, Cu3- (As04)2.Cu(OH)2, have all been pro- posed as insecticides but have not been used to any extent. Compounds similar to paris green made from organic acids other than acetic have also been tested but have not developed into commer- cial use. Several chemically different com- pounds are known as lead arsenate. Two of them are commonly used as in- secticides. Acid lead arsenate (dilead- ortho arsenate), PbHAs04, is formed by the action of arsenic acid on litharge salt. It is a white powder, insoluble in water. Basic lead arsenate (lead hy- droxy arsenate), Pb4(PbOH) (As04)3, also is a white insoluble powder. 219 Both forms should contain very little water-soluble arsenic pentoxide in order to minimize plant damage. Gen- erally they are much less apt to burn plant foliage than is calcium arsenate or paris green. The basic compound is safer to use on growing plants in some localities (for example, the foggy re- gions of California) than is the acid compound, but in general it is not so toxic to insects. Acid lead arsenate is used exten- sively to control chewing insects on fruits, such as apple and pear, on flowers, trees, and shrubs, and on vege- tables, such as potato and tomato. It also has extensive use in treating soil to control Japanese beetle and Asiatic garden beetle larvae and related soil- infesting forms. A number of United States patents cover processes for the manufacture of magnesium arsenates for use as insec- ticides. The magnesium arsenates tested as insecticides consisted gener- ally of the dimagnesium arsenate, MgHAs04, the trimagnesium arsenate, Mg3(As04)2, or the pyroarsenate, Mg2As207, with varying amounts of water of crystallization and excess mag- nesium oxide or hydroxide. Magnesium arsenate has been tested against a large number of insects af- fecting fruits and vegetables and at one time was recommended for the control of the Mexican bean beetle, but its use has declined. A crude manganese arsenate once was proposed as an insecticide for com- batting caterpillars on tobacco because its brown color made it less conspicu- ous on cured tobacco leaves than the white lead arsenate. Sodium arsenite is formed by dis- solving arsenic trioxide in sodium hydroxide solution. Depending on the ratios of the reacting materials, the products range from the monosodium compound, NaAs02, to the trisodium arsenite, Na3As03. A standard for- mula for making so-called liquid so- dium arsenite requires 4 pounds of white arsenic and 1 pound of sodium hydroxide per gallon of solution. 220 Sodium arsenite is not used as an insecticide on field crops because of its corrosive action. It is used as an in- gredient in poison baits for grasshop- pers, crickets, roaches, ants, and other insects, and in stock dips. It has been used extensively as a weed killer. Zinc meta arsenite, Zn(As02)2, is formed when a soluble zinc salt is re- acted with arsenious acid or white arsenic under carefully controlled con- ditions, as it is soluble in either acid or alkaline solutions. Zinc arsenite is used in wood pres- ervation but is not used in household insecticides or as a constituent of formulations to be used on field crops. Zinc arsenate, Zn3(As04) 2, has been proposed in place of lead arsenate in codling moth control, principally be- cause it avoids lead residues. Arsenates and arsenites of many of the other elements have been investi- gated for insecticidal use but none has been developed into satisfactory mate- rials. Organic arsenicals have likewise failed to find a place as insecticides. Barium carbonate, BaCOs, is a white, finely divided powder which is sometimes used as the toxic agent in rat poisons. Borax, Na2B407, and boric acid, H3BO3, have been used in roach pow- ders, but more effective compounds, such as sodium fluoride, DDT, and chlordane, are available now. Bordeaux, or bordeaux mixture, is the name applied to the compounds formed by reacting dilute solutions of copper sulfate with calcium hydroxide suspensions. If equivalent amounts of the two materials are used, an inti- mate mixture of the copper hydrox- ide, Cu(OH)2, and calcium sulfate, CaSC*4, is formed. This suspension has a blue color and leaves a bluish-white deposit on sprayed surfaces. Bordeaux mixture is primarily a fungicide but is often used in connec- tion with insecticides such as nicotine, lead arsenate, and calcium arsenate. It is sometimes used to control the potato leafhopper and as a repellent for flea beetles on various vegetables Yearbook of Agriculture 1952 and flowering plants. It is sometimes used also as an emulsifier for lubricat- ing-oil sprays applied to fruit trees, such as apple, pear, quince, and peach, when they are dormant. Several other copper compounds, including the oxide, oxychloride, phos- phate, quinolinolate, silicate, basic sul- fate, and cyanide are used as spray materials. They have little insecticidal value but are potent fungicides. Hydrated lime, or calcium hydroxide Ca(OH)2, is used in the manufacture of lime-sulfur, calcium arsenate, and bordeaux mixture. When limestone, CaC03, is heated, the carbon dioxide is driven off, leaving the product known as quicklime, CaO. When quicklime reacts with water, heat is evolved and the resulting product is hydrated lime, Ca(OH)2. Hydrated lime is not primarily an insecticide but is used as a safener with some of the arsenical sprays. Calcium cynanide, Ca(GN)2, reacts slowly with moisture in the air to liber- ate hydrocyanic acid gas, a highly toxic organic compound used as an insecti- cidal fumigant. Compounds that contain fluorine have been in use as insecticides since about 1890. Barium fluosilicate, BaSiF6, a white, finely divided powder, has been tested extensively as a substi- tute for arsenicals in the control of fruit and vegetable crop insects. It has some value in the control of flea beetles-, blister beetles, Mexican bean beetle, and others. Cryolite, or sodium fluoaluminate, Na3AlF6, is a white crystalline material. Natural cryolite (ice-stone) is mined in Greenland and imported into this country. Synthetic cryolite, of similar composition, has been manufactured and sold for insecticidal use. For most uses there is little difference in their effectiveness. Large quantities have been used on codling moth in the Pa- cific Northwest and on the tomato pin- worm, tomato fruitworm, lima-bean pod borer, corn earworm, Mexican bean beetle, walnut husk fly, pepper weevil, cabbage caterpillars, blister The Inorganic Insecticides beetles, and flea beetles. It is generally used as a spray but may be diluted with talc, pyrophyllite, or other diluents to form a dust. Sodium fluoride, NaF, is a white powder. Sometimes it is colored green or blue so it will not be mistaken for baking soda. It is used extensively as a roach powder and is effective against chicken and animal lice of various kinds. It causes serious damage on plants. Sodium fluosilicate, Na2SiF6, is a white crystalline powder much less sol- uble than sodium fluoride in water. It has been used as a dust and spray in the control of some insects on field crops, as a poison in cutworm, mole cricket, and grasshopper baits and is effective as a mothproofing agent for woolen fabrics. A large number of fluorine compounds, both inorganic and or- ganic, have been patented for use as mothproofing agents. Some compounds of mercury are used as insecticides. Mercuric chloride (corrosive sublimate), HgCl2, and mercurous chloride (calomel), HgCl, are used against fungus gnats, earth- worms, cabbage maggots, and onion maggots. Mercuric chloride is also used for the treatment of dormant gladiolus corms and as a fungicide and germi- cide. Formulations containing mercury compounds are sometimes used to con- trol insects affecting man and animals. Pastes containing elemental phos- phorus are made by grinding yellow phosphorus in the presence of water and then mixing with flour. Glycerin is sometimes used as an ingredient. Such pastes are effective against the American cockroach. Selenium compounds have been tested as insecticides, but because of their toxicity to man their use is not recommended on crops intended for human or animal consumption. Sodium selenate, Na2Se04, is a water-soluble salt. Plants can take it up from the soil in sufficient amounts to kill aphids feeding on the plants. A product containing selenium and sul- fur of the formula (KNFLSKSe has 221 been used in the Pacific Northwest to combat mites on apples and grapes. The use of elemental sulfur and al- kaline sulfides as insecticides and fungi- cides on field crops and in greenhouses dates back many years. The materials are elemental sulfur, sulfides, polysul- fides or salts of some of the oxygen acids of sulfur. Elemental sulfur is used alone as a dust or in combination with other insecticides with many of which it is compatible. The sulfur is reduced to a very fine state of subdivision by grinding, precipitation, or sublimation. Dusting sulfur, or conditioned sul- fur, is finely divided elemental sulfur made into a free-flowing powder by the admixture of i to 5 percent of clay, talc, gypsum, tri-calcium phosphate, or similar materials. Flotation sulfur, col- loidal sulfur, and precipitated sulfur refer to finely divided sulfur formed as a result of chemical reactions of sulfur-containing compounds with other compounds. Wettable sulfur is finely divided sulfur that has been treated with wetting agents of various kinds to render it wettable by water and thus susceptible to suspension in spray formulations. The alkaline sulfides and polysulfides, sometimes referred to as soluble sulfurs, are prepared by the re- duction of the salts of some of the oxy- gen acids of sulfur or by the action of alkaline solutions on elemental sulfur. The most important compounds of this class are the polysulfides of calcium, ammonium, barium, and sodium. Calcium monosulfide, CaS, has been used to a limited extent. It is formed by the reduction of calcium sulfate. Liquid lime-sulfur or calcium poly- sulfide, CaSx, is formed by the reactions between calcium hydroxide and ele- mental sulfur when they are boiled to- gether in water. It is assumed to con- tain a mixture of the sulfides up to and including the pentasulfide, CaS5. The theoretical reaction between 3 moles of hydrated lime, Ca(OH)2, and 12 moles of sulfur results in the formation of 2 moles of calcium pentasulfide, CaS5, 1 mole of calcium thiosulfate, CaS203, and 3 moles of water, HLO. 970134° — 52- -16 Dry lime-sulfur is made by adding a stabilizer such as cane sugar to liquid lime-sulfur and evaporating to dryness. Self-boiled lime-sulfur is made by utilizing the heat of hydration or slak- ing of quicklime, CaO, to carry on the reactions with sulfur. Ammonium polysulfide and sodium polysulfide are made by passing hydro- gen sulfide gas, H2S, into ammonium or sodium hydroxide containing excess sulfur. It is supposed that the chemical reactions are similar to those taking place in the preparation of lime-sulfur. Sulfur is used under some conditions for the control of potato leafhopper, the cotton fleahopper, tomato psyllid, mites, and plant bugs. Organic sulfur compounds, includ- ing thiocyanates, xanthates, and thi- uram disulfides, have some insecti- cidal properties although they are used largely as fungicides. Sulfur dioxide, SO,, made by burn- ing sulfur, is sometimes used to kill in- sects in closed spaces. Thallium sulfate, T1,S04, sometimes is used as the toxic agent in ant poisons. Several zinc compounds are in limited use as insecticides. Zinc sulfate, ZnS04, is sometimes used in place of copper sulfate in reactions with hy- drated lime to form a zinc bordeaux mixture that has special uses. Zinc chloride, ZnCl2, is used to protect against termites. R. H. Carter is a chemist in the Bureau of Entomology and Plant Quarantine, assigned to the division of insecticide investigations at the Agri- cultural Research Center at Beltsville, Md. After graduation from Morning- side College and the State University of Iowa, he was employed in chemical research in the Chemical Warfare Service for 10 years. Since joining the Department of Agriculture in 192J, he has been engaged in research in the development of insecticides, investiga- tions of spray residue problems, and toxicological investigations of the ef- fects of insecticide materials on farm animals. 222 Insecticides From Plants Louis Feinstein More than 2,000 species of plants are said to have some value as insect killers. They belong to 1 70-odd families. Com- mercial insecticides of plant origin are found in five families: Nicotine in the Solanaceae family; pyrethrum in Com- positae; derris, cube, and timbo in Leguminosae; hellebore in Liliaceae; and anabasine in Chenopodiaceae. Anabasine is also found in Solanaceae. Who first discovered the insecticidal value of plants is not known. The Ro- mans divided poisons into three groups, animal, plant, and mineral. They used two species of false hellebore in medi- cines and in rat and mice powders and insecticides. The Chinese discovered the insecticidal value of derris. Chemists in the Bureau of Entomol- ogy and Plant Quarantine since 1927 have conducted research on the prin- cipal insecticides of plant origin, such as nicotine, nornicotine, anabasine, ro- tenone, deguelin and related rotenoids, quassin, and the pyrethrins. They also have worked on more than 450 plants in an effort to discover new sources of these and other insecticides, as well as attractants, repellents, and adjuvants. They have learned that many of the species in the 1 70 families do not war- rant further investigation and that bo- tanical classification is not a depend- able guide in the search for insecticidal plants. Plant insecticides are only a small fraction of the insecticidal material used each year. Yet in the development of new insecticides they deserve careful consideration: Often they are highly effective against many insect enemies that are not successfully controlled by inorganic insecticides. The plant insec- ticides often arc relatively nontoxic to Insecticides From Plants man and other plants. Poisonous spray residues on fruits and vegetables may menace public health. The relative safety of plant insecticides to man helps to maintain their continued use. In this article I discuss the commer- cial plant insecticides and other plants that appear promising as insecticides. Included here are plants only of the higher orders (phanerogams). They are listed alphabetically according to plant family and genus. The plants are sufficiently promising to warrant inten- sive chemical and toxicological studies. The lower orders of plants (crypto- gams) include the algae, fungi, mosses, ferns, and horsetails. A more complete study of them may also prove to be worth while. Aesculaceae [Horse chestnut Fam- ily) . Aesculus calijornica is called the California buckeye. The horsechestnut is a highly prized street and lawn shrub and tree. The common horsechestnut casts the densest shade of almost any cultivated tree. George H. Vansell and his coworkers in California found that bees feeding on buckeye blossoms be- came paralyzed and died. Reports of other investigators, however, show that the insecticidal value of species of the horsechestnut family varies. Annonaceae [Custard- Apple Fam- ily) . The genus Annona includes some 90 species of trees and shrubs, mainly in tropical America. S. H. Harper, C. Potter, and E. M. Gillham in England extracted Annona reticulata and A. squamosa seeds and roots with ether. The petroleum ether solution of this ex- tract at o° C. precipitated out an in- secticidal material that was 50 to 100 times more potent than the original ether extract. Against some insects the concentrate had about the same toxic- ity as rotenone. More work should be done with the custard-apple. Apocynaceae [Dogbane Family). Haplophyton cimicidum, the cock- roach plant, has been used to combat cockroaches, flies, mosquitoes, fleas, lice, and other insects in Mexico. The dried leaves are toxic to the Mexican fruit fly. The water extract of the stems 223 of plants grown in Arizona is toxic to adult house flies. The crude alkaloid from this plant is effective against most insects. It is as toxic as pyrethrum to the squash bug. Boraginaceae [Borage Family) . He- liotropium peruvianum. The borage family contains many well-known gar- den plants and often is called the helio- trope family. The compound heliotro- pine was one of the best chemicals tested against the body louse, being apparently nontoxic to the skin and lasting more than 168 hours when used in cocoa butter. Tournefortia hirsutissima is used as a general insecticide in Haiti. Cannaceae [Canna Family). Mem- bers of this family mostly have tuberous rootstocks, stately, broad leaves, and showy flowers. The leaves and stems of canna plants contain an insecticide that gives results similar to tobacco in green- house fumigation. Celastraceae [Staff-Tree Family). Tripterygium wilfordii, the thunder- god vine, is a common insecticidal plant in southern China. The poison in it has been found in the root bark. Its chemistry has been investigated by M. Beroza, who reported that wilfor- dine is a mixture composed mainly of two similar alkaloids, a- and /?-wilfor- dine. Both are insecticidally active ester alkaloids. Powdered fresh small roots are toxic to first-stage larvae of the codling moth, the diamondback moth, and the imported cabbageworm. Alco- holic extracts of the roots are more toxic. Small roots, powdered, are about half as toxic as pyrethrum to the Amer- ican cockroach. The large and medium roots are nontoxic. Chenopodiaceae [Goosefoot Fam- ily) . Anabasis aphylla contains the al- kaloid anabasine, closely related to nicotine. It is the only commercial source for the alkaloid. It grows mainly in Russia and is not available in the United States. Anabasis aphylla is re- lated to the American tumbleweed. In this country my coworkers and I ex- tracted anabasine from Nicotiana slauca. 224 Clusiaceae (Balsam Tree Family). Mammea americana is known as mamey, "mamey de Santo Domingo." Harold K. Plank of the Federal Experi- ment Station at Mayaguez, P. R., be- lieves that this indigenous West Indian tree has greater insecticidal potential- ities than any other plant he examined. The active principle in the mature seeds, the most toxic part, is a type of substance somewhat similar in compo- sition and effect to pyrethrins. Plank found that six of the nine parts of the plant were appreciably or highly toxic to one or more insects. The bark has little toxic material. Cochlospermaceae. Cochlospermum gossypium. Kutira gum increases the effectiveness of nicotine sulfate sprays. The kutira appears to be a synergist to nicotine sulfate in its action against the bean aphid. Compositae (Thistle or Aster Fam- ily) . This large family of plants in- cludes thousands of herbs, vines, trees, and shrubs. The dahlia, chrysanthe- mum, coreopsis, marigold, aster, cos- mos, and many other garden flowers are composites. To the dried flowers of Chrysanthemum cinerariae folium the name pyrethrum is applied. Pyre- thrum, a safe and effective insecticide, is widely used in household sprays. Four compounds exist in pyrethrum — pyrethrins I and II and cinerins I and II. Pyrethrins are practically nontoxic to warm-blooded animals and can be safely used in the home. Heliopsis scabra is called oxeye. M. Jacobson, at the Agricultural Re- search Center, discovered that these plants contain compounds toxic to the house fly. Nearly all the toxic material is extracted by petroleum ether. Jacob- son purified the petroleum ether ex- tract and named one of the toxic mate- rials scabrin. W. A. Gersdorff and N. Mitlin, entomologists in the Depart- ment of Agriculture, reported that scabrin compares well with pyrethrum in killing value. Cucurbitaceae (Gourd Family) . The cucumber family is often called the gourd, melon, or squash family. Cu- Yearbook of Agriculture 1952 curbita pepo commonly is called pump- kin. Freshly cut pumpkin leaves rubbed on cattle and horses reputedly repel flies. Acetone extracts of pumpkin seeds killed mosquito larvae in experi- ments conducted by A. Hartzell and F. Wilcoxon of Boyce Thompson In- stitute. Euphorbiaceae (Spurge Family) . Croton tiglium contains croton oil. The plant is cultivated in China, where the seeds are the source of a home- made insecticide. The plant has insec- ticidal value against aphids. J. R. Spies, a chemist in the Department of Agri- culture, reported that an acetone ex- tract of the seeds was more toxic to goldfish than derris extract and that croton resin was more toxic than ro- tenone. Ricinus communis, the castor-bean plant, is said to have some insecticidal value. If that is true, the insecticidal principle is present only under certain conditions with respect to variety, cul- tural practice, and environment. A val- uable synergist is prepared from isobu- tylamine and undecylenic acid, which results from the chemical decomposi- tion by heat of castor oil. By the action of sulfuric acid on castor oil, we get a useful emulsifier for insecticidal oils. Flacourtiaceae. Ryania speciosa. The active principles of the plant are alka- loids and are effective in the control of the European corn borer. The roots and stems contain the insecticide, which is commercially prepared for use as dusts and sprays. Fagaceae (Beech Family). Castanea dentata is called the American chest- nut. F. W. Metzger and D. H. Grant found that a commercial dyeing and tanning extract of the American chest- nut was a good repellent against the Japanese beetle. Labiatae (Mint Family). Ocimum basilicum is known as common basil or sweet basil. Its oil killed 95 percent of the mosquito larvae tested at a con- centration of 50 parts per million, but an extract made from the whole plant killed none. H. D. Hively obtained a patent in 1940 for the use of the plant Insecticides From Plants as an insecticide. It is successful as a contact poison against flies, Colorado potato beetles, and many other insects. Salvia officinalis, or garden sage. Salvias are grown for their flowers and for their leaves. The leaves of some species are used for seasoning. Hartzell and Wilcoxon found that acetone ex- tracts of the leaves killed 80 percent and extracts of the roots killed 95 percent of the mosquito larvae they tested. Leguminosae (Pea Family) . The pea family is one of the most important group of garden plants in the world. Haematoxylon campechianum is called logwood. Hematoxylon is from the Greek for blood and wood, in allusion to the red wood. Metzger and Grant reported that two commercial extracts were good repellents against the Japa- nese beetle. Millettia pachycarpa, fish-poison climber, is worth further investigation. The ground seeds kill several species of insects. Alcoholic extracts of the roots from China paralyze the bean aphid. The plant contains a large amount of saponin and rotenone. The plant acts as a contact and stomach poison when it is mixed with soap. Mundulea sericea, or M. suberosa, is a promising insecticidal plant. It was discovered in the 1930's. It is a rote- none-yielding species. The plants from India are toxic, but those from various locations in Tanganyika and Zanzibar fall into two main divisions, those with smooth barks, which are toxic, and those with rough, corky barks, which are nontoxic. Pachyrhizus erosus, or the yam bean. In some tropical countries the seeds of the yam bean plant are used as an insecticide and fish poison. Tests in the United States by R. Hansberry and C. Lee gave promising results against the bean aphid and the Mexican bean beetle. Tephrosia virginiana is known as devils-shoestring. It is a pretty little native plant, which prefers dry, open, somewhat sandy places. It has long been known to possess insecticidal properties. The most toxic samples of 225 devils-shoestring were slightly more poisonous than pyrethrum, but less poisonous than derris. Against five species of insects the plants showed promise as a contact spray. Technical Bulletin No. 595 of the Department of Agriculture outlines studies of the pos- sibilities of devils-shoestring as a com- mercial source of insecticides. Liliaceae (Lily Family) . The foliage and rootstock of most species contain a poisonous juice. Amianthium mus- caetoxicum, crowpoison, shows prom- ise as an insecticide against the house fly, cockroaches, grasshoppers, and bees. It is inefficient against tent cater- pillars and aphids. The powdered bulbs and leaves are used as dusts. Water extracts show a slow but considerable insecticidal effect against Colorado potato beetle larvae and cockroaches. Melanthium virginicum, bunch- flower. L. H. Pammel in 191 1 stated that the bunchflower had long been used to poison flies. Schoenocaulon officinale is com- monly known as sabadilla. R. J. Dicke in a thesis submitted to the University of Wisconsin in 1943 reviewed 76 ref- erences on this plant, which has been used as an insecticide since the six- teenth century. The University of Wis- consin has patented a method for in- creasing the toxicity of sabadilla: Heating the powdered seed in kerosene or other solvent to 1500 C. for 1 hour. Sabadilla is effective against squash bugs, chinch bugs, harlequin bugs, and lygus bugs. Scientists in the Depart- ment of Agriculture in 1949 began a chemical study of the constituents of sabadilla seed. Veratrum. Three plants are popu- larly called hellebore — Veratrum al- bum, V. viride, and Helleborus niger. The term hellebore is incorrect when it is applied to the first two plants. The last, which is the true hellebore, grows in Europe and is not a commercial product in the United States. V. viride is the American plant. Powdered roots of the first two plants prevent the emer- gence of house flies from horse manure. Veratrum viride is often called 226 American false-hellebore, swamp helle- bore, Indian poke, and itchweed in the United States. Its active principles are alkaloids, which are toxic to man. Its value as an insecticide for the control of chewing insects on ripening fruit is due to its rapid loss of toxicity on ex- posure to light and air. Meliaceae (Mahogany Family). Melia azedarach is called chinaberry. Water extracts of the berries affect cockroaches slightly but are more toxic against honey bees. Leaves applied to the soil greatly reduce attacks of ter- mites. An alkaline extract of the fruits is effective against aphids. Cultivated plants sprayed with extracts of the chinaberry leaves are not touched by locusts. The active principle is soluble in hot water, alcohol, chloroform, or benzene but not in petroleum ether. Myrtaceae (Myrtle Family). Pi- menta racemosa is the bay-rum tree. The oil of the leaves is toxic to mos- quito larvae. Bay rum has been used in Venezuela to kill insects. A foreign patent covers its use in a mixture of several substances. Applied to summer garments, it protects the wearer against gnats. Effective as baits to attract Jap- anese beetles are 90 parts of geraniol and 10 parts of the leaf oil of a Pimenta species, or 90 parts of anethole and 10 parts of the oil. Pedaliaceae. Sesamum inducum, ses- ame. The seeds yield sesame oil, which contains sesamin, a powerful synergist for pyrethrum. In the Second World War the Armed Forces used more than 40 million aerosol bombs containing pyrethrum, liquefied gas, and sesame oil. The later bombs used 8 percent of the oil in the formula. Sesame oil also acts as a synergist for rotenone. Ranunculaceae (Crowfoot Family). Delphinium consolida is called field larkspur. The oil from larkspur seed tested as a contact spray (2-percent emulsion) was effective against spider mites and aphids but had little value against some other insects. The alka- loids of this plant were also effective against insects in various degrees. Rutaceae (Rue Family) . Phelloden- Yearbook of Agriculture 1952 dron amurense, the Amur corktree, is native to several Asiatic countries and was introduced into the United States in 1856. The unsaponifiable portion of the oil of the fruit is toxic to house flies in acetone solution but not in high- boiling kerosene. The residue of the fruit, the oil having been removed, is toxic to mosquito larvae, house flies, and larvae of codling moth. The ma- terial is a fast-acting poison like pyre- thrum and nicotine. Zanthoxylum clavaherculis, the southern prickly-ash, contains as- arinin, a compound structurally related to sesamin and, like it, a good synergist for pyrethrum against house flies. The southern prickly-ash also contains her- culin, a pungent substance highly toxic to house flies. It is closely related to several other isobutylamides previously isolated from plant materials. A trace of the active material, when placed on the tongue, produces an intense burn- ing, paralytic effect on the tongue and on the mucous membranes of the lips and mouth. Herculin has approxi- mately the same order of paralyzing action and toxicity to house flies as the pyrethrins. Sapindaceae (Soapberry Family). Sapindus marginatus. This tree, up to 30 feet high, is native in Florida. It is planted occasionally for interest or ornament. The word sapindus comes from the Latin for soap, combined with Indian, in allusion to the Indians' use of the berries for soap; the pulp lathers easily like soap. S. L. Hoover obtained a patent for the use of the berries of the tree as an insecticide or insectifuge. Three berries protected a bushel of wheat against infestation. In powdered or liquid form and mixed with dried foodstuffs, it repelled weevils and other insects. Simarubaceae (Ailanthus or Quassia Family) . This tree stands smoke and city conditions well, but the male flowers have a strong odor, which is offensive to some persons. The bark and wood contain insecticidal princi- ples, which are used on only a few crops. Insecticides From Plants Solanaceae {Nightshade or Potato Family). The potato family, often called the tobacco or tomato family, in- cludes vegetables of world-wide culti- vation, narcotics, drugs, tobacco, and a large number of garden flowers. Duboisia hopwoodii, called pituri, is an Australian species and often is men- tioned in discussions of nicotine. C. V. Bowen, a chemist in the Department of Agriculture, analyzed the dried leaves and larger stems and found the leaves to contain 3.3 percent and the larger stems 0.5 percent of nornicotine. H. H. Smith and C. R. Smith of the Depart- ment studied 29 wild species of Nico- tiana. They found that 5 species con- tained the alkaloid nornicotine only and 18 a mixture of nornicotine and nicotine. Against some insects, nornic- otine is superior to nicotine. Nornico- tine is more toxic to a nasturtium aphid and the pea aphid ; about equally toxic to the cabbage aphid, the citrus red mite, and other spider mites; but less toxic to the celery leaf tier, the large milkweed bug, and larvae of codling moth. Nicandra physalodes is also known as the Peruvian groundcherry or shoo- fly plant. It repels insects. In India it is used as an insecticide. Stories told about it are many: The plant distrib- uted around a room repels flies; in a greenhouse it causes the whitefly to dis- appear; a few hundred planted near a barn apparently keep the animals from being bothered by flies. Physalis mollis is commonly known as smooth groundcherry. Thomas A. Nuttall described it in 1834. It grows throughout Oklahoma. Before the de- velopment of prepared fly sprays, the fresh plant was used to control house flies. The bruised leaves and stems, mixed with a little water and sugar, killed flies. L. E. Harris of Ohio State University isolated a glycoside in an impure form; it was toxic to flies. He also isolated an alkaloid, but it was not toxic to flies in the small dosage used. Nicotiana glauca, tree tobacco, is a wild, fast-growing plant in Texas, Arizona, and California. Patrick J. 227 Hannan and I were granted patents covering two methods useful in extract- ing the alkaloids from Nicotiana species, including the alkaloid anaba- sine from Nicotiana glauca. Anabasine is a liquid alkaloid that closely re- sembles nicotine in its physical, chemi- cal, toxicological, and insecticidal properties. It has been reported to be four or five times as toxic as nicotine to certain aphids of economic impor- tance. Nicotiana spp. Tobacco and its chief alkaloid, nicotine, have been used since 1690 as insecticides. Nicotine forms salts with acids and most of the nico- tine used for insecticidal purposes in the United States is in the form of the sulfate. More than 29 species of Nico- tiana have been analyzed for their alkaloid content. Some American to- baccos used in making cigars of low nicotine content contain as much as 0.7 percent of nornicotine. One-eighth of the total alkaloids in certain samples of commercial nicotine sulfate solu- tions was nornicotine. Most species of aphids may be controlled with concen- trations of 1 part nicotine to 1,000 parts of water. Nicotine is recom- mended against only those insects that have soft bodies and those that are mi- nute in size, such as aphids, whiteflies, leafhoppers, psyllids, thrips, spider mites, and some external parasites on animals. Stemonaceae. Stemona tuberosa, or paipu, has long been known and used in China as an insecticide. Decoctions of the dried roots are said to be toxic to crickets, weevils, and the caterpillars of moths and butterflies. A 50-percent alcoholic extract of the plant is effec- tive against lice and fleas. Umbelliferae (Carrot Family). Carum carvi is called caraway and con- tains oil of caraway, which will help cure scaly-leg of poultry. Hartzell and Wilcoxon found that acetone extracts of the seed killed 90 percent of the mos- quito larvae they tested. Conium maculatum, poison hem- lock, contains an alkaloid, coniine, which is related to nicotine. 228 Coriandum sativum, or coriander, contains an oil that repels screw- worms. Applied in a 2-percent oil emulsion spray, it kills spider mites and cotton aphids. Coriander oil repels house flies, green bottle flies (Lucilia sericata) , and black blow flies. Pimpinella anisum is anise. Clothing treated with a soapy emulsion of anise %o\\ protects wearers from the sting of gnats. Anise oil repels black blow flies, house flies, and green bottle flies. Vitaceae {Grape Family). Parthe- nocissus quinquejolia, or Virginia creeper. An old reference to it states that a bunch of leaves rubbed on an infested area of an apple tree and crushing all the woolly apple aphids, made the tree entirely free of aphids a week later. Formerly the tree could not be kept free of aphids for any length of time. The plant world contains many interesting and useful insecticides that have not been investigated yet. Only a few have been mentioned here. The entomologists and chemists have passed by many thousands of plants in their search for an insecticide that kills in- sects but is safe to people and animals. Once a scientist discovers a plant useful as an insecticide, he must take the plant apart and discover the active principles in it. The discovery is only the first step toward the commercial usefulness of the plant. The next steps take time and effort. That a plant is poisonous to other animals or is a common weed rarely attacked by insects is not a positive indication of insecticidal properties. The insecticidal principles may be pres- ent in one or more of the following parts: Leaves and leaflets, flowers, petioles, seeds and seed hulls, fruits, twigs and stems, roots, bark, and wood. Often the plant will be insecticidal when it is ground up, but the extract of the material will not be poisonous. The farmer and the general public share in the discovery and development of new insecticides from plants. Grow- ing new plants for insecticides means 1 earbook of Agriculture 1952 new income to the farmer; the public gets farm products that are clean and free from insects and poisonous resi- dues. Since 1947 Department research on plant insecticides covering only six plants — tree tobacco, oxeye, sabadilla, devils-shoestring, thunder-god vine, and sesame — has led to the publication of more than 1 7 papers and the grant- ing of three public service patents. Louis Feinstein, a research chem- ist, joined the Department of Agricul- ture in IQ3Q. He holds degrees from Georgetown University and the Uni- versity of Pennsylvania. Dr. Feinstein has published papers on vitamins and nicotine alkaloids and holds patents on the extraction of alkaloids and other materials from plants. For further reference: G. T. Bottger and C. V. Bowen: Com- parative Toxicity Tests of Anabasine, Nor- nicotine, and Nicotine, Bureau of Entomol- ogy and Plant Quarantine publication E— yio. 1946. R. N. Chopra and R. L. Badhwar: Poi- sonous Plants in India, The Indian Journal of Agricultural Science, volume 10, pages 1-44. 1940. E. O. Eddy and C. M. Meadows: Karaya Gum in Nicotine Sprays, Journal of Eco- nomic Entomology, volume 30, pages 430- 432- 1937- W. A. Gersdorff and Norman Mitlin: In- secticidal Action of American Species of Heliopsis, Journal of Economic Entomology, volume 43, pages 554S55- '95°. H. L. Holier, E. R. McGovran, L. D. Goodhue, and W. N. Sullivan: The Syner- gistic Action of Sesamin With Pyrethrum Insecticides, The Journal of Organic Chem- istry, volume 7, pages 183—184. 1942. Roy Hansberry and Cecil Lee: The Yam Bean, Pachyrrhizus erosus Urban, As a Possible Insecticide, Journal of Economic Entomology (scientific note), volume 36, pages 35I~352. 1943- S. H. Harper, C. Potter, and E. M. Gill- ham: Annona Species as Insecticides, An- nals of Applied Biology, volume 34, pages 104-112. 1947. L. E. Harris: Chemical Studies in Okla- homa Plants. VI. Physallis mollis Nuttall — A Plant Insecticide, Journal of the Ameri- can Pharmaceutical Association, scientific edition, volume 37, pages 145-146. 1948. Albert Hartzell and Fredericka Wilcoxon: A Survey of Plant Products for Insecticidal Properties, Contributions from Boyce Thompson Institute, volume 12, 1941. Ralph E. Heal, Edward F. Rogers, Robert T. Wallace, and Ordway Starnes: A Survey of Plants for Insecticidal Activity, Lloydia, volume 13, pages 8Q-162. 1950. Martin Jacobson: Herculin, A Pungent Insecticidal Constituent of Southern Prickly Ash Bark, The Journal of the American Chemical Society, volume 70, pages 4234- 4237. 1948. F. B. LaForge: Constituents of Pyre- thrum Flowers. XX. The Partial Synthesis of Pyrethrins and Cinerins and Their Rela- tive Toxicities, with W. F. Barthel, The Journal of Organic Chemistry, volume 12, pages 199-202, 1947; The Presence of an Insecticidal Principle in the Bark of South- ern Prickly Ash, with H. L. Holler and W . H. Sullivan, The Journal of the American Chemical Society, volume 64, page 187. 1942. N. E. Mclndoo: The Castor-Bean Plant as a Source of Insecticides ; A Review of the Literature, Bureau of Entomology and Plant Quarantine publication E-666, 1945; Plants of Possible Insecticidal Value ; A Review of the Literature up to 1941, E-661, 1945; A Bibliography of Nicotine. Part II. The In- secticidal Uses of Nicotine and Tobacco, with R. C. Roark and R. L. Busbey, E-392, 1936; Plants Tested for or Reported to Possess Insecticidal Properties, with A. F. Sievers, U. S. D. A. Publication 1201. 1924. H. K. Plank: Insecticidal Properties of Some Plants Growing in Puerto Rico, Puerto Rico Federal Experiment Station (Mayaguez) Bulletin 49. 1950. R. C. Roark: Excerpts from Consular Correspondence Relating to Insecticidal and Fish-Poison Plants, United States Bu- reau of Chemistry and Soils, 1931 ; A Third Index of Patented Mothproofing Materials, with R. L. Busbey, Bureau of Entomology and Plant Quarantine. 1936. M. S. Schechter and H. L. Holler: The Insecticidal Principle in the Fruit of the Amur Corktree, The Journal of Organic Chemistry, volume 8, pages 194-197. 1943. E. H. Siegler and C. V. Bowen: Toxicity of Nicotine, Nornicotine, and Anabasine to Codling Moth Larvae, Journal of Economic Entomology, volume 39, pages 673—674. 1946. United States Patents: 1 ,619,258, Insec- ticide, patented by Sidney L. Hoover, March 1, 1927 (U. S. Patent Office Official Gazette, volume 356, page 132) ; 2,223,367, Insecticide, patented by Howard D. Hively, December 3, 1940 (volume 521, page 58) ; 2,525,784, Process for Extracting Alkaloi- dals from Plants with Aqueous Aluminum Sulfate, and 2,525,785, Process for Extract- ing Alkaloidals from Plants with Aqueous Ammonia-Ethylene Dichloride Mixture, patented by Louis Feinstein and Patrick J. Hannan, October 17, 1950 (volume 639, pages 719-720). Oil Sprays for Fruit Trees P. J. Chapman, L. A. Riehl G. W. Pearce Petroleum oils are used in several ways to control pests. Some kill insects and mites directly through their own action. Some supplement the action of other insecticides as co-toxicants, sol- vents and carriers, stickers, or stabil- izers. In the water-borne oil sprays com- monly applied to fruit trees, the oil usually is the sole or primary insectici- dal agent. That is also true of oils used to rid bodies of water of mosquitoes. Light petroleum fractions are widely used as solvents and carriers for many insecticides. The original fly sprays are a good example. The introduction of DDT and other organic insecticides has meant a great increase in the use of oil as the carrier for applying insec- ticides, especially the chemicals used to control household and building pests. These oil-insecticide mixtures usually are applied in the form of fine mists. With heat and a suitable gen- erator they can be applied also as thermal fogs, which remind one of military smoke screens. Often oils are added to insecticidal and fungicidal spray, dust, and poison- bait formulations as stickers, stabili- zers, and conditioning agents. In this chapter we discuss the water- borne oil sprays as they are used to con- trol pests of citrus and deciduous fruit trees. Kerosene was apparently the first petroleum product used for the control of plant pests in the United States. A. J. Cook of Michigan State College in- troduced in 1877 a kerosene-soap emul- sion which was widely employed to combat aphids and scale insects. Entomologists sought something more effective and turned to crude pe- 229 230 troleum. It proved to be too injurious to most plants. A search was then started for some fraction or series of fractions of petroleum that would be highly effective as insecticides, but rela- tively noninjurious to plants. Progress has been made in the search. Oil sprays are used most commonly in horticulture to control scale insects and mites, among which are many of our major fruit pests. Oil sprays are also used to control psyllids (pear psylla), plant bugs (apple red bugs), mealybugs, aleyrodids (whiteflies, cit- rus blackfiy), thrips, aphids (newly hatched), membracids (buffalo tree- hopper) , and others. Oil sprays readily destroy eggs of many lepidopterous pests, like the codling moth, oriental fruit moth, various leaf rollers, and cankerworms. Those insects are now more commonly controlled in the lar- val stage with the newer insecticides. More than 15 million gallons of oil are used annually in this country for horticultural sprays. Emulsified and di- luted to a 2-percent strength, that amount makes 750 million gallons of spray — enough to provide for the single coverage of 40 million to 50 million orange or apple trees. Tree spray oils are of two classes. Those intended for use on the hardy tree fruits during the dormant period are called dormant oils. Those applied to trees in foliage are the summer oils. The oils used on citrus in California may be classed as summer oils. The two groups differ chiefly in the degree of refinement of the oil and in its heavi- ness, or viscosity. Summer oils have been more highly refined and are of lighter weight than dormant oils. The classification is rather arbitrary, and be- cause the trend has been toward using the so-called dormant oils after growth starts and using more highly refined products, the distinction between dor- Y ear book of Agriculture 1952 mant and summer oils has had less and less meaning. The first major step in refining pe- troleum is its division into fractions by distillation. First to distill over are the low-boiling naphthas, then come in- creasingly higher-boiling lots, through gasoline, kerosene, fuel oils, and, finally, the lubricant fractions. Horti- cultural spray oils are derived from the fuel-oil and light-lubricant por- tions of petroleum; those from the lubricant portion predominate. Crude petroleums vary greatly in composition. Differences exist among crudes from the major production fields and even among wells in one field. We recognize three general types — paraffinic base, asphaltic or naphthenic base, and mixed base or midcontinent crudes. Spray oils have been prepared from all crude classes. Asphaltic-base crudes are utilized in California primarily because the local petroleum supply is generally of that class. East of the Rockies the midconti- nent crudes are more commonly used. Before we consider specifications for horticultural spray oils, it is well to have an understanding of their nature. The spray oils are composed es- sentially of hydrocarbons — compounds containing hydrogen and carbon. The arrangement of the atoms of the two elements in individual molecules is varied and complex. Only three basic classes of carbon structures occur, how- ever— paraffin chains, aromatic rings, and naphthene rings. It is possible by analysis to determine the approximate percentage of each structure-class in any oil. As will be brought out later, oil composition has an important bear- ing on both insecticidal efficiency and plant safety. The composition one might find for spray oils, manufac- tured from paraffinic and naphthenic crudes, is: Percentage of each structure Type of oil Refinement Paraffinic Conventional Naphthenic Moderate acid Naphthenic Conventional Paraffin chains 75 5° 45 Naphthene Aromatic rings rings 15 40 38 10 10 17 Oil Sprays for Fruit Trees Research workers learned long ago that the safeness of spray oils to plants in leaf is related to the aromatics and other unsaturates present. It is now generally agreed that oils can be made increasingly safer for use on evergreen plants and on deciduous plants in their growth period by lowering the aro- matic-ring content. That may be ac- complished in part in refining opera- tions by treating the oil with strong sulfuric acid or its equivalent. The aromatics and other unsaturated struc- tures react to form sulfonates, which can be separated from the remainder of the oil. The process has given rise to the term unsulfonated residue, or U. R. The term is widely used to in- dicate the degree of refinement of the oil or its degree of freedom from aro- matic structures. Oils intended for foliage sprays have U. R. values rang- ing from about 90 to 96 percent. Products used on deciduous trees in the dormant period may range from about 50 to 90 U. R. Until 1940 or so, oil composition was thought to have little practical relation to the insecticidal efficiency of horti- cultural spray oils. Since then, how- ever, studies made at the New York State Agricultural Experiment Station and elsewhere have established that efficiency is related to the paraffinicity of an oil. Thus efficiency increases as the paraffinic character in oils in- creases. The relationship has been dem- onstrated in the case of the major oil-susceptible pests of both deciduous and citrus fruit trees. It should not be inferred that the so-called naphthenic oils make unsatisfactory spray oils. They are in a sense simply low paraffin oils and consequently are used at greater strengths in the spray mixture to achieve results equal to those had with highly paraffinic items. Another factor affecting the insecti- cidal value of an oil is the size of its molecules. More familiar but less ac- curate terms for this property are vis- cosity and relative heaviness: Oils of small molecular size, such as kerosene, for example, have little separate value 231 in killing horticultural pests, but there seems to be no advantage in going above a certain molecular size. Viscosity and boiling-range data are the criteria most commonly used in commerce to indicate the molec- ular-size property of an oil. Viscosity is measured at certain temperatures by recording the time required in sec- onds for a sample to flow through a standard opening. It depends on the principle that molecular size controls flow speed, with the rate decreasing as size increases. In addition to size, however, flow rate is also affected by the shape of the molecules. That means that one cannot depend on viscosity measurements alone to clas- sify products as to their suitability for horticultural sprays among oils of dif- ferent origin. For example, a highly paraffinic oil having a viscosity of only 50 seconds Saybolt at ioo° F. may be more effective insecticidally than ex- tremely naphthenic products of 125- 130 seconds. Viscosity measurements are useful in indicating heaviness ranges among oils of common origin and manufacture. A more accurate indication of molecular size in an oil can be obtained from distillation- or boiling-range data. Moreover, that measure has two ad- vantages over viscosity data for spray- oil purposes: It indicates molecular- size range, and it permits a fairly close practical comparison, insecticidally, even among oils of different composi- tion. The California Department of Agri- culture in 1932 established distillation- range standards to regulate the sale of spray oils in that State. Five grades of summer or foliage-spray oils and three grades of dormant oils were set up, based on a minimum U. R., and the percentage of the product that distilled over at 6360 F. The system has not been adopted generally, but it has worked out well on the west coast, partly because spray oils in the area are made from the same general class of crude petroleum. 232 Oil sprays kill insects and mites by what appears to be essentially a smoth- ering action. By enveloping the pest with a continuous film of oil, the oil in- terferes with its respiration and ulti- mately causes death. That is the conclu- sion to be drawn from studies made by E. H. Smith and G. W. Pearce, who used eggs of the oriental fruit moth as test subjects. That the action is largely physical was shown by the ability of some eggs to survive 24-hour exposure to a lethal dosage of oil. In the experi- ment the oil was removed 24 hours after application through the use of an oil solvent. Similar results were obtained earlier by this technique on the winter eggs of the fruit tree leaf roller. In that in- stance, some eggs hatched after having been exposed a week to a deposit that would have killed all eggs had it not been removed. Oil sprays may kill hatched forms of insects in essentially the same manner as just described for eggs. Instead of a direct exchange of gases through the wall or shell, as in the case of an egg, however, respiration in hatched forms usually takes place through openings — spiracles — in the body wall connected with branching tubes extending in- wards (the tracheal system). Killing seems to be effected by oil flowing into the tracheae and plugging them, with death resulting from suffocation. In his studies on California red scale, Walter Ebeling found that the usual route of oil to the insect proper is under the scale covering from its edge. Some oil may penetrate directly through the armor. Besides killing the individuals that it touches, an oil film on the plant interferes with the successful establish- ment of the young that may hatch for some days following treatment. Such a residual effect is an important part of the total action achieved in the control of citrus mites and scale insects. Oil and water, despite the old say- ing, can be made to mix in the form of emulsions wherein the oil is dispersed as minute droplets throughout the Yearbook of Agriculture 1952 water. Oil is usually applied to fruit trees in the form of emulsions contain- ing about 1 percent to 4 percent oil. Emulsification is brought about by agi- tation and the addition of a substance, known as an emulsifying agent, that reduces interfacial tension. Oils are applied as emulsions pri- marily to regulate the amount of oil deposited on the plant. That is impor- tant : A rather direct relationship exists between oil deposit and both insecti- cidal efficiency and plant injury. The object is to lay down a deposit sufficient tc kill the pests present and yet below that which will cause plant injury. Often the operational margin is quite narrow. Oil-deposition rate is deter- mined chiefly by four factors: The oil strength in the spray mixture, the kind and amount of emulsifying agent used, the nature of the plant surface sprayed, and the amount of spray applied. The first requirement of an emulsi- fying agent, of course, is that it produce a satisfactory emulsion. It also should maintain a uniform concentration of oil throughout the batch of dilute emulsion in the spray tank. These con- ditions can be met by forming highly stable emulsions. Unfortunately stable emulsions generally lay down low oil deposits in spraying. To obtain at least moderate-deposition properties in the mixture, one must sacrifice some stabil- ity. Actually, agitation can largely off- set this disadvantage. Most modern spraying machines are equipped with agitation systems that permit the use of relatively unstable emulsions. The influence of the emulsifier on oil-deposition rate in spraying may be great. An emulsion prepared with one emulsifier may lay down as much oil on the plant at a 1 -percent strength as others used at 2-, 3-, or even 4-per- cent strengths. Further wide variations in deposition can be expected as the amount of any given emulsifier is varied. The deposition rate for a given emulsifier generally decreases as the amount used is increased. Another factor is the nature of the plant surface — whether bark, leaves, or Oil Sprays for Fruit Trees fruit, or, indeed, old and new bark, young and mature fruit, old and new leaves, and often the upper and lower surfaces of leaves. The surface factor is of less importance in treating decidu- ous fruit trees during the semidormant period, when relatively small varia- tions in bark surface are involved. The treatment of trees in leaf is something else again. The surface factor is of special importance in treating citrus trees for the control of pests like the California red scale, which occurs on all parts of the tree. If a pest must be controlled on two or more types of sur- faces, the amount of emulsifier should be so adjusted that at least a minimum effective dosage will be laid down on all surfaces. Thus, any recommendations for oil sprays should consider the concentra- tion of oil in the spray mixture and its oil-deposition rate as well. There has been a trend towards adjusting depo- sition rates to common standards. Probably no single oil-deposition standard will prove satisfactory for all purposes. When the oil deposit must be rigidly controlled, as in spraying oil- sensitive shade trees during dormancy and deciduous fruit trees in leaf, a relatively stable light-depositing emul- sion is indicated. But deciduous fruit trees are relatively tolerant of oil in the dormant period. Some overdosing of all or part of the tree then would be of little importance. Consequently less stable emulsions may then be used on fruit trees. Growers can buy a stock oil product in which the emulsifying agent is in- corporated or buy the straight oil and emulsifier separately and prepare the emulsion themselves in the spraying machine immediately before use. The latter is called tank mixing. Satisfactory spray-strength emulsions can be pre- pared by tank mixing as well as through the use of commercial stocks. Tank mixing costs less, but factory-made formulations offer convenience in han- dling and uniform performance. Commercial spray-oil stocks are of two classes, concentrated emulsions 233 and emulsible oils. Such terms as emul- sive oils, miscible oils, and soluble oils are also applied to the second type. Concentrated emulsions — preformed emulsions in a concentrated state — re- semble a thin, whitish paste and usually contain about 83 percent oil by volume. The concentrated emulsions will flow readily through the standard 2-inch bung for metal drums. The emulsible oils consist of oil in which one or more emulsifying agents have been dissolved. They usually con- tain 95 to 99 percent oil and often resemble straight oil in appearance. They are not emulsions in the state in which they are sold but produce emul- sions when added to water in the spray tank. They vary in the readiness with which an emulsion is formed in the tank. Some formulations produce an emulsion instantly; others first require some preliminary agitation in the pres- ence of only a small amount of water. Some authorities prefer to designate the former type of product as miscible oils, reserving the term emulsible oil for the latter. Although the so-called miscible oils emulsify readily, they lay down low oil deposits in spraying. The tank-mixing procedure is quite simple in principle. A 2-percent oil spray mixture can be prepared thus, in a high-pressure orchard-spraying machine equipped with a 400-gallon tank: With the engine running, just enough water is drawn into the tank to operate the pump — 15 to 25 gallons. The emulsifying agent is added, then the oil (which would be 8 gallons in this example). A spray gun directed into the tank is next opened and held open for 1 to 2 minutes. The circula- tion of water, oil, and emulsifier through the pump and its discharge or injection under high pressure into the tank effects emulsification. At this point the mixture should have a uni- form, creamy appearance. The final step is to fill the tank with water, and the mixture is ready for use. The foregoing procedure will pro- duce the most satisfactory type of tank- mixed emulsion, but it is not absolutely 234 necessary to pass the mixture through a spray gun. In the citrus area of Cali- fornia a general practice is to wait a minute or two before filling the tank for the agitators to create the emulsion. An improvement on the practice is to operate the pumps under full pressure during the prefill mixing. Many emulsifying agents may be used in tank mixing. Blood albumin has been widely used. In California a 25-percent product is used at the rate of 4 ounces for each 100 gallons of spray-strength emulsion. An 8-ounce rate is advised in New York. Application of more of the oil-spray mixture than may be needed to cover all or part of the tree usually causes no harmful effects. There is a limit to how much oil can be deposited in continu- ous spraying when most dilute emul- sions are used. It simply runs off be- yond this point. An important excep- tion is when part of a tree may be sprayed twice with a drying period be- tween. The situation may occur when growers follow the practice of spray- ing one side of the row when the wind, say, is in the west, and covering the east side several days later when the wind shifts. Almost twice as much oil will be deposited where the two coverages overlap as elsewhere on the tree. One should try to cover the whole tree in one operation. If each side of the row is sprayed separately, the opposite side should be treated 15 or 20 minutes later, or before the spray applied in the first half of the operation has dried. Petroleum-oil sprays have been used on citrus trees since about 1900. Commercial control of the major pests in most California citrus districts can be had with a single annual application of an oil spray. Such a program, the most economical of those available, has been widely followed in California. The dominant position of oil sprays on citrus is being challenged as the search continues for more efficient in- secticides and for ones without the objectionable effect on trees and fruit that is attributed to oil. Yearbook of Agriculture 1952 Different practices are followed with oil sprays on citrus in Florida because of differences in climate, cultural prac- tices, varieties, and in marketing. Flor- ida citrus trees are apparently more tolerant of oil sprays than are those of California — at least there seems to be greater latitude in the kinds of oil that can be used with relative safety on citrus in Florida. Growers in Florida in 1945 were using oils that ranged in viscosity from 69 to 108 seconds Say- bolt at ioo° F. and from 75 to 92 per- cent in U. R. Both naphthenic- and parafhnic-type products were em- ployed. Practices in California were more standardized ; the spray oils were prepared from much the same class of crude (California naphthenic-base) stock, refined to a U. R. of 90 percent or higher, and were available in a series of relatively narrow boiling fractions. The California Department of Agri- culture in 1932 established specifica- tions for spray oils that were based on certain U. R. and distillation stand- ards. The latter property was measured as the percentage of the product that distilled up to 6360 F. Five grades of oils were established for use on citrus fruit trees — light, lighf-medium, medium, heavy-me- dium, and heavy. The minimum re- quired standards for each grade are given in the accompanying table. Kero- sene and mineral-seal oil are also in- cluded because they have sometimes been applied on citrus. Citrus oil sprays are usually used in California at an actual oil concentra- tion of 1.66 to 1.75 percent — the rate used in tank mixing or in employing commercial cmulsible oil stocks. Con- centrated emulsion stocks — containing 80 to 85 percent of oil — are commonly used at a 2-percent strength. The gen- eral plan has been to keep the oil dos- age in the spray mixture more or less constant and to vary the oil heaviness to achieve the desired results in pest control or as tree tolerance may dic- tate. In general, pest-control efficiency as well as plant-injury hazards increase as oils of increasing heaviness are used. Oil Sprays for Fruit Trees Spray Oils Used on Citrus in California Mini- mum Dis- Viscosity U. R. tilled at in seconds (per- 636° F. Saybolt Grade {cent) percent) at 700° F. STANDARD GRADES OF FOLIAGE SPRAY OILS Light 90 64-79 55-65 Light-medium 92 52-61 60-75 Medium 92 40-49 70-85 Heavy-medium .... 92 28-37 80—95 Heavy 94 10-25 9°-io5 LIGHT OILS SOMETIMES USED ON CITRUS Kerosene 95 1 00 (}) Odorless kerosene. .98 100 (*) Mineral seal oil. ... 91 95 40-50 1 Viscosity values of oils lighter than min- eral seal oil are much lower; comparable determinations are difficult to obtain and relatively unimportant. The main consideration is selection of the proper grade, tree tolerance be- ing a limiting factor. One has to con- sider the kind of citrus fruit to be treated, the insect or mite present, the general district, previous experience with oil in the particular orchard or locality, and the season. The factors are interrelated. The tolerance of the California cit- rus fruits to oil sprays may be listed in the following decreasing order: Lemon, grapefruit, Valencia orange, navel orange, tangerine, and lime. A heavier grade oil is generally applied to lemons than to oranges. Mites and scale insects comprise the two major groups of oil-susceptible cit- rus pests. Unarmored scale insects, such as the black scale and the citricola scale, may be controlled with a lighter grade oil than armored species like the California red scale, yellow scale, and purple scale. A light-medium oil is con- sidered enough for unarmored ones. Growers who depend on a single annual treatment to control the ar- mored scales usually apply a medium- grade oil on oranges and a heavy- medium on lemons. Satisfactory con- trol of the California red scale and citrus red mite on lemons is obtained in some localities with two applications — 235 one in the spring and the other in the fall — of a light-medium oil. The oil sprays applied against scale insects will also control the citrus red mite and citrus bud mite. In fact, in certain localities oil sprays may be applied primarily for the control of mites. Oil heaviness is apparently not a factor in controlling the citrus bud mite, but against the red mite there is a correlation between the length of the protective period afforded by treat- ment and oil heaviness. Citrus is grown under three some- what distinct climatic zones, coastal, intermediate, and interior, in southern California. Citrus trees generally are more tolerant of oil spray in the cooler coastal zone than in the warmer, more arid interior zone. Experience has shown that oils heavier than light- medium should not be used on oranges in the interior, although lemons there will generally tolerate a medium-grade oil. By contrast, a heavy-medium oil (or, on occasion, a heavy oil) can be used on lemons in the coastal zone. Me- dium oil may be used on oranges in the intermediate zone and a heavy- medium grade on lemons. Oil sprays are most commonly ap- plied in California from late July through September. A time is selected when the younger, more susceptible stages of the scale insects predominate. Growers of lemons are inclined to delay treatment until October and Novem- ber to avoid high temperatures, which could result in fruit drop should they occur immediately after the applica- tion is made. Most of the serious effects encoun- tered in the early use of spray oils, such as leaf burn and heavy leaf and fruit drop, largely have been overcome by the use of better oils, minimum effec- tive dosages, and better timing of sprays. Certain more subtle effects re- main, however. It has been fairly well established, for example, that the juice of oranges from oil-sprayed trees will usually have a somewhat lower total content of soluble solids than that from untreated trees or trees fumigated with 236 hydrogen cyanide. Flavor of the fruit is linked to the soluble solids; any ap- preciable lowering of these constituents is undesirable. Other difficulties charged to the use of oil sprays include: Retarded fruit rind-color development; inhibition of "degreening" — development of color with ethylene gas — of the fruit after harvest; accentuation of the "water- spot" condition of navel oranges grown in certain districts in California; and in Florida a possible predisposition of trees to winter injury. Difficulties with rind-color development can be mini- mized by spraying at the recommended season and by avoiding applications immediately prior to harvest. Oil sprays have been suspected — wrongly — of lowering crop yields, of reducing the size of the fruit, of in- creasing the tendency of stems and small branches to die out, and of lower- ing the over-all vigor of the trees. Dinitro compounds, such as dinitro- o-cyclohexylphenol, and sulfur may be applied to citrus trees. Neither is com- patible with oil sprays and should not be used in combination with them. Be- sides, injury may result if an oil spray is applied within 2 weeks of a dinitro treatment or up to 2 months following the use of sulfur. Spraying should be done as closely after an irrigation as practicable. Spraying should be discontinued when it is evident that temperatures will rise above the safe maximum for the dis- trict. It may be possible to escape in- jury by working during the cooler parts of the day, but it is better to cease work when hot weather is forecast for sev- eral days in succession. The safe maxi- mum is 80 ° F. for the California coastal region and 95 ° for the interior district. Spraying should also be avoided during periods of very low humidity and possible frosts. The leaf- and fruit-drop difficulties can be greatly reduced by putting a minute amount of some growth-regu- lating substance, like 2,4-D, in the oil spray. The recommendation stems from research done by W. S. Stewart, Yearbook of Agriculture 1952 L. A. Riehl, and others at the Cali- fornia Citrus Experiment Station. Dosages suggested, in acid equivalent, are 4 parts per million of ester prepara- tions or 8 p. p. m. of metallic or alka- noamine salts. The amounts refer to the concentration of 2,4-D that will occur in the dilute spray mixture. The compound cryolite also may be added to oil sprays to save the cost of making a separate treatment. Cryolite is applied to control orange tortrix and similar species. Rotenone is said to im- prove the efficiency of oil spray against scale insects; rotenized oil sprays may be helpful against the black scale, but against California red scale it has not proved effective enough to warrant a general recommendation. Oil sprays are usually applied in California with high-pressure spraying equipment mounted on trucks. Spray- ing is done from the ground with 60- to 75-foot leads of hose and single- nozzle spray guns. The most satisfac- tory equipment also has an hydraulic- ally operated telescoping tower topped with a platform. It permits a man to work 30 feet from the ground so he can cover the tops of trees. A spray crew normally has two ground sprayers, a tower man, and the truck driver. To control red scale, particularly, the interior of the tree has to be sprayed as thoroughly as the outside. The growth of citrus trees often is dense, and the necessary coverage cannot be attained by spraying from the outside alone. For inside coverage, most spray men insert their spray gun through the foliage at four points around the cir- cumference. To cut labor costs, growers have been interested in the development of more mechanized means of spraying. Several new kinds of vertical spray booms have proved satisfactory, especially when the aim is to get fast outside coverage. Other equipment works on the prin- ciple of carrying the spray into the tree by means of an air-blast. The booms and air-blast-type machines, however, had not been adopted for general use on citrus in California in 1952. Oil Sprays for Fruit Trees Florida citrus, as we indicated, ap- parently is not so sensitive to the oil sprays as are citrus trees in California. Consequently growers can use a variety of oils. The usual concentration of oil in Florida is about 1.2 to 1.33 percent of actual oil. July is considered the best time to apply oil sprays. Treat- ments made then give good control of scale insects and avoid unfavorable ef- fects that may result from the misuse of oil sprays. High-pressure equipment and air-blast machines are widely used by citrus growers in Florida. Deciduous fruit trees are com- monly treated with oil sprays in spring, when the trees are semidormant. Highly refined oils used to be included in many summer sprays, but the prac- tice has been greatly curtailed since the introduction of DDT, parathion, and other new toxicants, which have largely replaced summer oils. Considerable differences exist in Eastern and Western States in the kind of oils and the ways they are used. Cus- tom accounts for some of the differ- ences. Other factors involve differences in petroleum supply, climate, and pest problems. In the Pacific Northwest oil sprays are applied in the spring before any green tissue appears in the buds. Later applications of dormant oils are not ad- vised because of the danger of injuring the buds. If oil is used alone in dor- mant sprays, it is at strengths of 3 to 4 percent. Such treatment is advised for San Jose scale, pear psylla, and the European red mite and clover mite, which overwinter as eggs. Another practice in the Northwest is to combine oil with liquid lime-sulfur for a dormant treatment; the oil is used at a 1- or 2-percent strength and the lime-sulfur at 3 percent. This com- bination spray is effective against scale insects, pear leaf blister mite, and apple rust mite; if 2 percent oil*is used, it will also destroy winter eggs of Euro- pean red mite and clover mite. The dormant spray oils used west of the Rockies are made from California 237 petroleum crudes. Specifications call for an oil of 100-120-second Saybolt viscosity at ioo° F. and allow an un- sulfonated residue value of 50 to 70 percent. Some Canadians have favored heav- ier viscosity oils than those generally advised in the United States. Products of 200—220 seconds viscosity are pre- ferred. The oils used in British Colum- bia are naphthenic, being produced from California crudes. Heavier vis- cosity oils are favored in British Colum- bia because they are thought safer and apparently more efficient than the 100- 1 20-second naphthenic oils. Fruit growers in Northeastern States commonly apply early-season oil sprays after some new growth has ap- peared in the buds rather than in the full dormant stage. On apples that avoids combining oil with dinitro in- secticides, which must be applied when the buds are dormant. Oil-dinitro mix- tures may cause serious bud injury. An- other reason for later spraying is that a higher kill of winter eggs of the Euro- pean red mite is had. The eggs become increasingly more susceptible to oil as their hatching period approaches. A higher kill of mite eggs may be ex- pected with a 2 percent oil spray ap- plied in the delayed-dormant stage than the same oil applied in the dor- mant period at a 4-percent strength. Apple trees are considered to be in the delayed-dormant stage when about a half inch of leaf tissue is exposed in blossom buds. To insure reasonable safety in dor- mant oils after new growth has ap- peared, oils, different from the ones formerly used in the Northeast (and still favored in the Northwest) were needed. Such oils, called superior dor- mant tree-spray oils, were perfected largely as the result of research by chemists and entomologists at the New York State Agricultural Experiment Station. The oils are widely used by orchardists in New York and are rap- idly gaining acceptance elsewhere in 970134°— 52- 17 238 the Northeast. They have these specifi- cations : Viscosity (Saybolt, at ioo° F.). Viscosity index (Kin- ematic). Gravity (A. P. I. de- grees ) . Unsulfonated residue (A.S.T.M.). Pour point Homogeneity 90—120 seconds 90 (minimum) 31 (minimum) 90 (minimum) Not greater than 30° F. A relatively narrow boiling distillate portion of petro- leum. The following methods of testing spray oils are to be used: Kinematic Viscosity, A. S. T. M. designation: D445-39T. Con- version to Saybolt Universal Viscosity, A. S. T. M. designation: D446-39. Kine- matic Viscosity Index, A. S. T. M. desig- nation: D567-40T. A. P. I. Gravity, A. S. T. M. designation: D287-39. Pour point, A. S. T. M. designation: D97-39. Unsul- fonated Residue, A. S. T. M. designation: D483-40. Those specifications define an oil of high paraffinic character and fairly low aromatic content. The paraffinicity re- lates primarily to insecticidal efficiency and the aromatics to plant safety con- siderations. Oils having a 90 percent unsulfonated residue rating or higher — roughly 10 percent or less aromatics — generally have proved safe to use on New York apple trees in the delayed- dormant bud stage. As we pointed out, all spray oils con- tain some paraffinic structures, but differ in degree of paraffinicity. The oil content of a spray, to achieve con- trol, may be decreased as products of increasing paraffinic content are used. This relationship has been shown for the following: San Jose scale, Euro- pean fruit lecanium, cottony peach scale, scurfy scale, apple red bug (eggs), eggs of the European red mite and probably the clover mite, fruit tree leaf roller and related species, and the eggs of codling moth, oriental fruit moth, grape berry moth, and eye- spotted bud moth. Superior dormant tree-spray oils are sold as straight oil for tank mixing or as commercial concentrated emulsions Yearbook of Agriculture 1952' or emulsible oils. Many New York growers tank-mix and use blood albu- min as the emulsifying agent. A fungicide is usually included in delayed-dormant applications of oil on apple trees in the Northeast to pro- vide protection against apple scab. Bordeaux mixture, 2-4-100, or its equivalent in a proprietary copper fungicide are commonly used. For the most resistant pests, such as scurfy scale, cottony peach scale, apple red bug, and fruit tree leaf roller, superior oils are employed at a 3-per- cent strength. A 2-percent concentra- tion is considered adequate under New York conditions for the control of San Jose scale, pear psylla, European fruit lecanium, and the European red mite. In areas south of New York, a 2-per- cent concentration is considered in- sufficient for the control of the San Jose scale. Strengths of 2.5 and 3 per- cent are advised there to combat the pest. The suggested spray concentra- tions are based on the oil-deposition properties imparted to an emulsion by blood albumin. Higher or lower dos- ages may be needed if the emulsions used differ greatly from blood albumin emulsions in oil-deposition rate. Dormant or semidormant treat- ments often are used to control the various species of aphids that are troublesome to the hardy fruits. They may be applied with the object of kill- ing either the overwintering eggs or the newly hatched aphids on the opening buds. Conventional spray oils are not particularly effective aphicides in either case. In the Pacific Northwest green-tip or delayed-dormant appli- cations of oil are suggested for the con- trol of fruit aphids. Growers in the East rely on dormant applications of dinitro insecticides to destroy the eggs or on the inclusion of nicotine sulfate or parathion in the de- layed-dormant spray. Apparently there is little relationship between paraffin- icity and the response of aphid eggs to oil sprays. If anything, the correlation lies between response and the aromatic content of oils. It is well known, for Oil Sprays for Fruit Trees example, that aphid eggs are highly susceptible to such aromatic products as cresylic acid and the tar oils. From this one might conclude that there would be an advantage in using oils of high aromatic content, that is, having low U. R. values. Unfortunately, such oils apparently cannot be depended upon alone to control aphids; further- more, their use must be restricted to dormant applications. The older types of summer oils have declined in popularity. Much of this situation can be attributed to their in- compatibility with fungicides and other insecticides. Sulfur has long been the stumbling block to the more exten- sive use of summer oils in the Eastern States. Serious direct foliage burn or delayed leaf drop may result from the use of oil and sulfur on the hardy tree fruits. Similar harmful effects have been noted with DDT and oil com- binations. No very definite specifications have been established for summer spray oils. A product meeting the following speci- fications should prove satisfactory for use in the East: A narrow-boiling- range product having a Saybolt viscos- ity at ioo° F. of 65-70 seconds, a min- imum U. R. of 92 percent, and an A. P. I. gravity of 33. Such an oil would be used at a 1 -percent concentration to combat summer infestations of mites, the cottony peach scale, and, combined v/ith nicotine sulfate or rotenone, the pear psylla. The use of oil sprays in the future depends on several considerations. Most of the objections to them re- volve around unfavorable plant re- sponses. If safer oils could be produced, particularly for use on the more sensi- tive plants, the use of oil should in- crease. It should be possible to produce safer and more efficient oils — synthetic oils and special fractions of petroleum, for example. Oils are less toxic than many other insecticidal materials to man. Their relative safety in that respect recom- mends them for wider use. Insects have shown a disturbing abil- 239 ity to develop resistance to some in- secticides, but so far not to oils. The way oils kill insects and mites, appar- ently through physical means, merits attention ; it may prove to be a valuable quality in the future use of chemical treatments for the control of pests. P. J. Chapman is professor of ento- mology and head of the division of en- tomology at the New York State Agri- cultural Experiment Station at Geneva, a unit of Cornell University. A native of California, he was trained at Stan- ford University, Oregon State College, and Cornell University. He was grant- ed a doctor's degree from Cornell in 1928. From 1923 to 1928 he engaged in extension activities at Cornell and from 1928 to 1930 served as entomol- ogist of the Virginia Truck Experiment Station at Norfolk. L. A. Riehl is an assistant entomol- ogist of the division of entomology at the University of California Citrus Ex- periment Station at Riverside. He was trained at the University of California at Berkeley and at Iowa State College, which granted him a doctor's degree in 1942. From 194.2 to 1945 he was em- ployed by the Rockefeller Foundation and served as a consultant on insect- borne diseases to the Surgeon General, United States Army. For those serv- ices he was awarded the bronze star and the medal of the United States of America Typhus Commission. From the Egyptian Government he received the Order of the Nile {Chevalier) and the Gambia Eradication Medal. G. W. Pearce in 1951 was appointed chief of the chemistry section of the technical development services in the Communicable Disease Center of the United States Public Health Service in Savannah, Ga. From 1930 to 1951 he was a member of the staff of the New York State Agricultural Experiment Station at Geneva. He holds three de- grees from Pennsylvania State College. Dr. Pearce has worked on investiga- tions of insecticides and fungicides, especially their analysis and chemistry in relation to their use on fruits. Aerosols and Insects W. N. Sullivan, R. A. Fulton Alfred H. Yeomans An aerosol, like fog or mist, is an as- semblage of particles suspended in air. An insecticidal aerosol has particles whose diameters range from i to 50 microns — from 1/25,400 to 50/25,400 inch. Insecticidal aerosols are dispersed in air by burning organic material, atom- izing mechanically, vaporizing with heat, or liberating through a small opening an insecticide that has been dissolved in a liquefied gas. In the last the liquefied gas evaporates and leaves small particles suspended in air. Many householders have become ac- quainted with aerosols in small con- tainers— so-called bombs, although of course they are not explosive. A more general application has been in use a long time. The Mono Indians of Cali- fornia knew the value of smoke in stupefying insects so that they could be easily collected for food. They pre- pared a smooth floor under trees con- taining the full-grown larvae of the pandora moth and built a smudge fire. The smoke caused the caterpillars to drop to the ground in countless num- bers. They were then raked into the fire, partly cooked, dried, and later eaten as a stew. Another example of an aerosol was seen in the Northeastern States one day in September 1950, when the sun turned an eerie purple and darkness came at 2 p. m. A mass of cold air had drifted down from northwestern Can- ada and brought along the smoke from forest fires in the Alberta and Mac- kenzie district — an illustration of how aerosols can be dispersed in air cur- rents from one point through large areas. Aerosol bombs were developed in 1 94 1, when L. D. Goodhue and W. N. Sullivan of the Bureau of Entomology and Plant Quarantine discovered that aerosols produced by spraying a solu- tion of liquefied gas and insecticide through a small hole into the air were highly toxic to mosquitos and flies. The aerosol solution was made by dis- solving pyrethrum and sesame oil in- secticides in a liquefied gas commonly used in household refrigerators and called dichlorodifluoromethane. The liquid has a vapor pressure of approxi- mately 75 pounds per square inch at room temperature. The aerosol solution is held in a strong steel container with an outlet tube to the bottom. In operation, the vapor pressure of the liquefied gas is sufficient to force the solution out of the tube and into the air through an orifice that may vary from 0.013 to 0.024 inch in diameter. The gas im- mediately evaporates and the tiny par- ticles of insecticide are dispersed as a fine mist. The scientists knew the gas was non- toxic and noninflammable, and they found it to be nontoxic to man and animals when they mixed it with in- secticide. The ease of application, the high concentration of insecticide, and the ability of the small aerosol particles to disperse and to stay suspended in the air for a long time fulfilled require- ments for a good household insecticide. A public service patent was issued on the invention and assigned to the Sec- retary of Agriculture for the free use of the people of the United States. Licenses are issued, royalty free, for the manufacture, use, and sale of products produced under the patent. So urgent was the need for a better way to kill mosquitoes and flies in war zones and so good was cooperation of the Department of Agriculture, the military, and industry that our troops used aerosol bombs within a year after they were discovered. Throughout the war the bombs were highly efficient against disease-carrying insects in bar- racks, mess halls, tents, and foxholes. They became standard equipment in 240 Aerosols and Insects long-distance airplanes, in which they were used to prevent the spread of hitchhiking insects. Occasionally one was used to cool the beer of a jungle fighter. In all, more than 40 million aerosol bombs were made for the Armed Forces. At the end of the war aerosol bombs were made for civilian use. Strong, in- expensive containers and suitable push- button valves were developed. New low-pressure propellants and solvents were perfected. The earlier formula- tions were modified to include combi- nations of pyrethrum, a pyrethrum syn- ergist, and DDT or methoxychlor. Making low-pressure aerosol con- tainers is a growing business. It has ex- panded to include deodorants, disin- fectants, and other products, besides insecticides. It amounted to 33 million dollars in 1949, with prospects of going above 100 million dollars in later years. The aerosol bomb is a good servant in kitchen, pantry, living room, bed- room, and cellar. Before it is used, the windows apd doors of a room should be closed. Pets, birds, fish bowls, and food should be removed or covered. The container should be held upright with the opening away from the face, so the aerosol will go toward the ceil- ing. The operator walks around the room to give a good initial distribution. The bomb should not be held closer than 3 feet to any object, or the aerosol may stain furniture, wallpaper, cur- tains, or draperies. It can be held 6 to 12 inches from baseboards and cracks where insects like roaches and ants crawl or hide. In treating the average room ( 1,000 cubic feet) to control such fliers as the mosquitoes, house flies, sand flies, black flies, gnats, and moths, the bomb valve should be opened to release the aerosol for about 6 seconds. This dos- age will also kill some types of ants, but is not effective against the larvae of clothes moths. A 15-second release will kill fleas, wasps, and hornets. Roaches can be decimated but it takes at least 2 minutes of spraying per room 241 and a whole bomb (12 ounces) for good results in the cellar. Spiders are hard to kill with aerosols. The room should be kept closed for 10 or 15 minutes after treatment for flying insects and ants, 30 minutes for fleas, and 1 hour or more for roaches. Then the room may be aired out, but that is not necessary. Gas-propelled aerosols are widely used to control insects in greenhouses. They cut the usual time for treating greenhouses from 48 man-hours to 10 minutes, eliminate black spot on roses, and can increase production 25 to 50 percent, depending on the degree of infestation. The formula originally developed for greenhouses contained 10 percent of hexaethyl tetraphosphate and 90 per- cent of methyl chloride. Parathion partly has replaced hexaethyl tetra- phosphate because it has lasting effect. Because parathion gives unsatisfactory control of resistant spider mites and aphids, new materials and formulas had to be found. Tetraethyl dithio- pyrophosphate came into use in local- ities where resistant insects have been found. A newer material, octamethyl pyrophosphoramide, applied as an aerosol, has appeared promising against resistant greenhouse insects. Liquefied gas aerosols have been used also against such insects as pea aphids on peas in the field. The lique- fied gas is released close to the peas through nozzles on a boom that has a shield above it. The aerosol thus is distributed so that much of it is held near the plants for a long enough time. The work that led to the develop- ment of the aerosol bomb started with a study of insecticidal smokes. Insects were subjected to a burning mixture of derris or pyrethrum, cornstalks, and sodium nitrate. The mixture burned like Fourth-of-July fireworks, and the smoke did kill insects, but such dis- persal of nonvolatile or slightly volatile insecticides was wasteful. The next step was to spray oil solu- tions of rotenone and pyrethrum on a hot plate. On contact with the heated 242 surface (about 3750 C), the droplets were partly vaporized and formed par- ticles of aerosol size. Aerosols so pro- duced are called heat-generated. It is an efficient way to produce insecticidal aerosols. Aerosols were then produced in the same way by spraying them onto the inner walls of a tube heated with electricity. After that, the hot exhaust of a small gasoline engine was used as the source of heat energy. From that came the suggestion that an Army smoke-screen generator be used to pro- duce insecticidal aerosols to treat large areas for mosquitoes and flies. Smoke was formed in the genera- tors by running a mixture containing a little water in oil through coils pass- ing through a combustion chamber, which was heated by an oil or gasoline burner. The oil-water mixture was completely volatilized by the heat and condensed into a smoke on contact with the outer air. The aerosol particle thus created was ideal for an Army screening smoke because it gave a good scattering of transmitted light and re- mained suspended in the air for a long time. It turned out, however, that the particle size was too small for efficient insect kill. A larger particle size and better insect kill was obtained by using a 50-50 mixture of water and oil and operating the machine at a lower tem- perature. Another Army smoke generator used incomplete combustion to produce smoke. It was later modified to an in- secticide aerosol generator by using a gasoline motor to drive a rotary air pump. The pumped air passes through and is heated in a gasoline-burning combustion chamber regulated at 4820 C. The hot air then passes through special nozzles into which the insecti- cide is injected. The particle size is regulated by the flow of insecticidal solution through the nozzle. Several methods have been used since the war to generate aerosols on a large scale. One machine uses a num- ber of spinning disks to break up the solution. One uses the exhaust gases Yearbook of Agriculture 1952 from a small pulse-jet engine. Another employs steam to atomize the solution as it issues from a nozzle. Many small indoor types of aerosol generators have been placed on the market. They use electrically driven spinning disks, rotors, and pressure pumps, electrically generated vapor- izers, and steam atomizers. One of the machines uses extremely high pressure generated by hand-pumping the liquid solution against a fixed charge of nitro- gen. There are also convenient pack- ages of mixtures for partly burning and releasing the insecticide as a fine aerosol smoke. For best results, the farmer or health official must study his problem in detail before applying an insecticide with an aerosol generator. The ma- chines can be set to produce different particle sizes. The choice of ma- chine depends on the use to which it is to be put, whether for insects in con- fined spaces, flying insects, or those that attack his field crops. These principles may guide the prospective purchaser: Aerosols are used indoors effectively as a way to control flying insects and to apply a light deposit on the top of exposed horizontal surfaces. The particle size has a bearing on the effectiveness of the aerosol. The particle size is critical for the amount that collects on an insect as it flies through the aerosol. Particles that are too small are deflected from the flying insect as smoke is from a moving auto- mobile. Particles that are too large settle rapidly, and their dispersion is poor; therefore their chance of touch- ing the insect is also poor. When an insect does collide with an oversized droplet, the excess insecticide is wasted. Our research has shown that the best particle size to use for flying insects is between 10 and 20 microns mass me- dian diameter. Aerosols are dispersed by air cur- rents. The particles will not be con- veyed into dead-end cracks or into material through which air does not circulate. The distance to which par- Aerosols and Insects tides will be carried depends generally on their settling rate. An oil particle i micron in diameter will settle 10 feet in 26.5 hours. A par- ticle of 15 microns will settle 10 feet in 15 minutes. In unheated buildings air currents are at a minimum, but heat- ing sets up air convection currents that are a great aid to dispersion. Some- times large-volume air blowers are used to aid dispersion. In an unheated room with a ceiling height of 8 feet, aerosols with a mass median diameter of 5 microns disperse fairly uniformly over an area 30 feet from the source, those of 15 microns over an area of 15 feet, and those of 25 microns less than 10 feet. The deposit as a result of an aerosol settling is about 95 percent on the top of horizontal surfaces and the rest on walls and ceiling. The amount of de- posit on a horizontal surface depends on the concentration of the aerosol above the surface so that if the aerosol is evenly dispersed throughout a room the resulting deposit will be propor- tional to the height above the surface. In large closed warehouses that con- tain packaged food, the problem of flying and exposed crawling insects can be controlled by aerosol treatments. Aerosols of various particle sizes were tested; a size of about 5 microns mass median diameter was selected as most effective and easiest to apply. These small particles are produced by ther- mal aerosol generators that can be op- erated outside the warehouse; the fine particles are introduced through an open door. The aerosol is carried first to the ceiling. By the time the treat- ment is complete, the aerosol is well distributed throughout the interior by convection currents. The door is then closed. Overnight the particles pene- trate into most of the cracks and crev- ices and settle on the top of exposed horizontal surfaces. It is sometimes necessary to limit the time of application in some closed in- teriors. The particle size then must be large enough to settle out in the time available. A 10- to 15-minute exposure 243 time is the minimum for satisfactory results. An aerosol having a mass me- dian diameter of 15 to 20 microns is sufficient for the short-exposure ap- plication. Equipment sometimes limits the par- ticle size. When heat from thermal gen- erators causes excess breakdown of the insecticide, equipment that produces larger particle sizes must be used. They then must be released from more than one point to cover adequately areas whose dimensions are larger than the distances of uniform deposit. Heated rooms will about double the dispersion area; rooms with high ceilings will add slightly to the dispersion. When treating greenhouses, it should be remembered that foliage injury can be caused by a particle size larger than the foliage can tolerate with the type of formulations used. Some formulas that we have used indoors are : ( 1 ) 1 pound of technical DDT dissolved in 7.5 pints of Sovacide 544C (Socony Vacuum) to make 1 gallon. (2) 1 pound of technical DDT dissolved in 2 quarts carbon tetrachlo- ride; to it are added 3.5 pints of No. 3 fuel oil to make 1 gallon. This formula is relatively safe from explosion. (3) 1 quart of 10 percent pyrethrum in deobase; 1 pint of piperonyl butoxide and 1 pint No. 3 fuel oil are added. Because of the explosion hazard when oil solutions are used indoors, not more than 1 gallon of the solutions should be used per 100,000 cubic feet. They should not be released near an open flame. Workers should wear proper respirators. The third formula, which contains pyrethrum, is recom- mended for use around foodstuffs. The formulations should contain a proportion of relatively nonvolatile oil to maintain the desired particle size while it is suspended in the air. In closed warehouses, 1 pound of DDT in 1 gallon of solution per 100,000 cubic feet of space, applied about every 2 weeks in summer, will provide pro- tection against insect infestation. The main problem in applying the aerosols outside, other than for tempo- 244 rary control of flying insects, is to put down a uniform deposit. To do that the aerosols are applied as wind-borne clouds. For best results the wind should be light, steady in direction, and mov- ing at ^2 to 8 miles an hour. The air temperature at ground level should be a little cooler than at 6 feet or more. This surface inversion keeps the aerosol cloud close to the ground; it is most important when low-growing crops are treated and least important for trees having a canopy of foliage. Good inversion usually occurs from i hour after sunset until sunrise but may exist all day if rain has cooled the ground. The dosage depends on how much has to be deposited on an acre to kill the insect. The deposit is heaviest near- est the point of release and decreases as the distance from the release point increases, because the larger particles settle first. Under the best conditions, only 25 to 50 percent of an aerosol con- taining particles less than 50 microns in average diameter is deposited over an open area in swaths up to 2,000 feet; most of it drifts beyond the area under treatment. In wooded places the deposit would be greater. When more than one swath is used, however, the dosage can be cut about 10 percent for each successive swath because of overlapping up to a total of 50 percent. The swath width should be selected according to the location of accessible roads, to places where the wheels of the machine will do the least damage to the crop, and places where oil de- posits will not injure the foliage. Some recommended particle sizes in microns mass median diameter for various swath widths and wind veloci- ties are as follows: Swath width in feet Wind velocity in miles per hour 13 5 100 40 70 90 200 30 50 65 300 25 40 55 5°° 20 35 40 1,000 15 20 30 1,500 10 20 25 7 100 75 65 5° 35 30 Yearbook of Agriculture 1952 At least one-fourth of the aerosol so- lution should be nonvolatile. Results are best with a concentrated solution. A popular formula is 5 to 7.5 pounds of DDT dissolved in 2 gallons of ben- zene or xylene plus 3 gallons of SAE 10 W motor oil or agricultural oil. Oil-soluble technical BHC may be used in the formula in place of DDT. The operators should wear protective masks and clothing. Aerosol generators are useful in military and civilian situations in which mosquitoes and flies create problems of public health. They make it easy to clean up infestations in towns or camps and are particularly effective along the seashore. They are less well suited for the control of agricultural pests out- doors. Some problems require the use of fungicides and insecticides together, and the fungicides may be too bulky for efficient handling in aerosol generators. Field-model aerosol machines using concentrated DDT solutions have been employed successfully against gypsy moths, lygus bugs, tarnished plant bugs, pentatomids, potato flea beetles, and leafhoppers. The aerosol machines are not suit- able for treating individual trees or areas of less than an acre because the aerosol fog is placed entirely by wind drift and the initial thrust of about 10 feet given by the generator is sufficient only to place the aerosol in the wind. W. N. Sullivan is an entomologist in the Bureau of Entomology and Plant Quarantine. He is a native of Massa- chusetts and a graduate of the Univer- sity of Massachusetts. R. A. Fulton, a chemist in the same Bureau, is a native of Oregon. He holds degrees from Oregon State College, the University of Wisconsin, and Stan- ford University. Alfred H. Yeomans, a technologist in the Bureau, has been engaged in in- vestigating and developing equipment used in insect control. He is a native of California and a graduate of Ohio State University. Applying Insecticides Using Insecticides Effectively E. J. Newcomer, W. E. Westlake B. J. Landis Merely to secure effective insecti- cides and apply them is not enough. The best material may fail if it is used at the wrong time or in the wrong way. The user will do well to learn some- thing about the habits of the pests he wants to get rid of, the physical prop- erties of insecticides, and the influence of weather and type of plant growth on their effectiveness. The contact insecticides — those that kill only the insects they touch — must be applied thoroughly. Because many materials retain their effectiveness for only a short time, the chance that the insects in moving about will later come into contact with them is not great. Also, many kinds of pests, such as some aphids and scale insects, cannot mcve about, although if the insecticide has some fumigating action it may suffice to place it within a short distance of the insect. Nicotine, for example, in hot weather will kill aphids not ac- tually wet by it. Thorough application of the stom- ach poisons, those that kill the insects that eat them, is less necessary. Many leaf-feeding insects move about. Lar- vae or caterpillars may travel from leaf to leaf on a plant; flying insects travel from plant to plant. Killing them at the earliest possible time may be unimportant. An application that reaches only one surface of the foliage or perhaps reaches only a part of the foliage may be enough. For a boring in- sect, such as the larva of the codling moth, however, thoroughness is essen- tial in order to destroy it before it gets out of reach of the poison. Consideration should be given to possible residual action of insecticides. Some are effective only for 24 hours or less. They evaporate or decompose rapidly, and often are particularly use- ful if a pest has to be dispatched shortly before harvest, as they do not leave harmful residues. Other insecticides disappear more slowly and are effective for a week or more. They are of value in arid farm- ing areas, for example, where row crops and orchards must be irrigated at in- tervals, during which time it may not be possible to apply insecticides. Still other insecticides, particularly the stomach poisons, may retain their effectiveness indefinitely. Thus, if the period during which control is needed is long, frequent ap- plications of an insecticide like tetra- ethyl pyrophosphate (TEPP), which decomposes rapidly, may be necessary. On the other hand, a single applica- tion of DDT in the soil has been known to control some soil-inhabiting pests for 5 years or more. The physical condition of insecti- cides affects their efficiency. Emulsions — droplets of oil sus- pended in water — are an example. The size of the droplets largely is governed by the kind and amount of emulsifier used in the mixture. If the size is large, the emulsion will not be stable; the oil tends to rise and float on the surface. 245 246 The droplets may be kept mixed with the water by vigorous agitation, but they separate rapidly when agitation is discontinued. Separation is likely to occur in the spray tank or in the sup- ply lines to the nozzles, and the result is that sometimes almost no oil will be present in the spray, whereas practi- cally undiluted oil may be sprayed when the tank is nearly empty. But if the oil droplets are very small, the emulsion may be so stable that virtually no separation will occur, even after standing for hours. For insec- ticidal use we require something in be- tween. It is desirable to have an emul- sion that is sufficiently stable to remain in suspension in the spray tank and supply lines but unstable enough to break immediately after leaving the nozzle, or upon contact with the sur- face being treated. Such an emulsion gives a much higher deposit of the oil than the more stable type. Wettable powders are widely used as insecticides. The insecticide and dilu- ent are combined into a dry powder, which is mixed with water before use to form a suspension. The particles must be less than 10 microns in size in order to give maximum efficiency. Larger particles do not adhere well to plant foliage and give less complete coverage, as there are fewer of them in a given quantity of material. Some commercial preparations even have an average particle size of 1 micron or less. The physical properties of insecti- cidal dusts, such as particle size, bulk density, and flowability, are important. A dust, carried to the plants by a blast of air, is easily diverted by natural air currents. Also, the adherence of the particles to the surface being treated depends partly on the velocity at which they strike the surfaces. Very small particles are not desirable in dusts since they tend to drift with the air currents and lose their velocity quickly after leaving the outlet of the machine. As a rule, dusts should pass through a 325- mesh screen, by washing with a suit- able medium. Such a screen will pass Yearbook of Agriculture 1952 particles as large as 44 microns in diameter. The usual dust mixture will contain particles much smaller than this, of course, but the average size may be far above that desirable for use in sprays. Particles in" excess of about 40 microns in diameter are not desirable because they will not adhere well to the foliage. Bulk density, the weight of the un- compacted powder per unit volume, may be expressed in pounds per cubic foot. The greater the weight the less tendency there will be for the dust to float or drift. Bulk density is particu- larly important in aircraft application because then the dust settles quickly to the ground to avoid excessive drift. Dusts so used should have a bulk den- sity of not less than 40 pounds per cubic foot. For application by ground equip- ment, 30 pounds per cubic foot may be enough. Mineral oil at the rate of 1 to 2 per- cent by weight aids in reducing the drift of dusts and probably increases adherence to plants. The addition of mineral oil to dusts for aircraft appli- cation is required in some areas. Dusts must resist packing in the hop- per well enough to permit them to flow freely and uniformly into the feed mechanism. A dust that packs will not give a uniform rate of flow from the machine and the coverage will be un- even. Flowability is determined by the diluent used. Some of the better dilu- ents from the standpoint of bulk den- sity do not flow well while some free- flowing materials are too light. Often a mixture of a small amount of a free- flowing material added to one of high bulk density is used to give the desired properties. Proper timing is important. If put on too early insecticides may be dissipated before the pest is present or in a sus- ceptible condition. If they are applied too late, the pest may already have caused injury. Timing is sometime de- termined by the stage of growth of the crop. For example, it is not desirable to treat crops when they are in bloom. Bees visiting the blossoms may be killed Using Insecticides Effectively and the yield of fruit or seed crops seri- ously reduced because of lack of pol- lination. The blossoms themselves may be injured. Sometimes a compromise must be made. For example, eggs of the European red mite often hatch just as apple trees are coming into bloom. An application of lime-sulfur just be- fore blossoming is apt to be too early; one made afterwards is too late for the early-hatching mites. If thorough con- trol is desired at this time both appli- cations must be made. Some pests, such as the green peach aphid or the two-spotted spider mite, which attack numerous crops, are pres- ent during most of the growing season. Effective and economical control of such pests depends on proper spacing of applications so that the crops are protected from injury with not too many sprayings or dustings. Control may be needed over a longer period in an early season than in a late one be- cause the active period of the pests is lengthened. The location of the pests on the plants also must be considered. If they feed chiefly on the lower surfaces of the leaves, those surfaces may have to be reached, especially if a contact mate- rial is being used. If they are on the roots, a soil insecticide is needed. If they climb the trees to feed on the foli- age, a treatment of the trunk may be indicated, or perhaps merely a me- chanical barrier will suffice. As to weather: The United States Weather Bureau has numerous stations and cooperative observers and can supply weather information for practically any farming area in the country. Information on temperature, rainfall, and wind is especially useful. Much of this information is given in the Yearbook of Agriculture for 1941, Climate and Man. By studying his local conditions over a period of years, a grower can learn to avoid bad weather or take advantage of good weather to a great extent when controlling insect pests. Wind often limits the application of 247 insecticides. If it is blowing more than 5 or 6 miles an hour, spraying is inter- fered with, although at times spraying may be necessary in windy weather in order to provide some protection from immediate insect attack. A study of the occurrence of wind may help to de- termine when to spray or dust. In the Yakima Valley of Washington, for ex- ample, the average percentages of good spraying weather during daylight hours in the spring when dormant sprays need to be applied to fruit trees are: March 1-15, 35 percent; March 16- 31, 22 percent; April 1-15, 15 percent; and April 16-30, 14 percent. Thus it is advantageous to get this spraying done early in this valley, because good spray- ing weather is less frequent as the sea- son advances. Application of dusts by aircraft should be made in the early morning or evening. Usually, after the sun has warmed the air, rising currents tend to carry the dust away from the crop. The same limitation often holds when dust- ing with machines on the ground. Per- fectly calm weather is not necessary, and a slight drift may be advantageous. An air movement of 1 to 8 miles an hour is preferred when aerosols are used. Temperature inversion, which oc- curs most often early in the morning or after sunset, should be taken advan- tage of when using aerosols. The tem- perature near the ground is then slightly cooler than at a height of sev- eral feet; the cloud of insecticide is kept from rising and therefore coats the plants more thoroughly. Air currents tend to carry away vola- tile insecticides, such as oils, and there- fore somewhat heavier dosages must be used outdoors than inside. Strong winds blow considerable quantities of dust from the plants, and they cause a loss of spray deposit by rubbing the leaves together or by rubbing leaves against the fruit. Temperature becomes important in the application of insecticides only when it is extreme. Most materials are effective at ordinary temperatures. 248 Nicotine sulfate is an exception, as it kills insects much better at tempera- tures above 75 ° F., partly because of increased fumigating action. There is danger of injury to fruit or foliage if some materials are used in hot weather, although their effectiveness is not les- sened. There is also danger of injury from oil sprays if followed by extremely cold weather. Rains may wash off insecticides that are soluble in water, and their effec- tiveness is lost. Most of our modern insecticides are not especially soluble in water. Most of them are available in a form that adheres well to fruit and foliage. Their effectiveness is not re- duced so much by ordinary rainfall as is sometimes supposed if the applica- tion has become thoroughly dry before it rains. Hard, driving rains, however, will remove much of the insecticide from exposed surfaces. This may be the reason why it is necessary, for example, to spray more often with DDT to con- trol the codling moth in the Midwest-, where rainstorms occur in summer, than in the drier parts of the North- west, where such storms are infrequent. Dusts are removed more extensively than sprays by rains. Trees or other plants may be sprayed when they are damp, but it is best not to spray them if they are dripping wet. The presence of dew or other moisture on plants is sometimes a help when dusting; it causes the dust to adhere better than if the surface is dry. A light spray of water has been used experi- mentally with a dust to increase its adherence. The condition of the plant itself may often influence the effectiveness of an insecticide. Some plants or fruits grow faster than others, and fast-grow- ing plants, such as potatoes, require more frequent applications than those that are growing slowly. Some grow more rapidly at one season than at an- other. The surface area of apples, for instance, may double within 2 weeks early in the season, although later on it may not double in less than 3 months. The surface, if it is to be kept covered Yearbook of Agriculture 1952 with an insecticide, must be sprayed or dusted more often in the early, part of the season than later. Any very smooth or waxy leaf or fruit surface is not easily coated with insecticides. Cabbage leaves are espe- cially difficult to cover thoroughly with liquids. Dusts often adhere better to such surfaces. Rough or hairy surfaces are more easily covered with either dusts or liquids. The shape and density of the foliage is a factor, too. Very heavy foliage interferes seriously at times with getting an insecticide to the pests. A person who is not familiar with insects and insecticides will do well to consult his county agricultural agent about the pest that is bothering him and the best insecticide for it. The home gardener also can get a great deal of practical information from the label of the insecticide he buys. Such directions usually suggest mixing certain amounts of the concen- trated insecticide material in a given amount of water. That may require mixing the insecticide on a weight or volume basis. Scales that weigh small amounts in ounces or a few pounds are useful. For measuring a given volume of dust or liquid, users should have on hand measuring equipment for tea- spoonfuls, tablespoonfuls, and cupfuls, pints, and quarts. For safety and con- venience, it is recommended that such measuring equipment be provided and used only for mixing insecticide ma- terials. Mixing large quantities of spray is usually just a matter of using one or more packages, the net weight being given on the label. For small amounts of spray there is no rule-of-thumb that may be followed because materials differ greatly in specific gravity. Some packages have a dilution table on the label. If there is none, perhaps the easiest way is to have the insecticide dealer determine the weight of a table- spoonful, cupful, or pint and record those details on the label for future reference. Tables of equivalents for Using Insecticides Effectively various quantities of spray will be found in the appendix to this volume. The old saying, "If a little is good, more is better" does not hold true in the use of insecticides. The use of ex- cessive amounts is wasteful, expensive, and often injurious to the plants, ani- mals, or soil. Sometimes one can combine various insecticides, insecticides and fungicides, or insecticides and fertilizers and save time and money in so doing. Again, though., it is well to read the labels of the preparations or consult official publications or competent authorities before making those mixtures at home. Many compounds are not compatible, and harmful combinations may be formed when they are mixed. The same equipment can be used for applying insecticides, fungicides, and herbicides, but one should be aware of the danger of using spray equipment for insecticides and fungicides that has been used to apply weed killers. Ordi- nary rinsing of the sprayer is not enough to remove the weed killer com- pletely. The sprayer, hose, and nozzle should be washed carefully with a sus- pension of activated charcoal, 1.3 ounces per gallon of water, or the sprayer should be filled with a solution of household ammonia, 2 tablespoon- fuls per quart of water, and allowed to soak for 24 hours. If that cannot be done, a separate sprayer should be used for herbicides. In summary, some points to be fol- lowed in using insecticides : If the insect is not known to you, find out what it is from the county agri- cultural agent, extension entomologist, or similar authority. Recommendations for the proper in- secticide to be used on a given pest may be obtained from those authorities, from State or Federal publications, from entomologists representing insec- ticide manufacturers, or from pest-con- trol operators. Prepare and use the insecticide in ac- cordance with recommendations. Read the label. Follow precautions and other directions carefully. 249 Do a thorough, careful, but not wasteful job of application. The proper insecticide, proper application, and proper timing are of equal import- ance. Store insecticides in a safe place. Clean up and care for dusting and spraying equipment as you would with any good piece of machinery. E. J. Newcomer is an entomologist in the Bureau of Entomology and Plant Quarantine. He has been associated with research on the control of the in- sect pests of deciduous fruit trees since his graduation from Stanford Univer- sity in igii, most of the time in the State of Washington. W. E. Westlake, a chemist in the Bureau of Entomology and Plant Quarantine, works chiefly on problems concerning insecticides for fruit insect control in the State of Washington. He has degrees from Montana State Col- lege and the University of Minnesota. B. J. Landis is an entomologist in the Bureau of Entomology and Plant Quarantine. Since IQ4I he has been in charge of the Union Gap, Wash., field laboratory of the division of truck crop and garden insect investigations, and has carried on investigations of potato bisects. Tarnished plant bug. From 0 to 5,000 in 34 Years Kenneth Messenger, W. L. Popham The airplane has become such a use- ful tool in the fight against insects that in 1952 more than 5,000 of them were equipped for that purpose in the United States. Attempts were made as early as 19 18 to control insects in this country by dumping poison dust from airplanes while flying over crops. But by 1921 a specially equipped airplane demon- strated its effectiveness in controlling an infestation of the catalpa sphinx near Dayton, Ohio. The lead arsenate dust that was released from a Curtiss biplane under the supervision of C. R. Neillie and J. S. Houser was unusually effective. The following year B. R. Coad, of the Bureau of Entomology and Plant Quarantine, borrowed two airplanes of the same type from the United States Air Service and applied dust to fields of cotton near Tallulah, La., for the control of the boll weevil, which then was destroying 250 million dollars worth of cotton annually. Dr. Coad reported that "the speed of operation was at least 1 00 times as fast as the best mule-drawn machine." Those demonstrations led to the commercial use of aircraft for insect control. The following year Huff- Daland Dusters, Inc., began commer- cial aircraft dusting in the Southern States. An industry was born. During the two decades that fol- lowed, many experiments were made with different types of aircraft, differ- ent installations, and different mate- rials, but few major improvements are recorded. Devices for wetting dusts as they were released, to make them ad- here better to foliage, were tried in the New England forests with only moder- ate success. Autogiros and a blimp were tested in the belief that their slower forward speed might improve forest coverage. But it was not until the early part of the Second World War that the airplane's real potential as a pest-con- trol vehicle became obvious. At that time the exposure of troops to insect- borne diseases challenged the initiative of entomologists, chemists, and mili- tary leaders. The answer to that chal- lenge - — broad sheets of DDT spray streaming from fast transport aircraft, blanketing otherwise inaccessible in- sect-breeding areas — saved countless lives and countless dollars. As early as 191 1 a German forest warden applied for a patent covering the use of aircraft in combatting forest pests. His effort apparently aroused no great interest in Germany until 1925 when several investigations, similar to those previously carried on in the United States, were undertaken. In that year a stand of mixed timber was treated to suppress an outbreak of the nun moth. Officials called the results "excellent, with no apparent harmful effects on birds or game." Airplane dusting to control locusts was tried about the same time by the Russians. In 1925, near Haguenau, France, a forest plantation was dusted by aircraft. In 1927, on Cape Breton Island, Nova Scotia, the airplane was used to dust spruce and balsam in an attempt to control a spruce budworm outbreak. Mosquito-breeding areas in South Africa, the United States, and other parts of the world were treated before 1930 with paris green. So encouraging were the results that it was forecast that the day would come when the airplane could be used to eradicate from entire continents the tsetse fly, carrier of sleep- ing sickness, and the malaria-carrying mosquitoes. At the Orlando, Fla., laboratory of the Bureau of Entomology and Plant Quarantine, E. F. Knipling and two of his associates, C. N. Husman and O. M. Longcoy, demonstrated that DDT in a concentrated solution ap- 250 From 0 to 5,000 in 34 Years plied from aircraft at the rate of i gallon or less per acre gave good con- trol of both mosquitoes and flies. Hus- man developed several devices for military trainer biplanes that made it possible to appraise accurately the effectiveness of this method of pest control. He continued this work during an assignment by the Navy in the South Pacific and developed improved equip- ment quite similar to that in use today. David G. Hall, also an employee of the Bureau, further developed and supervised methods of spraying mos- quito-infested areas while in the service of the Army Transport Command in the South Pacific. In this work, he equipped and directed C-47 transports with great effectiveness. Another type of equipment installed in aircraft for mosquito control was tested extensively by the Tennessee Valley Authority during 1945 and 1946. This consisted of an exhaust gen- erator which produced DDT aerosols. It gave a rather uniform coverage over wide swaths at exceedingly low rates of discharge. The installation was simple and inexpensive, but its use was limited to specific problems. Although it effec- tively penetrated heavy vegetation, the drop size of the spray was so small that it had very little residual value. Although the cost of aircraft appli- cations was fairly high in the postwar period, it has been progressively re- duced as a result of continued research in the development of concentrated materials and application equipment. The improvements and the growing realization of the versatility and effec- tiveness of the airplane resulted in a marked increase in the number used each year since the war. Despite shortcomings — the difficulty of controlling the distribution of an in- secticide from the air, the drift of fine sprays into nearby areas, the sometimes higher costs — nearly 500,000 hours are flown by pest-control aircraft annually. Illustrative of the importance of the airplane in suppressing large emer- gency outbreaks of insect pests are the following projects, conducted cooper- 25l atively by the Bureau of Entomology and Plant Quarantine, the Forest Service, States, and other organiza- tions. During the few years before 1952, hundreds of thousands of acres were sprayed by aircraft for the control of the gypsy moth in New England. In this work it is estimated that just one load of insecticide released by a C-47 airplane treats as large an area as could be covered by one truck-borne spray rig in 4 years — and more effec- tively. Also during recent years, several million acres in the Northwest have been sprayed with aircraft for the con- trol of the spruce budworm. In 1948 an outbreak of the tussock moth in the Northwest infested 450,000 acres. Within a period of a few weeks the entire area was sprayed so effectively that the treatment did not have to be repeated. Each year between 1949 and 1952, several hundred thousand acres were treated with baits and sprays to control grasshoppers on forage lands of Wyoming and Montana. Kenneth Messenger, a graduate of the University of California, has (Worked on agricultural pest-control programs since IQ33- He is in charge of the Aircraft and Special Equipment Center at Oklahoma City of the Bureau of Entomology and Plant Quarantine. W. L. Popham has taken part in large-scale plant disease and insect control programs since his graduation from Montana State College in 1924.. He has been assistant chief of the Bu- reau of Entomology and Plant Quar- antine since 1941. He received the de- gree of doctor of science from Mon- tana State College in 1948. More information about the use of aircraft for applying insecticides will be found in the following chapter and in many of the chapters in the second half of this book, which give details about types of aircraft and spray for- mulations used to control specific insects. Research on Aerial Spraying J.S.Yuill,D.A.Isler George D. Childress Some day it may be said that the air age in insect control arrived with the discovery of the unusual values of DDT during the Second World War. In the two preceding decades, the application of insecticides by airplane had been tried against various pests, but the method was not used exten- sively except over cotton fields. Its ad- vantages were recognized — large acre- ages could be covered quickly with no mechanical damage to the soil or to plants, and forest and swamps and other inaccessible places could be reached by air. There was one great limitation, though : The large quantity of insecticides that had to be applied for satisfactory control made the cost too high, even when the poisons were applied as undiluted dry powders. Liquid sprays, being less concentrated, required even greater quantities. DDT changed that situation. In their search for better insecticides for combatting malaria-carrying mosqui- toes in the Pacific and other war theaters, entomologists found that DDT in an oil-solution spray gave good results whert as little as one-fifth to one-fourth pound per acre was used. Engineers developed spraying appara- tus for several types of military planes, and before long entire islands were being sprayed as a routine protective measure against mosquitoes and flies. The end of the war brought a great demand for adapting aerial spraying to a variety of civil needs. Stimulating in- fluences were the publicity given war- time developments, the availability of war-surplus airplanes at low prices and former military pilots who wanted peacetime occupation in aviation, the discovery of other insecticides, and in- 252 creasing labor costs. Farmers, owners of timberlands, public health authori- ties, and we all became air-minded about insect control. Hopes were so high, in fact, that some people got the idea that airplanes and the new insec- ticides would quickly end all insect problems. But we soon learned that man's war with insects was not yet over. More was needed than a mere abundance of planes, pilots, and DDT. Much of the wartime development had been made in haste to meet specific military re- quirements, to get a job done, regard- less of cost ; in peacetime the idea is to do a job but to do it effectively and eco- nomically. After the war, therefore, it was necessary to do a great deal of re- search to reconvert wartime develop- ments to peacetime uses. Several Federal and State agencies and many commercial operators con- ducted the research or assisted by fur- nishing equipment for making experi- mental control tests. The investigations have centered on the development of more efficient distributing apparatus and more effective insecticide formula- tions and the improvement of aircraft for insect-control operations. Their methods have included gen- eralized observations or appraisals in the field, trial-and-error experiments of limited scope, and broader studies of the principles governing the dispersal and deposition of insecticides from air- craft. Their objective has been to de- velop wider uses for aerial application of insecticides and to apply them bet- ter, faster, and cheaper. Because sprays are less affected by wind and adhere better to foliage, the greater emphasis has been placed on spraying equip- ment. In the first year or two after the war a great variety of spray equipment was being used. Experimentation and experience, however, have gradually narrowed the field to three main types, boom and nozzles, rotary devices, and exhaust sprayers. The boom and nozzle sprayer was most commonly in use in 1952. Origi- nally developed for light planes, it has Research on Aerial Spraying been adapted to large transports. The sprayer usually consists of a'spray tank carried inside the plane from which the spray liquid flows to a wind-driven pump. The pump forces the liquids into a tubular boom mounted beneath the wing (beneath the lower wing of biplanes), from which it is discharged as a spray through atomizing nozzles. The spray is turned on or off by a quick-opening gate valve, and a con- stant pressure in the spray lines is maintained by the use of an adjustable pressure regulator installed in the line between the pump and gate valve. The chief advantages of this sprayer over others are the simplicity of its in- stallation and maintenance; the ease with which the degree of atomization of the spray or its rate of application can be changed (it is necessary only to changes the size or the number of the nozzles) ; the use of a pressure regula- tor, thereby insuring a constant pres- sure in the system and a uniform dis- charge rate; and the fact that any excess flow of liquid from the pump is returned to the tank through the by- pass from the pressure regulator, thus providing agitation or stirring action in the tank. Variations in design have been made for specialized jobs. For example, the boom has been placed inside the wing, and nozzles have been attached to it by short pipe connections that ex- tend vertically beneath the lower sur- face of the wing. In areas where uni- formity of spray coverage has not been a prime requirement, other modifica- tions have been to place the nozzles in clusters near the wing tips, on the rear edge of the wing, and on the tail as- sembly. Those installations improve the flight performance of the plane by re- ducing the air resistance, but they do not allow rapid adjustments of flow rate and atomization, which may be necessary for controlling different pests. Considerable work has been done on adapting standard pumps and develop- ing special pumps for the sprayers. 253 Both centrifugal and positive-displace- ment types have given satisfactory per- formance. The latter develop higher pressures but often are subject to ex- cessive wear when they are used with certain wettable powders that contain abrasive materials. Other developments include devices for driving pumps directly from the airplane engine, by hydraulic systems, or by electric motors; the substitution of aluminum for brass or iron to reduce weight; special pump bearings; and pump packing and rubber parts resist- ant to the solvent action of sprays. Some attempts have been made to eliminate the pump entirely and to depend on gravity flow of the spray liquid from the tank to the boom. It has been found, however, that gravity systems do not deliver at a uniform rate unless some means is provided to compensate for the decreasing hydro- static pressure as the tank empties. Many atomizing devices, such as nozzles, jets, slotted orifices, and small venturi tubes, have been tested with varying success. None has been de- veloped which will break up the liquid into drops of a uniform size. In general, though, nozzles that discharge the spray either in a hollow-cone pattern (similar to that of the common sprin- kling nozzle) or in a flat fanlike sheet have been the most satisfactory and are the ones most commonly used. Rotary sprayers were originally de- veloped for applying oils and concen- trated slurries of the older type of in- secticides, which were too thick to go through pumps. Later they were used for other materials. Some mechanical improvements have been made in post- war models for distributing the newer insecticides, but those sprayers have been less popular than the boom and nozzle type. The distinctive part of a rotary sprayer is the atomizing unit. It has a shaft with suitable housing and bear- ings; in front is a small, wind-driven propeller, and on the other end is a series of concave disks or circular wire brushes. The units may be placed on 970134°— 52- -18 254 the wings or on outriggers on the sides of the fuselage. Either way, the shaft is parallel to the fuselage. In flight the liquid flows by gravity from the tank to the center of the disks or brushes. It is then thrown outward by centrifu- gal force to the periphery of the rotat- ing units, where the passing air shears the liquid into drops. One can change the output by regulating the rate of flow of the material to the units. The speed of rotation, the number and spacing of disks or brushes, and, in the latter, the size of the individual bristles govern the atomization. Exhaust sprayers, first made for mos- quito control, were designed to pro- duce a cloud of spray like the mist sometimes applied inside buildings. The spray liquid is atomized by inject- ing it into the exhaust of the airplane engine. Usually the exhaust pipe is ex- tended somewhat beyond the engine, and the liquid is introduced into the throat of a venturi or into a special atomizing head on the end of the pipe. The apparatus must be carefully de- signed for each engine because any re- striction in the flow of the exhaust gases may create a dangerous back pressure against the engine. Since the war a few exhaust sprayers have been used in combatting some species of mosquitoes and other biting pests, but they have not come into general use for two rea- sons: The application rate is too low to kill many kinds of insects, and the spray is so fine that much of it may be carried away by wind or may evaporate before reaching the ground. Work with dusting equipment has been directed mainly toward getting a wider and more uniform distribution of the materials beneath the plane. The materials usually are discharged from a spreader, like a venturi, on the under side of the fuselage. Consequently a heavy deposit frequently forms along the flight lines and the lateral spread is limited. Efforts have been made to correct the condition, chiefly by chang- ing the design of the spreaders. Some redesigned spreaders have wide open- ings at the discharge ends or longitu- Yearbook of Agriculture 1952 dinal deflecting vanes so arranged that the dust is thrown outward on a diag- onal. Other spreaders are bifurcated, each branch being curved outward for the same effect. In the project for de- velopment of an agricultural airplane, described later, plans have been made to try building streamlined dusting units into the wings. One advance in a special field is the development of equipment to distrib- ute grasshopper bait from a multi-en- gine plane. Such poisoned baits have been applied by small aircraft, but the planes cover only limited areas. In order to combat extensive outbreaks, therefore, a bait spreader was designed for a C-47 transport plane. A large hopper holding 8,000 pounds of dry bait was built into the cargo space. A large air duct on each side of the fuse- lage extends from an opening near the leading edge of the wing, along the floor of the cargo space, and opens to the outside again near the rear of the fuselage. In operation, vaned rollers feed the bait from the hopper into the ducts. The flow of air carries it to the outside. Such a plane can treat 10,000 acres in a day, compared to 1,000 acres for the biplanes commonly used in crop work. The equipment has been modified to permit its use for applying sprays as well as baits by installing re- movable tanks inside the bait hopper. The new distributing apparatus has given reasonably good performance in the control of a number of insect pests, but there is need for improvement. Some progress can be made by refine- ments in existing equipment, but in the long run the maximum efficiency can be had only by developing equipment on the basis of the fundamental factors that govern distribution of insecticides from the air. Research projects have been started to study those factors, particularly the effect of the aerodynamic forces created by the plane, the size of the spray drops or dust particles, and weather conditions. We have evidence that, aside from the effect of wind, the aerodynamic Research on Aerial Spraying forces created by the airplane, particu- larly in the wake behind the plane, are largely responsible for the way the in- secticide is finally distributed on the ground. This conclusion is based on results from several investigations, which we summarize here : 1. When a plane with a full-span spray boom is flown 3 to 10 feet from the ground, the spray swath laid down has about the same width as the wing span. When the altitude approximately equals the wing span, however, the swath width is increased four or five times. A further increase in altitude does not further increase the swath. 2. At the low altitude, the spray is driven downward, as evidenced by its reaching the less exposed parts of the plants, but in the higher-altitude flights the spray has very little downward force when it reaches the ground and does not penetrate a dense ground cover. The same effect has been ob- served in the application of dusts. 3. Increasing the discharge rate of the spray does not increase the swath width but merely deposits a greater amount within the swath. 4. In either the low- or high-altitude flight, the swath width is greater when the outlets are spaced over half or more of the wing span. Releasing the spray from a short boom or single outlet directly beneath the fuselage gives a very narrow swath, but extending the boom beyond the wing tips has not given any greater swath than the full- span boom. 5. When repeated tests are made under carefully selected weather con- ditions, certain random variations al- ways occur in the amount of spray deposited that cannot be accounted for by weather conditions alone. Aeronautical engineers have long known that the flow of air created by an airplane in flight is turbulent and spreads backward, outward, and down- ward. The paths of this airflow are complex and have not been completely worked out, but it seems certain that their general direction governs the dif- ferences in swath width we described 255 and that the turbulence of the air flow causes the random irregularities in deposit. Some research has been done to find out how the size of spray drops or dust particles affects the deposition of the insecticides. The aim is to determine the most effective size. None of the practical atomizing devices known to- day, whether used on the ground or in the air, will produce uniform spray drops. The average size of the drops can be made large or small, of course, but there is always a range of sizes above and below the average; the larger the average size the greater is the range. Therefore the terms coarse, medium, or fine, when applied to sprays, are only relative expressions. Flight tests have shown that the de- gree of spray atomization markedly affects the width of the spray swath and the distribution of the deposit within the swath, especially when the plane is flown at an altitude equal to or greater than the wing span. A very fine spray of an average drop size of about 50 microns ( 1 micron equals about 0.00004 mcri) gives a wider swath and a more uniform deposit than a coarse spray in which the drops average about 200 microns. Because the larger, heav- ier drops in the latter are less affected by the outward forces in the wake of the plane, they fall more nearly ver- tically and hence tend to concentrate the spray in the center of the swath. On the other hand, very fine sprays have much greater loss of drops by wind and evaporation. There may also be a difference in the insecticidal efficiency in the de- posits of coarse and fine sprays. The loss of the fine sprays is much greater, but field observations indicate that they penetrate crop or forest foliage better than do coarse sprays. Therefore it would seem that they should be more effective in reaching insects at the base of crop plants or those living beneath a forest canopy. As a matter of fact, in laboratory tests on certain species of insects, deposits of fine sprays have given somewhat higher mortality than 256 equal deposits of coarse sprays. On the other hand, the coarse sprays have had a longer residual effect. Similar tests of residual effectiveness with aerial ap- plications in the field have not given conclusive results on this point. Some studies on the effect of particle size on distribution have been made with dusts, but the extent of the work has been limited because the particles, being irregular in shape and consider- erably smaller than spray drops, are much harder to collect and measure. As with spray drops, fine dust particles give a wider swath and cover foliage better than coarse ones. They are more subject to the effects of wind, however. In the case of diluted dusts, if the car- rier or diluent (usually a clay, lime, or talc) is made up of particles different in size from those in the active ingredi- ent, the two components may separate in the air and give a very irregular de- posit of poison on the plants. The least controllable factors that limit the effectiveness of aerial appli- cations are air movement (wind and convection), temperature, and humid- ity— factors that may change greatly within seconds and over only a few hundred feet. Wind is surely the most important. It may cause irregular coverage of the treated plants and may cause the spray or dust to drift beyond the treated area. The amount of loss by drift will not de- pend on the wind velocity alone, how- ever. Size of the drops and altitude of flight also affect the loss. For example, in a wind of 1 mile an hour, a 200- micron drop released 10 feet above the ground will drift about 6 feet. But if the drop is released in a 10-mile-an- hour wind from an altitude of 50 feet it will be carried about 300 feet. Under the same conditions, a 20-micron drop will travel some 3.5 miles. Aside from reducing the amount of insecticide reaching the insects, drift may cause most of the material to strike the plant horizontally. That may result in an un- even distribution on the foliage of plants or trees. The effect of the wind on spray dis- Yearbook of Agriculture 1952 tribution from planes is especially im- portant when the more potent new insecticides are used. It may cause the spray or dust to drift from the area being treated, thereby contaminating nearby crops sufficiently to be a hazard to people and livestock. The maximum permissible wind velocity when treat- ing crops or spraying for mosquito con- trol generally should not exceed 10 to 15 miles an hour, and when treating forest it should not be greater than 8 miles an hour. Local conditions, of course, may be such that even those velocities are too high. Convection is another factor. It is an upward flow of air that takes place when the ground temperature is higher than the air temperature. Convection currents usually become noticeable as the sun warms the ground during the morning. They may develop consider- able force by afternoon. They affect sprays and dusts much as wind does. They are more variable than wind but, unlike wind, they carry the drops of particles upward instead of horizon- tally. We have no simple way to deter- mine the amount of convection in an area during spraying or dusting opera- tions. A fairly workable rule of thumb is that the operations should be stopped when the pilot finds the air is becoming bumpy, or when the lighter parts of the spray or dust cloud show a tendency to rise. Temperature and humidity affect spray distribution indirectly. Increasing temperature promotes convection and increases evaporation rate. The latter may be particularly important when finely atomized, highly volatile mate- rials are being applied. Humidity is chiefly important for its effect on the evaporation of water spray. We know of at least one instance in which a finely atomized water spray, applied on a hot, dry morning, evaporated before it reached the ground. The best time to apply insecticides from the air is usually from daylight to 9 or 10 o'clock in the morning. Air movement and temperature then are at their lowest and humidity at the Research on Aerial Spraying highest. In some localities the short time just before sunset also is satisfac- tory. The fundamental factors interact closely. No one can be isolated and studied by itself. Furthermore, when the control of an insect is being studied the effects of all factors must be con- sidered in respect to its habits and en- vironment, and the insecticide to be used. Many of the new organic insecti- cides, primarily the chlorinated hydro- carbons and hydrocarbons containing phosphorus, are highly effective in small amounts. Their development has advanced all methods, air and ground, that employ low volumes of concen- trated insecticides. The discovery of each has made it necessary to work out spray or dust formulas for each pest by testing the new compound in com- bination with other compounds, with different solvents or dust carriers, and at different concentrations. Some research has been directed toward finding how the physical prop- erties of spray liquids affect dispersal and the efficiency in killing insects. An example is the finding that liquids of high viscosity are more coarsely atomized than less viscous ones and therefore give a narrower spray swath. The more volatile the spray, however, the more rapidly it evaporates; there- fore a smaller amount reaches the in- sects. Compared to the amount of research on distributing apparatus and insecti- cides for use in aircraft, the research on developing aircraft especially for spraying and dusting has been very limited. Most of the need for planes immediately after the war was met by adapting war-surplus biplane trainers because they were cheap and sturdy and could carry up to 1,200 pounds of insecticide. Many an operator, how- ever, has preferred small, two-place, high-wing monoplanes, particularly for treating small acreages. Their per- formance has been improved. Some spray and dust equipment has been designed so it can be removed easily 257 and the plane can be used for other purposes. One builder of aircraft dispensing equipment has designed a spray tank in the shape of a seat, the back and bot- tom having a capacity of about 30 gal- lons. Such tanks need not be removed. The helicopter was designed for general-purpose uses, but its ability to fly low and slow has been of particular advantage in spraying and dusting. It is well suited for treating small, in- accessible areas. It can be landed near or in the field being treated, and con- siderable ferry time is saved. The heli- copter is said to be more effective than fixed-wing aircraft in giving thorough coverage to plants, but its first cost and operating costs are quite high. In order to fill the need for a fixed- wing aircraft which would be more suitable for agricultural purposes than war-surplus trainers, the National Fly- ing Farmers Association, Texas Agri- cultural and Mechanical College, the Civil Aeronautics Administration, and the United States Department of Agri- culture sponsored the development of such a plane. The prototype model was constructed at the Personal Aircraft Research Center of the Texas Agricul- tural and Mechanical College. The de- sign was based on findings in a survey among commercial operators and re- search organizations to determine the essential characteristics that it should have. Safety was given special considera- tion. The cockpit is located so as to give the pilot excellent visibility. For protection in a forced landing, all loads and heavy masses are located in the wings or forward of the cockpit, and the pilot has a special seat, safety belt, and shoulder harness. The leading edge of the landing gear is sharp so it can cut wires it might accidentally touch in flight. Two structural members and a crash tripod over the cockpit give ad- ditional protection. The all-metal, low- wing monoplane can carry 1,200 pounds of insecticide. It operates at speeds up to ioo miles an hour. Spe- cially designed flaps and ailerons give excellent slow-flight characteristics and slow landing speeds, less than 40 miles an hour. Special types of distributing appa- ratus were developed. Space for the equipment is in the fuselage and the wing, which was made extra thick for the purpose. The sprayer has tanks and a boom in the wing and an engine- driven pump in the fuselage. The fuse- lage has a dust hopper with a conven- tional spreader underneath. On the drawing boards were plans for other types of distributing apparatus, par- ticularly duster units mounted in the wing to give a wider and more uniform swath and equipment for distributing seeds and fertilizers. The plane was flown successfully in 1950. J. S. Yuill has been an entomolo- gist in the division of forest insect in- vestigations, Bureau of Entomology and Plant Quarantine, since 1935- Since 1946 he has been engaged in the development of aerial spraying for forest-insect control. He attended the University of Arizona and the Uni- versity of California. D. A. Isler is a senior agricultural engineer in the division of farm ma- chinery, Bureau of Plant Industry, Soils, and Agricultural Engineering. A graduate of Ohio State University, he joined the Department of Agricul- ture in iQ2y and has worked on the development of equipment for control of various insect pests. Since 1945 he has worked at the Agricultural Re- search Center at Beltsville, Md., on the development of aerial spraying equip- ment for control of forest pests. George D. Childress is chief of the aviation extension division in the Office of Aviation Development, Civil Aero- nautics Administration. A native of Virginia, he engaged in commercial airport and flying school operations in and around Roanoke from ig2j to J939> when he joined CAA as an assistant aeronautical inspector. 258 Machines for Applying Insecticides Howard lngerson, Frank Irons Machines for applying insecticides are available in many makes, models, types, and sizes. They offer a wide range of selection for different con- ditions and uses. They save labor and provide more efficient ways to combat pests. Power equipment is of six types — high-pressure sprayers, low-pressure sprayers, air-type sprayers, mist spray- ers, dusters, and fog applicators. The power source usually has been gasoline engines, either by separate en- gine or through power take-off from a tractor. The trend during the past few years has been toward air-cooled en- gines for the engine-powered units be- cause they weigh less and are more compact than the water-cooled types. Water-cooled engines are used partic- ularly for the higher horsepower re- quirements, however, because suitable air-cooled engines have not been avail- able for the larger machines. The en- gines in use range from one-horse- power, air-cooled types to the large in- dustrial water-cooled engines of 75 horsepower or more. Tractors, besides hauling the equip- ment, furnish power to operate the ma- chine. A standard power take-off at- tachment extends from the rear of the tractor through a power shaft and is connected with the drive shaft of the sprayer or duster. The vehicles and mountings for carrying the application equipment are: Trailer type, tractor-mounted, motor-truck-mounted, self - propelled, and wheelbarrow and pushcart types. Trailer-type and tractor-drawn ma- chines commonly are used in orchards and on row crops, especially when heavy machines are required. Machines for Applying Insecticides Tractor-mounted dusters and low- gallonage sprayers are widely used for field and row-crop applications. This type of mounting is limited to the weight-carrying capacity of the trac- tor and tires. Motor trucks are some- times used for carrying orchard and row-crop equipment and they are regu- larly employed for carrying mist spray- ers for shade-tree spraying. Wheelbar- row- and pushcart-type power sprayers and dusters have come into common use around greenhouses, farm build- ings, small truck farms, and estates. Special self-propelled, high-clear- ance sprayers and dusters have been developed for treating corn for Euro- pean corn borer and corn earworm. Some of the machines are adaptations of the detasseling vehicles used in the production of hybrid seed corn and have a clearance of 4 feet to 7 feet. The high-pressure sprayers, com- monly spoken of as hydraulic, are de- signed for working at pressures of 100 to 600 pounds per square inch and are rated in terms of the number of gallons per minute that the pump discharges at a given pressure. The hydraulic pumps have one to four cylinders and are vertical or horizontal. Some are of open-type design. Others are com- pletely enclosed with oil-bath lubrica- tion comparable to tractor and auto- motive engine design. The tanks are of wood or steel and usually are from 10 to 20 times the capacity of the pump — for example, a pump of 7 gallons per minute capacity might be used with a tank of 150 gallons capacity; a 20- gallon pump is used with a 300- or 400- gallon tank. Uniform and complete agitation of unstable mixtures is essential for satis- factory results. Most sprayers have a mechanical agitator, a power-driven shaft, which extends through the tank and has several paddles on it. The pumps and other parts that come in contact with the chemicals must resist their corrosive and abra- sive properties. Hence, they usually are made of brass, bronze, rubber, stain- less steel, and porcelain. 259 Large, stationary, and high-pressure spraying systems have been installed in some orchards that are too hilly for portable machines. Pipe lines, under- ground or elevated, carry the spray un- der controlled pressure to all parts of the orchard. Outlets are provided at intervals along the pipe system for con- necting hand-operated spray guns with long-lead hose. Improvements are being made all the time in the distributing attach- ments. To replace the one or two nozzles on the end of a 10- to 14-foot spray rod, the adjustable spray gun was in- vented and is in general commercial use. The multinozzle spray gun, com- monly called a broom or spray head, later came into general use. Now we have attachments to make the spraying of large orchards automatic. They are best suited to sprayers with a pump capacity of at least 20 gallons a minute. High-pressure sprayers equipped with special booms have been perfected for row crops, including potatoes and tomatoes. The booms are arranged to cover 2 to 30 rows or swaths of 6 to 40 feet. The number of nozzles per row and their arrangement on the boom depend on the crop, plant growth, and coverage needed. Livestock spraying requires high pressure to drive the spray material through the hair or wool and to cover the animals evenly and completely. Adjustable-type spray guns are used. Clusters of nozzles that direct the spray from underneath the animals complete the operation. High-pressure sprayers are used also for spraying shade trees and sanita- tion spraying to control flies and mos- quitoes. The pumps used in low-pressure spraying are mostly of the gear type. They generally have either bronze or brass parts, resist corrosion, and are suitable for spraying solutions and emulsions. They are not suited for use with suspensions that carry abrasive 260 chemicals. Such sprayers have been in- troduced into the cotton areas. They are mounted directly on a tractor and operated by power take-off. Air sprayers use air as the carrier for the spray chemicals. The air blast replaces the water-carrying power of high-pressure and high-gallonage ma- chines. The fans or blowers in air spray- ers are of three types — axial, radial, and centrifugal. The requirements of air spraying are: Proper balance between the vol- ume and velocity of air; nozzles adapted to the particular air velocity and volume; the proper placement of the nozzles in relation to the air stream; and the arrangement of the air-dis- charge outlet so as to direct the air that carries the spray chemicals so that it will cover the plants. Air sprayers are rated in terms of air capacity in cubic feet per minute and velocity in miles per hour. They range from 250 cubic feet a minute at 150 miles an hour to 45,000 cubic feet a minute at 100 miles an hour. Some air sprayers are designed for use with di- lute spray materials and for applying semiconcentrated and concentrated materials. Other types are designed solely for applying concentrated mate- rials and often are designated as mist sprayers. Mist sprayers are used more and more to control flies and mosqui- toes indoors and outdoors and have made sanitation spraying practical and Yearbook of Agriculture 1952 economical. Mist sprayers have been adapted for use on shade trees to save labor and materials. The air-blast machines blow finely divided spray into the trees, and their air-moving capacity should be sufficient to agitate all the air within the tree and displace much of it. Ralph V. Newcomb and Arthur D. Borden, working in California, deter- mined the discharge volume of spray in cubic feet per minute needed to spray trees of various sizes when the machine is traveling at various speci- fied rates of speed and is spraying trees on both sides of the line of travel. The volumes are given in the table below. These investigators tell us that a relatively low air velocity works better than a high velocity and causes less in- jury to the fruit and foliage. With the rates of travel and the discharge vol- umes shown in the table, an air velocity of 90 to 1 10 miles per hour at the nozzles is enough. Nozzles should be used which will break the spray up into droplets 30 to 50 microns in size. Such droplets will produce uniform coating of spray on fruit and foliage with very little runoff. James Marshall, in British Colum- bia, and others have developed smaller machines, spraying to one side only, which can be operated partially under- neath the overhanging branches of the trees. The discharge volume of these machines needs to be only half that of the two-way machines and are there- Discharge Volumes of Sprays Travel Miles per hour Feet per minute H 44 1 88 1% 132 2 176 2% 220 3 264 3% 308 4 352 Discharge volume of spray needed to spray trees measuring- IO'xW'xIO' 20'x20'x20' 30'x30'x25' (1,000 (8,000 (22,500 cubic feet) cubic feet) cubic feet) Cubic feet per minute 4,400 17,600 32,850 8, 800 35, 200 65, 700 13,200 52,800 98,550 17, 600 70, 400 22, 000 26, 400 30, 800 35. 200 Machines for Applying Insecticides 261 Power Equipment for Applying Insecticides Uses Shade trees. Livestock . General farm live- stock and weed control. Power sprayer recommendations Type Orchards (apples, pears, peaches, cherries, citrus and other tree fruits, and nuts). Row crops (potatoes, melons, tomatoes, celery, beans, onions). Units protected Acres of mature trees i-3 4-10 10-20 20-40 40-75 75-200 Acres r-10 10-20 20-50 50-100 100-200 Number of trees fi, 000-3, °°° 1 3, 000-5, °°° 1 1, 000-3, 00° [3, 000-5, °°° Number of mature cattle 10-30 30-100 I 1 00-500 [ 500-5, 000 110-50 50-150 150-500 Type Pump capacity GPM 3-4 5-7 10-20 20-35 High pressure . . . do do High pressure or small air type. High pressure or 35_6o medium air type. Large air type .... 50-65 High pressure 6-7 do 7-20 do 20-35 do 35-°° do 60 High pressure . do Mist sprayer . . do High pressure . do do 35 60 3-5 5-6 3-4 3-4 6-7 .do 15-20 3-5 4-7 7-20 Low pressure . High pressure . . .. .do Tank capacity Gallons 50-100 50-150 200-400 300-500 400-600 400-600 100-150 150-200 200-400 400-600 400-600 400-500 500-600 50-100 100-150 10-15 15-50 100-150 200-300 5° 50-150 150-300 Price Dollars 400-475 500-750 800-1, 400 1 , 500-2, 500 2, 50°-3> 5°o 4, 000-6, 000 650-800 750-1,650 1, 700-2, 000 2, 000-2, 5OO 2, 200-2, 80O 2, 000-2, 500 3, 000-3, 5°o 1, 5OO-2, OOO 2, 000-2, 5OO 2OO-25O 2OO-45O 60O-75O 200-I, 80O I 75-250 450-750 60O-I, 65O fore smaller and less expensive. Fruit growers in the Pacific Northwest have had satisfactory results with machines having a somewhat lower discharge volume than indicated in the table. Many insecticides may be used in the air-blast machines at 2 to 5 times the concentration normally employed with spray guns. Much less water is needed. For example, in mature apple orchards in the Pacific Northwest it is sometimes necessary to use 20 pounds of 50 per- cent DDT per acre per application to control the codling moth. Applied with spray guns and using the DDT at 1 pound per 100 gallons, 2,000 gallons would be needed. With an air-blast ma- chine, the same acre could be sprayed by applying the 20 pounds of DDT in 500 gallons of water. Some saving of insecticide can even be made with the latter method because when such a machine is properly used much less of the spray drips to the ground than when spray guns are used. The cost is also much less. One orchardist in Washington reduced his per acre cost from $164 in 1947, when he applied lead arsenate and oil with guns, to $42 in 1949, when he applied DDT and parathion with an air-blast machine. He had only 5 percent of cull apples in 1949 as compared with 10 percent in 1947. Dusters use air to distribute dry materials. They are used in nearly all fields of insect control as the sole means of treatment or to supplement sprayers. They are of two general types. Single-outlet dusters are used primarily for dusting in orchards. Multiple-out- let dusters with flexible conductor tubes and spreader nozzles at the ends of the tubes are designed for direct applica- tion of dust to field and row crops. Dusters can be used only when there is little air movement. The dusts adhere less well than liquids. Dusters have the advantages of being lighter in weight and they can carry enough insecticide for long periods of operation. Dusting is popular in places where water sup- plies are limited. Fog applicators were developed dur- ing the Second World War primar- ily for spraying enclosed space. Some have been used to control flies and mos- quitoes in buildings and outdoor areas. Few are used by farmers; they are gen- erally considered unsatisfactory for providing adequate residual deposits and coverage because they rely on air drift to carry the insecticide. Sprayers and dusters have to be cleaned and oiled systematically. The corrosive and abrasive properties of spray and dust materials make it es- sential that tanks and dust hoppers be emptied and cleaned at the end of each day of use. Pumps and all accessories should be washed out to reduce corro- sion and nozzle-clogging troubles. Pumps and engines must be drained during freezing weather. Lubrication of all wearing parts according to the manufacturer's instructions will allow the equipment to operate smoothly and increase its useful life. Replacement or repair of wearing parts as needed is good economy. Howard Ingerson joined the Bu- reau of Entomology and Plant Quar- antine upon graduation from Pennsyl- vania State College. Later he managed commercial orchards in Ohio. Since 1935 he has been agricultural sales manager and research representative of a firm in Lansing, Mich. Frank Irons is an agricultural en- gineer in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He is leader of a research project that studies machinery used to control pests and plant diseases. He is stationed in Toledo, Ohio. Choosing and Using Hand Equipment T. E. Bronson, Earl D. Anderson Hand-operated sprayers and dusters are suitable for applying insecticides around the home and garden ; in stores, restaurants, hospitals, and other com- mercial and public buildings; and on farms for protecting livestock, poultry, and buildings. Small fields of 5 or 10 acres or less, depending on the type of crop, may be treated with hand or traction equipment, although power machines are generally desirable in larger fields, especially if time is a factor and if labor costs are high. Hand equipment is used also to supplement power equipment in large fields — for spot treatment of localized infestations, for instance. It was to combat the ravages of one species of insect, known only as a mu- seum specimen from 18 19 to about 1850, that the modern sprayer had its beginning. As the pioneers of this country moved westward, attracted by free land and the discoveries of gold, some of the pioneers stopped in the foothills of the Rockies and planted crops, including potatoes. Between 1850 and i860 many of the early settlers were threatened by starvation as hordes of the insects, with a pleasing new source of food, rapidly increased in population as they de- voured one field of potatoes after an- other. By the time the migration of the insects reached the older settled At- lantic coast area, experimentation had proved that the insect was vulnerable to the poison later known as paris green. Impatient with such makeshift methods of application as whisk brooms and hand dusting for protect- ing their own potato crops, early in- ventors, such as John Bean of Cali- fornia, D. B. Smith of New York, and 262 Choosing and Using Hand Equipment 263 Brandt Brothers of Minnesota, devel- oped and improved the first hand sprayers. Thus it was that a small in- sect, the Colorado potato beetle, was largely responsible for the early de- velopment of suitable equipment for applying insecticides. Hand-operated equipment includes household sprayers, electric sprayers, general utility sprayers (compressed- air sprayers, knapsack sprayers, wheel- barrow sprayers, and hand spray pumps and accessories), and dusters (plunger dusters, crank dusters, knap- sack dusters, and wheelbarrow dust- ers) . Of each there may be many sizes, models, and types. Some are best suited for one particular type of job. Others have features of design that adapt them for several different uses. In any event, the insecticide has to be applied prop- erly if it is to be most effective. The main function of a sprayer is to break the liquid into droplets of effec- tive size and distribute them uniformly over the surface or space to be pro- tected. Another function is to regulate the amount of insecticide to avoid ex- cessive application that might prove harmful or wasteful. Dusters have similar functions. Dust- ing is not a suitable method of knock- ing down insects in flight, but it is used to control crawling insects in the home, garden, and field. Properly applied dusts and sprays usually are equally effective. Dusts cost more but they need no mixing by the user. For home use, a small plunger-type duster ready to take to the garden on the daily inspection trip is desirable. If more than one in- secticide is often used, it is well to have two of these small dusters. The sprayer or duster best suited for a specific job can be determined more readily when the basic requirements for chemical control of insects are con- sidered. The bothersome insects that we wish to control in and around build- ings are either crawling insects or flying insects. Crawling insects are generally con- trolled by applying a residual coating of an insecticide to the surface upon which the insects may crawl or rest, such as the floor, wall, or ceiling of a structure, the bodies of animals, or the foliage of a plant. The insects are killed by coming in contact with the chemical deposit or by ingesting it. Chemical dusts and sprays are used for the purpose, although, of course, only sprays are used for treating walls and ceilings of structures. In choosing a duster, size to fit the job is the pri- mary consideration, but for some ap- plications there is a choice between units that provide intermittent or con- tinuous discharge of the dust. A sprayer that delivers droplets large enough to wet the surface read- ily should be used for proper applica- tion of surface or residual sprays. Ex- tremely fine droplets tend to be di- verted by air currents and be wasted. To control flying insects, one can use residual sprays on surfaces where the insects may rest or discharge a knock- down type of insecticide into the air in which the insects are flying, killing them upon contact. A sprayer is needed that will produce a fine mist or a fog, which will stay suspended in the air for a time. One chart shows some of the equip- ment for the control of insects pests of lawn, garden, or field, which feed on vegetation or live in it. Such insects also may be classified as sucking insects or chewing insects. Sucking insects gen- erally are controlled by applying a con- tact insecticide, which kills the insect by absorption through the respiratory system or through the body wall. In the liquid form, the contact insecticide should be applied by a sprayer that will produce a fine-droplet mist or fog. The size of duster to be used to apply the dry form of the insecticide should be chosen to fit the size of the job. The chewing insects may be controlled by the use of either a contact insecticide, such as used for controlling sucking in- sects, or a residual or surface type of insecticide, of the kind used to control crawling insects in and around build- ings. Household sprayers of the hand- 264 Yearbook of Agriculture 1952 Choosing and Using Hand Equipment 265 operated plunger type are used princi- pally for applying sediment-free liquid insecticides in the home to control flies, moths, mosquitoes, and other pests. They are also used for applying insec- ticide or disinfectant sprays in stores, restaurants, and dairy barns. They are the simplest and least expensive sprayers. The typical sprayer consists of a tank holding several ounces to about 3 quarts of liquid. Air pressure from the built-in plunger pump breaks up the liquid into droplets. The sprayers usually are made of tin and may cost as little as 25 cents. Plunger sprayers are of two types — the intermittent, or single, action, and the continuous action. The intermittent sprayer discharges the spray material only with each for- ward stroke of the pump. It delivers a finely atomized spray and is designed for applying space or knock-down in- secticides to kill flying insects in closed rooms. The continuous-type sprayers pro- duce a constant discharge while the pump is being operated. Some have twin nozzles, one of which is used to produce the fine-droplet space sprays and the other to produce the coarser- droplet surface or residual sprays. An- other type has a single nozzle, which may be quickly adjusted from one type of spray to the other. Compression-sprayer performance is obtained with some of the larger sprayers — usually the 3-quart size — which are equipped with a lever- operated cut-off valve. This feature permits pumping up a head of air pressure while the tank is placed on the floor or ground. The spray is released by depressing the valve lever. These sprayers are used extensively for spray- ing small dairy herds and for the larger spraying jobs around the home, such as control of clothes moths and flies. Electrically operated household sprayers are used extensively in restau- rants, factories, and public buildings by professional pest-control operators. They are used also in the home and in farm buildings. The most common type has a metal or glass supply tank hold- ing about a pint or a quart of insecti- cide. The force for expelling the liquid is supplied by an electrically driven rotary-type air compressor or by a piston-type liquid pump operated by an electric vibrator. The sprayers have adjustable nozzles for the application of either space- or residual-type sprays. With proper attachments they can be used for spraying paint and applying insecticidal dusts. The cost varies from about 5 to 10 dollars for a home type unit and up to 50 dollars or more for the industrial type, which is equipped with a time switch. General utility sprayers fill a variety of needs for which the house- hold sprayer, because of its small size and capacity, is inadequate. They are designed particularly for applying sprays in the yard, garden, and dairy barns. They may be used for spraying small trees and for applying residual insecticides around the home, in farm buildings, on livestock, or on public health projects involving insect control. Their nozzle design permits the use of a wide range of insecticide formula- tions, including emulsions and wetta- ble powders. The compressed-air sprayer is a com- mon tool in gardens and farmyards. The newer insecticides, which are ap- plied in highly concentrated form, ex- tend the area of usefulness of this unit to the treatment of small acreages of field crops, such as cotton. It is also the mainstay of most public health projects involving insect control the world over because of its simple design and operation and relatively low cost. Compressed-air sprayers consist es- sentially of a 2- to 5-gallon tank, air- tight filler cap, air pump for com- pressing the air in the tank above the liquid, and outlet connection. The dis- charge hose is fitted with control valve, strainer, and nozzle; usually it has an extension tube to permit easier cover- age. Several different nozzle disks, hav- 266 Yearbook of Agriculture 1952 H V Z CU o Cu T3 D, OJ H u < o c* Dh s c/3 £ "> r^l r U\ 1 ^ I <3 ( J * »^i o ■^ Choosing and Using Hand Equipment 267 ing different size and shape of aperture, are supplied to provide a selection of spray patterns. These usually include solid cone and hollow cone (both coarse and fine) and flat fan and solid stream. Most of these sprayers are made of galvanized steel, although some are made of copper, brass, or stainless steel for industrial uses or for use by professional pest-control opera- tors. They cost about 5 to 25 dollars. These sprayers should be rilled not more than three-fourths full of liquid, so that space is left for building up a head of air pressure. Occasional pump- ings maintain the normal operating pressure of 30 to 50 pounds. The sprayer should be shaken occasionally to prevent the material from settling. When equipped with the usual cone or flat-fan nozzle disk, the nozzle is nor- mally held about 6 to 18 inches from the surface to be sprayed. Some models of compressed-air sprayers are equipped with refillable C02 cylinders, which eliminate hand pumping of the sprayer. The cylinder or cartridge holds enough gas to expell three or four tankfuls of spray material at a constant pressure. Knapsack sprayers differ from the compressed-air sprayers primarily with respect to the method of carrying, type of pump used, and the provision for agitation. These sprayers are carried on the back like a knapsack and are held in place by two shoulder straps. They are equipped with liquid-type pumps, which can develop maximum pressures of 80 to 180 pounds per square inch, a feature that broadens their field of use and permits the spray- ing of higher trees, for example. An internal pump of the piston or dia- phragm type is commonly used and with it an air chamber to develop a steady spraying pressure. Spray agita- tion is also provided either by means of a bypass stream from the pump or by a paddle connected to the pump handle. On some makes the pump handle may be attached to either side to permit right- or left-hand operation. Some models are equipped with a double-acting external slide-type pump. The pump of the knapsack sprayer, regardless of type, is operated continuously while spraying. The tanks of these sprayers are usually made of galvanized steel and have a capacity of 4 to 6 gallons. Special models are available with tanks of copper or stain- less steel. Several different nozzle disks are usually furnished with the conven- tional nozzle, or the sprayer may be equipped with an adjustable nozzle to provide a wide range of spray patterns such as a fine mist for tender flowers or a coarser spray to wet the surface of a building for residual fly control. Because these sprayers develop higher pressures and have greater capacity than most compressed-air sprayers, they are better adapted for use on larger spraying jobs such as for large gardens or truck farms. The cost varies from about 15 to 50 dollars for the different models. Wheelbarrow sprayers are the largest of the hand-operated spray units. They are particularly useful where the amount of spraying is too great to be handled adequately by a knapsack or compressed-air sprayer. Large gardens, greenhouses, small acreages of truck crops, and farm buildings where the cost of power equipment may not be warranted are some of the places where wheelbarrow sprayers are particularly suitable. This equipment is also satis- factory for spraying small numbers of fruit trees, because of its adequate capacity and pressure. The tanks have capacities of 12 to 18 gallons. A barrel- type piston pump mounted in the tank develops pressures up to 250 pounds. The tank unit is mounted on a wheel- barrow frame fitted with one or two wheels with either steel or rubber tires for convenience in moving from place to place. This sprayer is equipped with a mechanical agitator and the usual discharge hose, extension tube, shut-off valve, and nozzle with an assortment of disks. Special equipment available with some models includes an air-pressure 268 tank and pressure gage supplied with the necessary fittings and extra length of hose to permit one-man operation. After pumping up pressure in the air tank the operator can spray some dis- tance from the machine returning oc- casionally to rebuild the pressure and move the machine as the work pro- gresses. Wheelbarrow sprayers sell for 30 to 80 dollars or so. Hand spray pumps are inexpensive and efficient for occasional sprayings where more expensive equipment may not be justified. Sufficient pressure may be developed with these pumps to spray average-size fruit trees. No spray container is furnished with them. They are of three types — bucket, barrel, and slide. Bucket pumps are of the positive- acting plunger type equipped with an air chamber for constant pressure and with a discharge hose, nozzle, and strainer. A foot rest or bracket is pro- vided for supporting the pump. As they develop pressures up to 250 pounds, they can be used for a number of spray jobs. Because of their simplicity in de- sign and operation, the entire malaria- control program in some countries is planned largely around this piece of equipment for insecticidal application. Barrel pumps are similar to bucket pumps but are larger and of heavier construction. They have clamps for at- tachment to a barrel and a mechanical agitator. A slide pump consists of a telescop- ing-type pump which serves as part of the discharge system, a fixed or adjust- able nozzle, a supply hose, and an in- take strainer. These pumps develop pressures up to 180 pounds. The range in price of these sprayers is about as follows : Bucket pumps, 5 to 10 dollars; barrel pumps, 10 to 25 dollars; and slide pumps, 5 to 15 dollars. Accessories and fittings of many kinds for special spraying needs are provided by manufacturers — special nozzles, discharge extension tubes, and multiple-nozzle booms, to mention a few. There is a wide variety of nozzles Yearbook of Agriculture 1952 adapted to special uses, i, e., angle nozzles for under-leaf spraying, swivel- head nozzles, fan-type nozzles, and ad- justable nozzles that provide a spray ranging from a fine mist to a solid stream. Special disk inserts are avail- able for use in standard nozzles to give different droplet sizes or patterns such as flat fan, solid cone, or hollow cone. The multiple-nozzle boom is an acces- sory which greatly speeds up the usual spraying operation. It is especially use- ful for fly and mosquito control in buildings and around the home grounds and as special equipment on knapsack sprayers to be used in areas inaccessible to power sprayers. Another useful accessory is the sprayer cart which provides mobility to compressed- air sprayers and is handy for transport- ing the larger size sprayers, especially if the source of water is some distance from the area to be sprayed. Plunger dusters are the most practical for use around the average home. The smaller household dusters are especially convenient for placing insecticidal dusts in kitchens, pantries, and basements for control of ants and other crawling insects. The larger gar- den-size duster is ideal for applying dusts for the control of insects and plant diseases in the home garden. The principal parts of the duster consist of the chamber for holding the dust, the air pump for supplying the air blast, and the delivery tube and nozzle. The dust chamber may be of glass or metal construction. The nozzles on some dusters may be adjusted at vari- ous angles to obtain better under-leaf coverage. These dusters sell for 50 cents to 2 or 3 dollars. Crank and knapsack dusters are de- signed for use on estates, in large gar- dens or small acreages of truck crops, and on field crops such as cotton or tobacco. They are also useful for con- trolling spot infestations in larger fields before making a general treatment with large equipment. The crank and knapsack dusters are provided with dust hoppers holding Choosing and Using Hand Equipment 269 from 5 to 25 pounds of dust, an agi- tator to prevent the dust from packing, and a mechanism to feed the dust uni- formly into the outlet. An extension tube may be fitted to this outlet for dusting small fruit trees. The crank duster is suspended in front of the operator by means of shoulder straps. An air blast is gen- erated by a manually operated rotary fan or blower, which provides a con- tinuous discharge of dust. To treat row crops, the dust may be put out through one or more tubes arranged to cover either one or two rows in front of or behind the operator. The knapsack duster develops an air blast by means of a bellows rather than by a fan or blower. The dust is therefore discharged intermittently and for this reason the knapsack duster is particularly adapted for treating crops planted in hills or for spot dust- ing of crops in which the plants are widely spaced in the row. By continu- ous operation, however, this duster may be used for applying dust to other crops. Wheelbarrow or traction dusters are intermediate between hand and power equipment. They are used for continu- ous dusting of small acreages of row crops. Because of their light weight and mobility, they may often supple- ment power dusters when fields are soft or rough. These dusters are like the crank dust- ers but are of heavier construction and are mounted on wheelbarrow frames. Power is derived from the traction wheel of the unit ; the fan is driven by a chain or belt. Wheelbarrow dusters are usually designed for two-row cover- age and use two discharge nozzles per row. The nozzles are adjustable for crops of different height or for apply- ing dust to the under sides of the leaves. The hoppers hold 15 to 50 pounds of dust and the discharge rate may be varied from about 5 to 45 pounds per acre. A clutch is provided for intermittent use in the field. Some larger models have a front hitch for use in attaching to horse, mule, or gar- 970134°— 52 19 den tractor when larger fields are to be dusted. Crank and knapsack dusters range in price from 20 to 50 dollars and wheelbarrow dusters from about 75 to 1 25 dollars. Household sprayers require little maintenance because of their simple design. They should be stored in a dry place. If the pump should lose its com- pression, a little lubricant should be ap- plied in the air hole at the end of the cylinder to soften the pump leather and keep the seal with the cylinder wall. Before the beginning of the spraying season, the general utility sprayer should be disassembled and inspected to make sure that it is in good shape. Worn parts, which may interfere with the operation of the unit, should be re- placed. Repair or replacement parts can be bought from the local retail dealer or from the manufacturer. The sprayers should be cleaned after each use. After safe disposal of any insecti- cides remaining in the sprayer, the tank should be rinsed with clean water. Then the nozzle should be disassem- bled and cleaned and some clean water should be forced from the tank through the discharge line to remove any for- eign deposits. The tank should then be dried or inverted for drainage and the whole unit stored in a dry place. Before dusters are placed in seasonal storage, the dust reservoir should be emptied and thoroughly cleaned. Ex- tension tubes should also be disassem- bled and all the joints cleaned of any dust deposits. Working parts on the larger dusters should be cared for in the manner prescribed by the manu- facturer. T. E. Bronson, an entomologist, was associated with the Bureau of En- tomology and Plant Quarantine from 1924 until 1951, when he resigned to join a chemical company. He is a grad- uate of the University of Wisconsin. Earl D. Anderson is secretary of the National Sprayer and Duster Asso- ciation. He has degrees in agricultural engineering from Iowa State College. 270 Yearbook of Agriculture 1952 Strawberry leaf beetle. Bean leaf beetle. Tortoise beetle. Harpalus pennsylvanicus, a ground beetle. qrfjni -«■ "-■"'-it} Pales weevil. Striped blister beetle. Warnings as to Insecticides The Safe Use of Insecticides F. C. Bishopp, John L. Horsfall Insecticides kill insects because they affect a life process like respiration, di- gestion, circulation, and nerve reac- tions. A person also might experience some effect on his life processes if enough of the chemicals should get into the body by mouth, with or with- out food ; through the nose, by breath- ing vapors or particles of dusts or liq- uids; or through the skin, by absorp- tion. Any person who plans to use an in- secticide should inform himself there- fore of its characteristics. What man, in his right mind, would attempt to fly an airplane without first learning how to do so? Many sources give the characteris- tics of the various insecticides and di- rections for using them safely. The De- partment of Agriculture, county agri- cultural agents, most of the State agricultural colleges, agricultural ex- periment stations, and extension serv- ices can furnish, on request, printed matter that describes the characteris- tics of insecticides and the precautions to be observed in using them. Another important source of infor- mation is the label on the container. Every user, before he opens the pack- age, should read all the statements, di- rections, and warnings on it because it relates specifically to the material in the package. The instructions are there for good reason — the user's safety. The Insecticide, Fungicide, and Ro- denticide Act requires that insecticides entering interstate commerce be reg- istered and that labels on them carry information to safeguard the user and the public. The Food and Drug Admin- istration, a unit of the Federal Security Agency, establishes tolerances; that is, levels of insecticidal residues that are safe on foods. Many States have laws requiring appropriate labels and con- trolling the application of pesticides by custom applicators. Insecticides differ in degree of tox- icity— the amount that would harm livestock or man: Man could tolerate pyrethrum and sulfur in rather large quantities, but small amounts of cal- cium arsenate or sodium fluoride would be dangerous. Insecticides differ also in the way they act. Some (like nicotine sulfate) may be very poisonous, but show little cumulative effect. Others (like lead arsenate) are less acutely poisonous but build up in the system and produce ill effects if they are taken repeatedly into the body. Some insecticides (like hexaethyl tetraphosphate) may be poisonous to insects and higher animals when first applied but lose their strength quickly. Others (like DDT) are less poisonous but persist for considerable periods. Thus very poisonous materials must be handled and applied carefully. In the use of the less toxic but more per- sistent materials the hazards from residues must receive the major at- tention. The danger always exists that a per- 271 272 son may exercise great care when first using an insecticide and, experiencing no ill effects, may become more and more careless. The home gardener who needs to control pests in his back yard would select insecticides from the group which need only a minimum of care in their use — pyrethrum and rotenone, for ex- ample. The main precautions for him ?re to avoid getting the insecticides into the eyes or mouth or on the skin. Like all other insecticides, they should be stored in a place where children cannot touch them and where they cannot contaminate food. A nonhazardous compound usu- ally does not carry a poison label. If it includes a compound in solution with a propellant and is to be applied as a fine mist or if it is dissolved in deodor- ized kerosene and is to be applied as a spray to control household pests, a pre- caution on the label would state that contamination of foods should be avoided. If the spray contains a kero- sene solvent, it should not be applied near an open flame. It is harmful if swallowed. Insecticides with limited hazards are used commercially and many are recommended for the home gardener. A person who uses them should avoid breathing dust or spray mist, avoid contamination of feed and foodstuffs, keep the insecticides away from chil- dren and domestic animals, and wash himself thoroughly after using the ma- terials. The home gardener who uses a hand duster or sprayer, by observing the wind direction, can avoid breathing the dust or mist. If he happens to be particularly susceptible to inhalation of dusts of any kind he can obtain pro- tection by using an inexpensive respira- tor of the type having a cloth filter pad. A handkerchief tied over the nose and mouth will give some protection. Commercial operators who are ex- posed to inhalation of spray mists or dusts day after day should use a simple pad respirator. The warning not to Yearbook of Agriculture 1952 contaminate feed and foodstuffs should make it obvious that this group of in- secticides may leave residues on fruit and vegetables if they are used too near harvest. Other insecticide formulations in this group may have vapors that should not be inhaled. They may have a slight hazard because of possible skin absorp- tion or irritation. They may be harm- ful if swallowed. If the label indicates a hazard from breathing the vapors, a respirator hav- ing a cartridge filter through which the air passes will furnish protection. If danger of skin absorption is in- dicated on the label, contact with it should be avoided. Rubber gloves will be useful when handling it. Some sol- vents affect synthetic rubber quickly, and it is best to use gloves of natural rubber. If the insecticide might harm the eyes, goggles should be worn. After spraying or dusting, clothing should be changed and the body carefully bathed. Clothing considerably con- taminated should be laundered before being worn again. Some of the insecticides of this class are used to control household pests. Even though the risk is not unduly great, the instructions on the labels should be followed. Hazardous compounds may or may not bear the word "poison" on the la- bel, depending on their concentration, but all are labeled to show that they are dangerous and may cause death if swal- lowed. Empty containers should be promptly destroyed or buried. They should never be left where children or domestic animals can get at them. Such insecticides should not be stored where contamination of food or feed can take place. The user should famil- iarize himself with the antidote men- tioned on the label for accidental poi- soning. Nearly all these insecticides, at the concentrations indicated, are used chiefly by commercial growers or those who apply insecticides on a contract basis. The Safe Use of Insecticides Certain concentrated volatile insec- ticides are intended for mite control in chicken houses. Anyone using them should take special care to avoid breathing the vapors and letting them touch the skin. Users of hazardous sprays should wash thoroughly and change clothing after working with the material for any length of time. A person who uses dusts day after day should use a good respira- tor and change the pads frequently. Particular hazards are associated with the commercial use of organic phosphorus compounds. They are haz- ardous if swallowed, inhaled, or ab- sorbed through the skin or eyes. Protec- tive gloves, clothing, goggles, and a respirator with a special canister ca- pable of absorbing the vapors should be worn. Users should wash thoroughly with soap and water after each day's operation. Aerosol bombs may contain pyre- thrum, allethrin, or DDT as the insect- killing agent. The use of amounts indi- cated on the label creates no hazard, but it is best to close a treated room and remain outside for 1 5 minutes or more. DDT solutions used in the form of thermal aerosols or fogs to control flies and mosquitoes over large areas cause practically no hazard to the operator or to persons in the fogged area who might breathe the mists for a short time. The air dilution and short expo- sure are protective factors. To remain in such aerosol clouds for long periods is inadvisable, however. Aerosols are sometimes used to de- stroy insects on vegetation. The insecti- cide is dissolved in an organic solvent in the aerosol formulation. This may present some hazard from skin absorp- tion, and care should be taken when handling such solutions. Gloves of natural rubber should be worn. Hands and skin should be washed if there is contact with the solution. Goggles and a respirator should be worn to avoid breathing the fumes. Organic phosphorus insecticides are not recommended for use in these fogs 273 for outdoor-area treatment. Aerosols in which they are combined with a propellant are released from cylinders for controlling greenhouse pests. The operator should wear protective cloth- ing, gloves, and a face mask equipped with a universal-type N-canister. Treated greenhouses should be posted and locked, and no one should enter them until they are thoroughly aired. With the increased use of insecticides since 1945, however, more precautions have become necessary to protect pilots and flagmen as well as other people, livestock, and wildlife in the treated and adjacent areas. Special care must be observed when pilots or helpers apply sprays or dusts containing organic phosphorus com- pounds. It is necessary to avoid breath- ing the dust, vapor, or spray mist and to avoid skin contact. Loaders must wear a full-face mask provided with a universal-type N-canister, because of their severe exposure to the insecticide concentrates. Pilots and helpers should wear respirators with a fume-type fil- ter and chemical-absorbing cartridge. Freshly laundered, waterproof, or pro- tective clothing that covers all exposed skin surfaces should be worn. The clothing should be changed daily. The equipment should be checked before loading to make sure all connections are tight and the system is functioning properly. The pilot should lay out his course so as to avoid flying back through an insecticide cloud. Clean, natural-rubber or rubber- dipped gloves should be worn. They should be replaced frequently. The pilot and helpers should bathe thor- oughly and change clothing after fly- ing operations. Fumigants, which kill insects in more or less enclosed spaces, have dif- ferent degrees of hazards. Fumigants that present minimum hazards, such as ethylene dichloride-carbon tetra- chloride mixture, may be used safely by farmers to treat stored grain. Others, which require greater precautions, like 274 carbon disulfide, should be used only by commercial pest-control operators. The most hazardous fumigants, such as hydrocyanic acid gas, should never be used by the novice. The pest-control operator is trained to handle them and knows the proper precautions. Any fumigant that is toxic to insects is also toxic to human beings. Before any fumigant is used, the var- ious sources of information should be consulted and labels should be care- fully read. Antidotes and first-aid treat- ments should be noted. Two principal precautions that should be taken when a fumigant is used, regardless of its degree of toxic- ity, are to avoid exposure to a heavy concentration or inhalation of vapors for a long time and to avoid spilling the fumigant on the skin or clothing. A farmer who may be using a limited amount to treat one or two grain bins and who is using a fumigant recom- mended for farm use is exposed to it only for a short time. Protection from breathing the fumes is usually not nec- essary, as he can avoid prolonged in- halation. When the fumigant is used repeatedly over a long period and in enclosed spaces, protection against in- halation is necessary. If there is expo- sure to high concentrations, a full-face mask should be worn. The mask should have a canister suitable for the fumi- gant used; not every canister will af- ford protection against all gases. Be- cause the life of a canister is limited, new ones must be supplied whenever those in use show signs of weakness. Gas masks are not designed for protec- tion against prolonged exposure to heavy gas concentrations, and exposure should be limited to the short period necessary to release the fumigant and to open the building for airing. Many fumigants readily penetrate the skin and may be taken up by the blood stream. Gloves resistant to the fumigant should be worn, particularly if large quantities are involved. If ma- terial is spilled on the skin or work clothes, a bath with soap and water should be taken and the clothing Yearbook of Agriculture 1952 changed. This precaution should be followed immediately with the most dangerous fumigants. Persons not actually engaged in ap- plying the fumigant should be pro- tected from drift or leakage or from gaining entrance to the enclosed space during the fumigation. The buildings should be locked and posted. Keeping watchmen on duty during the exposure period is desirable. Some fumigants, such as carbon di- sulfide, have a fire or explosion hazard, and care should be taken to avoid any spark or flame near the vapors. Contamination of food products being fumigated directly or when stored in warehouses receiving a gen- eral fumigation must be avoided. Cer- tain fumigants, such as hydrocyanic acid gas and methyl bromide, are ab- sorbed, especially in moist materials, and therefore such products should not be used as food until thoroughly aired and found to contain no appreciable residue. Damp mattresses and clothing may absorb the gas and thorough airing before they are used is important. Drift of insecticides outside of the area being treated may create a danger, especially if extremely poisonous mate- rials are being applied. People living or working in the line of drift may be made seriously ill in extreme cases. A few insecticides, such as hexaethyl pyrophosphate and parathion, present hazards because of their vapors or dusts. They might cause serious conse- quences if large quantities drifted over areas where human beings, domestic animals, or fowl might be subjected to concentrations of the vapor or dusts for considerable time. Areas to be treated should be vacated until after the vapors or dusts have dissipated, unless one is certain that no hazard would exist. Sprays are less likely to drift than dusts. Certain insecticides are stomach poisons of low solubility in water. If they should drift to gardens of leafy vegetables or small fruits the residue might be dangerous. Some insecticides might remain on the plants for some The Safe Use of Insecticides time and be difficult to wash off. It would not be safe to eat products from such gardens soon afterwards. Drift of such insecticides over pastures might create a hazard to grazing stock — more because of the amount they take in with their food than because of actual contact of the material with their bodies. Similar drift to fields of hay crops may leave deposits sufficient to make the resulting hay injurious to livestock or indirectly to the public through the contamination of milk or meat. Obviously one has to consider the general situation before starting to ap- ply the insecticides. If possible, the ap- plication should be made at a time when hazards from drift will not be created, regardless of the kind of in- secticide used. Applications from the air are more likely to contaminate ad- jacent fields, gardens, and pastures than ground applications. Therefore only competent and reliable pilots should be employed to do such work, fields should be posted, and the owner should be on hand to supervise the operation. Canopies or hoods on ground equipment help to keep the materials from drifting and so add to the effectiveness of the application. Insecticides or repellents often are applied to livestock and household pets. Pyrethrum and rotenone, for ex- ample, present no hazards if kept out of the animal's eyes, but solutions and emulsions containing toxaphene, ben- zene hexachloride, and chlordane may be harmful if the concentration is too high. Formulations containing para- thion and hexaethyl pyrophosphate should never be used on animals, re- gardless of the formulation. Solvents such as oils and xylene in themselves are irritating and may cause loss of hair and scaling of the skin. They may also facilitate the penetration of the insecticide into the animal's body. For those reasons only insecticides and formulations recommended by proper authorities and labeled with specific in- 275 structions should be used on livestock or pets. Precautions on the label should be observed. Because some crops may be injured by insecticides, labels should indicate susceptible crops and the insecticide concentration that should be employed against pests. Solvents or other in- gredients may be safe on one crop but injure another. For example, it has been shown that cucumbers and squash are likely to be injured by DDT insec- ticides, although the same strength and dosage would be perfectly safe on most crops. Even certain varieties of a given vegetable may be easily injured by an insecticide that would not affect others. Some insecticides are poisonous to fish, toads, lizards, and snakes. For ex- ample, DDT and most of the new chlorinated materials, particularly toxaphene, kill fish at a very low con- centration. Care must be taken to avoid insecticidal treatments or drift over open water, such as wide rivers and lakes, since wind may concentrate the material along a margin in sufficient quantity to kill fish. Remnants of spray from tanks and spray equipment should be drained and washed into a hole in the earth where they will not gain ac- cess to streams and ponds. Insecticides should never be mixed on or near wells. Awareness of hazards and adoption of safeguards in using and storing pes- ticides are urgently needed. The object of instructions to that end is not to frighten people so that they will not use pesticides but to get them to ob- serve proper precautions. The intelli- gent use of pesticides will enable users to derive the greatest good with the least chance of adverse effect. F. C. Bishopp is an assistant chief of the Bureau of Entomology and Plant Quarantine. John L. Horsfall, a graduate of the State University of Iowa, is chief entomologist of the American Cyan- amid Company. He has been engaged in research and development of chem- icals for pest control since IQ20. Toxicity to Livestock R. D. Radeleff, R. C. Bushland H. V. Claborn An insecticide for use against para- sites of livestock must meet several specifications. It should be effective against the parasites, but it must not immediately harm the host when used as recommended, nor should it cause injury if the treatment is repeated fre- quently over a long period. It should be safe even when somewhat carelessly used. The ideal insecticide should not be stored in the body of the treated ani- mal nor appear in the milk of lactating animals. Materials that are safe at first even in large quantities are often the worst problem from the standpoint of storage in meat and secretion in milk; because such residues are so important to public health they are a significant part of the study of the toxicology of insecticides. So far, we do not have an insecticide that meets all the requirements. Some very safe materials destroy some para- sites. Some destroy many parasites but are not entirely safe when they are ap- plied to animals. Petroleum oils have been exten- sively used as insecticides or as com- ponents of insecticide formulations. Applied to the skin, they are harmless in small amounts, but in large amounts (4 ounces or more per animal) they cause severe reactions — blistering, ex- cessive salivation, difficult breathing, loss of appetite, depression, and death in cattle, horses, sheep, and goats. The effect of the oils usually is ob- served in the first few days following application and may continue for some time. Many cases of poisoning of live- stock have been attributed to the in- secticide dissolved in the oil because of the failure to recognize the dangers associated with the oil itself. 276 Straight oil solutions of insecticides therefore are never recommended for use on livestock other than as mist sprays applied at rates not exceeding 2 ounces per cow. In producing emul- sion concentrates intended for use on livestock, manufacturers must devise their formulas so the use of recom- mended amounts of the product will not lead to excessive doses of the oils. The stockman should use only the rec- ommended dosages lest he increase the dosage of oil to a toxic level, although he may be using safe limits of the dis- solved insecticide. Solvents are used in nearly all liquid preparations of insecticides. They may be oils, in which case we know their reaction, or they may be higher alcohols or special synthetic products, whose reaction is not known. The solvent may or may not be actively toxic but it should be considered when losses of animals occur following treat- ment. Although solvents will influence the speed of absorption of insecticides into the body, it is significant that the total absorption is essentially the same, regardless of the solvent. Some solvents, notably xylene and toluene, cause itching and burning for a short time after application in the hot sunshine, even though used in small amounts. If the concentration is high enough (6 percent), the animal may become dizzy or even be anesthetized. If still higher ( 25 percent) , death may result. Each manufacturer is responsible for making certain that the solvents he uses are not toxic in the amounts rec- ommended. Labeling laws governing the interstate shipment of insecticides as well as most State labeling laws do not require a statement on the package of the solvent content. The livestock man therefore often has no way of choosing his materials on the basis of the solvent used. Insecticides produced from plants generally are safe for use upon live- stock. They are neither acutely toxic Toxicity to Livestock nor capable of being stored within the animals sufficiently to create a hazard to humans. Pyrethrum and rotenone are notable examples of safety. An exception is nicotine, which, in the form of nicotine sulfate, is used principally to control mange or scab. As nicotine sulfate is most commonly used by regulatory officials, who are skilled in its use and have a reliable test for the strength of the dip, poison- ing from it as a result of dipping is uncommon. Animals poisoned by nico- tine sulfate show tremors, nausea, and disturbed respiration and finally enter a comatose condition, in which they may die. Some plant products are irritants and cause discomfort when they are applied to animals, but rarely is an animal killed by an insecticide derived from plants. Sulfur, lime-sulfur, and arsenic have been used to treat livestock. Sul- fur, used externally, is almost com- pletely nontoxic to mammals. Lime- sulfur, which is actually a complex of sulfides, may cause irritation, general discomfort, and even severe burning. Rarely does it kill an animal. Arsenic, as used in cattle dips, is extremely poisonous. It has given re- markable control of the cattle tick. The many losses of livestock, in deaths and in injuries as a result of burning and blistering after dipping, amply illus- trate the toxic nature of arsenical dips. Arsenic is absorbed through the un- broken skin and stored in tissues but is not excreted or secreted in the milk of lactating animals in detectable amounts. Acute arsenical poisoning causes death in i or 2 days from the time of treatment. At autopsy, the intestinal tract shows marked inflammation, the liver and other organs may be swollen, and the lungs may be severely con- gested. Less acute poisoning may cause blistering; cracking and peeling of the skin; profuse diarrhea, possibly with free blood; rapid emaciation; poor appetite; and obvious pain. 277 Poisoning by arsenical dips is not always the result of excessive dosage. Even a normally safe dosage may pro- duce burning or death if the animals are treated in wet weather or they are overheated. Because arsenical dips are primarily solutions of arsenic in water and an accurate test is available, losses have been less than if the dipping solutions could not be easily checked. The many arsenical compounds used in treating field crops may be poisonous to» livestock that eat them. Poisoning frequently has resulted from dusts that drift across fields into pastures and dusts remaining in containers carelessly left on premises occupied by livestock. Sometimes a dipping vat is emptied on an unprotected pasture. Some animals seem to crave arsenic and will seek out spots contaminated with it. The synthetic organic materials have simplified parasite control on livestock, but they also have hazards. The most important are the cumulative effects of repeated exposure and the problems of residues in meat and milk. As each of the new insecticides is a study in itself, it is best to discuss each one individually. DDT is a relatively safe insecticide. All livestock can tolerate single appli- cations of 8 percent DDT. As many as 10 applications of 2 percent DDT at 2-week intervals have failed to produce clinical changes. Cattle have also toler- ated 36 applications of 0.5 percent DDT at the same intervals. Cattle, horses, sheep, goats, and hogs all tol- erated 8 treatments with 1.5 percent DDT at 4-day intervals. DDT also is safe for dogs, but it must be used sparingly on cats, as they may be poisoned by relatively small amounts. Chickens should not be sprayed with DDT or dipped in it. Mice and rats are very susceptible to DDT, as is seen in the number of dead rodents found in barns that were treated with DDT. DDT occurs in the milk of cattle soon after spraying. Scientists at the 278 Oklahoma Agricultural and Mechani- cal College discovered that in 1947. Immediately the Department of Agri- culture began a study of samples of milk taken weekly from dairy herds that were sprayed once a month with 0.5 percent DDT. All the samples contained 0.1 to 2.0 p. p. m. (parts per million) of DDT. The average was between 0.6 to 0.7 p. p. m. Similar studies were made in 1948 on milk from dairy cattle sprayed with 0.5 per- cent DDT only as needed to control horn flies. The average DDT content of that milk was 0.25 p. p. m. Addi- tional tests made under controlled con- ditions with individual cows thor- oughly sprayed with 0.5 percent DDT indicated that a maximum of 2.6 p. p. m. was reached the second day after spraying and that the figure grad- ually dropped to 0.3 after 21 days. In some tests an increase of DDT was found in milk after barns were sprayed. The contamination varied in intensity. Even when the spraying was done most carefully some DDT was found in the milk of cows later fed in the barn, except when the feed troughs were completely protected during spraying or washed after spraying. DDT may be^tored in the fat of sprayed cattle or of cattle fed con- taminated feed. In a series of experi- ments at the Department's laboratory at Kerrville, Tex., Hereford cows with sucking calves were sprayed five times with 0.5 percent DDT at 4-week in- tervals. One-half the calves were sprayed each time and the others re- ceived no treatment. Two weeks after the fifth treatment, the fat of the cows contained an average of 15 p. p. m. of DDT. The unsprayed calves that sucked the sprayed cows averaged 25 p. p. m. Sprayed calves that sucked the sprayed cows averaged 52 p. p. m. of DDT. Yearling Hereford steers were sprayed at 3-week intervals with 0.5 percent DDT emulsion. Three weeks after one application their fat con- tained 18 p. p. m. Three weeks after the second treatment the average was Yearbook of Agriculture 1952 31 p. p. m. After the fourth it was 32.8, and after the sixth, 35.2. The steers gradually lost the DDT, having 4.7 p. p. m. in the fat 24 weeks after the last spraying. Yearling steers sprayed once with 0.5 percent DDT showed 1 1.2 p. p. m. 2 weeks later, but after 22 weeks they showed 2.9 p. p. m. Yearling Herefords fed 10 p. p. m. of DDT in all items of their feed for 30 days showed 6.8 p. p. m. in the fat on the last day of the feeding. Sheep fed the same diet showed 3.1 p. p. m. in the fat at the end of the feeding period and 1.2 p. p. m. 90 days after the feeding of the insecticide was dis- continued. Only the gamma isomer of tech- nical benzene hexachloride is useful against pests of livestock. The gamma isomer may be sepa- rated from the other isomers of tech- nical benzene hexachloride and a purified product obtained, which is composed of 99 percent or more of the gamma isomer. This purified product is known as lindane. Most farm animals are resistant to poisoning by gamma benzene hexachloride. In single treat- ments, adult cattle and horses can withstand sprays or dips containing 0.25 percent gamma isomer. Sheep, goats, and hogs can withstand 0.5 per- cent. Young calves are quite suscepti- ble, however, and the gamma isomer must be used with caution on them. Experiments so far have not established a fixed point of danger, but enough young Jersey calves have been poisoned by 0.05-percent sprays to cause such a dose to be set as slightly above the maximum that is safe. No deaths or poisonings resulted from the use of 0.03-percent sprays or dips on thou- sands of calves. The gamma isomer of benzene hexa- chloride has little danger as a chronic toxicant. The use of 0.2-percent wet- table powders on range cattle resulted in no clinical disturbances, although the dose was repeated 10 times at 2- week intervals. Benzene hexachloride is stored in Toxicity to Livestock the body of animals as a mixture of isomers. Yearling Hereford cattle were sprayed 12 times at 2-week intervals with a 0.25-percent emulsion of benzene hexachloride (0.03 gamma isomer) ; 2 weeks after the final spray- ing there appeared to be 31 parts per million of benzene hexachloride pres- ent in their fat. Ten weeks after the final spraying no residue was present in the fat. This storage, in the light of later studies, was due mostly to isomers other than the gamma. The pure gamma isomer (lindane is 99+ percent gamma isomer) is readily stored but is eliminated just as readily. Steers sprayed once with 0.03 percent lindane showed no residue 2 weeks after spraying. Steers sprayed six times at 3-week intervals with 0.03 percent lindane sprays showed no detectable residues 3 weeks after each treatment. Lindane is also secreted in milk fol- lowing spraying. Dairy cows sprayed with 0.05 percent lindane showed 1.0 p. p. m. of lindane in the milk on the day after they were sprayed. The milk was negative for lindane after 7 days. Cows sprayed with 0.1 percent sprays showed 1.5 p. p. m. lindane in the milk on the day after spraying. The milk was negative for lindane by the fourteenth day. Hereford cattle fed 10 p. p. m. of lindane in every item of feed stored 8 p. p. m. in their fat after 70 days of feeding. Cattle fed 100 p. p. m. stored 98 p. p. m. in their fat. This stored material disappeared rapidly, the fat being negative for the 10 p. p. m. in 6 to 10 weeks and 10 to 14 weeks for those fed 100 p. p. m. Benzene hexachloride is safe for dogs and cats at 0.5 percent gamma isomer in dry dusts. It should not be used ex- cessively on cats, because they are sus- ceptible to poisoning. If benzene hexa- chloride is used as a poultry-house treatment, care should be taken to avoid wetting the birds, as they are easily poisoned. Chlordane is relatively safe as to acute toxicity. Most farm animals, ex- 279 cept young calves, can withstand 2.0- percent sprays and dips. Young Jersey calves are killed occasionally by 1.0- percent sprays. Chlordane is not so safe as to its chronic effects. Cattle can withstand one or two treatments of 2.0 percent chlordane at 2-week intervals but are killed by three such applications. That seems to be true of other farm animals. We are not sure whether chlordane appears in the milk of treated cattle. Studies of milk from dairy cattle sprayed with 0.5 percent chlordane in- dicate a possible contamination of less than 1 p. p. m. of chlordane. Conclu- sive statements can only be made when more specific analytical techniques be- come available. Chlordane does appear to be stored readily in the fat of treated animals. Steers sprayed 12 times with 0.5 per- cent chlordane showed the equivalent of 20 p. p. m. chlordane in their fat 2 weeks after the twelfth application. No detectable residue was found 10 weeks after the twelfth spraying. A single spraying with 0.5 percent chlordane left a possible 2.5 p. p. m. of chlordane in the fat 2 weeks after spraying. Yearling Hereford cattle fed 25 p. p. m. of chlordane in every item of feed for 56 days showed only 1 2 p. p. m. in the fat on the twenty-eighth day and 19 p. p. m. on the fifty-sixth day of feeding. Only 5 p. p. m. of the insecti- cide remained in the fat 1 2 weeks after the feeding was discontinued and it had disappeared completely after 20 weeks. Delaine sheep fed the same diet showed only 7 p. p. m. at the twenty- eighth day and 12 p. p. m. on the fifty-sixth day. No insecticide residue was found in the fat 4 weeks after the feeding was discontinued. Yearling Herefords fed 10 p. p. m. of chlordane in every item of the diet for 112 days had 1 1 p. p. m. in the fat at the end of the feeding period. Fat of Delaine sheep fed on the same diet contained 9 p. p. m. in the fat after the same feeding period. Chlordane is a safe insecticide for dogs and cats if it is used in recom- 280 mended amounts according to the manufacturer's directions. As with DDT, many dead mice and rats will be found after treatment of premises. Toxaphene is reasonably safe when it is used as a spray for farm animals. Dip formulations have caused trouble, however, and the cause of the diffi- culties has been hard to determine. As an acute toxicant, toxaphene is most dangerous for the young calf. Concentrations of i.o percent toxa- phene have killed very young calves. Adult cattle, sheep, goats, horses, and hogs withstand 2.0-percent concen- trations. We have no indication that toxa- phene is chronically toxic to farm animals when it is used at recom- mended strengths. Cattle treated 10 times with 2.0-percent sprays displayed no clinical disturbances. Nor have we a specific method for determining the presence or absence of toxaphene in milk. The evidence in 1952 indicated that less than 1 p. p. m. would appear in milk following the spraying of a cow with 0.5 percent toxaphene. Toxaphene has little tendency to be stored in the fat of animals. Hereford steers under feed-lot conditions were sprayed with 0.5 percent toxaphene every 2 weeks for 24 weeks. One week after the twelfth spraying no detectable residue existed in the fat. During the sprayings, the level may have gone as high as 5 p. p. m. In a repetition of this experiment, except with the cattle kept under range conditions, approxi- mately 8 p. p. m. of toxaphene was present in the fat 2 weeks after the twelfth spraying. Steers and sheep fed 10 p. p. m. of toxaphene in all items of their feed showed no toxaphene in the fat on the thirtieth day of feeding, as indicated by organic chloride analysis. Toxaphene is highly toxic to dogs. Its toxicity to cats has not been studied. Pets should not be treated with it. Toxaphene should not be used directly on chickens. Yearbook of Agriculture 1952 Methoxychlor is safe from all standpoints. Even calves 1 week old have tolerated 8.0-percent sprays, and young chickens were not affected by 4.0-percent dips. Methoxychlor is secreted in the milk of cows, reaching 0.4 p. p. m. 1 day after treatment with 0.5-percent sprays. Because of the low toxicity of methoxychlor, the presence of small amounts of it in milk is not considered cause for alarm. Methoxychlor does not tend to be stored in the fat of cattle or sheep in large quantities. Two weeks after a single spraying of 0.5 percent methox- ychlor, steers showed only 2.8 p. p. m. in the fat. When cattle were sprayed six times at 3-week intervals with 0.5- percent sprays and the fat sampled 3 weeks after the sprayings, the fat con- tent was as follows: One spray, 1.5 p. p. m.; two sprays, 1.5 p. p. m. ; six sprays, 2.4 p. p. m. All residue had dis- appeared 12 weeks after the sixth spraying. Yearling Hereford cattle fed 10 p. p. m. of methoxychlor in every item of the diet showed no methoxychlor on the thirtieth day of feeding. Delaine sheep fed the same diet also showed no methoxychlor in the fat after the same feeding period. TDE is practically like DDT in toxicity to animals, storage in fat, and secretion in milk. Dieldrin is toxic to week-old calves at 0.25 percent concentration, to cattle at 2.0 percent, and to sheep and goats at 3.0 percent. Hogs appear able to stand about 4.0 percent. It is not so safe when it is used repeatedly. Cattle sprayed three times at 2-week inter- vals with 0.5-percent material showed clinical symptoms of poisoning. Dieldrin may be stored in the fat of cattle. Two sprayings at 0.25 percent at 3-week intervals produced 17 p. p. m. in the fat 3 weeks after the second ap- plication; eight applications produced 14 p. p. m. in the fat 3 weeks after final treatment. It took 13 weeks for a resi- due of 14 p. p. m. to disappear. Toxicity to Livestock Yearling Hereford cattle fed 25 p. p. m. of dieldrin in every item of feed showed 75 p. p. m. in the fat on the twenty-eighth day of feeding, and 74 p. p. m. on the fifty-sixth day. Delaine sheep fed the same diet showed 43 p. p. m. in the fat after 28 days of feeding, and 69 p. p. m. after 56 days of feeding. Detectable amounts of diel- drin were still present in the fat of both sheep and calves 32 weeks after the feeding was discontinued. Dieldrin is secreted in the milk of treated cattle. Dairy cows treated with 0.5-percent sprays showed a maximum of 7 p. p. m. in the milk on the third day after spraying. The milk was prac- tically free of dieldrin 21 days after treatment. Aldrin appears to be slightly less toxic than dieldrin. Week-old calves seem to be able to stand nearly 0.25- percent concentrations. Very little is known of the toxicity of aldrin for other animals. Hereford cattle fed 25 p. p. m. of aldrin in every item of feed showed 49 p. p. m. in the fat after 28 days of feed- ing and 78 p. p. m. after 56 days. De- laine sheep fed on the same diet showed 60 p. p. m. on the twenty- eighth day of feeding and 78 p. p. m. on the fifty-sixth day. Detectable amounts of aldrin were still present in the fat of sheep and calves 32 weeks after feeding was discontinued. Here- ford cattle fed 10 p. p. m. of aldrin in every item of the diet for 1 1 2 days had 49 p. p. m. in the fat at the end of the feeding period. Delaine sheep fed on the same diet showed 55 p. p. m. in the fat at the end of the feeding period. Some explanation of how experi- ments are conducted to determine the foregoing facts is in order. To determine the amount of insect- icide which may kill, several animals of each particular species are treated with sprays or dips, beginning at a high concentration and working up or down as may seem desirable. A suffi- cient number of animals and of differ- 281 ent concentrations are used to produce poisoning and to find a maximum safe dose and a minimum toxic dose. In every instance, the animals are completely saturated with the spray or dip. They are allowed to be under normal conditions and free to roll or lie down and to lick themselves or one another. All animals are observed at close intervals for 48 hours or longer for symptoms of poisoning. If symptoms are seen they are recorded in detail to establish a means of recognizing the particular poisoning in other animals. Records are kept of the many possible variables, time of application, type of equipment, details of formulation, time of onset of symptoms, time of death, and any other indicated data that might be needed. If an animal dies from poisoning, an autopsy is performed promptly. Records are made of any lesions and samples of tissue are preserved for study under the microscope. We have mentioned toxic doses. Those doses, however, did not neces- sarily kill the animal in question. Toxic doses are those that cause some ab- normal activities or otherwise affect the normal health of the most suscep- tible of the animals treated. When minimum toxic doses are mentioned, it is understood that at that dosage only one, or at most a few, out of the treated group became affected. Studies of chronic effects are made in the same way; the applications and observations are simply repeated. Contamination of milk is studied by thoroughly spraying or dipping lactat- ing animals and collecting milk sam- ples at intervals. Generally samples of milk are taken for several days before the treatment and at frequent intervals after treatment. The samples may be taken as a portion of the total milk drawn by a mechanical milker into individual buckets or may be drawn by inserting a canula into the teat and drawing the milk into closed con- tainers through flexible tubing. In all cases, every piece of equip- 282 merit is specially cleaned, scoured, and dried. The animal's udder is cleaned and dried. From inside the udder until the chemist completes his analysis there is no chance given for outside insecticides to contaminate the milk to be studied. To determine the amounts of insec- ticides in animal tissue, two methods are used. Appropriate amounts may be taken of various tissues during au- topsies after death from poisoning or following slaughter. Samples taken after slaughter may be expensive, be- cause if the residues be large the meat is not sold for human consumption and must be destroyed. The method also involves the use of many animals. In studies of autopsy samples from cattle and sheep, it was found that even when the insecticide residues reached several hundred parts per million in the fat there were less than 2 p. p. m. in muscle tissue. It was then decided that fat would be the tissue of choice for later analytical work. The values given in this paper are all for fat — the amount of insecticide in a given cut of meat will be in proportion to the percentage of fat contained in that meat. The values we give, there- fore, are actually the extremes that would be found under the conditions of treatment stated. A biopsy technique was devised to eliminate waste of animals and to pro- vide better data. Treated animals are cast, suitably anesthetized, and a 2- ounce sample of fat is then taken from the caul through an abdominal in- cision. The process is much like an appendectomy. The method allows samples to be taken before treatment as well as several times after, or dur- ing, treatment. It also allows us to make sure that there are no measurable deposits of insecticide within an ani- mal at the end of an experiment. The method allows one animal to provide the data that six or seven did by the autopsy method. It also produces more valuable data. Observations on the behavior of treated animals also are of value. Yearbook of Agriculture 1952 The chlorinated hydrocarbons out- wardly show their effect on an animal by various nervous disturbances. No two animals poisoned by a given in- secticide will show exactly the same chain of symptoms, yet the symptoms are enough alike to enable one to identify them. An affected animal will generally first become excitable and a little more alert to its surroundings. Twitches of various muscles soon follow, begin- ning usually at the head and going backward along the body. The twitches may increase in intensity until there are spasms and, finally, convulsions. In addition, the animal might assume abnormal attitudes, such as standing with the head between the forelegs and under the body, a sternal position with the hind legs in standing position, and persistent chewing movements. Occa- sionally the animal attacks any moving object. There is usually profuse saliva- tion, rolling of the eyes, dribbling of urine, and bawling. The body tempera- ture may climb to 1 140 F. Some animals show none of these ac- tive symptoms; instead they are de- pressed and unaware of their surround- ings. Some animals are alternately de- pressed and excited. Severity of symp- toms is no index of the likelihood of death or survival. Death may occur an hour or several weeks after exposure. Most cases run their course within 72 hours. Findings at autopsy are somewhat variable. Alone, they never are diag- nostic of poisoning by these insecti- cides. There will usually be cyanosis (blue-colored skin and membranes), congestion, and small hemorrhages of various organs, most frequently on the heart. The lungs usually are congested, heavy, and dark in color, suggesting primary stages of pneumonia. Often an excess of fluid occurs in and around the brain and spinal cord. If the animal was affected over a long period, the carcass may be thin and lacking in moisture. The liver and kidneys may show abnormal consist- encies. Toxicity to Livestock Microscopic lesions in animals dying quickly are few, other than those men- tioned, as observable at autopsy. In prolonged cases, there are fatty changes in the liver and kidney, some degenera- tion in those organs, and degeneration in the brain. Otherwise, few if any sig- nificant changes can be seen. We prevent untold numbers of deaths of livestock and costly economic losses by making these experiments be- fore new materials are widely used. Some animals have to be sacrificed in carefully controlled experiments in or- der that safe methods may be worked out enabling the livestock grower to use the new chemicals for improved pest control. R. D. Radeleff., a veterinarian in the Bureau of Animal Industry, is sta- tioned at Kerrville, Tex. He is a grad- uate of Schreiner Institute and the Agricultural and Mechanical College of Texas. Since 1947 he has engaged in work on the problems of insecticide toxicology. R. C. Bushland is in charge of the Kerrville laboratory of the Bu- reau of Entomology and Plant Quaran- tine. After studying at South Dakota State College and Kansas State Col- lege, he joined the Bureau in 1935. Most of his research has been on insec- ticides. At the Bureau's Orlando labo- ratory during the Second World War, he was one of the group who first in- vestigated the chlorinated hydrocarbon insecticides to establish their value in the field of medical entomology. H. V. Claborn,, a chemist of the Bureau of Entomology and Plant Quarantine, is also stationed at Kerr- ville. Since 1929 he has worked for the division of insecticide investigations, the research laboratories of the Bureau of Dairy Industry, and the Food and Drug Administration. He is a native of Arkansas and a graduate of George Washington University. For further reference: R. C. Bushland, H. V. Claborn, H. F. Beckman, R. D. Radeleff, and R. W. Wells: Contamination of Meat and Milk by Chlor- 283 inated Hydrocarbon Insecticides Used for Livestock Pest Control, Journal of Eco- nomic Entomology, volume 43, pages 649— 652. 1950. R. C. Bushland, R. W. Wells, and R. D. Radeleff : Effect on Livestock of Sprays and Dips Containing New Chlorinated Insecti- cides, Journal of Economic Entomology, volume 41, pages 642-645. 1948. R. H. Carter: Estimation of DDT in Milk by Determination of Organic Chlo- rine, Analytical Chemistry, volume 19, page 54, 1947; The Chlorinated Hydrocarbon Content of Milk from Cattle Sprayed for Control of Horn Flies, with R. W. Wells, R. D. Radeleff, C. L. Smith, P. E. Hubanks, and H. D. Mann, Journal of Economic En- tomology, volume 42, pages 116-118, 1949. H. V. Claborn, H. F. Beckman, and R. W. Wells: Excretion of DDT and TDE in Milk from Cows Treated With These In- secticides, Journal of Economic Entomol- ogy, volume 43, pages 850-852, 1950; Con- tamination of Milk from DDT Sprays Ap- plied to Dairy Barns, Journal of Economic Entomology, volume 43, pages 723-724, 1950. D. E. Howell, H. W. Cave, V. G. Heller, and W. G. Gross: The Amount of DDT Found in Milk of Cows Following Spraying, Journal of Dairy Science, volume 30, pages 717-721. 1947. R. D. Radeleff: Chlordane Poisoning: Symptomatology and Pathology, Veterinary Medicine, volume 43, pages 342-347, 1948 ; Toxaphene Poisoning; Symptomology and Pathology, Veterinary Medicine, volume 44, pages 436—442, 1949; Omentectomy of Cattle for Studying Insecticide Residue in the Body, Veterinary Medicine, volume 45, pages 125-128, 1950; Acute Toxicity of Chlorinated Insecticides Applied to Live- stock, with R. C. Bushland, Journal of Eco- nomic Entomology, volume 43, pages 358— 364, 1950. Milton S. Schechter, Milton A. Pogorel- skin, and H. L. Haller: Colorimetric Deter- mination of DDT in Milk and Fatty Mate- rials, Analytical Chemistry, volume 19, pages 51-53. 1947- The mole cricket, like the mole, has devel- oped strong forelegs for tunneling in the ground, where it feeds upon the roots of plants. Residues, Soils, and Plants Victor R. Boswell Since the mid-1940's there has been a renewed and heightened interest in questions involving residues of agri- cultural chemicals in soil. This has oc- curred because agriculture has been recently supplied with a number of en- tirely new synthetic compounds for pest control, of largely unknown stabil- ity and toxicity to plants when present in the soil. During the Second World War, when DDT first came into use in this country, one of its properties that es- pecially intrigued us was its unusual persistence. It seemed almost too good to be true that a single application of a small amount of DDT solution upon window or door screens, walls, or trim would remain effective as a fly killer for many weeks. There were hopes that it would prove highly persistent in the soil for the killing of harmful insects there. It appeared highly desirable that we should have an insecticide of such persistence that a single treatment of the soil for insect control would remain effective for years. DDT has proved to be just that kind of remarkable sub- stance. Entomologists in the eastern United States have found that DDT applied to the soil at 25 pounds per acre in 1945 was still effective against Japanese beetle grubs in 1950. In cer- tain tropical and other soils, however, DDT loses its effectiveness relatively rapidly. This stability or persistence of an in- secticide in the soil that may be highly desirable for insect control can, how- ever, turn out to be a serious disadvan- tage under some conditions. Is it harm- ful to plants, to soil bacteria, or fungi? How long will this or that insecticide remain in the soil without leaching out, 284 without gradually vaporizing, or with- out being rendered inactive to plants or insects by chemical change or by the action of soil micro-organisms? If any amount is harmful to plants or to the soil bacteria and fungi, is it also so per- sistent that residues reaching the soil after dusting or spraying crops will ac- cumulate to an extent that will harm plant growth? If residues can accumu- late, how fast? How often and how heavily can a particular substance be used without danger of lowering the productivity of the soil after some years? Can a harmful amount of such residue be removed or corrected by special treatment of the soil? These are questions that must be answered not only for insecticides but for any agri- cultural chemical that is purposely ap- plied to or that incidentally reaches the soil. One of the first reports of crop injury believed to be due to an accu- mulation of an insecticide in the soil appeared in 1908. Symptoms of injury were evident in apple trees in an or- chard in Colorado that had been sprayed repeatedly with lead arsenate. Soil analysis showed 61 p. p. m. (parts per million) of arsenic in the soil of the orchard, much more than the amount naturally present. Later work suggests that the injury in question was not due directly to the arsenic in the surface soil of the orchard. The old work, however, did demonstrate that ordinary use of an arsenical insecti- cide in an orchard results in an arsenic accumulation in the surface soil. During the years 1930 to 1933, in South Carolina, several investigators reported a series of observations and studies of toxicity of arsenic in the soil to crop plants. They found that on heavy soils — Cecil clay and Davidson clay in those instances — cereals and cotton were uninjured by thousands of pounds of calcium arsenate per acre. Vetch and cowpeas were injured by 1,000 to 1,500 pounds per acre or more. On the clay soil soybeans were uninjured by large amounts, but on Residues, Soils, and Plants Norfolk sandy loam 200 to 300 pounds per acre caused serious injury. They found that the crops on soils low in iron content were injured by much smaller amounts of arsenic in the soil than when they were grown on soils high in iron. Soybeans were seriously injured on Norfolk sandy loam follow- ing only 3 years of cotton dusted with calcium arsenate to control boll weevil. In Louisiana, about the same time others also found that a given amount of calcium arsenate caused much more injury on a light soil than on a heavy one. On Crowley silty clay neither 50 nor 150 pounds per acre had any effect on rice. On Crowley very fine sandy loam, however, 50 pounds reduced the rice yield 45 percent and 150 pounds reduced it 65 percent. In the 1940's, work in central New Jersey showed that most vegetables are sensitive to arsenic. Lima bean, snap bean, and turnip were especially sensitive; they were killed by 1,000 or 2,000 pounds per acre in the surface 3 inches of soil. Early growth of all crops was retarded, and some crops very seriously, but if the plants sur- vived long enough for the roots to pen- etrate below the treated zone, they made considerable recovery. The greatest accumulations of ar- senic residues from the spraying of crops have been noted in the orchard soils of the Pacific Northwest, especially Washington, where heavy lead arsenate sprays have been applied annually for many years. Most of the arsenic residue is confined to the surface 6 to 8 inches of soil, the amounts found below 8 inches rarely exceeding those occurring naturally in the soil. The old orchard trees have been apparently uninjured by the great accumulations sometimes found because the arsenic accumulated only in the surface 6 to 8 inches after most of the tree roots were well estab- lished in the deeper levels of the soil. In some orchards amounts up to ap- proximately 1,400 pounds per acre of accumulated arsenic trioxide have been determined in the surface 8 inches. Arsenic up to 30 times and lead 40 285 times the amount occurring naturally have been found. Although a large amount of arsenic in the surface soil may not harm the trees, it is definitely harmful to many kinds of cover crops, and cover crops are essential to profit- able tree yields over a long period of time. As lead arsenate has accumu- lated, legume cover crops have become progressively poorer in many orchards. During the depression years of the 1930's many of the less productive or- chards in Washington were pulled out, and efforts made to grow alfalfa or annual crops on the old orchard sites. Alfalfa and beans ofteYi died on those high-arsenic tracts although they thrived on immediately adjacent sites that received no spray residues. Of the vegetable gardens observed on numer- ous old orchard sites, none was entirely successful and many were failures. Several years after heavily sprayed trees had been removed, rye and pota- toes grew fairly well but beans and peas still showed marked sensitivity. After several years tomatoes, asparagus, and grapes showed intermediate sensitivity, but they grew poorly on land from which the trees had been recently removed. If the roots of orchard trees en- counter the high accumulations of arsenic found in the upper layers of some old orchard soils, the trees may be definitely injured. Specific arsenic toxicity symptoms have appeared in peach and apricot trees planted on land from which old sprayed apple orchards had been removed. As the years pass, the arsenic toxicity of the former orchard soils is gradually decreasing. With the cessation of ap- plications of arsenicals, with the grad- ual leaching effects of rainfall and irrigation, and with continued culture of the less sensitive crops, the produc- tivity of those soils in the Pacific North- west should be ultimately restored after many years. In South Carolina also it was noted that after the arsenic applications (to cotton) were stopped, productivity gradually returned to those soils that had been damaged. 970134°— 52- -20 286 Since 1945 large quantities of new synthetic organic insecticides have been used. The most important of these can be classified in one or another of two general groups of substances: ( 1 ) chlorinated hydrocarbons, such as DDT and BHC; and (2) phosphorus compounds such as parathion and HETP. With no background of earlier experience with these or related sub- stances for use as insecticides, there was no basis for knowing whether any one of them would prove to be more persistent or toxic in the soil to plants, than lead arsenate, for example, or less so. The almost unbelievable insect- killing power of some of them, the apparent stability of DDT, and even the "newness" of these materials all combined to give a genuine urgency to the questions that arose about them. The instances of damage to orchard and cotton soil by arsenic accumula- tions stood as warnings of what might result from long-continued use. If one of these new substances should, when mixed in the soil, happen to be many times as toxic as lead arsenate is to common crop plants (some are many times as toxic as lead arsenate to insects in the soil) and as persistent as lead arsenate or more so, there would surely be trouble ahead. From the first, some feared that under certain conditions of use farmers might encounter dam- aging effects to crops by soil residues in a shorter time and of a more trouble- some character than had resulted from using lead arsenate. Soon there were not just one or two compounds to con- sider, but a dozen of them; and it was important to find out as soon as pos- sible what their potential long-time effects might be. Perhaps one of the most striking observations on plant response is that various insecticides, when present in the soil in appreciable amounts, may definitely reduce rate of growth, total growth, and yield (as of seed or fruit) without producing above ground any symptoms of injury whatever. This in- ability to detect any harmful effect by inspection of the above-ground parts Yearbook of Agriculture 1952 of the plant may very well result in overlooking many instances of unsus- pected insecticide residue injury. The crop may appear entirely normal, but if there are no exactly comparable plants nearby on residue-free soil, the retarded growth cannot be detected unless it is rather severe. Definite symp- toms of injury show up, usually, only when growth has been retarded so severely that it would be noticed whether other symptoms were present or not. Leaf and stem discolorations and malformations are among the last symptoms to appear. Much harm may be done before they become evident. Below ground, the situation is not quite so difficult to detect. In general, plants that are retarded in growth by DDT or BHC will show root abnor- malities although the tops appear nor- mal. The moderately affected plants may show only somewhat stunted and shortened root systems. In more severe cases the roots are sometimes discol- ored and abnormally short, numerous, and virtually without root hairs. Ex- treme injury is characterized by very numerous short, thickened, stubby roots. The roots appear to have been stopped in growth soon after starting, with successive flushes of roots emerg- ing only to suffer the same fate. Strangely, DDT in the soil seems to have little effect on germination and emergence although many plants are rather highly sensitive to it after emer- gence. Technical BHC, on the other hand, has shown a consistent and very harmful effect on germination and emergence as well as later growth. Many investigators have described the abnormal seedlings of various seeds germinated in media containing BHC. The thickening and distortion of tissue in extreme cases is suggestive of that induced by some of the so-called growth-regulating substances such as 2,4-D which are effective in extremely small amounts. Seeds of many crops sown soon after mixing chlordane ( 25 pounds per acre or more) with the soil also give poor stands of plants. The amounts of various substances required Residues, Soils, and Plants to produce such marked effects are discussed later. Determinations of rates of ac- cumulation and of persistence of in- secticides in the soil obviously involve many years of work. On the other hand, the relative sensitivity of different kinds of plants to various insecticides can be found in a much shorter time. It very large quantities of certain in- secticides in the soil are harmless to a wide range of plants, there is little con- cern over any potential dangers from their accumulation- — assuming that they do accumulate and that their de- composition products are not harmful to crop growth or quality. If mixtures representing relatively small accumu- lations in the soil produce undesired effects on any plants, it immediately becomes important to know what plants differ in sensitivity, and what amounts of the substance can be tol- erated without harm. In studying the tolerance of plants to various insecticides in the soil the investigator usually treats a series of plots with successively heavier applica- tions, most of the treatments purposely much larger than would ever be ap- plied at once, or even in a year, in prac- tice. He not only wants to know that plants may be uninjured at ordinary rates of use, but also wants to know how much of a given substance is re- quired to produce injury. It is impor- tant to know not only what amounts are safe, but also what amounts are unsafe for normal plant growth. It is for these reasons that many experi- ments mentioned in these pages involve some truly massive dosages. Growth habit of a particular plant and the cultural conditions under which it grows may determine whether or not its roots come into contact with an insecticide to which it is known to be — or may be — sensitive. As in the instance of arsenic, DDT accumulates in the surface soil. It is highly insolu- ble, stable, and does not move down into the zone where most of the tree roots are. DDT has been applied to the 287 surface of undisturbed soil beneath large apple trees at rates as high as 3,000 pounds per acre without affect- ing the tree. Up to 50 pounds of DDT per acre for the destruction of Japanese beetle grubs has been applied to the surface of the soil in which a very wide range of woody nursery plants were established without injuring them. Established peach trees are apparently unharmed by years of accumulation of DDT in the surface soil, but roots of peach seedlings are definitely sensitive to amounts of 100 pounds per acre or more of DDT in the soil in which they are planted. In general, trees and bushes are probably no more resistant to one or another of these potent com- pounds than are the annual crops that have been tested, but they apparently escape injury because the toxic mate- rial does not reach their roots. We have seen a few instances of par- tial recovery of annual plants from mild injury that appear to be due to the fact that deep roots finally pene- trated below the toxic surface soil into noncontaminated soil. Shallow-rooting species or seriously injured plants in highly toxic soils are usually unable to establish sufficient roots below the toxic zone to make any recovery. Not only do species of plants differ widely in their sensitivity to this or that insecticide in the soil, but varieties within species sometimes show big dif- ferences. The conventional botanical relationships are not always a safe basis for predicting how a given species or variety will react. The effects of DDT and BHC have been studied more extensively than those of other new insecticides and thus afford more examples that can be cited. By no means have all important crops been tested extensively enough with most of the new compounds to permit a classification according to sensitivity to each one. Corn and other cereal plants that have been tested are generally tol- erant to relatively large doses of DDT 288 in the soil, although there are some striking exceptions. In fact, frequent exceptions occur in the general be- havior of many plant groups in re- sponse to various insecticides mixed with soil. Few generalizations appear safe at this stage of our knowledge. Instances of reduced germination and stand of corn have been reported as due to dosages of ioo to 400 pounds per acre of DDT in the soil but most observations show no effect of such amounts upon either germination and stand or upon later growth. Two inves- tigators have reported that DDT ap- pears to stimulate growth of corn slightly, quite aside from any insectici- dal effect. Dosages as high as 1,000 pounds per acre have been without ef- fect in some tests. Wheat is generally tolerant, as are most of the few varie- ties of barley and oats tested. Some var- ieties of rye, especially Abruzzi and Rosen, are highly sensitive to DDT. Dosages of 50 to 100 pounds per acre markedly reduce growth of Abruzzi rye, and under some conditions as little as 25 pounds is sometimes harmful. Residue accumulation in a peach or- chard after only 4 years of normal use (about 100 pounds total per acre) se- riously interfered with the growth of Abruzzi rye as a winter cover. The potato is another important crop that, so far, appears to be tolerant to rather large amounts of DDT in the soil — up to 400 pounds per acre, possi- bly more on some soils. Members of the cabbage family are also much more tolerant than some other crops. The few varieties of cabbage, broccoli, col- lards, and turnips tested have shown no effects on young growth from dos- ages up to 400 pounds per acre. To- bacco has tolerated 100 pounds. Cot- ton, soybeans, peanuts, and many other major crops have been tested so little that their classification is uncertain. In the few tests reported, however, they appear somewhat sensitive but not highly so. Some members of the pea family are highly sensitive to DDT while others are not. In general, snap beans and Yearbook of Agriculture 1952 lima beans are sensitive, some varieties extremely so; Stringless Black Valen- tine snap bean is probably as sensitive as any variety of a common crop plant observed to date. As little as 25 to 50 pounds per acre may affect its growth. Under some field conditions, 100 pounds has markedly reduced growth and 200 pounds has reduced yields by one-half. Other common varieties ap- pear somewhat less sensitive. Spinach, beet, and tomato also are highly sensi- tive to DDT in the soil, as little as 25 pounds per acre producing noticeable depression in growth under some conditions. In general, the members of the pumpkin family are sensitive to very sensitive, but here again there are ex- ceptions. Summer squash and pump- kins of the same species (Cucurbita pepo) are extremely sensitive and cu- cumbers are moderately so while musk- melon appears rather tolerant. Among the fruits, only peach and strawberry have been tested in such a way that the feeding roots were ex- posed to soil containing DDT. Peach is sensitive. Strawberry is highly sensi- tive. Strawberry is so very sensitive that merely dusting the rows of young mother plants as for insect control leaves enough DDT in the surface soil to interfere seriously with the forma- tion of daughter plants. Apparently rooting at the nodes of the runners is markedly reduced by the DDT in the surface soil through which the young roots must pass if they are to become established and support a daughter plant. How various plants react to DDT in the soil is no indication whatever as to how they will react to BHC. In fact, many of the relative differences among crops in response to DDT are reversed with BHC. For example, corn is either unaffected or slightly stimulated by DDT while snap beans are highly sen- sitive; but corn is sometimes nearly killed by concentrations of BHC that snap beans tolerate with no measurable harm. Despite the high sensitivity of Residues, Soils, and Plants some plants to DDT, many important ones appear highly tolerant. None, however, will tolerate anywhere near as much BHC as DDT is tolerated by some, and most of those tested are either highly susceptible or susceptible to BHC. Honey Dew muskmelon is one of the most sensitive plants tested in soil containing BHC. Strawberries, on the other hand, highly sensitive to DDT, appear unusually tolerant to BHC. A large number of observations on effects of BHC has been reported both from Europe and here in America. One of the most striking features of these diverse reports is the consistently harmful effects of getting BHC con- centrated in close proximity to, or con- tact with, seeds upon planting. Because of its remarkable control of many soil- inhabiting insects that attack seeds or seedlings before emergence, a number of experiments have been conducted in which BHC was dusted on the seed in the open row or drill in the soil before covering the seed with the soil. Marked injury usually resulted although only a pound or two up to 5 or 6 pounds of technical BHC per acre was applied. Coating bean and corn seed with 4 ounces of technical BHC per bushel, with a "sticker," resulted in serious injury. As little as 3 parts per million (about 6 pounds per acre of soil 62/$ inches deep) of technical BHC was somewhat harmful to red clover, soybean, and vetch, while 30 parts per million caused serious injury. Up to 15 pounds per acre thoroughly mixed with the soil has been reported harmless to grain crops while reports on other trials under other conditions showed 20 to 50 pounds harmful to wheat, oats, and barley. Dosages of 50 pounds per acre or more have been generally harmful to most crops grown the same year the technical BHC was applied. Fifty to 80 pounds, however, was not harmful to cotton or tobacco the following year. Experimental applications of 100 to 200 pounds of technical BHC per acre — in efforts to find upper limits of 289 tolerance of "resistant" crops — have consistently ruined the plantings. Some potato varieties have made normal-appearing growth and yield on soil treated with as much as 80 pounds, while others under different conditions have been injured supposedly by only 20 pounds. Regardless of the effect on growth, it is now well known that BHC must never be used in growing pota- toes, sweetpotatoes, carrots, beets, other root crops, or peanuts — any crop of which the edible part develops in con- tact with the soil — because the insecti- cide spoils the flavor or imparts a bad odor to the product. When a single application of BHC is no greater than recommended for the control of a specific pest in the soil and when it is thoroughly mixed in the soil so that it will not be too concen- trated near the seed and young plant, it is by no means always harmful. It is highly toxic to the germinating seeds and roots of most plants, however, and must be used with discretion. Investigators in India and in Eu- rope, after studying the abnormalities of plant tissues and plant cells that were caused by contact with BHC, have gone so far as to suggest the pos- sibility that repeated growing and seed saving of a single stock of a sensitive crop on soil containing appreciable amounts of BHC may cause the stock to deteriorate genetically. While this appears to be an extreme view, only time can tell whether or not it is well founded. Lindane, a relatively pure prepara- tion of the gamma isomer of BHC, con- tains very little of the several odorous impurities present in technical BHC that have little or no insecticidal value but that are nevertheless highly toxic to plants. Lindane is of particular in- terest because only about one-eighth as much is required for insect control as is required of the technical BHC, and because it is less odorous. Several work- ers have shown that although it has little if any harmful effect on germi- nation and stand it tends to be about as toxic to later growth, pound for 290 pound, as technical BHC. In practice, of course, the probabilities of adding large amounts of lindane in a short time are remote — especially since the cost per unit of gamma isomer is higher in the form of lindane than in technical BHC. Chlordane and toxaphene have been used less extensively than DDT and BHC and their possible toxic effects on plants have been studied less. In the field, at applications up to 20 pounds per acre chlordane has been harmless to tobacco, cotton, soybean, cowpeas, corn, and rye. In other fields 28 pounds stunted beans slightly when they were planted soon after applying the chlordane to the soil. Twenty to 25 pounds per acre had no effect on a wide range of species of grasses but as much as 40 to 80 pounds of chlordane applied to lawns or sods caused some temporary injury. In greenhouse tests at 200 pounds per acre chlordane was more toxic to four varieties of sorghum than to most varieties of cereals tested. Strawberries appeared highly tolerant. The effects of chlordane on the ger- mination and stand of vegetable crops have been rather variable. Some in- vestigators have found 20 pounds per acre to have no effect on germination of a wide range of vegetables while oth- ers have found beans, beets, tomatoes, and members of the pumpkin family to be injured by 20 to 25 pounds or more per acre. A hundred pounds or more hurt germination of those crops seri- ously. On the other hand, germination of lima bean, corn, and members of the cabbage family was affected but little by 100 pounds. Effects of chlordane on later growth of vegetables have also been variable. As little as 5 pounds per acre has been reported to affect the growth of sensi- tive varieties of squash in some tests but 25 pounds or more has not been harmful in others. Sometimes other vegetables apparently are harmed after emergence by no more than 20 pounds per acre, and in other instances un- Yearbook of Agriculture 1952 harmed by 100 pounds per acre. Musk- melon, especially Honey Dew, some varieties of squash, and cucumber, however, have appeared rather highly sensitive. Four pounds of chlordane per acre applied in the furrow before covering sugarcane seed pieces slightly stim- ulated the emergence of the shoots and caused no injury. Of the few reports on the effects of toxaphene none show any harmful effect on germination and stand or upon growth after plant emergence, where no more than 25 pounds per acre have been used. One investigator found tomato germination and the post-emergence growth of pumpkin, squash, and watermelon to be de- pressed about a third by 100 pounds per acre of toxaphene. Two hundred pounds depressed the growth of sor- ghum nearly 50 percent, and of beans and tomatoes about 30 percent. Most cereals and lima beans tested were little affected at this rate. Two other insecticides, aldrin and dieldrin, are so new that they have been used and studied even less than those discussed in the preceding pages. The limited data available indicate that to most crop plants they are more toxic than DDT, chlordane, and toxa- phene, pound for pound, but less toxic than BHC and lindane. Dieldrin ap- pears generally more toxic than aldrin. Because of their great potency the rec- ommended dosages are very small. Therefore their accumulation as soil residues harmful to plants does not now seem likely to occur in a short time, if ever. Parathion in the soil, up to 50 to 100 pounds per acre, has produced no harmful effect on growth of vegetables with the possible exception of snap beans and muskmelons, planted soon after treating the soil. Plantings made later than 3 to 4 months after treating the soil were not harmed. There was some depression of germination soon after heavy treatments were applied, but later plantings were unaffected. Residues, Soils, and Plants Parathion stimulated the growth of strawberries. Extensive tests of DDT with many crops show that certain acid muck soils render large amounts of DDT nontoxic to plants that are seri- ously injured by equal amounts that are applied to certain mineral soils. Mineral soils show some differences in the degree of injury that is produced by a given amount of DDT mixed in them. From the little now known, it appears that on light, sandy soils low in clay, silt, or organic matter, plants sensitive to DDT will be harmed the most. On loamy and claylike soils they will be injured a little less, and on cer- tain muck soils little or no injury is expected to most crops when treated with less than 400 pounds per acre. Many factors besides texture doubtless are involved in these differences among soils. Bush squash, a highly sensitive crop, was harmed little on muck at 400 pounds, but on mineral soils was in- jured at 100 pounds. Beans, also highly sensitive on mineral soil were un- harmed by 400 pounds on muck. As yet there is no adequate evidence regarding the effect of soil conditions on the plant toxicity of residues of organic insecticides other than DDT. It is logical to suppose, however, that mineral colloids and organic matter may well affect plant response to resi- dues of some other compounds some- what as they have to DDT and arsenic. As YET THERE HAS BEEN no good evidence that a plant will absorb DDT from the soil and translocate it into its edible parts. Numerous analyses of various parts of plants from DDT- treated soil have always failed to show DDT within the plant. There is ample evidence, however, that BHC in the soil does contaminate those edible parts of plants that de- velop below the soil surface. Some po- tato growers have suffered heavy loss — and some consumers and merchants have also suffered loss — because the 291 BHC that was added to potato fields to control insects imparted a disagree- able flavor and odor to the potatoes. This bad odor and flavor of BHC per- sists in the soil for a considerable time, certainly for more than a year, but for how long is not known. Some fields erroneously treated with BHC may be unfit for the growing of potatoes or edible root crops for several years. There have been a few instances of spoiling the flavor and odor of peanuts either by treating the soil or dusting the young plants in such a way that the BHC directly reached the soil in which the pods developed. Chemical analysis has revealed that when BHC is put in the fertilizer for peanuts it may enter the seeds in measurable amounts. Chlordane also has been found as a contaminant of root crops harvested from soil treated with chlordane to control soil-inhabiting insects. Amounts of some insecticides in the soil too small to harm plant growth may make some products grown therein unfit for food or feed. This whole question of absorption of insecticides from the soil by plants is of major practical importance to consumers, to the food industry, to the manufacturers of insecticides, and to research and regulatory agencies alike. Although a persistent insecti- cidal residue in the soil may show no immediately harmful direct effect on crop plants, it might possibly affect some of the soil bacteria, fungi, or other micro-organisms either for good or ill. The few microbiological studies that have been made with such insecticides as DDT, BHC, chlordane, and toxa- phene show that the soil micro-organ- isms tolerate these chemicals better than crop plants do. DDT up to about 250 pounds per acre had no significant effect on numbers of organisms, or the power of organisms to produce nitrates or ammonia in the soil. BHC, on the other hand, kills off certain soil fungi and nitrifying bac- teria for some months at 100 to 500 292 pounds per acre. Twenty pounds of BHC had no marked effect on nitri- fying or ammonifying bacteria. Chlordane is somewhat fungicidal in large doses in the soil and depressed nitrate formation at 100 to 500 pounds, but at 20 pounds had no clearly signifi- cant effect. Chlordane produced as great effects after the treated soil had been stored in the laboratory for a year as it did soon after treatment. Toxaphene is unusual, among the chlorinated hydrocarbon insecticides studied so far, in that it appears to be attacked by soil micro-organisms and used as a source of food by them. From all that is now known, DDT is an unusually stable organic com- pound when in the soil. Of large amounts added to certain soils only about 5 percent per year is decom- posed. Reference has been made to its phenomenal persistence in the control of Japanese beetle grubs. In the soil its toxicity to plants appears to be just as persistent as its insecticidal qualities, or more so. And, unhappily, it also ap- pears that some of its probable decom- position products are also persistent and toxic to plants. How long those de- composition products might persist in the soil, no one knows. Some small plots treated with different amounts of DDT in 1945 appeared in 1951 to be prac- tically as toxic as at first. It is hardly conceivable that the soil will show no decrease in toxicity in the foreseeable future, but there is not now any basis for estimating how long it will take for the toxicity of 100, 200, or 400 pounds per acre to disappear. Soil analyses in orchards sprayed with DDT, using organic chlorine con- tent as an index, have shown that the amounts of DDT accumulated in the surface soil beneath the trees roughly approximate the total number of pounds per acre used during the years it has been applied to those trees. Ex- perience has shown that this accumu- lation will interfere with the growth Yearbook of Agriculture 1952 of rye cover crops in as little as 4 to 5 years under a heavy spray schedule. Pound for pound, BHC is more toxic to more crops than is DDT. For- tunately, however, evidence is develop- ing that it is definitely less persistent than DDT. Large dosages in the soil decomposed at the rate of about 10 percent or more per year. Mild injury from BHC in the field in the season it was added to the soil has sometimes failed to recur. This may be due, in part, to further dilution of the BHC by mixing it with a larger volume of soil as the field is plowed and fitted for another year's crops. Chemical analysis of soils years after a large amount of BHC has been applied, or after it has been applied an- nually for years, has detected substan- tially lower proportions of the total amount supplied than has been found with DDT. Furthermore, some small carefully controlled plots that received massive doses, 100 and 200 pounds per acre in 1946, were showing marked loss of toxicity by 1950. After 3 years both the 100- and 200-pound plots were still toxic to beans, a moderately sensitive crop. By 1950 beans grew equally well on both the treatments and on the controls, indicating a sub- stantial disappearance of the BHC. When planted to very sensitive corn, however, immediately after the less sensitive beans were removed, it be- came clear that a relatively large pro- portion of the 200-pound treatment still persisted. The corn in the 100- pound plots was nearly equal to the control, but in the 200-pound plots, growth stopped at about 8 inches in height, and the plants were badly yel- lowed. BHC, therefore, does disappear slowly from the soil but much work will need to be done to determine how much can be dissipated or destroyed per year in different kinds of soil in different climates. Under the condi- tions of the experiment cited, it ap- pears that as much as 15 to 20 pounds of the material toxic to plant growth might become dissipated under some Residues, Soils, and Plants conditions but that is only a rough indication. Information on the persistence of chlordane is still meager. Its insecti- cidal persistence is much less than that of DDT. Of heavy dosages in soil about 15 to 20 percent per year disappears, while large percentages of small dos- ages appear to be lost. While its toxicity to plants is almost certainly less per- sistent than DDT, it should perhaps be considered as having potentialities for developing accumulations in the soil where it is used heavily and repeatedly. Toxaphene also is much less stable than DDT. The meager data available suggest that large dosages disappear at about the same rate as chlordane. Resi- dues reaching the soil following foliage applications, however, appear to ac- cumulate faster than chlordane and BHC and slower than DDT. Aldrin and dieldrin appear inter- mediate in their persistence and tend- ency to accumulate, but because of the small dosages used will accumulate only slowly. Following foliage applica- tions, aldrin accumulates about like chlordane, and dieldrin more rapidly. The new phosphorus compounds such as parathion are known to be highly unstable and thus are no cause for concern as potential harmful resi- dues in the soil. The insecticides manufactured from plants, such as derris and pyrethrum, are natural plant products that are presumed to decompose readily in the soil. There is no evidence that they either do or do not contribute any harmful residue to the soil, but it is hardly conceivable that they should, considering their origin and make-up. Fumigants such as D-D mixture, methyl bromide, chloropicrin, and oth- ers are all highly toxic to plants. It is necessary to delay planting for several days after treatment to allow time for the toxic vapors to diffuse out of the soil. Since, however, these substances are rapidly and completely vaporized there is no fear of a residue problem with them. Some fumigants temporar- 293 ily kill off certain beneficial bacteria, fungi, and other soil life, or at least up- set micro-life temporarily. There is no evidence, as yet, that their repeated use leads to any accumulative persistent undesirable effects. Bearing in mind the relative toxic- ity to plants and the apparent persist- ence of various insecticides in the soil, rate of use must be considered in order to estimate the probability that any one substance will lead to trouble under a given set of conditions. Attempting to predict, with our present small knowl- edge, is admittedly hazardous. We be- lieve, however, that failure to be guided by the best estimates we can make may be even more hazardous. Most of the so-called field crops that are sprayed or dusted with DDT receive relatively small amounts in any one year, only 1 or 2 up to 4 or 5 pounds per acre per year. Ordinarily the field crops grown on a given tract over a period of years do not all require treatment so that DDT is not applied to such fields every year. Supposing that in a 20-year interval half the crops required treatment and that amounts near the maximum customary rate of use were applied, the maximum total application would probably be about 50 pounds. Even if all this reached the soil and no loss occurred in 20 years — hardly probable — that amount would represent no important hazard to any field crop we have tested. On some field crops, larger amounts of DDT are used : Up to 8 or 9 pounds per acre for European corn borer on corn, aphids on peas, and leafhoppers on beets, and 10 to 12 pounds on cot- ton. It is probable that in one of the more intensive general farming sys- tems involving several such crops over a 20-year period the total DDT used would be closer to 100 pounds than to 50 pounds. This 100 pounds may well represent a borderline level for injury of moderately sensitive crops but not for tolerant ones. Before DDT has been so used for 20 years we will know much 294 more about its limitations than we know now and will be able to guard against excessive use on our principal farm crops. Our cause for immediate concern about accumulations of harmful resi- dues of DDT in the soil is not its use at i to 5 pounds per acre on staple farm crops in some years, but its heavy use every year on the same tracts of land planted to orchards or to truck or other special crops. As with arsenicals, DDT goes into the soils of orchards much more heavily than into soils growing other crops. As much as 50 to 60 or more pounds per acre per year is used in some orchards. All evidence points to early trouble in the growing of sus- ceptible cover crops such as Abruzzi rye and some legumes in such orchards. Although most grasses are tolerant, and there have been few, if any, instances of suspected injury to sod covers, ac- cumulations that will harm orchard sods should not be considered impos- sible. Furthermore, a given tract does not remain in orchard indefinitely. What use can be made of it after an orchard is removed following 10 or 20 years' treatment? Heavy repeated dos- ages are used on sweet corn and some other truck crops in some districts with the probability of developing enough residue in the soil to harm sensitive crops. The potato crop receives up to about 20 pounds of DDT per acre per year in some districts, more often about 6 to 10 pounds. The potato is highly tol- erant to DDT, but many of the crops grown in rotation with it are not. In intensive potato districts, especially in which DDT is used on crops in rota- tion with potatoes, there is the possibil- ity of adverse effects on some crops within 10 years. For most truck crops commonly treated with DDT only 2 to 5 pounds per acre per crop is recommended. In the milder parts of the country two and even three truck crops requiring insect control with DDT may be grown annu- ally, raising the not improbable use of DDT to 6 to 15 pounds per acre or Yearbook of Agriculture 1952 more annually. We must remember, too, that many farmers tend to use in- secticides, fungicides, fertilizers, and seed, at rates much higher than neces- sary, "just for good measure." Misuse must be avoided. Despite its great value and its firm place for many purposes, some current uses of DDT seem to have real poten- tialities for impairing the usefulness of the soils on which it is being used heavily. Furthermore, there is no as- surance as to how rapidly a toxic level of DDT in the soil will decrease to a harmless level. Indications are that it will be very slow. Because the disagreeable flavor and odor of BHC are readily imparted to food products it is unsafe to apply it to the tops of food plants after the above-ground edible parts have de- veloped appreciably; and it should never be put in the soil before planting any crop, the edible part of which de- velops below the soil surface. This characteristic of BHC has sharply lim- ited its use in the growing of food crops. BHC is used rather heavily, early in the season, for controlling certain fruit insects such as plum curculio- — 3 to 6 pounds of the gamma isomer or equiva- lent per acre in a mature orchard. In the form of lindane this represents but 3 to 6 pounds; but in the form of the less expensive technical BHC it repre- sents 25 to 50 pounds. About 40 to 50 pounds per acre of technical BHC are used very extensively to control cotton insects and 5 to 10 pounds per acre are extensively used in the soil to control soil-borne pests of various grain and other farm crops. From the rates of disappearance of BHC from the soils that we have observed it seems probable that soil applications of 5 to 10 pounds of tech- nical grade (or 1 to 2 pounds of lin- dane) at intervals of 1 or 2 years will rarely if ever develop residues that im- pair the growth or yield of crops. These small amounts, however, will probably contaminate foods that de- velop below the soil surface. Residues, Soils, and Plants As much as 50 pounds of BHC per acre added directly to the soil year after year will almost certainly build up an amount in 5 years or less that will be definitely harmful to several important crops. BHC is somewhat volatile, however, loses its plant and insect toxicity in the soil much less slowly than DDT, and is believed to "weather away" and decompose ap- preciably after it is applied to the foli- age of plants. It has, therefore, ap- peared unlikely that as large a pro- portion of the amount applied will actually reach the soil as occurs with DDT. Experiments only 2 years in progress (1951) tend to confirm this view. While BHC actually accumulates in the soil following heavy applica- tions to the foliage of crops, such accumulation appears to be substan- tially less than with DDT with which it is being compared. At heavy rates, however, it accumu- lates, and it is toxic to plants. Chlordane is rarely used at rates in excess of 10 pounds per acre per crop, generally at 6 pounds or less. Ten- and 20-pound applications di- rectly to the soil have shown consider- able loss of insecticidal value after 1 year. Chemical analysis of plots so treated indicated that only 4 to 5 pounds of chlordane remained a year after application. Thus, although chlordane does seem to be only moder- ately persistent, some of a normal ap- plication persists more than a year, in- dicating that if used repeatedly at in- tervals of about a year a slow accumu- lation of a residue may be expected. These statements on persistence are admittedly based on meagre evidence and may have to be modified later. Nevertheless, chlordane seems to pre- sent a definitely less immediate and potentially serious problem than does DDT. As indicated on preceding pages, toxaphene, and parathion and other phosphorus compounds appear to in- volve no hazard through the accumu- lating of residues in the soil. 295 Methoxychlor, TDE, aldrin, diel- drin, heptachlor, and other new syn- thetic insecticides have appeared so re- cently that too little has been learned about their plant toxicity and persist- ence in the soil to permit specific state- ments about them as this is written (1952). Early results, however, indi- cate that methoxyclor, TDE, and DDT are rather similar in persistence and that DDT is more toxic than the first two. Chlordane, aldrin, and dieldrin appear persistent but somewhat less so than DDT. Since it has been shown that some of these remarkably efficient and eco- nomical insecticides may and do ac- cumulate to an undesirable degree in the soil under certain conditions, we may confidently look to the research chemist and to the chemical manufac- turer for still newer compounds that will have the advantages of those now in use but without the disadvantages of too great stability and toxicity to plants in the soil. Since a few good insecti- cides are already known that appear now to present no soil-residue problem, others doubtless will be produced. It is true that those now known to be relatively unstable and less toxic to plants may not be as effective or eco- nomical for controlling certain farm pests as some of the more persistent ones are. There is no reason, however, to suppose that it will always be so. Highly effective nonaccumulative in- secticides will surely be developed for use where desirable. Their further de- velopment, production, and use to re- place the too persistent ones should be urged. Victor R. Boswell is head of the division of vegetable crops and dis- eases at the Plant Industry Station at Beltsville, Md. He grew up on a small farm in southwest Missouri and studied horticulture at the Univer- sity of Missouri and the University of Maryland. After 6 years of teaching and research experience in Maryland he entered the Bureau of Plant Indus- try of the Department of Agriculture 296 in 1928, with responsibility for the vegetable crop investigations in that Bureau, which responsibility he still holds. He became assistant head of the division of fruit and vegetable crops and diseases in 1941. Dr. Boswell has written many publications on a wide range of problems relating to the pro- duction, growth, development, yield, and quality of vegetable crops. He was assigned to the War Department in I945~4^ as a member of the National Resources Section of SCAP in Tokyo where he was responsible for the pro- duction branch in the agriculture divi- sion. He was president of the American Society for Horticultural Science in 1939- For further reference: J. d' A guitar and P. Grison: Premieres etudes sur le probleme des taupins en Bretagne, Comptes Rendus Hebdomadaires des Seances de I'Academie d' Agriculture de France, volume 34, pages 261-267. 1948. W. B. Albert: Arsenic Toxicity in Soils, Forty-sixth Annual Report of the South Carolina Agricultural Experiment Station, pages 44-45- ^933- Earle C. Blodgett: A Systemic Arsenic Toxicity of Peach and Apricot on Old Apple Land, Plant Disease Reporter, volume 25, pages 549S51- 194'- M. L. Bonnemaison: Essais preliminaires de traitements contre les taupins, Comptes Rendus Hebdomadaires des Seance de I'Academie d' Agriculture de France, volume 33, pages 556-559- J947- B. A. Bourne: Effects of Benzene Hexa- chloride and Chlordane on the Germina- tion of Sugarcane Cuttings, Sugar Journal, volume 10, number 8, pages 3—4, 20. 1948. T. A. Brindley, R. Schopp, and F. G. Hinman: Effect of Initial High Dosages of DDT on Yields of Peas and Wheat, Journal of Economic Entomology, volume 43, pages 565-567- 1950. J. W. Brooks and L. D. Anderson: Tox- icity Tests of Some New Insecticides, Jour- nal of Economic Entomology, volume 40, pages 220-228. 1947. R. K. Chapman and T. C. Allen: Stimu- lation and Suppression of Some Vegetable Plants by DDT, Journal of Economic En- tomology, volume 41 , pages 616—623. 1948. Karl Chulski: The Effect of Benzene- Hexachloride on Some Crops Grown on Various Soil Types, Michigan Agricultural Experiment Station Quarterly Bulletin, vol- ume 31 , pages 170—177. 1948. H. P. Cooper, W. R. Paden, E. E. Hall, and others: Effect of Calcium Arsenate on Yearbook of Agriculture 1952 the Productivity of Certain Soil Types, Forty-fourth Annual Report of the South Carolina Agricultural Experiment Station, pages 28-36, 1 93 1 ; Soils Differ Markedly in Their Response to Additions of Calcium Arsenate, Forty-fifth Annual Report of the South Carolina Agricultural Experiment Station, pages 23-28, 1932. W . E. Fleming: Chlordan for Control of Japanese Beetle Larvae, Journal of Eco- nomic Entomology, volume 41 , pages 905- 912, 1948; Effect on Plants of DDT Ap- plied to Soil for the Destruction of Japanese Beetle Larvae, Bureau of Entomology and Plant Quarantine publication E—737, 1948; Persistence of Effect of DDT on Japanese Beetle Larvae in New Jersey Soils, Journal of Economic Entomology, volume 43, pages 87-89, 1950; Effect of Lead Arsenate in Soil on Vegetables, with F. E. Baker and L. Koblitsky, Journal of Economic Ento- mology, volume 36, pages 231—233, 1943. Arthur C. Foster: Some Plant Responses / to Certain Insecticides in the Soil, U. S. D. A. Circular 862. 1951. M. C. Goldsworthy, in Plant Disease Re- porter, volume 32: Effect of Soil Applica- tions of Various Chlorinated Hydrocarbons on the Top Growth of Blakemore Straw- berry Plants, pages 186-188; Effect of Technical DDT, Incorporated in Quartz Sand and Soils, on the Growth of Pear Trees, pages 437-441 ; The Effect of Incor- porating Technical DDT in Soil on the Growth of Blakemore Strawberry Plants, with J. C. Dunegan, pages 139-143; Effect of Soil Applications of "Parathion" on the Top Growth of Blakemore Strawberry Plants, with R. A. Wilson, pages 388-390. J. M. Grayson and F. W. Poos: Southern Corn Rootworm as a Pest of Peanuts, Jour- nal of Economic Entomology, volume 40, pages 251-256. 1947. Wm. P. Headden: Arsenical Poisoning of Fruit Trees, Colorado Agricultural Experi- ment Station Bulletin 131. 1908. B. Hocking: On the Effect of Crude Ben- zene Hexachloride on Cereal Seedlings, Scientific Agriculture, volume 30, pages 183-193- *950- H . R. Jameson, F. J. D. Thomas, and R. C. Woodward: The Practical Control of Wireworm by y-Benzene Hexachloride ('Gammexane') : Comparisons with Di- chlorodiphenyltrichlorethane (D. D. T.), Annals of Applied Biology, volume 34, pages 346-356. J947- J. S. Jones and M. B. Hatch, in Soil Science: The Significance of Inorganic Spray Residue Accumulations in Orchard Soils, volume 44, pages 37-62, 1937; Spray Residues and Assimilation of Arsenic and Lead, volume 60, pages 277-288. 1945. L. W. Jones: Are Insecticides Toxic to Soil Microorganisms? Farm and Home Science (Utah Agricultural Experiment Station), volume 11, pages 58-59. 1950. W. M. Kulash, in Journal of Economic Entomology: Soil Treatment for Wire- worms and Cutworms, volume 40, pages 851-854, 1947 ; Further Tests With Soil In- secticides to Control Southern Corn Root- worm, volume 42, pages 558-559, 1949. M. C. Lane, M. W. Stone, H. P. Lan- chester, E. W. Jones, and K. E. Gibson: Studies with DDT as a Control for Wire- worms in Irrigated Lands — Progress Re- port, Bureau of Entomology and Plant Quarantine publication E-765. 1948. W.S.McLeod: Effect of Hexachlorocyclo- hexane on Onion Seedlings, Journal of Eco- nomic Entomology {scientific note), volume 39, page 815. 1946. H. E. Morrison, H. H. Crowell, S. E. Crumb, Jr., and R. W. Lauderdale: The Effects of Certain New Soil Insecticides on Plants, Journal of Economic Entomology, volume 41, pages 374-378. 1948. B. B. Pepper, C. A. Wilson, and J. C. Campbell: Benzene Hexachloride and Other Compounds for Control of Wireworms In- fecting Potatoes, Journal of Economic En- tomology, volume 40, pages 727—730. 1947. P. T. Riherd: DDT and Benzene Hexa- chloride to Control Southern Corn Root- worm, Journal of Economic Entomology, volume 42, pages 992-993- 1949- K. Sakimura: Residual Toxicity of Hexa- chlorocyclohexane Incorporated in Soil, Journal of Economic Entomology, volume 41, pages 665-666. 1948. M. S. Smith: Persistence of D. D. T. and Benzene Hexachloride in Soils, Annals of Applied Biology, volume 35, pages 494—504. Nathan R. Smith and Marie E. Wenzel: Soil Microorganisms Are Affected by Some of the New Insecticides, Soil Science So- ciety of America Proceedings, volume 12, pages 227-233. 1947. L. L. Stitt and James Evanson: Photo- toxicity and Off-quality of Vegetables Grown in Soil Treated With Insecticides, Journal of Economic Entomology, volume 42, pages 614-617. 1949. Rosemary I. Stoker: The Phytotoxicity of D D. T. and Benzene Hexachloride, An- nals of Applied Biology, volume 35, pages 110-122. 1948. C. L. Vincent: Problems in Vegetable and Small Fruit Production on Toxic Or- chard Soils of Central Washington, Amer- ican Society for Horticultural Science, Pro- ceedings, volume 37, pages 680-684. l939- J. K. Wilson and R. S. Choudri: Effects of DDT on Certain Microbiological Proc- esses in the Soil, Journal of Economic En- tomology, volume 39, pages 537-538, 1946; The Effect of Benzene Hexachloride on Soil Organisms, Journal of Agricultural Re- search, volume 77, pages 25-32. 1948. H. C. Young and J. B. Gill: Soil Treat- ments with DDT to Control the White- Fringed Beetle, Bureau of Entomology and Plant Quarantine publication E-750. 1948. Residues on Fruits and Vegetables B.A.Porter, J. E.Fahey How to use a chemical to control in- sects on fruits and vegetables without harming the person who eats them re- mains a serious problem. It concerns the chemists who de- velop insecticides, the growers who use them, food officials of State and Fed- eral agencies, and home gardeners, who might not always treat the poisons with the respect due them. Many scientists, who realize that worms and insect debgis in fruits and vegetables would lower their value to grower and consumer, have done a tremendous amount of research on ways to keep the foods free of dangerous contami- nation with insecticides. It seemed that the problem was settled in 1880, when in reporting the first official tests of arsenicals, the in- vestigator, A. J. Cook, of Michigan, took into account the possible effect of the insecticide on the consumer. Mate- rials then available were paris green and london purple. The results of the analyses were taken to mean that there was no danger that injurious quantities of poison could reach the consumer when those insecticides were used. Further reassurance was given 1 1 years later by another official worker, C. P. Gillette, of the Iowa Agricultural Ex- periment Station. He also studied the matter carefully and announced that a person would have to eat at one sit- ting 30 cabbages that had been dusted with paris green to get enough poison to hurt him. Evidently he assumed that the insecticide would be evenly dis- tributed and the loose leaves trimmed off. In those early days, spraying and dusting were light compared with later practice, and the insecticides in use had poor sticking qualities. The conclusions 297 298 of the early investigators were prob- ably correct at the time. But the problem since has become intensified. Our cropping areas have become more concentrated. Insect pests have become more abundant and hard to control. Spray and dust pro- grams have included more and heavier applications. Fears increased that with the growing use of insecticides the fruits and vegetables on the consum- er's table might have excessive residues on them. Men in the Department of Agriculture therefore, in surveys in 1915 to 1919, analyzed hundreds of samples of peaches, cherries, plums, apples, pears, grapes, cranberries, to- matoes, celery, and cucumbers for lead, arsenic and copper. The investigators concluded that only little spray residue remained on fruit or vegetables that had been sprayed according to stand- ard recommendations. They reported, however, that excessive residues re- mained on fruits or vegetables that had been oversprayed or sprayed too close to harvesttime. Another complication has arisen since then. At first the possible immedi- ate effect of spray residues on consum- ers was chiefly considered. Less thought was given to the possible cumulative effect of taking in extremely small quantities of poison day after day. Such effects are hard to detect and easy to confuse with other conditions. Cases of sickness developing immedi- ately after the poisons were taken in have been rare, if they have occurred at all. But since the 1920's the belief has grown that the gradual accumula- tion of poisons in the system might have unfavorable effects. The problems of spray residue on vegetable crops have been met in sev- eral ways. Many insects that attack vegetable crops, such as the Mexican bean beetle and several kinds of cab- bage caterpillars, can be controlled by the use of pyrethrum or materials that contain rotenone. Residues of both are considered unobjectionable. Both are also effective against most of the in- sects that attack small fruits like cur- Yearbook of Agriculture 1952 rants, raspberries, and cranberries. Commercial practices often eliminate residues of insecticides on vegetable crops. For instance, the part of the cabbage plant that has been exposed to the insecticide is often trimmed off in preparing the cabbages for the mar- ket. The use of insecticides can be lim- ited to the early stages of the growth of the plant when the part to be eaten has not yet formed. Many of the insects that affect tree fruits can be controlled without caus- ing excessive spray residue. The plum curculio, which also feeds extensively on peach and apple, lays most of its eggs and causes the most damage in most localities in late spring or early summer. Insecticides can be used freely then, because only the smallest traces will remain on the fruit at picking time. Many insects attack tree fruits throughout the growing season, how- ever. The most important of these has been the codling moth on apples and pears. Since this problem received ma- jor attention from the early 1920's un- til the early 1940's, we review its his- tory during that period in some detail. The codling moth is often called the appleworm. Many have had the ex- perience of biting into an apple con- taining a worm or into an apple in which a worm had lived. Wormy apples rot quickly in storage or during shipment. Uncontrolled, the worms can ruin 50 to 90 percent of the crop. A crop that is even 50-percent wormy has little commercial value, since the cost of handling and sorting it is often more than the value of the part that can be salvaged. Such apples are fit only for immediate local use, or for low-grade byproducts. Lead arsenate was the standard in- secticide against the codling moth from early in the present century until 1945. As the worms became more and more abundant and hard to control, the number of spray applications, the strength of the spray mixture, and the number of gallons applied per tree steadily increased. The amount of lead and arsenic on the fruit at harvesttime Residues on Fruits and Vegetables also increased steadily. Western pears in 19 1 9 were condemned by the Bos- ton Board of Health because of exces- sive residues of arsenic. A few years later British health authorities ob- jected to shipments of American apples for the same reason. By 1925 it had become evident that the use of lead arsenate sprays had increased to the point where American apples and pears were carrying quantities of resi- dues that were at least potentially dan- gerous to public health. As soon as the serious nature of the problem became clear, the Department of Agriculture moved to carry out its responsibility for the enforcement of the Food and Drug Act. An admin- istrative tolerance for arsenic (as of As203) of 0.025 grain per pound of fruit (about 3.5 parts per million) was established, following a conference of health authorities. Efforts were made to reduce the residues below this figure, and by 1932 the amount of arsenic permitted on apples and pears under the administrative tolerances had been progressively reduced to 0.0 1 grain per pound of fruit (about 1.4 parts per million) . During the 1920's attention was focused entirely on the arsenic portion of lead arsenate, on the assumption that if the arsenic were reduced to safe quantities, the lead would also be eliminated as a hazard. It was found later that the lead did not always weather off as rapidly as the arsenic and that it was not always so com- pletely removed by washing. Since the early 1930's residues of lead arsenate have therefore been judged largely by their lead content. Although both lead and arsenic have long been known to be serious poisons, precise information was lacking on the quantities of those materials that could be taken into the human body in the form of spray residues without harm. To shed light on this problem, the United States Public Health Service carried on an investigation from 1937 to 1940 around Wenatchee, Wash., a leading apple-growing section, where 299 during the 1930's as much as 7 million pounds of lead arsenate was used an- nually. A careful study was made of 1,231 persons, many of whom worked or lived in or close to apple orchards and were thus extensively exposed to lead arsenate, both in their diet and their surroundings. The following is quoted from the report of the studies : "Only six men and one woman had a combination of clinical and laboratory findings directly referable to the absorption of lead arsenate. Some physicians may interpret these cases as minimal lead arsenate intoxication. However, as regards lead, these cases do not come up to the criteria of the Committee on Lead Poisoning of the American Public Health Association for lead intoxication, incipient plumb- ism or lead poisoning. These subjects were all or^hardists and ranged in age from 23 to 68 years." Although such a study is less exact than experimentation on guinea pigs, mice, or other laboratory animals, it gives an approximate indication of what actually happens to human beings exposed regularly to lead arsenate. Many of the persons examined had undoubtedly taken much greater quan- tities of lead arsenate than the general consuming public. On the basis of the results of the study, the administrative tolerances for apples and pears were increased to 0.025 grain of arsenic (as arsenic trioxide) per pound and 0.05 grain of lead per pound of fruit (about 3.5 and 7 parts per million, respec- tively) . That eased the situation greatly from the standpoint of the growers. It was then possible for most growers in the East and Midwest to market apples without washing them and for the growers in the Northwest to clean their fruit satisfactorily with mild washing programs. During the 1920's and ever since, vigorous efforts have been made to solve the problem of spray residues on apples and pears. These efforts have taken two directions: The develop- ment of methods and equipment for removing the excessive residue after 300 the fruit was harvested, and the de- velopment of effective insecticides or other methods of control less objec- tionable from the standpoint of res- idues. Effective washing methods and ma- chinery were promptly developed for removing most of the residues before the fruit is marketed. In the washing process the apples are passed through dilute hydrochloric acid and then through a spray of clean water to rinse off the acid and the poison. In extreme cases a double wash is used ; the apples are passed through a dilute alkaline solution, rinsed, passed through a di- lute acid, and then rinsed again. To aid in removal, the wash solutions are sometimes heated. The development of such equipment permitted the safe marketing of the apples, but the cost of installing, maintaining, and operat- ing the machinery came at a bad time. Prices during the 1930's were abnor- mally low, and many growers were having difficulty in meeting their ob- ligations. Washing the fruit added one more item of production cost. It was a real hardship to many growers, and some of them even lost their orchards. The ultimate solution of the problem evidently should be sought in another direction. The old-time practice of trapping the worms in bands placed around the trunks of the trees was revived and improved by a chemical treatment of the bands. Traps baited with fermenting sugar solutions with the addition of attractive chemicals were developed. The possibilities in light traps, orchard clean-up, and parasites were explored. Many of those practices had control value, but they did not reduce infestations to a point where the spray program could be materially shortened. The first efforts to develop new in- secticides produced few practical re- sults. With all its shortcomings, lead arsenate was a good insecticide, and finding a better one proved to be a hard job. During the 1930's several new materials nearly met the needs, but not quite. Nicotine bentonite and other Yearbook of Agriculture 1952 nicotine mixtures were used effectively in a number of midwestern orchards. Cryolite, a compound containing fluo- rine, proved to be about equal to lead arsenate in the Northwest, but ineffec- tive or undependable elsewhere and objectionable from the standpoint of residues. DDT, the first of the new, complex organic compounds, since 1945 has largely replaced lead arsenate in the control of codling moth. Less DDT is needed and in many areas fewer appli- cations are necessary. As a result, resi- dues from DDT programs are much less than the former residues from lead arsenate. But residues of DDT are hard to remove. If they are found to be too great, the problem will have to be met by adjustments in the spray program or by the substitution of less objection- able spray materials during the latter part of the growing season. The codling moth is only one of many insect pests that the grower must control. Now that the codling moth has been reduced to a comparatively minor status, other pests have assumed greater importance. Some, such as the orchard mites or spider mites, have become more abundant and destructive follow- ing the use of DDT. To meet this situ- ation the development of additional new pesticides is being pushed by De- partment of Agriculture and State workers and insecticide companies, who have had to conduct extensive research in several fields. The work of the chemists and entomologists in de- veloping, testing, and evaluating in- secticides is only the beginning. A chemical that shows promise must be evaluated as to residues, particularly whether the residues are likely to be a potential hazard if the material is used commercially. The pharmacologist must carry on experiments with the new product on laboratory animals. The determination of the dosages that are fatal or that cause obvious distress is the simplest part of the job. The po- tential danger of taking in tiny quan- tities of an insecticide day after day must also be determined. Any effect on Residues on Fruits and Vegetables consumers of spray residues would come about by repeated consumption of small quantities. As indicated earlier, the quantities found on marketed fruits and vegetables are rarely, if ever, great enough to have any immediate effect. To secure information on cumulative effects, experiments must extend over many months, as much as 2 years. The investigations have created some problems for the chemists. The amounts to be measured are so small that they are expressed as parts per million. During the interval between the last insecticide application and har- vest the residue may be reduced in three ways — by crop growth, by phys- ical weathering, and by chemical breakdown of the insecticide — so that the residues at harvest, expressed in parts per million, are lower than they were immediately after application of the spray or dust. To determine such tiny quantities of insecticides, the chemist must de- velop highly sensitive methods of analysis. Such metods are rarely avail- able for use with the new insecticides, and the development of the required highly accurate methods is often dif- ficult. Many of the new insecticides are chlorinated hydrocarbons. If it is defi- nitely known what compound is pres- ent, all that has to be done is to deter- mine the amount of organic chlorine in it and then compute from that figure the amount of the original compound that is present. For example, DDT contains 50 percent of chlorine. If DDT is known to be the only insecti- cide that has been used, the amount of chlorine present is determined. The resulting figure is then multiplied by 2, which gives the amount of DDT pres- ent. The problem is rarely that simple, however. Sometimes the analysis of untreated fruits or vegetables shows considerable amounts of natural or- ganic chlorine that must be allowed for. Sometimes two chlorinated hydro- carbon insecticides are used on the same fruit or vegetable. It is then im- 070134°— 52 21 301 possible to compute the amounts of each from the organic chlorine deter- mination, and special methods have to be worked out to measure the differ- ent compounds separately. Some of the chemical determinations are made by comparing the color of a solution containing poison dissolved from the sprayed fruit with the color of a solu- tion containing a known quantity of the chemical. Some fruits or vege- tables contain substances that produce colors similar to those being measured. These natural color changes interfere with the accuracy of the analysis, and must be eliminated or allowed for. The solution of these difficulties requires great ingenuity on the part of the chemist. Although the establishment of tol- erances as a result of hearings con- ducted by the Federal Security Agency in 1950 may have a stabilizing influ- ence, the situation will never be a static one. Insect-control problems are changing continually. New insecticides will continue to appear and create new problems. The residue factor will always be an important consideration. We are confident that steady progress will be made, and the public will be insured of a safe, adequate, and con- tinuous supply of fruits and vegetables of high quality. B. A. Porter is in charge of the divi- sion of fruit insect investigations in the Bureau of Entomology and Plant Quarantine. He joined the Depart- ment in 191J, and for many years con- ducted field investigations of various orchard-insect problems in Connecti- cut and Indiana. He is a graduate of Massachusetts Agricultural College. J. E. Fahey is a chemist in the Bureau of Entomology and Plant Quarantine. Since 1934 he has been in charge of the chemical work at the Department's fruit insect laboratory at Vincennes, Ind. He has made special studies of spray residues on fruits and other agricultural products. He was graduated from Oregon State College in 1928. State Pesticide Laws Allen B. Lemmon Ordinarily the user of a pest-control material cannot himself investigate or test the effectiveness of the product. He must rely on the manufacturer's warranties. Various State laws give him — as well as the manufacturers and the public — a measure of protection. The first pesticide law was adopted by New York State in 1898 to regulate the sale of paris green, then the most important insecticide. Similar laws were adopted the following year by Oregon and Texas, and in 1901 by California, Louisiana, and Washing- ton. In 191 o, the Federal Insecticide Act was enacted. It was supplanted in 1947 by the Federal Insecticide, Fungi- cide, and Rodenticide Act, which pro- vides coverage of additional types of pest-control materials. The scope of the State pesticide laws was similarly increased as the impor- tance of new materials has been real- ized. Before the Second World War, many of the State pesticide laws fol- lowed the general pattern set by the Federal Insecticide Act of 19 10. Some of them pertained only to certain types of pest-control chemicals. With the de- velopment and widespread use of DDT and other synthetic organic pesticides, the need for control of the labeling and sale of these economically important and potentially injurious materials has been reflected by the enactment and amendment of State laws. The scope of many laws has now been extended to include any substance intended to be used for preventing, destroying, repelling, or controlling any insects, fungi, bacteria, weeds, rodents, preda- tory animals, or any other form of plant or animal life which is a pest. It would serve no useful purpose to 302 tabulate the details of individual State laws now in effect because this type of legislation is particularly active, and amendments anticipated in many States would soon render the informa- tion obsolete. The table gives a sum- mary of the general characteristics of State pesticide laws and the names and addresses of the agencies enforcing them at the end of 195 1 . A uniform State act has been adopted by 19 States. One of these dropped the registration procedure and several have omitted the provision with regard to registration under protest. Eight States have no economic poisons law. Two others have such limited cov- erage as to be in the class with those that have none. This leaves 19 States with various laws, some of which are more inclusive than the uniform act, and others that will probably be brought up to date as legislators have time. Requirements of the individual laws and information with regard to their administration may be had by writing to the agency concerned. At the time the Federal law was modernized it was realized that corre- sponding action should be taken to modernize the different State pest-con- trol laws. At the request of the National Association of Commissioners, Secre- taries, and Directors of Agriculture, the Council of State Governments devel- oped a proposed State insecticide, fun- gicide, and rodenticide act. This was drafted for the convenience of States that might wish to consider legislation to protect the public against mis- branded or adulterated pesticides and to establish uniform State and Federal requirements for the marketing of these materials. When State laws and regulations covering insecticides and other eco- nomic poisons vary greatly from State to State, it is difficult for a manufac- turer to prepare a label that will meet both Federal and State requirements. If he has national distribution, several different labels may be required for the same product, and the possibility al- ways exists that improperly labeled ma- State Pesticide Laws terial will be sent into a State that re- quires a different type of label. This places a heavy burden on the manufac- turers who operate in more than one State. The result is an increase in the costs of doing business and ultimately in higher prices paid by farmers and other users. The uniform State insecticide, fun- gicide, and rodenticide act provides a basis for uniform action by the States. As used in the act the term "economic poison" means any substance or mix- ture of substances intended for pre- venting, destroying, repelling, or miti- gating any insects, rodents, fungi, weeds, or other forms of plant or ani- mal life or viruses (except viruses on or in living man or other animals) that the commissioner declares to be a pest. To avoid the implication that all eco- nomic poisons are highly toxic to hu- man beings, or that the scope of the law is restricted to poisons, as that term is commonly used, it is becoming cus- tomary to refer to pest-control chemi- cals as pesticides. An important provision of the law is with regard to registration of pesticides before they are offered for sale in a State. Registration serves as a screen to prevent ineffective, fraudulent, or dangerous economic poisons from be- ing marketed in the State. It helps en- forcement and permits correction of unsatisfactory or illegal labeling before a product enters trade. Some State laws provide that regis- tration of any pesticide may be refused or canceled after a hearing, if the product is of little or no value for the purpose for which it is intended or is detrimental to vegetation (except weeds), domestic animals, or public health and safety when properly used. Similar action may be taken if false or misleading statements concerning the product are made or implied by the firm or its agent, orally or in writing or in advertising. In the States where laws do not pro- vide for refusal of registration and there is disagreement between the ap- plicant and the enforcement agency 30.3 with regard to the acceptability of a product, the applicant may demand "registration under protest." This is a controversial provision in the uniform State insecticide, fungicide, and ro- denticide act and many, believing that an official should not be required to register a questionable product under protest, prefer a hearing procedure whereby the facts can be determined and registration absolutely refused for a product that is worthless or too haz- ardous to use. The registrant is pro- tected against misjudgment or arbi- trary action of the administration in that he can bring court action if he be- lieves the official's actions are in vio- lation of law. Registration of new pesticides is a difficult administrative problem. New products are constantly being de- veloped, involving new chemicals, new combinations of chemicals, or new uses for chemicals. (In California, for ex- ample, the number of pesticides reg- istered for sale has doubled every 10 years, and approximately 10,000 prod- ucts are registered now.) Before an economic poison can be accepted for registration, adequate data must be available to demonstrate its effective- ness for the purpose intended, and that the proposed handling and use do not present any intolerable hazard. The necessary data depend somewhat upon the particular type of product involved. Although not all the items are perti- nent to a specific product, the type of information that may be needed to establish the eligibility of the product for general sale is suggested by the following outline: Chemical and physical: 1. Chemical name. 2. Chemical formula. 3. Chemical structure. 4. Melting point. 5. Boiling point. 6. Vapor pressures at various tem- peratures. 7. Solubilities in various solvents. 8. Odor. 9. Density. (This is important for 3°4 some liquid products to compare dos- ages by volume and by weight.) 10. Corrosive action on metals. 1 1 . Flammability. 12. Stability (hydrolysis, oxidation, sunlight, explosion hazard) . 13. Compatibility with other eco- nomic poisons. 14. Suitable diluents. 15. Purities, grades, or mixtures to be available commercially. Proposed usage: 1 . Name or names of the pest, pests, or type of pest for which the product affords control. 2. Name or names of the plants, crops, animals, or places to which prod- uct is to be applied. 3. Dilution recommended. (For ex- ample, "Use without dilution." "Use 1 gallon with 99 gallons water to make 100 gallons of spray.") 4. Preparation for use. (For ex- ample, "Fill the tank one-quarter full of water. Start agitator running. Slowly add the emulsion and then fill the tank." "Maintain agitation while using.") 5. Method of application. (For ex- ample, "Apply as a spray, using par- ticular care to wet thoroughly the lower side of the leaves." "Dust plants thor- oughly to touch as many of the insects as possible.") 6. Rate of application. (For exam- ple, "Apply 15 gallons per tree." "Use 5 pounds per thousand square feet of lawn." "Apply 25 pounds per acre." "Apply 200 pounds per acre and disk into the soil.") 7. Time of application. (For exam- ple, "Apply when buds begin to swell in spring." "Apply when jackets are falling from the fruits." "Apply when insects first appear." "Apply before plants start to head." "Do not apply during blooming period." 8. Frequency of application. (For example, "Dust plants thoroughly at 3 -week intervals during growing sea- son." "Do not spray oftener than twice a year.") Effectiveness: Experimental data available to dem- Yearbook of Agriculture 1952 onstrate the effectiveness and suit- ability of the product for the intended usage. This must include pests treated on specific crops under climatic and soil conditions similar to that of State where registration is requested. Hazards and cautions: 1. The primary hazards to human beings who handle the compound, the particular parts of the body affected, the symptoms of poisoning, and their duration. 2. The acute toxicity to the particu- lar species of animals on which it has been determined by inhalation, inges- tion, skin absorption. 3. The chronic toxicity to the par- ticular species of animals on which it has been determined by inhalation, in- gestion, skin absorption. 4. Information on first aid or medi- cal treatment of injured persons or animals. 5. Toxicity or harmfulness to valu- able plants or animals on which it might be used. (a) Are certain plants sensitive? (For example, cantaloups and apricots are sensitive to dusting sulfur. White clover in lawns may be injured by 2,4-D. Beans may be injured by arsen- icals.) (b) Are certain animals sensitive? (For example, cats may be injured by coal-tar dips or some of the chlorinated hydrocarbons. Calves may be injured by oil sprays. Caged birds may be in- jured by dusts.) (c) Is it injurious to plants under certain conditions? (For example, pe- troleum-oil sprays may injure plants if applied when plants are abnormally dry or when temperatures are above 90° F. Coal-tar products may be suit- able for application to dormant de- ciduous trees but they are injurious to foliage.) 6. Other possible hazards. (a) Does it leave a stain or un- sightly residue where these might be objectionable? (For example, aerosols may stain walls or furnishings if the applicator is held too close to the sur- face. Bordeaux mixture, lime-sulfur, State Pesticide Laws ferric dimethyl dithiocarbamate, and some other products may leave objec- tionable residues on ornamental plants, flowers, or fruits. Oil sprays may blem- ish table grapes or plums.) (b) Does it impart an obnoxious taste to prepared foods, food crops, or to meat animals, as benzene hexa- chloride insecticides or coal-tar disin- fectants do? (c) Does it injure asphalt-tile floors as kerosene-base household sprays do? (d) Does it present a hazard to honey bees? (e) Is it particularly injurious to cats, fish, or caged birds? (f) Does it persist in the soil and injure crops subsequently planted? (g) Is it absorbed by dairy cattle and excreted in the milk? (h) Does it corrode or otherwise in- jure spray equipment? (i) Is it absorbed in treated food- stuffs, as parathion is absorbed in citrus peel and in mature olives? (j) Are precautions necessary in disposal of empty containers to avoid possible injury? (k) Are any special precautions necessary in cleaning spraying or dust- ing equipment? Analytical methods: i . Analytical methods available for : ( a ) The technical material. (b) Commercial products contain- ing it with other ingredients. (c) Spray and dust residues or other minute amounts on foodstuffs or other contaminated material. 2. If analytical methods are not available, how is the quality of the manufactured product controlled? Spray residue: If it is to be applied to foodstuffs, how may residues be removed? A troublesome problem to all con- cerned has been that of fees charged for registration. In many States it is the general policy of the Government to charge fees against the industries regulated in sufficient amount to carry the cost of enforcement work. In other States the costs of operation are 3°5 charged against the general taxes and no special fees are collected. In 1951, 32 States had economic-poisons regis- tration fees of one type or another. Six- teen States had no fee of any type. The fees vary from a nominal amount of $2 an item in New Mexico to those in Florida which has a license fee of $1 25 with an additional registration fee of $2.50 an item. If a manufacturer were to register 12 different economic poisons in all States requiring fees (in- cluding Hawaii) , his total cost for reg- istration fees would be approximately $2,000. Manufacturers have pointed out that in many cases the fee is excessive in re- lation to the amount of business done and acts as a handicap to a manufac- turer doing a small business in several States. On the other hand, the amounts colfected in some States are so small that adequate enforcement work is not possible. It is generally agreed that the money collected should be expended for enforcement work, and if fees are collected with no enforcement work, proper protection is not afforded. Another step toward simplification would be more uniformity as to the registration period. If all renewals were required at one time, for example January 1 or July 1 , instead of differ- ent months throughout the year for different States, the manufacturer could take care of all his registration applications at one time, with con- siderable savings in his handling of registration. Most laws are in agreement that pesticides should be sold only in the unbroken original package of a regis- trant. In some cases sales out of open packages have been permitted under State laws, but to prevent adulteration and to fix responsibility, the uniform act requires products to be sold only in the original package of a registrant. It is hazardous to permit highly toxic materials to be sold in broken pack- ages, and possibilities of misuse or ac- cidents are greatly increased when a material is handled in bulk without proper labeling. 3°6 Yearbook of Agriculture 1952 <1> O 5f> •I5 >- y o S ; s J2 .53 o M o-r ca 3 -a r3 pq _; "O ■-_; M £ a a Co o Si fl a co iJU I si. ° J «' « ° e V V -G fl AS ^ t/5 CO - <-> o u iO 10 a - a Q u 3^ z •£« S 3 £_, 5 "C « ■« bo£ Pr1 «5 .g" a -s a £ .y s» U S U c CO CB to S to "U 3^ £< £ S i! 3 •- a v v P.05 1- c cb •.- fro. 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'_^ '~ : V Sh ■- 0 rt - £ o O .23 E"S Is 11 2 o C Oi pq-a 4J 3 m "3 <— 1 bo ii 3 ■M > Q *i a a v e li c - = c • - 5 c w'x c 2 « - C (N ~ T3 O •2 S .a o E^ > 1 , o h ^ ^ ^ ^ S State Pesticide Laws Careful attention has been given to the definition of the term "ingredient statement." There are two options for an ingredient statement on the label of pesticides as set forth in the uniform act. Option i, which is preferred, con- sists of a statement of the name and percentage of each active ingredient, together with the total percentage of inert ingredients in the economic poi- son. Option 2 is a statement of the name of each active ingredient to- gether with the name of each inert ingredient and the total percentage of inert ingredients, if there are any in the economic poison. The act provides that Option i shall apply if the prepa- ration is highly toxic to man and, fur- thermore, in case an economic poison contains arsenic in any form, there must be shown a statement of the per- centage of total arsenic expressed as metallic and water-soluble arsenic ex- pressed as metallic. This requirement is the same as that in the Federal act. Because many pesticides can cause serious injury if improperly handled, precautions or appropriate informa- tion concerning the particular hazards is a necessary part of adequate labeling. Sometimes the existence or the sever- ity of a hazard is not discovered until accidents occur after a product has been marketed. The complete toxico- logical properties of all pesticides are not available, and not infrequently labeling of products requires revision in light of newly developed informa- tion. As individual States attempt to get adequate precautionary labeling of hazardous materials, there is need for uniform action to avoid conflicting requirements and multiplication of the manufacturer's problems. Several man- ufacturers' organizations have realized that development of proper labeling of hazardous chemicals is a part of good business procedure. They have found that attacking the problem themselves does much to forestall the need for corrective action by State administra- tions, therefore avoiding troublesome conflicts that arise when each State at- tempts to solve the problem separately. 309 The Manufacturing Chemists' Associa- tion has studied the problem and issued a guide for the preparation of warning labels. The problem of precautionary label- ing of pesticides is further complicated by the fact that, in many States, some poisonous or caustic pesticides come under the jurisdiction of a pharmacy act as well as a pesticide act. In other States the labeling of hazardous pesti- cides is left solely to one administra- tion. Some laws require protective color- ing of certain poisonous materials. In general, the requirements pertain only to calcium arsenate, lead arsenate, and sodium fluoride, but additional chem- icals may be included. The need for such legislation, and the consequent problem of uniform action by different States, is avoided when manufacturers on their own initiative color the chemi- cals that might be confused with food- stuffs. One of the purposes of pesticide laws is to assure delivery of materials that conform to the guaranteed anal- ysis shown on the label. This requires sampling products offered for sale in the State and performing the neces- sary examinations and chemical and physical analyses. The sampling should be on a scale large enough to provide an adequate analysis of the materials used and to make certain that they comply with the law. Publication of the official findings is a benefit of a well-functioning law. In the administration of laws cover- ing such a highly technical and rapidly expanding field as economic poisons, there should be free exchange of in- formation among regulatory officials, Federal and State. An Association of Economic Poisons Control Officials has been formed. Its executive committee meets regularly with representatives of the Federal office to consider matters of policy, particularly with regard to proper labeling requirements. Such co- operation, recognized by provisions in both the uniform State act and the Federal Insecticide, Fungicide, and Rodenticide Act of 1947, has been beneficial to users of pest-control mate- rials as well as to manufacturers. Many things, such as information with regard to the toxicity of a particular chemical, can best be developed by a Federal agency and then distributed to the State officials. California has a law with regard to prohibiting the sale of fresh or dried fruits or vegetables carrying deleteri- ous spray residue. Although other States did not have such a specific law, the same control may be exercised through the different State food and drug acts. All in all^ economic poisons prob- ably are as closely regulated as any other class of materials generally sold. Besides Federal registration before a product can be shipped in interstate commerce, State laws require regis- tration in the individual States. In the enforcement of these laws, official samples are drawn from mate- rials offered for sale, and analyses are made to determine if the products cor- respond to guarantee. Sale of a de- ficient material is a violation which is usually considered a misdemeanor. Also, publishing the analyses of official samples serves as a strong deterrent to sale of adulterated materials. Effective enforcement by regulatory offices assures purchasers of properly labeled pest-control materials that con- form to the guarantee stated on the label by the manufacturer. Allen B. Lemmon is chief of the Bureau of Chemistry, California De- partment of Agriculture. The Bureau administers laws relating to the label- ing and sale in California of agricul- tural chemicals, including economic poisons. Mr. Lemmon received a bach- elor's degree in engineering from Stan- ford University in 1930 and a degree of engineer in 1932. He joined the California Department of Agriculture in 1933> as an inspector of economic poisons and fertilizers, and advanced to chief of the Bureau of Chemistry in 1946. 310 The Federal Act of 1947 W. G. Reed The Federal Insecticide, Fungicide, and Rodenticide Act regulates the marketing in interstate commerce of economic poisons — insecticides, fungi- cides, rodenticides, and herbicides. Products intended for trapping, de- stroying, repelling, or mitigating in- sects or rodents, or destroying, repel- ling, or mitigating fungi, are also sub- ject to the act. The law was enacted June 25, 1947, and became fully effective June 25, 1948. It replaced and expanded the protection afforded by the Insecticide Act of 19 10. The old act regulated only insecticides and fungicides and became effective at a time when such products were comparatively simple, consisting mainly of paris green, pyrethrum, bordeaux mixtures, and like goods. The purpose of the 1947 Act is to protect the public in the use of eco- nomic poisons, many of which are dangerous and all of which are subject to limitations in application. Products subject to the Act must be registered with the United States Department of Agriculture prior to introduction in interstate commerce. Provision is made for testing products as they are en- countered in the regular channels of trade to determine whether they are in compliance with the law. Those that are found to be in violation may be seized by the Government to remove them from the channels of trade. The manufacturer may be prosecuted if such action is warranted. To furnish necessary information to the users of economic poisons and guide them in using such materials safely and effectively, the Act provides that every product must have attached to it a label showing: The Federal Act of ig4j 1. The name and address of the manufacturer or person for whom manufactured. 2. The name, brand, or trade-mark under which the article is sold. 3. The net contents. 4. An ingredient declaration. 5. An appropriate warning or cau- tion statement, when necessary, to pre- vent injury to man, animals, vegeta- tion, and useful invertebrate animals. In addition, labels on highly toxic products must contain the word POI- SON in red, the skull and crossbones, and a statement of an antidote. Ade- quate directions for use must accom- pany each product. It is unlawful to market in inter- state commerce any economic poison that has not been registered with the Department or one that is misbranded or adulterated. A product is adulter- ated if its strength or purity falls below the professed standard or quality as ex- pressed on its labeling or under which it is sold, or if any substance has been substituted wholly or in part for the ar- ticle, or if any valuable constituent of the article has been wholly or in part abstracted. An economic poison or de- vice is misbranded if its labeling bears any statement, design, or graphic rep- resentation relative thereto or to its in- gredients which is false or misleading in any particular. An economic poison is misbranded : 1 . If it is an imitation of or is offered for sale under the name of another eco- nomic poison. 2. If its labeling bears any reference to registration under the Act. 3. If the labeling accompanying it does not contain directions for use that are necessary and, if complied with, adequate for the protection of the public. 4. If the label does not contain a warning or caution statement which may be necessary and, if complied with, adequate to prevent injury to living man and other vertebrate animals, veg- etation, and useful invertebrate ani- mals. 5. If the label does not bear an in- 311 grcdient statement on that part of the immediate container and on the out- side container or wrapper, if there is one, through which the ingredient statement on the immediate container cannot be clearly read, of the retail package which is presented or dis- played under customary conditions of purchase. (The Secretary of Agricul- ture may permit the ingredient state- ment to appear prominently on some other part of the container, if the size or form of the container makes it im- practicable to place it on the part of the retail package which is presented or displayed under customary conditions of purchase.) 6. If any word, statement, or other information required by or under au- thority of this Act to appear on the label or labeling is not prominently placed thereon with such conspicuous- ness (as compared with other words, statements, designs, or graphic matter in the labeling) and in such terms as to render it likely to be read and under- stood by the ordinary individual under customary conditions of purchase and use. 7. If in the case of an insecticide, fungicide, or herbicide when used as directed or in accordance with com- monly recognized practice it shall be injurious to living man or other verte- brate animals, or vegetation, except weeds, to which it is applied, or to the person applying such economic poison. One of the most important provi- sions of the law is the one that requires insecticides and other economic poi- sons to be registered with the United States Department of Agriculture prior to introduction into interstate com- merce. This gives the public additional protection and assists manufacturers and distributors in complying with other provisions of the law. To obtain registration, an application, together with information concerning the com- position and proposed labeling, must be submitted to the Department. The material is carefully scrutinized by specially trained scientific personnel in the various fields involved, and if the 312 product, its labeling, and other mate- rial appear to be in compliance with the law, a registration notice is issued. Occasionally proposed labeling con- tains recommendations for uses about which there is little or no published information. In such instances the ap- plicant is requested, if he has not already done so, to furnish a full de- scription of the tests that have been made with the product and the results thereof upon which the claims are based. That information is reviewed; if it appears to warrant the claims and the product and its labeling are other- wise in compliance with the law, the product is registered. If it does not appear that the article is such as to warrant the proposed claims, or if it and the labeling or other material re- quired to be submitted do not comply with the law, the applicant is notified of the deficiencies and given an oppor- tunity to make necessary corrections. If he then insists, in writing, that such corrections are not necessary and re- quests that the article be registered, the law requires that it be registered under protest. In general, manufacturers and distributors have cooperated in making whatever changes in formulations or labeling the Department has consid- ered necessary. Also, because of the higher penalties that are imposed un- der the law if a person or firm is found guilty in court of marketing a mis- branded or adulterated product that has been registered under protest, few requests for this type of registration have been received. Before a new economic poison is offered for registration and general distribution, usually experiments by qualified people have been carried on to determine how it can be used safely and effectively. Frequently laboratory and field tests are conducted over a period of several years before enough information can be obtained to prepare adequate directions for use and appro- priate precautionary labeling. During this period of research and investiga- tion, materials may be transported in Yearbook of Agriculture 1952 interstate commerce without legal re- strictions if they are intended solely for experimental use by or under the super- vision of a Federal or State agency authorized by law to conduct research in the field of economic poisons or by others if a permit from the Depart- ment has been obtained before ship- ment. To obtain an experimental permit, the shipper or person making the de- livery is required to submit a signed application showing: The name and address of the shipper and place or places from which the shipment or shipments will be made; the proposed date of shipment or proposed shipping period, not to exceed i year; the iden- tification of material to be covered by the permit; the approximate quantity to be shipped and types of tests that will be made; whether the product is to be sold or delivered without cost; that the economic poison is intended for experimental use only ; and the pro- posed labeling, which, besides other statements, must state that the product is for experimental use only. The Department of Agriculture carries on investigations continually throughout the country to determine whether economic poisons that are be- ing marketed in interstate commerce are properly registered and otherwise in compliance with the law. Investiga- tors visit retail stores, warehouses, and other places to investigate shipments and collect samples. The samples are selected from unopened packages and proof of interstate shipment is ob- tained. Each sample is sealed by the investigator and sent to one of the Department's laboratories. Usually it is first analyzed chemically to learn if the composition and net contents are as declared on the label. In addition, other types of laboratory tests and field trials are frequently necessary to see whether the product will perform satis- factorily when the label directions are followed. For instance, an agricultural insecticide or fungicide may be tested in an orchard or on growing crops to The Federal Act of 194J see if it is effective for the purposes in- tended and not injurious to the vegeta- tion to which it is applied. A weed killer may be tested in the greenhouse or in a field to find out whether it will give satisfactory control under actual conditions of use. Products recom- mended for use in controlling livestock pests are tested on the species of ani- mals they are intended to protect, and rodenticides are tested on rodents. Sometimes an insecticide is an effec- tive bug killer but it will burn or shrivel the vegetation to which it is applied or give it an off-flavor. Effects such as these cannot be detected by chemical analysis and are apparent only after field trials have been con- ducted. If, as a result of its investigations, the Department finds a sample to be in violation of the law, action may be taken leading to prosecution of the manufacturer or shipper. Federal court action can be taken to seize the mate- rial in question to remove it from the market. If prosecution is contemplated, the law provides that before such ac- tion is taken notice must be given to the person against whom legal action is contemplated and such person given an opportunity to present his views re- garding the alleged violation. If the facts appear to warrant prosecution, they are certified to the proper United States Attorney. The law also provides that an eco- nomic poison subject to it may be seized if it is adulterated or mis- branded ; if it has not been registered in accordance with the provisions of the Act; if it fails to bear on its label the information required by the Act; or if it is a white powder that is not colored or discolored as required by the Act. The owner may, of course, contest the seizure in Federal court before a judge and jury. The Government must prove that the goods are in violation to sustain the seizure and maintain the control of the goods. If the Govern- ment fails to do that, the goods are returned to the owner. If an article is condemned under the seizure provi- 313 sion, it must be disposed of by destruc- tion or sale as the court may direct, but it cannot be sold contrary to the pro- visions of the law. However, upon pay- ment of the costs of the libel proceed- ings and the execution and delivery of a good and sufficient bond conditioned that the article shall not be sold or otherwise disposed of contrary to the provisions of any laws having jurisdic- tion over it, the court may direct that such article be delivered to the owner. The owner may then relabel or other- wise treat the article to bring it into compliance with the law, after which it may be released by the court and the bond cancelled. Persons or firms found guilty of mar- keting an economic poison without registration, or of making claims dif- ferent in substance from those made at time of registration, upon conviction may be fined not more than $1,000. That penalty applies if the composi- tion of the product differs from that represented in connection with regis- tration. With respect to products reg- istered under protest, in each instance, upon conviction for an offense con- cerning which a registrant had been warned, the person may be fined not to exceed $1,000 or imprisoned for not more than 1 year, or both such fine and imprisonment. Persons guilty of violating other pro- visions of the law may be fined up to $500 for the first offense, and on con- viction for each subsequent offense, may be fined up to $1,000 or impris- oned for not more than 1 year, or both such fine and imprisonment. Economic poisons imported into the United States are subject to the same requirements, including registration, as those produced in this country. Sam- ples of imports are examined. If they appear to be in violation of the law or are otherwise dangerous to public health or are of a kind forbidden entry into or forbidden to be sold or restricted in sale in the country in which they are made or from which exported, they may be refused admission into the United States. Manufacturers and distributors of economic poisons in the past have found it difficult to prepare labeling that would comply with the various State and Federal laws under which they must operate. The present Fed- eral law recognizes the difficulty and authorizes administrative officials to cooperate with State regulatory agen- cies in carrying out the provisions of the law and in securing uniformity of regulations. Full advantage is taken of this authorization both through co- operative arrangements for enforce- ment work and by conferences with representative groups of State officials. The Federal Insecticide, Fungicide, and Rodenticide Act is a relatively new law and the benefits that have resulted from it cannot yet be fully evaluated. However, it can be stated that farmers and other users of economic poisons and the general public are now being given better protection than ever before against worthless, dangerous, and in- adequately labeled economic poisons. W. G. Reed has been engaged in regulatory work on insecticides and other economic poisons since IQ45, when he was made chief of the insec- ticide division, which is now in the livestock branch of the Production and Marketing Administration of the De- partment. This division administers the Federal Insecticide, Fungicide, and Rodenticide Act. Dr. Reed, a native of Iowa, first joi?ied the Department in the Meat Inspection Service in ig2g. He holds a degree of Doctor of Veter- inary Medicine from the Chicago Vet- erinary College. Insecticides and the Pure Food Law Readers may be interested in the chapter "The Insecticide Industry," page 450, in which the author discusses the work undertaken by manufacturers of insecticides to insure the safe use of their products and the interest of the industry in supporting State and Fed- eral legislation to regulate the distribu- tion and use of insecticides, fungicides, and related products. 314 P. B. Dunbar In this paper I discuss the provisions of the Federal Food, Drug, and Cos- metic Act of June 25, 1938, as they concern insecticidal residues, and re- cent activities under these provisions. First, a few fundamentals : ( 1 ) Con- gress, in passing the law, recognized that the use of insecticides is necessary, both to bring many agricultural food crops to maturity in a condition suit- able for human consumption and to protect many foods against insect dep- redations during manufacturing oper- ations and storage. (2) By and large, insecticides are poisons, their toxicity varying only in degree. ( 3 ) The terms of the law do not preclude the use of insecticides, but they make provisions which guarantee that when they are used the health of consumers eating foods so treated shall be protected. The House Committee on Interstate and Foreign Commerce of the 75th Congress, in a report on the bill which became the law in 1938, made the fol- lowing comment on section 406, which relates specifically to insecticidal resi- dues: "This subsection first prohibits the unnecessary addition of poisons. Where such additions are necessary, the estab- lishment of tolerances is authorized, based upon the practical necessities for the use of poisonous substances. It is well recognized that an adequate fruit and vegetable supply could not be brought to maturity without the use of toxic insecticides and fungicides. But the situation is made extremely com- plex by the number of poisonous sub- stances used for different crops in dif- ferent localities, and by contaminations which unavoidably occur in many manufacturing processes. The purpose Insecticides and the Pure Food Law 3i5 of the subsection is to insure that the total amount of poisons the consumer receives will not be sufficient to jeop- ardize health. The needs of each branch of the food-producing industry can be met and the public health can be adequately protected." The law attacks the problem of pro- tecting the public against poisons in foods by defining any food as adulter- ated if it contains a poisonous or de- leterious substance that may render it injurious to health, or if it contains any such substance that is not required in the production of the food or that can be avoided by good manufacturing practice, or (where it is so required or cannot be so avoided ) if it exceeds tol- erances prescribed by the Federal Se- curity Administrator after public hear- ing. In prescribing a tolerance, the law directs the Administrator to take into account the extent to which the poison- ous or deleterious substance is required or cannot be avoided in the production of each food and the other ways in which the consumer may be affected by the same or other deleterious sub- stances. In any event, the Administra- tor is enjoined by the statute to pre- scribe the tolerances at such levels that the public health will be protected. This law has been on the statute books for more than a decade. Un- doubtedly numerous tolerances would have been established long before this had it not been for the intervention of the Second World War and the result- ant preoccupation of all concerned with other urgent matters. During the war period and immediately after- wards, many new and potent insecti- cides were developed. Scientists knew little about their toxicity, either to the person who applied the sprays or to the consumer who ate the finished food product. In some instances accurate methods for the estimation of the re- sidual spray left on or absorbed by the food product were lacking. It was not known whether the residues remained intact, whether they were altered by weathering to nontoxic or more toxic residues, whether they could be re- moved by washing, or whether they were absorbed into the plant structures and therefore could not be removed. It is a commentary on the changing attitude of the times that with the mul- tiplication of new spray substances manufacturing groups, growers' organ- izations, entomologists, plant patholo- gists, and physicians, as well as con- sumer groups, began to recognize that it was high time to attack the residue problem and the question of safe toler- ances in a fundamental fashion. The Bureau of Entomology and Plant Quarantine and other bureaus of the Department of Agriculture, the Inter- departmental Committee on Pest Con- trol, organized groups in the insecti- cide-manufacturing industry, scientific workers in entomology and plant in- dustry, food manufacturers using the raw materials of agriculture, and the growers themselves have shown a con- structive interest in reaching some kind of a sound conclusion on the subject of spray-residue tolerances. And so, in January 1950, it seemed that everyone was ready to begin the hearings on statutory tolerances. The hearings, called by the Federal Security Administrator, were in session, with occasional recesses, from January 17 to September 15, 1950. Those who at- tended them were impressed with the spirit of cooperation and good will manifested throughout the sessions. The hearings were limited to the tolerances on fresh fruits and vege- tables. Testimony on the necessity for using any particular insecticide or fungicide on any particular fruit or vegetable came first. Next, the ques- tion of which pesticides are poisonous or deleterious in themselves was thor- oughly explored. Subsequent sessions dealt with "the amounts of these sub- stances which are poisonous or delete- rious, which are received from all sources by consumers," and with "the toxicity of the substances for which limits are to be established." The final session was devoted to relevant evi- dence not previously covered. Evidence was also taken at that time on amend- 316 ing the fluorine tolerance which had been duly promulgated in 1944 and soon after nullified on a legal tech- nicality. The record consisted of 9,000 pages of testimony (by 255 witnesses) and nearly 1,300 exhibits. It encompassed the investigations of scientific workers in many fields, the considered opinions of medical and toxicological minds, and factual information bearing on the aggregate intake by the consuming public of agricultural and other poi- sons. Many have repeated the opinion that the value and completeness of the record is unequaled anywhere. An editorial in "Agricultural Chem- icals" for April 1950 said: "Assimilation of this material may take a long time, but we have a feeling that eventually it will be the means of correcting much of the confusion which has existed in the field for several years. The establishment of tolerances for pesticides old and new will set univer- sally-recognized standards to guide fu- ture planning. "It is doubtful that without the hear- ing, such a collection of data would ever have been assembled. It was a job too big for the industry itself to have undertaken; both from the standpoint of prohibitive cost and because of a lack of proper coordination. Also, the findings of an investigation conducted entirely by manufacturers could be re- garded as biased." Tolerances are sure to exert an enormous stabilizing effect. But it would be the worst sort of Pollyanna philosophy to relax into an attitude of complacency. Insects do not stand still ; in fact, some of them seem to meet the situation by breeding poison-resistant strains. Teams of entomologists and chemists, in and out of Government, are constantly tailoring new insecti- cides to measure. Among the other scientists, who are by no means idle, we must give credit to the toxicologists, food technologists, analytical chemists, and others who are all fighting the in- sect menace, without losing sight of Yearbook of Agriculture 1952 their primary obligation to safeguard the public health. What of the future? In our fight against an implacable enemy, perhaps we will learn how to kill with a rapier instead of a bludgeon. Perhaps, too, we can develop rapiers that will do their job on insects but will not affect man and his domestic animals. The syn- thesis of additional naturally occurring organic agricultural poisons is one of the hopeful approaches. After all, most of these natural products seem to be relatively benign in their potential threat to consumers, as well as to soils and crops. It is the men of vision who will win this battle. Too often in the past scien- tific thinking has had but a single ob- jective among the many aspects of the insect war. Such slogans as: "Kill all the insects," "Produce fruit without spot or blemish," "An enormous yield is everything," "We must do something quick; never mind the future," and the like, have all borne bitter fruit. There is no one answer to the "insect menace" any more than to the other problems of modern civilization. "Of nothing too much" was the Greek motto, and it's worth remembering now. P. B. Dunbar was Commissioner of Food and Drugs in the Food and Drug Administration of the Federal Security Agency until his retirement in May ig^i , after 44 years of Federal service. W&mM '-~^3#^: Firefly. Resistance to Insecticides Insects Are Harder To Kill B. A. Porter Some people have discovered that "insects are getting harder and harder to kill." They are partly right. A few of the hundreds of pests that farmers must control have developed resistance to insecticides. That means that an insect can sur- vive and thrive in the presence of a chemical that is supposed to kill it. It does not mean that every insect of the kind involved will survive the appli- cation of the insecticide. If a sizable proportion can survive the insecticide at a practical strength, the pest is said to be resistant. Sometimes insects from one source are harder to kill than those of the same kind from another source. The group harder to kill usually is re- ferred to as a resistant strain. The development of resistance by an insect can be explained simply in a general way, but the actual details of the process in a given kind of insect, with respect to a given insecticide, are complex and not understood fully. A few principles are basic. No two living creatures are exactly alike. Among people there are differences in color of hair and eyes, height, weight, health, and many other details. People vary also in the effect that diseases have on them. An epidemic of disease in a community may attack some per- sons seriously but not touch others. 070134' -52- -22 So it is with insects. A speck of an insecticide may kill one insect but leave another of the same kind unaffected. If the amount of the insecticide to which the insects are exposed is great enough, all will be killed, of course, but often the amount applied is in- sufficient to kill all the insects present. The least resistant are killed at once. The resistant ones survive. Apparently what has happened when insects have developed increased resistance is that the offspring of the resistant survivors have a similar de- gree of resistance. After that selective process has gone on for several gen- erations, most of the insects that are easily killed have been eliminated and only the resistant ones are left. More applications or greater quantities of the insecticides are needed then for adequate control. The point finally is reached where the particular insecti- cide becomes so ineffective that some other material or method of control must be developed. The men who have made a special study of the development of resistance do not agree entirely on the technical details of just what happens. Exact in- formation on the subject is essential from a scientific standpoint but is not needed for our understanding of the general subject as discussed in this article. The possibility of differences in the susceptibility of insects of the same kind to insecticides has been recog- nized for more than a half century. As early as 1897, John B. Smith, entomologist of the New Jersey Agri- cultural Experiment Station, men- tioned variations in results in the 317 3i8 control of the San Jose scale and other insects. He commented on "an out- standing difference in the amount of resistance to poisons, either external or internal." The possibility of growing resistance to insecticides was first pointed out in 1 9 14 by A. L. Melander, then profes- sor of entomology in Washington State College. His studies were made on the San Jose scale, which for some years apparently had been well con- trolled by spraying with a strong solu- tion of lime-sulfur. At the time, the San Jose scale was much harder to kill with lime-sulfur in the Clarkston area of Washington than it was in the Wenatchee and Yakima valleys or else- where. A similar situation developed in the early 1920's in southern Illinois, south- ern Indiana, northwestern Arkansas, and elsewhere in the Midwest. Lime- sulfur suddenly seemed to have little effect on the San Jose scale in many orchards although previously it had given good control. The insect killed the trees in several thousand acres of fine orchards despite careful and lib- eral applications of lime-sulfur. The work of Melander and others suggested that the San Jose scale might have developed resistance in some lo- calities, but their studies did not en- tirely rule out the possibility that dif- ferences in resistance were caused by seasonal or local conditions. The de- velopment of resistance among other insects was demonstrated within a few years, however. Perhaps the first clear-cut demon- stration of strains of insects differing in resistance and the possibility that their average resistance could increase was reported by H. J. Quayle, of the California Citrus Experiment Station at Riverside. In 19 16 he published an article, "Are Scales Becoming Resistant to Fumigation?" A standard method of controlling scale insects on citrus in California for many years was to place canvas tents over the trees and fumigate them with hydrocyanic acid. Quayle brought to Yearbook of Agriculture 1952 Riverside scale insects from Corona, Riverside County, Calif., where there had been serious difficulty in getting control by fumigation, and other scale insects from Orange County, where control was easy. Both lots of insects were fumigated under the same tent over an artificial tree. The results par- alleled those obtained at the places where the stocks of scale originated. Five percent of the scale insects from Corona survived, but fewer than 1 per- cent of those from Orange County sur- vived. Quayle and his associates studied the problem at least 25 years. Since 1930 the Department's citrus insect laboratory at Whittier, Calif., has also carried on studies of the re- sistance of the California red scale to hydrocyanic acid. Several strains of the scale insect have been reared in the laboratory through many generations and have been fumigated in various ways. Their resistance has been deter- mined from time to time. The results of the research can thus be summarized: Some strains of the California red scale differ greatly in their resistance to fumigation with hydrocyanic acid. The differences per- sist through many generations when the scales are reared under laboratory conditions. If the California red scale is subjected to repeated fumigation in the laboratory, the strain developed from the survivors is much more re- sistant than the original stock and requires many times the original dose of cyanide to give control equal to that obtained on the nonresistant strain. Parallel studies, but much less ex- tensive, have been made with the black scale and the citricola scale, species that seem also to have developed resistance to fumigation with hydrocyanic acid. One practical result in California citrus orchards has been a marked re- duction in use of fumigation against the California red scale and other scale insects. Growers have found it neces- sary to resort to spraying with oil, sometimes supplemented with ground derris or cube root, or with extracts from them. Insects Are Harder To Kill The next major insect pest known to have developed resistance was the codling moth, or appleworm, against which lead arsenate was the chief insecticide for more than 40 years. It has always been hard to control in many of the drier western regions and in some eastern localities where the growing season is long and hot. In other places in the East, control was not especially difficult in the early decades of the century, although even there the number of applications of lead arsenate needed for adequate worm control had steadily increased. The differences in the efforts required to control the worms generally were attributed to differences in climate and other conditions. W. S. Hough, of the Virginia Agri- cultural Experiment Station, was the first to attack the problem. He started in the late 1920's. He carried on his studies in an insectary — a screened shelter — where conditions were like those in the shade of an apple tree. He brought together strains of codling moth from areas that needed different degrees of spraying. Apples from un- sprayed trees were thoroughly sprayed with lead arsenate. Then he allowed newly hatched worms to try to chew their way in, as they do in the orchard. In the experiments Hough eliminated any differences due to locality, spray practices, or abundance of codling moth. Differences in the proportion of the worms that could get through the spray covering without being killed would indicate differences in resistance to the insecticide. Hough compared worms from Vir- ginia orchards, in which three or four sprays gave almost complete control, with worms from near Grand Junction, Colo., where the insect was notoriously hard to control. In the first season's tests, 31 to 39 percent of the Colorado worms got through the poison success- fully, but only 5 to 7 percent of the Virginia worms survived. Hough raised both strains of worms in the insectary through 14 or more generations and continued to find the same differences. 3*9 He later found that appleworms from Virginia orchards that had been regularly well sprayed with lead ar- senate entered sprayed fruit in much greater numbers than those from un- sprayed or poorly sprayed orchards. Strains from various Virginia orchards fed through successive generations in the insectary on sprayed fruit became more and more resistant to lead arse- nate and were able to enter sprayed fruit in increasing numbers. L. F. Steiner and associates at the Department's fruit insect laboratory at Vincennes, Ind., later made a similar study and reached similar general con- clusions. They found that codling moth stocks from different orchards in the Ohio Valley differed greatly in ability to enter sprayed fruit. The greatest re- sistance was in codling moth worms from an orchard that had been heavily sprayed with lead arsenate the preced- ing 5 years. Worms from a similar or- chard that had been unsprayed for 5 years were much more readily killed. It is not always easy to prove that an insect has developed resistance. Two or more strains of the insects have to be kept under the same conditions, al- though separated from each other in such a way that the strains cannot be- come mixed, before differences in the results with an insecticide can be said to be caused by differences in the in- sect itself. The control results in one year may be quite different from those obtained in another. Such differences do not necessarily mean resistance. They could as well be caused by sea- sonal or local factors. On the other hand, the development of resistance has sometimes gone un- recognized or has been minimized by skeptics who have felt that some other factor was responsible: Even after Hough's first results were published, some workers in other areas averred that the trouble was that Colorado growers did not know how to spray. Resistance is most likely to develop when all insects in a given situation are exposed to the insecticide and there is little reinfestation by insects not ex- posed. For instance, when a citrus grove is fumigated for the California red scale, the cyanide gas reaches all parts of the trees, and there is little movement of scales from untreated to the treated area. When an orchard is sprayed for the codling moth, an effort is made to spray every tree thoroughly. Repeated applications are made the same season. That means that almost all of the California red scales or codling moth worms in an orchard are exposed to the action of the insecti- cides. But with many of our common insects, only a small part of the total number present in an area may be ex- posed to insecticides. For example, only a small portion of the food plants attacked by the Japanese beetle in a given area are usually covered with an insecticide. Many home gardeners in cities do little spraying; shade trees on private property and many on public land rarely are well sprayed. Often they are not sprayed at all. The area surrounding the city usually has many beetles in wasteland or in other situa- tions where spraying is impractical or unprofitable. Up to 1952 there had been little or no indication that the Japanese beetle had built up resistance to DDT, the insecticide most com- monly used in its control. The development of resistance to standard insecticides has practical sig- nificance. Such would be the testimony of growers in Colorado and elsewhere who gave up trying to raise apples be- cause they could not get rid of the worms with lead arsenate. Such devel- opments have made it necessary for entomologists to develop new insecti- cides and alternate methods of control. B. A. Porter is in charge of the di- vision of fruit insect investigations in the Bureau of Entomology and Plant Quarantine. He joined the Depart- ment in igiy and for many years con- ducted field investigations of various orchard insect problems in Connecti- cut and Indiana. He was graduated from Massachusetts Agricultural Col- lege in igi4- 320 Insecticides and Flies W. N. Bruce Today spray applications that almost eradicated house flies on treated prem- ises a few years ago are not noticeably reducing the fly populations found in the field. Heavy and frequent treat- ments with DDT, methoxyclor, chlor- dane, dieldrin, and lindane have failed to give satisfactory control of certain field strains of flies. Resistance to insecticides was prob- ably first noticed by the farmer or the field observer as a failure in the con- trol of house flies, but it was not proved until several laboratory methods of de- termining the degrees of resistance were perfected. The methods are of two kinds — those that treat individual flies and those that treat large numbers of flies in one operation. The first is represented by the long-used microsyringe method of testing, which is used by the California Citrus Experiment Station and the Illinois Natural History Survey, and by the micro-loop method, which is used by the Department of Agriculture and the Public Health Service. The second is represented by the spray-chamber method, used by Department of Agri- culture workers, and by the residual panel test method, which is used quite extensively by the Public Health Service. The topical, or local, application with a microsyringe is a good labora- tory method of obtaining quantitative data on the amounts of insecticides needed to kill adult house flies. In- dividual females are selected from flies anesthetized with carbon dioxide and treated with acetone solutions of the insecticide. The actual treatment is ac- complished within a carbon dioxide anesthetizing chamber, in which a Insecticides and Flies 0.25-milliliter syringe is actuated by a micrometer caliper. A minute, meas- ured amount of insecticidal solution is applied to the prothorax of the fly. Treated flies are placed in clean paper containers and fed. The numbers of dead and live flies are recorded 24 hours later and the percentage of mor- tality is calculated. The micro-loop method also is valu- able in computing the amounts of in- secticides needed to kill adult house flies. A micro-loop is a very small loop made on the end of a piece of fine, noncorrosive wire. The loop is dipped into the insecticidal solution. The liquid retained in the loop is trans- ferred to an anesthetized fly. Treated flies are fed and retained for a 24-hour mortality count. We also can get the relative degrees of resistance of groups of house flies by using a spray chamber. The usual procedure is to place caged flies in the chamber, which we then fill with mist of the insecticidal solution by means of a small atomizer. We then transfer the flies to clean cages to be fed and retained for the 24-hour mortality count. A comparison between mortal- ity produced in cages containing flies of standard laboratory strains and mor- tality in cages containing flies of ques- tionable resistance reveals the relative degrees of resistance. By adjusting spray concentrations, we can measure very high or low degrees of fly resist- ance. From a practical standpoint, the rel- ative resistance can best be determined by exposing the groups to panels treated with the insecticide. Panel tests cannot be used to determine quantita- tively the amount of insecticide needed to kill flies but rather gives the prac- tical answer to the question of effective kill by surface treatments. Degrees of relative resistance are determined by varying the length of fly exposure to the treated panels and sometimes by varying the amount of insecticide on the panel. After flies are exposed to a treated panel, they are fed and held for the 24-hour mortality counts. 321 Often in comparing the effectiveness of insecticides we use the term "median lethal dosage (LD-50)" to express relative toxicity values. In studies on resistant flies, a median lethal dosage is the amount of insecticide in micro- grams per fly required to kill one-half of the sample of flies treated. LD-50 values of insecticides on flies are influenced significantly by the room temperature during and after treat- ment. In cool holding temperatures, flies are more easily killed by DDT-like compounds and less easily by chlordane or dieldrin. Flies used in computing the LD-50 values in tables 1 and 2 were retained at 80 °. Flies used for test re- sults shown in tables 3 and 4 were held at 60 °. The temperatures explain dif- ferences in the LD-50 values of the various insecticides in relation to stand- ard laboratory strains of flies. The first recognized occurrence of DDT-resistant flies was reported in 1947 by Giuseppe Sacca and A. Mis- siroli of Italy. The first widespread use of DDT was made by American occu- pation forces in Italy. Dr. R. Weis- mann, also in 1947, reported a strain of flies in Sweden that exhibited a significant amount of resistance to DDT. In 1948 an alarming amount of DDT resistance was discovered among flies infesting southern California and scattered places in the Southern, East- ern, and Central States. By the end of the 1949 growing season, resistance to DDT had become prevalent among flies in most parts of the United States. A survey conducted in 1949 by the Illi- nois Natural History Survey showed that 87 percent of the farms in Illinois were infested with DDT-resistant flies. A survey in 1950 revealed the presence of DDT resistance in all populations of wild flies that were tested. The surveys gave evidence that the wild susceptible strains were becoming resistant to DDT over a period of 2 or 3 years. The actual trends in development of DDT-resistant strains on two Illinois farms from 1945 to 1950 are shown in 322 table I. The 1950 levels of DDT re- sistance for the farms are significantly higher than the 1948 or 1949 levels, even in the absence of applications of DDT. Investigators in California ob- served the same phenomenon after the use of DDT for fly control had been discontinued for 2 years. Several investigators have attempted since 1947 to produce DDT-resistant flies by exposing successive generations of susceptible flies to DDT in the lab- oratory. Richard Fay and his associ- ates of the Public Health Service ex- posed adult flies in partially treated stock cages to produce a strain of a rather low order of resistance in 45 generations of adults. Starting in 1946, W. V. King and the staff of the labora- tory in Orlando, Fla., produced a strain of flies highly resistant to DDT by ex- posing 55 generations of adults to sprays of DDT solutions. At the Illinois Natural History Survey laboratory, George C. Decker and I got spectacu- lar results by exposing both larvae and adults to DDT. We contaminated the larval media and treated the adult stock cages with near-lethal dosages of DDT solutions. In that way we could select strains highly resistant to DDT in 9 to 18 generations from the stand- ard laboratory strain. We attempted to simulate field conditions in which barn surfaces and manure piles are treated with insecticides. As to the nature of the trend in the acquisition of DDT resistance by the standard laboratory strain when both larvae and adults were selected by Yearbook of Agriculture 1952 DDT treatment, it appears that the process of segregation or the initial es- tablishment of resistance to DDT is slow, but, when resistance is once es- tablished, its intensification is rapid and proceeds to a maximum level, which is reached when the DDT found in the environment no longer acts as a selec- tive agent. Research results that I reported in December 1949 revealed the devel- opment of strains of flies resistant to dieldrin, chlordane, lindane, toxa- phene, methoxychlor, pyrethrins, para- oxon, and a mixture of all the effective chlorinated hydrocarbons. Since that report, all of the resistant laboratory strains except the para-oxon and py- rethrins strains have reached a maxi- mum resistance point or have risen as high as is selectively possible by the method I used. A strain that showed a threefold increase in tolerance for diel- drin in November 1949 had risen to a 2,000-fold tolerance by July 1950. The dosages needed to kill individuals of these strains far exceed those which could be applied in the control of field populations of house flies. Three kinds of trends characterized increased tolerance or resistance in experiments in 1948 and 1949. The DDT type of acquisition trend, in which a susceptible fly strain slowly developed characters that permitted a more rapid selection in succeeding gen- erations, was characteristic of dieldrin, methoxychlor, and chlordane, as well as DDT. 1. LD-50 Value of DDT on Two Field Strains Farm Treatment on farm 1945 DDT 1946 DDT 1947 DDT 1948 DDT 1 949 Dieldrin 1 1 950 Dieldrin 3 1 Barns completely insulated and old DDT covered. 2 Much DDT still adhering to the walls, etc. 3 Dieldrin-resistant flies began to develop. 4 Lindane-resistant flies began to develop. Farm B ~~** Treatment "■ LD-50 on farm LD So O. 2 DDT 0. 18 •3 DDT •4 .8 DDT • 7 8. 1 DDT q. 0 4.0 Lindane 2 8.5 18.0 Lindane 4 42. 0 Insecticides and Flies 0> > 0) X, ■M O X M i> a, (SO 0 n u c o "3 0 u d w h-I u U u c -G ►H C <4H c T3 11 a c 0 lf> 1 u Q h-i Q u 3 '11 0 "**l -G Uh ^ U (N 1/5 3 O X rt <-l-l 0 a, 6 c U bo c * 0 X c/j V) C 0 •-* w n u Oh Ch < p^ re U c- 0 H *te. O e> oo co o ~ (N -Q -Q -a £ B iS _- « _ X _ JH Q S-g o ■4" -< A A A A 3 • a H -a .- n . H -5 ■§ 2 -S Q aj c JJ C x iS " °--c -C b c * f w r 2 5 ° s js V ^ « 4-» c ■— ■* CD 2 g _o o o •— 1 c .S > > 2«H 22 3 n £ S is .2 Q Oh CL, S j 5 S S 3 3 323 324 A second type, a lindane type, in which a susceptible fly strain gradually attained a higher and higher degree of tolerance, but in which there was no noticeable abrupt change in the rate of acquisition, was characteristic of para- oxon and toxaphene, as well as lindane. The third type was the one exhibited by strains of flies already resistant to one or more compounds. In this case the DDT- or methoxychlor-resistant strains rapidly took on resistance to other related or nonrelated chlorinated compounds. Thus, DDT-resistant house flies developed a high degree of resistance to lindane in one-half to one-third the number of generations that were required to develop the same degree of resistance from a susceptible strain of house flies. Our laboratory studies forecast rather accurately events in the field. That a fly strain resistant to one in- secticide has a small but significant amount of resistance to other insecti- cides may explain the rapid build-up of resistance to other insecticides by the DDT-resistant flies. In all probability such DDT-resistant strains possess se- lected genetical characters associated with the development of resistance to other chlorinated insecticides not found in susceptible strains. Field strains of DDT-resistant flies are found to have acquired resistance to the newer chemicals (chlordane, dieldrin, and lindane) used for fly con- trol in place of DDT. These strains ac- quired resistance to the new insecti- cides much faster than they had ini- tially acquired resistance to DDT. Field observations agreed with labora- tory findings. Increased resistance to dieldrin, toxaphene, and lindane was reported by Ralph March and Robert Metcalf, of the California Citrus Ex- periment Station, to have occurred in the DDT-resistant Pollard strains fol- lowing just three applications of lin- dane. Similar findings were reported by Kenneth D. Quarterman of the Savannah, Ga., laboratories of the Public Health Service, and by E. F. Yearbook of Agriculture 1952 Knipling, of the Bureau of Entomology and Plant Quarantine. In Illinois such an added resistance or tolerance occurred after two seasons of application of lindane or dieldrin. In studying this added dieldrin and lindane resistance acquired by field strains, investigators in California and Illinois noted that the levels of DDT resistance also had increased, appar- ently by the selective action of lindane or dieldrin. March and Metcalf discovered that DDT-resistant field strains of house flies usually are quite resistant to DDT-like compounds or analogs. They also reported that DDT-resistant flies subjected to lindane treatment devel- oped tolerances for dieldrin, hepta- chlor, aldrin, chlordane, and toxa- phene, as well as the expected lindane resistance. The Pollard DDT-resistant strain became resistant to all of the chlorinated hydrocarbons that might be used for fly control after only three applications of lindane. Apparently these multiple-resistant field strains were not resistant to the nitro-paraffin derivatives of DDT. Decker and I were able to develop, from standard laboratory stock, flies that were highly resistant to lindane and only slightly resistant to the other toxicants. We also produced methoxy- chlor-resistant flies that were suscepti- ble to DDT. These are two known ex- ceptions to the classification proposed by March and Metcalf that there are in the field two types of resistant house flies — those resistant to DDT and its analogs ( Bellflower strain, table 3 ) and those resistant to all the chlorinated hydrocarbons (Pollard strain, table 3) which are used as residual applications. Although flies can be developed that resist a specific insecticide, changes are associated with their development that make the strains moderately resistant to other insecticides related chemically or by mode of action. A small amount of tolerance also seems to be acquired for unrelated compounds. An exami- nation of tables 2, 3, and 4 will show Insecticides and Flies 3. Measured Drop Tests Showing Compar- ative 24-Hour Topical LD-50's in Micro- grams per Female Fly for Laboratory, Bellflower, and Pollard Strains 24-hour LD—50's Compound Laboratory Bellflower Pollard DDT DFDT DTDT 1 DEtDT 2 Methoxychlor . DDD Lindane Heptachlor3 . . Aldrin Dieldrin Toxaphene . . . Parathion Pyrethrins .... Allethrin 4 . . . . 0.033 . 10 . 16 . 1 1 .068 • 13 . 010 .032 .044 .031 . 22 .015 1. o •43 6. 1 > 100 o 1.2 70 2.7 3 2. 7 96 1.4 >ioo 080 060 076 050 62 020 94 97 •25 '•5 .78 .86 3-4 • °23 1.6 ' •5° 1 2,2-bis(/>-tolyl)-i,i,i-trichloroethane. 2 2,2-bis (/>-ethylphenyl)- 1 ,1,1 -trichloroeth- ane, 3 The most toxic ingredient of technical chlordane. 4 Allyl analog of Cinerin I 4. Measured Drop Tests Showing Compar- ative 24-Hour Topical LD-50's in Micro- grams per Female Fly for Laboratory, Bellflower, and Pollard Strains 24-hour LD-^o's Labo- Bell- Pol- Compound ratory flower lard i,i-bis(/>-chloro- phenyl)-2-nitro- propane l o. 095 o. 1 5 o. 1 1 1, 1- bis (/>-chloro- phenyl)-2-nitro- butane * 15 .18 .11 DDT 033 11 1 00 1 A mixture of 2 parts of the first compound and 1 part of the second compound is market- ed as Dilan. Data from Ralph B. March and Robert Metcalf of the California Citrus Experiment Station. the magnitudes of these conferred tolerances. The magnitude of the degrees of tolerance for toxicants acquired by house fly populations precludes the pos- 325 sibility of any important individual adaptation, but rather indicates a real change in the genetic make-up of the insecticide-resistant populations. Such a genetical change probably is brought about by a gradual selection of adap- tive mutations which alter the physio- logical processes in such a way that the insecticides are more quickly inacti- vated or detoxified. Attempts to learn the genetics of re- sistance show that both males and fe- males carry DDT-resistant characters and that crosses produce what would be called physiological blends. In other words, the first hybrid generation pos- sesses an intermediate degree of re- sistance. From the standpoint of genetics, we have little hope of eliminating resist- ance by crossing resistant flies with sus- ceptible flies, because resistance prob- ably results from several gene changes, which cannot easily be defined as domi- nant or recessive. The resistant flies, al- though well adapted to a DDT en- vironment, however, may not be so well adapted as susceptible flies to a DDT-free environment. Along with the advantageous mutation occurring during the development of resistant stock, there are probably many disad- vantageous mutations that may be re- flected in a decreased rate of repro- duction. Several possible changes might make field strains of house flies more and more difficult to control : The per- meability of the cuticula might change, the rate of detoxification might in- crease, the toxicant might repel the in- sect, selection might produce flies with protective habits of resting in places not ordinarily treated, and morpholog- ical changes in the dimensions of the fly might occur. We easily can demonstrate some of these changes. Workers in Illinois found that their Multi I strain of DDT- resistant flies decomposed DDT as quickly as it entered the body to produce a nontoxic DDE [2,2-bis {p- chlorophenyl) - 1,1 - dichloroethylene], 326 whereas susceptible strains did not. Other DDT-resistant strains of flies were found to break down only part of the DDT that enters the body of the fly. Because not all the DDT can be recovered as DDT or DDE, an unidentified fraction remains. Flies of the DDT-resistant laboratory strain from the Orlando laboratories are ap- parently larger than susceptible flies. There also seems be a close correlation between the rate of larval develop- ment and resistance to insecticides. Insecticide-resistant house flies grow more slowly than the susceptible flies. Field observers have noticed changes in resting-site habits of some resistant flies. Those insects rested on floors and lower parts of buildings but not on treated ceilings and walls. Some field strains of house flies are paralyzed by DDT more rapidly than are susceptible laboratory strains, but they completely recover from the initial paralysis of DDT poisoning. The quick paralysis and complete recovery serves as a pro- tective mechanism in the field by pre- venting flies from resting on walls or ceilings long enough to absorb a lethal dose of poison. Resistance to insecticides seems to be closely correlated with the ability of the fly to degrade chemically the toxi- cant to a nontoxic chemical. The site of the degradation appears to be in the hypodermal layer of the body wall of the insect. We do not know the actual mechanics of the process. What is the solution to our prob- lem? If we had an easy solution we would have no serious problem. A solu- tion is needed, and to that end I offer four items for investigation and thought. 1. The usefulness of DDT and other chlorinated compounds is coming to an end in the control of house flies unless some promising chemicals can be found that will prevent the flies from chemically degrading or evading the toxic effects of them. The Public Health Service has tested a large num- ber of DDT activators and found Yearbook of Agriculture 1952 that i,i-bis(/;-chlorophenyl) ethanol greatly increased the effectiveness of DDT on DDT-resistant house flies. It remains to be seen whether flies will become resistant to the combination. 2. New insecticides will have to be developed that are not chemically re- lated to the chlorinated hydrocarbons. Some of the new phosphate compounds may become useful in controlling re- sistant house flies now found in the field. Dilan, a nitro-paraffin deriva- tive of i,i-bis(p-chlorophenyl), has shown some promise in field and labo- ratory tests, according to Ralph March. But the usefulness of Dilan may be short-lived because Dilan-resistant flies have been developed from a multi- resistant strain in five generations. 3. Some investigators believe that field strains will revert to susceptibility in the absence of treatment. Field strains tested by workers in California and Illinois and investigators of the Public Health Service showed no loss of DDT resistance 2 years after DDT was discontinued as a residual treat- ment for fly control. DDT-resistant strains retained in the laboratory in the absence of DDT retained their high levels of tolerance for 30 to 50 genera- tions. On the other hand, some investi- gators have reported instances of re- version of DDT-resistant flies to more susceptible strains of flies. We of the Illinois Natural History Survey have laboratory data that indicate that re- sistant flies may produce fewer prog- eny and in some cases have a longer life cycle than susceptible flies. Gor- don Bender, of the University of Illi- nois, working with fly-muscle prepara- tions, found that respiration rates were higher in resistant than in the suscep- tible flies. These two facts suggest that resistant flies may not be so well adapted physiologically to their en- vironment and are not so efficient as susceptible flies. If so, then one can ex- pect a gradual loss of resistance to in- secticides among field strains of house- flies, as the susceptible strains literally outproduce the resistant strains. If the flies in the field lose their resistance to the insecticides, when treatments have been discontinued and residues on walls and in the soil disappear, then the chlorinated hydrocarbons will again become useful as chemicals to reduce fly populations. 4. Still greater emphasis should be placed on sanitation. Insecticides should be used only to augment the fly control obtained by strict sanita- tion. There is no substitute for sani- tation. W. N. Bruce, a native of Nebraska and a graduate of the University of Nebraska, is associate entomologist for the Illinois Natural History Survey. He worked at Iowa State College for 2 years as instructor and research as- sociate and has conducted research on insects affecting man and animals for the Illinois Natural History Survey since IQ45- For further reading on resistant flies, Mr. Bruce suggests his articles, Latest Report on Fly Control, Pests, volume ij, number 6, pages y, 28 (1949), and House Fly Toler- ance for Insecticides, with G. C. Decker, Soap and Sanitary Chemicals, volume 26, number 3, pages 122-125, 145-142 (1950), and articles by — W. V. King and J. B. Gahan: Failure of DDT to Control House Flies, Journal of Economic Entomology, volume 42, pages 405-409. 1949. Arthur W. Lindquist and H. G. Wilson: Development of a Strain of Houseflies Re- sistant to DDT, Science, volume ioy, page 276. 1948. Ralph B. March and Robert L. Metcalf: Insecticide-Resistant Flies, Soap and Sani- tary Chemicals, number 7, pages 121, 123, 125, i39: }95?. A. Missiroli: Riduzione o eradicazione degli anofeli? Rivista di Parassitologia, vol- ume 8, number 2/3, pages 141-169. 1947. K. D. Quarterman: The Status of Fly Resistance to Insecticides in the Savannah Area and Its Implications in the General Problems of Fly Control, C. D. C. Bulletin, volume 9, number 11, pages 3-7. 1950. Giuseppe Sacca: Sull'esistenza di mosche domestiche resistenti al DDT, Rivista di Parassitologia, volume 8, number 2/3, pages 127-128. 1947. James Sternburg, C. W. Kearns, and W. N. Bruce: Absorption and Metabolism of DDT by Resistant and Susceptible House Flies, Journal of Economic Entomology, volume 43, pages 214-219. 1950. Mosquitoes and DDT W. V. King Increased resistance to DDT has been recorded for several species of mosquitoes in widely separated parts of the world. Included are the house mos- quitoes, Culex pipiens in Italy and C. quinquefasciatus in India; two salt- marsh species, Aedes taeniorhynchus and A. sollicitans, in Florida; and two floodwater species, Aedes nigromaculis and A. dorsalis, as well as Culex tarsalis in California. An encouraging fact is that two species of Anopheles failed to show increased resistance in areas where they had been exposed for sev- eral years to DDT residual treatment in buildings. E. Mosna was apparently the first to report increased resistance in a species of mosquito, Culex pipiens autogenicus (molestus) from the Pon- tine marshes in Italy. He found many live specimens of the species in bed- rooms of houses in May 1947, where for the second year 5 percent DDT in kerosene had been applied as a residue for the control of Anopheles. Speci- mens he collected from the interiors were exposed to the treated walls and were alive after 48 to 72 hours, but specimens from a laboratory strain died within 3 to 5 hours. He thought it pos- sible that two races of this variety of mosquito might exist, distinguished basically by the different grade of re- sistance to DDT. Laboratory tests with the eighth generation reared from re- sistant material showed that the resist- ance was transmitted through eight generations without marked diminu- tion. From preliminary laboratory and field tests with chlordane and benzene hexachloride, Mosna learned that the insecticides had residual action lasting more than 4 months and were there- 327 328 fore suited to practical control of Culex that are resistant to DDT. In India, from experiments con- ducted for 10 months, J. F. Newman and others learned that successive gen- erations of the southern house mos- quito exposed in the laboratory to DDT residues showed a marked increase in resistance to DDT. A 20-minute expo- sure caused 100 percent rnortality of females originally, but no mortality re- sulted from 30-minute exposures a few months later. A similar resistance to benzene hexachloride also was shown. The failure of DDT sprays to give satisfactory control of the common salt-marsh' mosquito and another salt- marsh species, Aedes taeniorhynchus, in Broward County in Florida, was first noticed in 1947 in Hollywood, where much DDT had been applied in pre- vious years to control heavy infesta- tions. The failure was observed again in 1948 and 1949, when similar difficulty was experienced in Brevard County near Cocoa Beach and the Banana River Airbase, where an extensive salt marsh had been treated repeatedly with DDT sprays the previous 4 years. In June 1949 the results of aerial spray- ing operations in the area were checked by members of the Orlando laboratory of the Bureau of Entomology and Plant Quarantine. It became evident that satisfactory reduction of adults of the two salt-marsh species A. taeniorhyn- chus and A. sollicitans was not ob- tained with the standard dosage of 0.2 pound of DDT per acre. Even twice that dosage failed to give as good con- trol as had been obtained with the standard dosage. This indication of in- creased resistance was confirmed by laboratory tests in which larvae and reared adults of A. taeniorhynchus and A. sollicitans were compared for sus- ceptibility to DDT with similar speci- men material of A. taeniorhynchus from other areas in the same county that were not known to have received DDT applications previously or only an occasional treatment for adult con- trol. A. sollicitans were not present in Yearbook of Agriculture 1952 the untreated areas at the time the col- lections were made for the tests. The evidence from the laboratory tests demonstrated the increased toler- ance of the specimens from the treated areas. In the larvicide tests, the mortal- ity of fourth-stage larvae averaged about 16 percent, compared with .an average of nearly 90 percent for the control larvae. Similarly, in space-spray tests with 1 percent DDT solutions against reared females, the comparable figures were 18 percent and 83 per- cent. The results indicated a fourfold increase in tolerance or more. Larvae of Aedes taeniorhynchus collected in 1949 from a treated area in Sarasota County on the Gulf Coast also showed increased resistance. '* In tests on mosquito specimens from Brevard County, chlordane and ben- zene hexachloride, both technical and refined (lindane), produced about the same mortalities of larvae and adults from the treated areas as from un- treated areas at similar dosages. That was true also of parathion in larvicide tests. Parathion was not included against the adults. TDE, like DDT, was much less toxic to the specimens from the treated area. Toxaphene was somewhat less toxic. Lindane was by far the most toxic compound to adults, and parathion to the larvae from all areas. Aerial spray tests with several in- secticides were also carried out against the DDT-resistant mosquitoes. Lin- dane, the most effective of the insecti- cides tested, gave good control of adults at dosages of 0.05 and 0.1 pound per acre. Technical benzene hexachloride (12 percent gamma) at 0.2 and 0.4 pound and dieldrin and parathion at 0.05 and o. 1 pound gave results nearly equal to lindane. Chlordane and DDT at 0.2 and 0.3 pound per acre and toxaphene at 0.2 pound were not highly effective in most tests. Larvicidal tests on small plots were conducted with several insecticides ap- plied as emulsions. In the Cocoa Beach area DDT was much less effec- tive than in untreated areas, but the Mosquitoes and DDT other materials — dieldrin, parathion, lindane, technical benzene hexachlo- ride, and toxaphene — all gave good and approximately similar results in both the treated and untreated marshes. Dieldrin and parathion were the most effective at dosages of 0.025 and 0.05 pound per acre, closely fol- lowed by lindane and toxaphene. Indications of increased resistance to the effects of DDT in larvae of Aedes nigromaculis and A. dorsalis in Kern County, Calif., were noticed in the fall of 1947 and early in 1948 on a large ranch that had been regularly treated by truck and plane and had also been used for experiments on the applica- tions of DDT emulsion siphoned into the irrigation water. The dosage was increased from 0.15 to 0.25 parts per million with continued failure. Later a part of the fields was treated with DDT by plane at the rate of 0.4 pound per acre and part with toxaphene at 0.3 pound. The toxaphene killed all stages of larvae, but the DDT failed to kill even the first stages. Complaints that DDT was not giving good control of the larvae of Culex tarsalis were also received in the district at about the same time. R. M. Bohart and W. D. Murray reported that unsatisfactory results in the control of Aedes nigromaculis was experienced in Tulare and Merced Counties in 1949. To confirm the field observations, laboratory tests were made with larvae of the species col- lected in three pastures, which had previously received repeated DDT lar- vicide applications, in the mosquito- abatement district in Tulare County. They compared the larvae with larvae from three pastures in Kings County not known to have been previously treated with DDT. Based on the dos- ages required to cause 50 percent mor- tality, the average for the larvae from Tulare County was more than 10 times that for the control larvae. The least resistant of the larval lots from the treated fields required about three times as much DDT as the most resist- ant lot from the control area. In com- 329 parative tests between DDT and tox- aphene, the latter was considerably the more toxic to the DDT-resistant larvae but less toxic to the control lots. DDT residues applied to walls of living quarters and other buildings have been widely used in different countries to control carriers of malaria. Tests to determine whether an increase in tolerance had occurred were carried out in two areas where this method of control had been in operation for sev- eral years. The results were negative. In the Mexican village of Temixco, DDT sprays were applied to the inte- rior wall surfaces of all houses and other buildings once in early spring each year from 1945 to 1948. The sprays reduced markedly the numbers of Anopheles pseudopunctipennis in the village and in the surrounding rice fields. Laboratory tests were run in 1948 under the direction of J. B. Gahan and Wilbur G. Downs to de- termine the relative susceptibility of adults that had been collected in the village and the untreated village of San Jose, about 10 miles away. In June and July 96 tests were conducted with about 2,000 adult mosquitoes from each village. The insects were reared from gravid females collected in the two places and were tested by exposure to cloth panels impregnated with DDT. The average mortality was somewhat higher for the mosquitoes from the treated village than it was for those from the untreated village (56 percent versus 43 percent for the two sexes combined). The finding seemed to demonstrate that no loss of suscep- tibility had occurred. G. F. Ludvik and others reported in 1950 on the first year of a study of DDT resistance in Anopheles quad- rimaculatus in the Tennessee River Valley after 5 years of routine treat- ment, in which were used DDT resi- dues against larvae and adults. They subjected specimen material to a va- riety of tests in comparison with sim- ilar material from untreated areas. The comparisons consisted of labora- tory larvicidal tests in suspensions of 330 DDT, exposures of larvae in pans to aerial DDT sprays, exposures of adults to DDT-treated panels, and release of adults in residue-treated rooms. The mosquitoes from treated areas showed slightly greater tolerance to DDT in some of the tests, but the workers con- cluded from their preliminary studies that they had not developed an out- standing resistance. R. W. Fay and others have reported the results of preliminary experiments to determine the possible development of a resistant strain of Anopheles quad- rimaculatus. Adults of an insectary- reared colony of the species were ex- posed for four successive generations to DDT-treated panels for enough time to give mortalities of about 66 percent. Eggs from the surviving females then were obtained for rearing. In tests of susceptibility to DDT of the exposed strains, the mean mortality showed a slight but statistically significant drop in the first generation. No change oc- curred during the next three genera- tions but was followed by an increase to the original level in the first genera- tion after discontinuance of exposure to DDT. In tests against other insecti- cides, a similar loss of susceptibility was shown to methoxychlor but not to chlordane, benzene hexachloride, al- drin, or TDE (DDD). Because the pattern of increase and decrease in re- sistance was basically different in these tests from that reported for house flies (in which the changes in each direc- tion were much more gradual) further confirmation of these results seems nec- essary before conclusions can be drawn. W. V. King is a technical consultant in the Orlando laboratory of the divi- sion of insects affecting man and ani- mals, Bureau of Entomology and Plant Quarantine. The work at the labora- tory is supported by funds allotted by the Secretary for Defense for investi- gations of entomological problems of medical importance to the military forces and the development of methods of control of the insects involved. Dr. King has been with the Bureau most of Yearbook of Agriculture 1952 the time since igi2 and his work has been chiefly on insects affecting man. As a special agent of the Rockefeller Foundation, he spent 3 years in the Philippines on investigations of malaria mosquitoes. On active duty in the Sani- tary Corps of the Army during the Sec- ond World War, he spent nearly 3 years in New Guinea and other parts of the western Pacific on malaria control and mosquito investigations. He was in charge of the Orlando laboratory until 1951 , when he relinquished his admin- istrative duties. Dr. King cites the following articles for some of the information in his article and suggests them for further reading: J. H. Bertholf: DDT Resistant Mosqui- toes in Broward County, Fla., Florida Anti- Mosquito Association Proceedings, pages 80-83. /Q5°- R. M. Bohart and W. D. Murray: DDT Resistance in Aedes nigromaculis Larvae, Proceedings of the 18th Annual Conference of the California Mosquito Control Associa- tion, pages 20-21. 1950. Thomas L. Cain, Jr.: Observations on DDT-resistant Species of Mosquitoes Found in Brevard County, Florida Anti- Mosquito Association Proceedings, pages 84-85. 1950. C. C. Deonier: Aerial Spray Tests on Adult Salt-Marsh Mosquitoes Resistant to DDT, with T. L. Cain, Jr., and W. C. Mc- Duffie, Journal of Economic Entomology, volume 43, pages 506-510, 1950; Resistance of Salt-Marsh Mosquitoes to DDT and Other Insecticides, with I. H. Gilbert, Mos- quito News, volume 10, pages 138-143. 1950. R. W. Fay, W. C. Baker, and M. M. Grainger: Laboratory Studies of the Re- sistance of Anopheles quadrimaculatus to DDT and Other Insecticides, National Malaria Society Journal, volume 8, pages 137-146. 1949. James B. Gahan, Wilbur G. Downs, and Heliodoro Celis S.: Control of Anopheles pseudopunctipennis in Mexico with DDT Residual Sprays Applied in Buildings. Part II, American Journal of Hygiene, volume 49, pages 285-289. 1949. W. V. King: DDT-Resistant House Flies and Mosquitoes, Journal of Economic Ento- mology, volume 43, pages 527-532. 1950. G. F. Ludvik, W . E. Snow, and W. B. Hawkins: The Susceptibility of Anopheles quadrimaculatus to DDT after Five Years of Routine Treatment in the Tennessee River Valley, National Malaria Society Journal, volume 10, pages 23-34. I95I- Fumigants Nature and Uses of Fumigants Robert D. Chisholm Fumigants are chemicals that give off poisonous vapors. Their value for killing insects was known to the Greeks and Romans, Homer referred to the use of sulfur for the purpose. About 200 B. C. Cato mentioned that the fumes from a mixture of sulfur and asphalt would kill tree-infesting insects. Since then many compounds have been found to be valuable as fumigants for a large number of species — for some, in fact, fumigation offers the only prac- tical means of control. For others it provides an alternate means to sup- plement spraying or dusting. The selection of the right fumigant depends on several factors besides its ability to kill. It must not injure the commodity or thing attacked by the insects or nearby objects. It must not leave a residue that is toxic to humans or that imparts an unpleasant odor or taste if it is used on foodstuffs. Its cost must be less than the value of the ma- terials saved from the insects. It must have certain properties — adequate vapor pressure or rate of vaporization, ability to penetrate the commodities fumigated, little sorption by the com- modities, and chemical stability. Because of the large number of in- sect species and the variety of their environments, the ideal fumigant has not been discovered. Fumigants are not equally effective against all insect species. A fumigant, furthermore, that can be used to kill insects in one en- vironment perhaps cannot be used in another environment. Fumigants usually are applied in en- closed spaces — vaults, houses, ware- houses, mills, ships, bins, tanks, tents, or vacuum chambers, all of sufficiently tight construction to prevent undue loss of the vapors. Air presure is used sometimes to test for tightness. Losses from enclosed spaces that are protected from wind are much less than the losses from exposed places. Within practical limits, cracks or other open- ings should be sealed before a fumigant is applied. Sometimes fumigants are applied to soils. Some fumigants are gases at ordi- nary room temperatures (about 70 ° F.). Others are liquids or solids that vaporize slowly at ordinary tempera- tures or require heating for effective use. The gases are usually compressed in cylinders, from which they are re- leased as gases or liquids. (An excep- tion is sulfur dioxide, which is gener- ated by the burning of sulfur.) Fumi- gants that are liquids can be sprayed or sprinkled onto the commodity or throughout the enclosed space. Some are vaporized from pans, often with the aid of heat. Liquids used for soil fumigation are injected into the soil or applied as solutions or water emulsions. Solid fumigants can be vaporized by heating or scattered on and through- out the commodity or within the space, where they vaporize slowly. Successful and economical fumiga- tion depends on uniform distribution of the vapors. Some vapors are lighter than air and tend to concentrate in 331 332 the upper level in an enclosed space, particularly if they are released near the top. Others, heavier than air, may stratify in the lower levels. Such unde- sirable features may be overcome part- ly if light vapors are released near the bottom and the heavy ones near the top. More uniform distribution can be had by using a circulating fan to mix the vapors with the air. After mixing, stratification at different levels is often of little importance. Distribution in soils depends on the structure, moisture content, and tem- perature of the soils and the uniform- ity and depth of application. The vapors of many compounds will kill insects, but relatively few are used for the purpose. Carbon disulfide (CS2) is a liquid at ordinary room temperatures. The chemically pure form is colorless, but commercial grades are slightly yellow. It has a disagreeable odor. Other prop- erties: Boiling point 46.3 ° C, melt- ing point — 1 1 1.6° O, specific gravity 1. 26 1 22°/20° O, and a vapor pres- sure of 297.5 millimeters at 20° C. It evaporates rapidly at ordinary temper- atures and its vapors are 2.6 times as heavy as air (calculated from molec- ular weights) . The vapors of. carbon disulfide are explosive when mixed with 1 to 99 volume of air. At 1470 C. it ignites spontaneously. The mixtures may be exploded on contact with flames, live coals, sparks from electrical fixtures, or hot steam pipes. Great caution in its use is essential. People should avoid lengthy expo- sure to the vapors. Short exposures may cause only headache or nausea. If exposure is continued, the symptoms become severe as a result of pathologi- cal changes due to the solubility of lip- oids in carbon disulfide. Such changes may cause death. Carbon disulfide has been employed as a fumigant since 1854. Its use has increased steadily until recent years. Other fumigants or a mixture of 1 part of carbon disulfide with 4 parts Yearbook of Agriculture 1952 of carbon tetrachloride have largely replaced it. It has been used exten- sively for the fumigation of houses, warehouses, and stored products such as grains. Because it is toxic to all forms of life, it cannot be used for the fumigation of plants in greenhouses. Grains fumigated with carbon disul- fide when they are moist are apt to germinate poorly. Carbon disulfide is used also as a soil fumigant, originally having been found useful for the treatment of soil about the roots of grapes infested with root aphids. Later it was used in emul- sions for the control of larvae of the Japanese beetle and other soil-inhab- iting insects. Carbon tetrachloride, or tetra- chloromethane (CC14), is a liquid at ordinary room temperatures. It smells like chloroform. Other properties: Boiling point 76.8 ° C, melting point — 23. o° C, specific gravity 1.595 200/ 40 C., vapor pressure 159.6 millime- ters 200 C., and vapor weight about 5.3 times that of air. Its vapors are noninflammable — it is safe for use where a fire hazard is present. In liquid or vapor form, carbon tet- rachloride is toxic to humans. Symp- toms of poisoning may be produced by absorption through the skin. Some of the characteristic symptoms are fa- tigue, backache, burning of the eyes, stomach disturbances, and liver injury. A constant exposure to more than 100 parts per million of air is considered dangerous. Exposure to 10,000 parts per million or less for an hour may produce symptoms that last only a short time. In the presence of an open flame, carbon tetrachloride is con- verted to phosgene and its toxicity is increased greatly. Carbon tetrachloride has rather low insecticidal value. Consequently its cost is too high for many purposes. Its use is largely limited to operations where a fire hazard is present or in small-scale fumigations where cost is unimportant. Its principal use is in Nature and Uses of Fumigants mixture with other fumigants, such as carbon disulfide or ethylene dichloride, to reduce fire hazard. It is used also as a diluent for more toxic fumigants, such as methyl bromide or ethylene dibromide, to assist in the distribution of the vapors of the more toxic com- pounds. Large quantities are used in such mixtures, particularly in grain fumigation. Chloropicrin, or trichloronitro- methane (CC13N02), is a colorless liquid at room temperatures. It causes vomiting and intense irritation of the eyes and throat at relatively low con- centrations. Other properties: Boiling point 112.40 C, melting point — 64 ° C, specific gravity 1.651 20°/4° G., and a vapor pressure of 18.3 milli- meters at 200 C. Its vapors are about 5.7 times as heavy as air. It is noninflammable and is sub- stantially free of fire or explosion haz- ards. In that respect it excels certain other fumigants, such as carbon disul- fide or ethylene oxide. It is toxic to humans. It was used in mixture with other more toxic gases during the First World War and was known as vomiting gas — soldiers who removed their masks were thus ex- posed to higher concentrations of the other gas in the mixture. It is some- times added to hydrocyanic acid and methyl bromide as a warning agent. To fumigate stored products, it may be poured or sprayed on the infested material. Because it has a low rate of volatility, it is often mixed with carbon tetrachloride or ethylene dichloride to promote vaporization and distribution of the vapors. It has the disadvantage of being retained by the fumigated product and can only be removed by prolonged airing. It is apt to injure living plants and seeds. To control soil-inhabiting insects, it is. usually injected into the soil in mix- ture with the diluents previously named or with xylene. Sometimes it is emulsified in water, and the emulsion is sprinkled on the surface or poured into holes. Such treatments will control 333 certain species of fungi, nematodes, and weeds, but it must not be used where plants are growing. D-D mixture is essentially a mixture of 1,3-dichloropropylene and 1,2-di- chloropropane obtained as a byproduct in the manufacture of allyl alcohol from petroleum. Its composition is somewhat vari- able. A typical lot contained 30 to 33 percent of low-boiling and 30 to 33 percent of high-boiling 1,3-dichloro- propylene, 30 to 35 percent 1,2-di- chloropropane, and about 5 percent of heavy trichlorides of propane. It is a dark-colored liquid at ordinary tem- peratures and has a sharp, disagreeable odor. A typical lot had a boiling point of 930 C. On distillation 95 percent was recovered at 1420 C. and dryness resulted at 1630. It had a specific grav- ity of 1 . 1 98 20°/4° C, a vapor pressure of about 31.3 millimeters at 200 C, and a flash point (Tag. open cup) of 8o° F. D-D mixture is inflammable and is dangerous to use in enclosed spaces in the presence of sparks or open flames. It is dangerous to humans. Pro- longed breathing of its vapors may cause the symptoms associated with the inhalation of the vapors of chlorinated hydrocarbons. The seriousness of such symptoms depends on the concentra- tion of the vapors and the length of the exposure. It is very dangerous if spilled on the skin, shoes, or clothing and is likely to cause irritation, a burning sen- sation, and blistering. If it is spilled on clothes, the garment should be removed immediately and the skin in contact with it washed thoroughly. The gar- ment should be washed and aired until the odor of D-D mixture can no longer be detected. The chief use of D-D mixture is to fumigate soil against wireworms, gar- den centipedes, and such. It is highly effective against nematodes and re- duces the populations of fungi and bac- teria. For small-scale use it is poured into holes or furrows. For large-scale use it is applied in a continuous stream 97(ii:j4°— 52- -23 334 in the bottom of the furrow while plow- ing or by a mechanical trailer applica- tor, which injects the material under pressure at the desired depth and fills the furrows. D-D mixture is apt to cause injury to plants and is therefore used almost en- tirely before planting. The soil should be thoroughly aerated 10 to 14 days after treatment. If that is done proper- ly, most crops can be planted 3 to 4 weeks after treatment. DlCHLOROETHYL ETHER Or I- chloro-2- (/?-chloroethoxy) ethane (C4HSC120) is used as an insect fumi- gant. It is a colorless liquid at ordinary temperatures. It has a mild, distinct, but not particularly objectionable odor. Other properties: Boiling point 178° O, melting point — 500 C., specific gravity 1.222 20°/4° C, vapor pressure 0.7 millimeter at 200 C, vapor weight about 4.9 times that of air. Dichloroethyl ether is a safe fumi- gant as to danger from explosion. It should not be used near open flames as its decomposition products are danger- ous if people breathe them. At high concentrations the vapors are irritat- ing to the eyes, nose, and throat. If such exposure is continued for a long period, anesthesia followed by death may result. At low concentrations there is little irritation. It is valuable as a fumigant for many soil-inhabiting insects in lawns and gardens and is useful for the treatment of soils in greenhouses. Its high boil- ing point and low vapor pressure allow such fumigations to proceed over a long period and provide for the re- tention of toxic concentrations of the vapors in the soil for extended periods. The compound will kill a number of insect species, but it may also injure growing plants. It is best used where no plants are growing. The soil should be aerated before planting. Some plants, roses and carnations among them, are more susceptible to injury than grasses are. Orthodichlorobenzene, or 1,2- Yearbook of Agriculture 1952 dichlorobenzene (CoHjClo), is a color- less liquid at ordinary temperatures. It has a strong, characteristic odor. Other properties: Boiling point 180-30 C., melting point ~i1-$° C, specific gravity 1.3048 20°/4° C, and vapor weight about 5 times that of air. Orthodichlorobenzene can support combustion with difficulty and burns with a sooty flame. Under many condi- tions it is free from fire or explosion hazards. It is poisonous to people. Prolonged breathing of its vapors should be avoided. It may be absorbed through the skin. If spilled on the person, the wet clothing should be removed at once and the affected part should be washed thoroughly with soap and water. It injures growing plants. Its prin- cipal use is to treat logs or trees in- fested with bark beetles, such as the Black Hills beetle and the Engelmann spruce beetle. Such trees are usually in a dead or dying condition. The insecti- cide is applied to prevent the spread of the beetles to healthy trees. During the Second World War it was used as a fly larvicide, usually diluted with fuel oil, for the treatment of pit latrines and cadavers. Paradichlorobenzene, or 1,4-di- chlorobenzene (C0H4CL), is a white crystalline compound at room temper- atures. It has a characteristic odor that at low concentrations is not unpleas- ant. Other properties: Boiling point 173.40 C, melting point 530 C, spe- cific gravity 1.4581 20°/4° C, vapor pressure 0.64 millimeter at 20° C, vapor weight about 5 times that of air. Paradichlorobenzene is safe from fire and explosion hazards under most conditions of use. It is harmful to humans. Prolonged breathing of its vapors should be avoided. At high concentrations the vapors cause smarting of the eyes and some throat irritation. It is used in large amounts against many species, notably the peach tree borer. It is placed in a shallow trench around the tree trunk at a distance of Nature and Uses of Fumigants about 2 inches and then covered with soil. As a household fumigant, partic- ularly for clothes moths, it can be scattered on the shelves or suspended in small cloth bags from the hangers in closets. That way allows the heavy va- pors to be more uniformly distributed through the closet when the door is closed — better than spreading the crys- tals on the floor. It may be scattered on and under carpets, under furniture cushions, and in closed containers used to store blankets and other woolens. No air should circulate in the space being fumigated for at least 24 hours. Living quarters should be thoroughly aired out before they are used. Open flames should be kept away. Ethylene dibromide, or 1,2-dibro- moethane (CH2BrCH2Br), is a color- less liquid at room temperatures. It has a sharp, chloroformlike odor. Other properties : Boiling point 131. 6° C, melting point io° O, specific grav- ity 2.1 701 25°/4° C, and vapor weight about 6.5 times that of air. Ethylene dibromide has neither a flash point nor a fire point. There is no danger of fire or explosion. It is highly toxic to humans. Prolonged breathing of its vapors even at low concentrations should be avoided. It may be absorbed through the skin. Any clothing that it touches should be removed immediately. Parts of the body wet with it should be washed thoroughly with soap and water. Symptoms of poisoning include head- ache and nausea. Unconsciousness or death may occur several hours after prolonged exposure. Reddening or blistering may result from contact of the liquid with the skin. It is used for the control of many insects. It is effective against nema- todes, Japanese beetle larvae, wire- worms, and other soil-inhabiting spe- cies. It may be diluted with a light petroleum fraction or xylene and drilled into the soil, using the same methods described earlier for D-D mixture. For other purposes, such as the control of the Japanese beetle 335 where those methods are not practical, it is emulsified with water and applied to the surface of the soil, or infested plant balls may be dipped in a dilute water solution. Ethylene dibromide is toxic to many plants, and soils treated with it should be aired thoroughly before planting. It is effective against more than 50 insect species that infest grain in stor- age or grain mills. For that purpose it is generally mixed with other liquids, such as ethylene dichloride, carbon tetrachloride, carbon disulfide, or methylene chloride. The solutions are sprayed on top of the grain in bins, which should have tight-fitting covers. In grain mills the solutions are sprayed or poured into various parts of the ma- chinery and splashed around the inside of empty bins. Ethylene dichloride, or 1,2-di- chloroethane (CH2C1CH2C1) , is a col- orless liquid at ordinary temperatures. It has an odor like that of chloroform. Other properties: Boiling point 83. 7 ° O, melting point — 35. 30 O, specific gravity 1.257 20°/4° C, vapor pres- sure 62.9 millimeters at 200 C. Its vapors are about 3.5 times as heavy as air. Ethylene dichloride supports com- bustion with difficulty and burns with a smoky flame. Under many conditions it is not dangerously explosive. Mix- tures of 6 to 16 percent with air are in- flammable. To eliminate such hazards, it is often mixed with 3 volumes of car- bon tetrachloride. It is toxic to humans. Prolonged breathing of its vapors should be avoided. Some of the symptoms of poisoning are dizziness, headache, or nausea. Exposure to high concentra- tions may produce unconsciousness and death. It is widely used to control many in- sect species, usually mixed with other fumigants, as mentioned previously. It is an effective general-purpose fumi- gant in buildings and in vaults, where it may be evaporated from shallow pans, placed preferably in elevated lo- 33^ cations in the enclosed space. Evapora- tion may be hastened by heating elec- trically and by blowing a stream of air over the liquid. Often it is sprayed on the surface of grain bins. Grains and seeds may be so fumigated with little danger to germination. Foodstuffs, es- pecially those having a high fat con- tent, may retain a disagreeable taste and odor after fumigation. Emulsions of ethylene dichloride are used as soil fumigants. For the peach tree borer, an emulsion is poured on the ground close to (but not touching) the tree and covered with soil. Dosage and concentration are regulated according to the age of the tree. Water solutions are used against Japanese beetle larvae; infested plant balls are dipped in the solution, or it is poured on the soil of potted plants. Ethylene dichloride is toxic to cer- tain plants. The degree of toxicity ap- pears to be related to plant species and to the type and moisture content of the soil. Ethylene oxide, or 1,2-epoxy ethane ( ( CH2 ) 20 ) , is a gas at ordinary room temperatures. It has a mild odor at low concentrations and a more dis- tinct'one when the concentration is in- creased. Other properties: Boiling point 10. 70 C, melting point — 1 1 1.30 C, specific gravity 0.887 7°/4° C., vapor pressure 760 millimeters at 10. 70 C, and vapor weight about 1.5 times that of air. The vapors of ethylene oxide are in- flammable and at concentrations of 3 to 80 percent can form explosive mixtures with air. To reduce such haz- ards it is usually mixed with carbon dioxide before application or with dry ice at the time of application. A com- mercial mixture, containing ethylene oxide at the rate of 1 pound to 9 pounds of carbon dioxide, is available in metal cylinders. Ethylene oxide can harm people. Prolonged breathing of its vapors should be avoided. At low concentra- tions its effect may be hardly detect- able. High concentrations cause severe Yearbook of Agriculture 1952 irritation of the eyes, nose, and throat, and serious injury may result from such exposure. It is highly effective for destroying insect life in many kinds of stored prod- ucts, especially packaged cereals, bagged rice, tobacco, clothing, and furs in vaults. For such purposes it is ideal because it is highly effective and leaves no odor, flavor, or deleterious residue in the fumigated product. It may, however, injure foods like nuts, dried fruits, and such fresh fruits as raspberries, blackberries, and bananas. It may lower the germination of grains and of other seeds. It does not affect the milling qualities of grains. Another important use is to fumi- gate historical documents in the Fed- eral Archives Building in Washington. It is often best employed in vacuum fumigation chambers or it may be re- leased from cylinders or mixed with dry ice to form slush that is added to grain as the bin or elevator is being filled. Another use is to destroy molds, fungi, and other plant life in spices. Hydrocyanic acid, or hydrogen cy- anide (HCN), is a colorless gas at room temperatures. For most people it has a strong, characteristic odor like that of bitter almonds, but some people cannot detect it. Other properties: Boiling point 26 ° C, melting point — 140 C, specific gravity 0.697 i8°/4° C, vapor pressure 610 millimeters at 200 C. Its vapors are slightly lighter than air. Hydrocyanic acid is inflammable. At concentrations between 5.6 and 40 percent it forms explosive mixtures with air, but in fumigations the con- centrations are so low that little danger of explosion in the presence of sparks exists. It is extremely toxic to humans and is very dangerous to use. It may be breathed into the system or absorbed through the skin. Conse- quently fumigators must wear efficient gas masks and proper protective cloth- ing. Inexperienced persons should not attempt to use it. Its action is so fast Nature and Uses of Fumigants that one may have no warning symp- toms before unconsciousness or death occurs. It unites with the hemoglobin of the blood and prevents the tissues from absorbing the oxygen transported to them. Despite the dangers, hydrocyanic acid is widely used. For some purposes it is measured from cylinders with the aid of air pressure. For others it is generated by addition of its sodium or potassium salts to a mixture of sulfuric acid and water. For special uses it is packaged with an absorbent such as felt or diatomaceous earth. The gas is released on distribution of the absorb- ent in the space to be fumigated. It also is released on distributing granular calcium cyanide in the presence of moist air. Its first important use as a fumigant was developed in California in the 1880's for the control of scale insects on citrus trees. At first the gas was gen- erated under a tent covering the tree. Liquid hydrocyanic acid, made avail- able since, has become the most im- portant source of the fumigant. The method can be used without serious injury to the trees in the drier parts of California, but it has not been success- ful in more humid areas, such as Florida and Louisiana. It long has been used as a fumigant for plants in greenhouses. Such fumi- gations, and those of citrus trees, are made at night to reduce the hazard of plant injury; during daylight hours, plants absorb the gas more readily. The plants must be dry when fumigated, or the gas will dissolve in the moisture on the plant and may cause injury. Houses, warehouses, flour mills, stor- age vaults, and ships are often fumi- gated with hydrocyanic acid to de- stroy insects. It is very important that all be thoroughly aired out before any- one enters them. Things like mattresses and pillows require long airing. It is absorbed so rapidly that it may not pen- etrate throughout large bulks of milled cereals. Fresh fruits and vegetables, dried fruits, grain, flour, and other foodstuffs have been fumigated with 337 hydrocyanic acid. Most of those that have hard rinds or skins do not absorb dangerous amounts of the fumigant. Others, such as green vegetables, im- mature potatoes, or bananas, may be injured severely. After airing, the amounts absorbed from ordinary fumi- gation concentrations and exposures are not considered to be unduly hazard- ous to humans. Methyl bromide, or bromometh- ane (CH3Br), is a colorless gas at room temperatures. It is almost odor- less. Its boiling point is 4. 6° C, melting point — 930 C, specific gravity 1.732 o°/o° C, and vapor pressure 760 milli- meters at 4.6 ° C. Its vapors are about 3.3 times as heavy as air. It is noninflammable. Its vapors mixed with air cannot be ignited by a flame. It is therefore useful as a fire ex- tinguisher. Specific conditions can be established to provide for ignition by means of an intense electric spark, but they are not encountered during ordi- nary fumigations. Methyl bromide is toxic to humans. Prolonged breathing of its vapors should be avoided. The lack of a warn- ing odor makes it especially dangerous. Poisoning, which may not be apparent until hours or a day after exposure, may result from breathing its vapors. A gas mask should always be worn when exposure is possible. In mild cases the symptoms are disturbance of the equilibrium, double vision, head- ache, and vertigo. In acute cases delir- ium, loss of consciousness, convulsions, and sometimes death occur. Its use as a fumigant has increased rapidly since 1932, when its usefulness was first reported. Its many advantages make it one of the most widely used of all fumigants. It is highly toxic to many kinds of insects in all stages of development. It is chemically stable, is only slightly sol- uble in water, and (at the concentra- tions required) has no deleterious ef- fect on most plant tissues. It imparts no objectionable taste or odor to food- stuffs, and usually leaves no danger- 338 ous residue. It is convenient to handle, for it is readily liquefied, and yet it va- porizes at temperatures encountered in fumigating. The action of methyl bromide on in- sects may be slow. Certain species, such as larvae of the Japanese beetle, may show little evidence of being affected after fumigation; death may occur a week or more later. With other fumi- gants, such as hydrocyanic acid, death generally occurs during fumigation or shortly thereafter or the apparently dead insect may recover. Some of the important uses of methyl bromide are for the fumigation of warehouses, flour mills, ships, and resi- dences, and flour, grains, seeds, fruits, vegetables, tobacco, and bulbs in vaults, bins, tanks, or railroad cars. It is also highly effective as a soil fumi- gant; often it is used to treat nursery stock infested with certain insects against which quarantines have been established. Besides, it is used to rid many varieties of potted greenhouse plants of various insect species. All plants and vegetables will not tolerate methyl bromide fumigation. A few varieties of azaleas and ever- greens are apt to be injured. The ripen- ing of tomatoes may be delayed and sweetpotatoes may spoil. Naphthalene (C1iiH3) is a white crystalline compound at room temper- atures. It has a strong, characteristic odor. Other properties : Melting point 80.220 C, boiling point 217.90 C, density 1 . 1 45, vapor pressure 0.08 millimeter at 200 C. Its vapors are about 4.4 times as heavy as air. Its vapors burn with a luminous but smoky flame. It is one of the safer fumi- gants with reference to explosion haz- ard. Because specific mixtures of its vapors and air can be ignited, it should not be used near open flames. Under ordinary fumigating condi- tions, naphthalene is not dangerous to humans. Its strong odor at high con- centrations and the irritating effect of its vapors on the eyes and nose offer ample warning to prevent the breath- Yearbook of Agriculture 1952 ing of injurious amounts. Prolonged breathing of its vapors may cause a de- lirious condition. If the crystals are in- gested, the symptoms include excessive vomiting, purging, and great abdomi- nal pain, followed by nephritis. The main use of naphthalene as a fumigant is for the protection of woolen goods and furs, and it is commonly known as moth balls or moth flakes. Paradichlorobenzene in cake or crys- tal form is often used in place of it. It has been used to a limited extent as a soil fumigant for the control of wireworms or the larvae of certain in- sect species, such as the Japanese beetle. Newer and more effective fumi- gants include ethylene dibromide and methyl bromide. Nicotine, or 1 -methyl-2- ( 3-pyridyl ) pyrolidine (Ci0H14N2), is the chief alkaloid of tobacco. It is an oily, color- less liquid. It is almost odorless when it is pure but develops a tobaccolike smell on standing and rapidly turns brown when it is exposed to air. Other properties: Boiling point 247.3 ° C, melting point below — 8o° C, specific gravity 1.00093 20°/4° C, vapor pres- sures 0.08 millimeter at 200 C. and 7.00 millimeters at ioo0 C. Its vapors are about 5.6 times as heavy as air. As far as fire and explosion dangers are concerned, nicotine is one of the safer fumigants. Only 0.04 pound of it is required to saturate 1,000 cubic feet of air at 68° F. Much lower concen- trations are required to kill certain in- sects. It is inflammable, so it is best to avoid using it near open flames. Nicotine is one of the most virulent poisons known to man. Serious or fatal poisoning may result from ingestion of very small amounts of it, from absorp- tion through the skin or from breath- ing its vapors. Its strong odor, very irri- tating to the nose, provides warning of its presence during fumigations. If it is spilled on the skin, it should be washed off immediately. It causes contraction of the blood vessels with marked in- crease in the blood pressure, followed, after larger doses, with vascular dila- Nature and Uses of Fumigants tion and fall of pressure. The pulse rate is lowered at first and later becomes rapid. The principal use of nicotine as a fumigant is for the control of insects in greenhouses. It was first used for the purpose about 1825, tobacco being Wireworm. House fly larva. burned in an enclosed space. Later it was volatilized from water solutions, or the alkaloid was heated and the vapors distributed by means of a fan. Nicotine aerosols have been highly ef- fective. Very likely the effectiveness of nicotine in sprays and dusts applied to trees and other plants is due partly to fumigation and partly to absorption or ingestion. Sulfur dioxide (S02) is a colorless gas at room temperatures. It has a strong, characteristic odor. Its boiling point is — io° C, melting point — 72. 70 C, and liquid density 1.434. It is about 2.2 times as heavy as air. It is noninfiammable and can be used without danger from fire or ex- plosion. It is highly toxic to humans, but its vapors at low concentrations are so irritating to the eyes, nose, and throat that acute poisoning is rare. At high concentrations it is absorbed by the moist surfaces of the respiratory tract and results in inflammation and swell- ing. A person is usually unable to re- main long enough in a space being fumigated with sulfur dioxide to re- ceive a toxic dose. 339 It is one of the first known of the fumigants, but other fumigants have largely taken its place. It is soluble in water, forming sulfurous acid, which is corrosive, and is a powerful bleaching agent. Under moist conditions, it is apt to tarnish metals and cause injury to colored wallpaper and fabrics. In moist or dry climate it is toxic to many in- sects. It has long been used as a home fumigant or in other locations where the hazards mentioned are not impor- tant or are preferable to the presence of the insects. It is a practical fumigant for refrigerator cars in which fresh fruit is shipped. It may be distributed in enclosed spaces by burning sulfur or by releasing it from commercial cylinders. Sulfur dioxide is injurious to grow- ing plants, many kinds of fruits and vegetables, and wheat and flour for bread making. It adversely affects seed germination. Other fumigants used in limited amounts to meet specific insect-control problems include acrylonitrile, 1,1- dichloro- 1 -nitroethane, 1 , 1 -dichloro- 1 - nitropropane, ethyl formate, methallyl chloride, methylene chloride, methyl formate, propylene dichloride, and tetrachloroethane. Robert D. Chisholm is a chemist in charge of the Moorestown, N. J., laboratory of the division of insecticide investigations, Bureau of Entomology and Plant Quarantine. After gradua- tion from the University of Massachu- setts, he was engaged for about 15 years as a chemist in the commercial manufacture of insecticides and fungi- cides. He joined the Department in I935- Much of his work has been on the development of insecticides, at- tractants, and equipment for the con- trol of the Japanese beetle and of bark beetles. The references given for insecticides in the bibliography in the appendix are recom- mended for further reading on fumigants. Fumigating Soils and Plants Randall Latta, M. C. Lane Fumigants differ from other types of insecticides in that the fumes must be confined so that the insect is exposed to a considerable concentration for some time. The length of exposure and the strength of the concentration are inter- related— the higher the concentration, the shorter the lethal exposure; the lower the concentration, the longer the exposure. Fumigants therefore are not well adapted for controlling insects on growing crops. They are often the only efficient method for treating plant com- modities, however, when the insect is protected within seeds, pulp, or stems or is in the soil around the roots and when the treatment must be effective within hours so that the commodity can be moved into commerce. Fumigants serve three general pur- poses: To treat growing crops, to de- stroy insects in soil, and to treat plant commodities. An example of the use of a fumigant on a growing crop is the treatment of citrus trees for controlling scale insects and other citrus pests. Hydrocyanic acid gas, HCN, has long been utilized for that purpose. Rows of trees are covered with tents. The dosage, in pro- portion to the tree size, is injected or blown under the edge of each tent in the row, and the trees are exposed to the fumes for an hour. The tents are moved to the next line of trees, and the process is repeated. The fumigation is carried on when there is little or no air movement, usually in the late evening. The method has been adopted for the control of Hall scale, an insect on stone- fruit trees. Fumigation of growing plants in greenhouses and mushroom houses is 340 an old practice. HCN evolved from granular calcium cyanide and gas evolved from nicotine compounds heated, burned, or painted on hot- water pipes are often used. Many other fumigants have been tried, but none has been so widely accepted as those two. Organic phosphate insecticides — such as hexaethyl tetraphosphate (HETP), tetraethyl pyrophosphate (TEPP), tetraethyl dithiopyrophos- phate, and parathion — and other or- ganic materials, such as lindane, can be applied as aerosols in greenhouses to give a combined contact and fumiga- tion effect. The vapors from these materials are toxic to insects in ex- tremely low concentrations. Fumigant vapors may be retained for a long time in the soil, and they might be quite toxic to insects and other organisms living there. Probably the first such fumigant to have wide- spread use was carbon disulfide. It was employed to kill the grape phylloxera, a root-louse that was threatening the grape and wine industry in France. The chemical was tested against wire- worms in the United States as early as 1 89 1 and was recommended for use against various soil insects until re- cently. Paradichlorobcnzene (applied in crystalline form in soil around tree trunks to control the peach borer) and napthalene flakes (worked into surface soil for wireworms) were other early soil fumigants. Chloropicrin, calcium cyanide, and many other fumigants have been used in attempting to con- trol soil insects and nematodes. Most have been too costly or too difficult to apply to be practical on any large scale on the farm. All have limited use in greenhouses or seedbeds. Because the damage caused by wire- worms, symphilids, and nematodes is so great, many fumigants have been tested for use in soil. In some years, wireworm damage to the potato crop of the Pacific Northwest alone has caused losses of 4 million dollars to farmers. Soil pests have caused losses Fumigating Soils and Plants to the lima bean crop of several mil- lion dollars annually for many years in California. The damage to other crops probably has been proportionally as great. During the late 1930's and early 194.0's research for better fumigants was intensified. Many new organic chemicals came on the market. One, a mixture of the two chemicals dichloro- propane and dichloropropylene and known as D-D, was used in 1943 against the pineapple mealybug in Ha- waii and was found to be a potent agent against mealybugs and nema- todes. Later tests in California dem- onstrated its effectiveness against wireworms. Another material, dichlo- ronitroethane, proved to be more effective and suitable when soil tem- peratures were low. Ethylene dibromide was found to be an efficient and economical fumigant for wireworms. An increased interest in soil fumi- gation has led to an improvement in testing procedures. Until recently only the fumigants that had proved success- ful for fumigating grain or households were tested, and much the same test- ing methods were followed. Research workers later began testing fumigants in the presence of soil instead of ex- posing the insect alone. The newer tests disclosed that more fumigant was required when it was applied in soil, that it must be active enough to move around freely in the soil, and that it must not be too strongly absorbed by the soil. Soil physicists have investigated the movement of gases in the soil. Some of their work can be applied to soil fumi- gation. The scientists agree that gases enter and leave and move around in the soil mass by diffusion. Diffusion is slow, especially in compact soil. Experiments revealed that dichlo- ronitroethane moved in compact soil about 24 inches in 16 days, or at an average rate of 1.5 inches a day. The rate is increased when the soil is loosened; the movement depends on the amount of free air space. Harrow- 341 ing, rolling, or anything that reduces the free air space slows down the rate of diffusion. Plowing and disking, which loosen the soil, increase the rate of diffusion. The amount of water in the soil also influences diffusion, as water fills the air spaces and slows down the movement of gases. Experiments in 1949 showed that 10 to 30 times the amount of fumigant actually needed to kill wireworms must be applied to field soil for successful control. This large excess of fumigant is absorbed by the soil or escapes into the air. The best and most widely used soil fumigants are ethylene dibromide and a mixture of dichloropropane and di- chloropropylene. Both have been used successfully against wireworms and nematodes. Ethylene dibromide, a heavy liquid with a rather low rate of evaporation, moves slowly through the soil. Its rate of escape from the surface also is slow. It should be used in loose soil to speed diffusion. Some sort of surface seal (such as provided by a light rolling or harrowing with a spike harrow) is de- sirable. It is about as efficient in cold soils, down to nearly freezing, as in warm soils. It is not greatly affected by soil moisture if the soil is not saturated with water. The wireworms common in the Pacific Northwest can be con- trolled by a dosage of about 2 gallons of ethylene dibromide to the acre. That amount is actually diluted with a highly refined light oil, such as paint thinner, because the available equip- ment does not readily measure or apply less than 6 to 8 gallons an acre. Larger doses are needed to control nematodes. The dichloropropane-dichloropro- pylene mixture, also a liquid, is much lighter than ethylene dibromide and not nearly so toxic to insects. About 25 gallons to the acre are required to con- trol wireworms in the Pacific North- west. It is much more volatile than ethylene dibromide, and the soil sur- face must be sealed by harrowing or rolling after the fumigant is applied. Neither fumigant should be used in saturated soil. They should be given a 342 week to 10 days to permeate the soil and kill the insects. Thereafter, if the odor of the fumigant is still strong in the soil, heavy disking or spring tooth- ing will open it up and allow the fumi- gant to escape. With these new and more practical fumigants has come the development of machines for applying them. The old hand-operated, single-row injec- tion machines of earlier days were im- practical on the large acreages that needed treatment in the Western States and Hawaii. The many different machines that have been used are mostly of the power-injection or gravity-feed types. For the larger acreages the trailer- or tractor-mounted types are satisfactory. They can cover 10 to 40 acres a day. The liquid fumigant is released into the soil under pressure through tubes fastened to the rear of soil-chisel shanks mounted on draw bars so that the fumigant is injected at the best depths for maximum penetration. The chisel shanks are usually set 12 inches apart, and there may be 5 to 14 on a machine according to the power avail- able to pull them through the soil. Some drawbacks of injection machines are their high cost, their inability to work well except on a prepared soil bed, and the rapid escape of the gas through the apertures left by the chisel shanks. Thousands of acres have been treated since 1945 with the machines, using D-D and ethylene dibromide. The gravity-feed applicators are more suitable for the average farmer on a small acreage. Also known as plow applicators, they can be made on the farm from a second-hand gasoline tank, some quarter-inch copper tub- ing, and a valve or two attached to a standard tractor or plow. The fumi- gant is discharged by gravity just ahead of the plow or plows onto the exposed plow sole, where it is covered immediately by the soil of the next fur- row. Needle valves regulate the flow according to the speed of the tractor and width of the furrow. The equip- ment is low in cost. The soil does not Yearbook of Agriculture 1952 have to be prepared beforehand. If the surface is harrowed lightly after plow- ing, the toxic vapors are held in the soil long enough to give the most effi- cient diffusion of the fumigant. Lindane and parathion are effective for treating soil to destroy insects in greenhouses and plant nurseries where plants are growing. Small amounts of the materials added to soil in the green- house bench will control symphilids. The effect is a combination of contact and fumigation. Officials who enforce plant quar- antines are interested in soil fumigation that will free restricted areas from a particular insect so that plants can be grown or stored there without hazard of infestation and subsequent dissemi- nation of the pest. Under the Japanese beetle quaran- tine, several methods were perfected. One is fumigating under tar paper or tarpaulin covers with carbon disulfide injected in holes 1 foot apart each way. Another is sprinkling a water solution of methyl bromide or a mixture of ethylene dibromide and ethylene di- chloride over the soil surface. A third is treating the soil that would make the ball of a balled and bur- lapped nursery plant. Before the plant is dug, a quantity of emulsions or so- lutions containing carbon disulfide, methyl bromide, or a mixture of ethyl- ene dibromide and ethylene dichloride is applied to the area around the plant in the nursery row. A modification of the method is to dip the soil or root balls of nursery plants after digging in emulsions or solutions of carbon di- sulfide, ethylene dichloride, a mixture of ethylene dibromide and ethylene di- chloride, or a mixture of a fumigant and a contact insecticide — ethylene di- bromide and chlordane. The white-fringed beetle, the object of another quarantine, is more resistant than the Japanese beetle in the young or larval stage. Fumigation under tar paper of plant-free soil areas with methyl bromide is effective in destroy- ing larvae. It is used by itself or dis- Fumigating Soils and Plants solved in an organic solvent such as ethylene dichloride and injected into the soil at spaced intervals. However, the various emulsions or solutions that are used around plants or as dips for the Japanese beetle treatments are not tolerated by nursery plants at the stronger concentrations needed to kill white-fringed beetles. The incorporation of such stable in- secticides as DDT and chlordane in the nursery soil has proved successful and has obviated much of the necessity for soil fumigation in relation to plant quarantine measures. Fumigation of potting soil is likewise important in enforcing quarantines. Carbon disulfide, chloropicrin, and methyl bromide are used frequently to fumigate potting soil that is to be used under certified conditions. Under the regulations that apply to plant products imported into the United States, fumigation is widely utilized — on shrubs, trees, corms, bulbs, roots, tubers, cut flowers, seeds, re- stricted fruit and vegetable products, cotton byproducts, and broomcorn. The summary we give below of im- ported material treated in 1949 and 1950, mostly by fumigation, indicates the scope of such fumigation. Because of infestations with living giant African snails, even shiploads of steel scrap from islands in the South Pacific have been fumigated. In 1 9 18 quarantine inspectors be- gan to fumigate American railway cars on their return from Mexico. Many of the cars were used to haul cottonseed or another cotton product in Mexico and became contaminated with Mexi- 343 can cottonseed, which might contain live larvae of the pink bollworm. Large fumigation houses, ranging in capacity from 2 to 20 freight cars, were maintained at six border points to treat all returning cars. The largest house had a capacity of more than 200,000 cubic feet and required 80 to 120 pounds of liquid hydrocyanic acid for one fumigation. In 1949 the practice was discontinued except for special reasons, because the precautions taken in Mexico reduced to a negligible point the probability of contamination of cars with infested cottonseed. Fumigation is also used under vari- ous domestic plant quarantines. Under Japanese beetle quarantine regula- tions, methyl bromide fumigation of fruits and vegetables to eliminate any live adults was the preferred practice for many years. As many as 5,000 freight-car loads a year were so treated in the 1940's. Since then the use of DDT dust has replaced some of this fumigation. One to two million nurs- ery plants were also fumigated. Under white-fringed beetle quarantines, fumi- gation of balled and burlapped nursery plants has been practiced since 1939. Fumigation is used also to treat white potatoes, peanut hay, and lupine seed. To combat the sweetpotato weevil, fumigation has been required for table- stock sweetpotatoes that move from quarantined areas to other growing areas in the South. Fumigation with methyl bromide has been adapted to the treatment of Christmas trees and greens cut in lo- calities infested with gypsy moth. It destroys dormant egg clusters on the Imported Materials Fumigated in 2 Years Cotton lint, linters, and bagging bales Cottonseed cake and meal pounds .... Cotton samples number . . . Fruits and vegetables cases Chestnuts, cipollini bulbs, and pigeon peas containers. Broomcorn bales Plants, cuttings, bulbs, roots {containers .' f containers . Seeds Miscellaneous plant products lots . \pounds '949 1950 500, 000 850, 000 40, 000 9, 324, 000 17, 200 25, 5°° 8, 400 60, 000 47, 5°° 53, 5°° 36, 200 35° 2. 585> 5°° 4= 5 1 0, 000 5, 000 7, 264 1,500 67, 300 48, 700 52, 200 2 1 , 500 1 9, 000 344 branches. Formerly the greens and trees had to be inspected one piece at a time. Methyl bromide fumigation is used for treating cottonseed as an alterna- tive to the long-standard heat treat- ment. Sacked cottonseed for planting can be fumigated in the manner usually practiced for most commodities, but cottonseed in bulk has to be fumigated under conditions of forced circulation in order to distribute the fumigant. To do that, special apparatus was devised to fit in with the normal handling practices of cottonseed. Large steel tanks, holding up to 600 tons of cot- tonseed, have blowers and a duct sys- tem that draw the fumigant down through seed more than 40 feet deep. The tanks are loaded and unloaded by mechanical conveyors commonly used in handling cottonseed. Fumigation is also done in freight cars by connecting a portable forced-circulation system, which operates outside the car, to flex- ible ducts attached to the floor and ceiling levels. More than 73,000 tons of cottonseed were fumigated in 1950 in storage tanks and more than 300 car- loads were fumigated on a railroad sid- ing. That is only a small part of the total amount of cottonseed treated for pink bollworm, but fumigation is a val- uable alternative to heat treatment in newly discovered areas of infestation where heat-treating equipment is not available. Fumigation likewise is used for treating commodities regulated by var- ious State plant quarantines. Several States require the fumigation of white potatoes originating in California be- cause of the potato tuberworm. Thou- sands of carloads of potatoes are fumi- gated with methyl bromide in compli- ance with those quarantines. Cali- fornia and Arizona require the fumiga- tion of many plant-propagating mate- rials as a condition of entry. When Cal- ifornia removed restrictions on Texas citrus fruit because citrus canker was no longer found in Texas, many car- loads of grapefruit were fumigated to destroy such surface insects as scales, Yearbook of Agriculture 1952 to meet other California requirements. In 1940 or so, before the oriental fruit moth was found in Pacific Coast States, nursery-plant hosts that might carry overwintering larvae were allowed en- try into Western States if they were fumigated before shipping; a large trade developed between midwestern nurseries and western fruit growers. When the insect was discovered on the west coast, fruit and fruit boxes were fumigated before moving from local quarantined areas to noninfested areas. When Hawaii was quarantined be- cause of the presence of the oriental fruit fly, two products were fumigated to permit movement to mainland markets. Millions of Vanda orchid flowers were fumigated in 1949 and 1950; later research established that the fruit fly could not finish its life cycle on the flowers even though eggs and young larvae were found on them. Then the restrictions were removed. Pineapples are shipped to mainland markets following fumigation to de- stroy eggs or larvae of the fruit fly at- tached to or embedded in the skin of the fruit. Randall Latta was leader from IQ42 to 1951 of a project to develop treatments for plants and plant prod- ucts regulated by plant quarantines. His staff works with plant quarantine units in the Bureau of Entomology and Plant Quarantine to develop proper dosage schedules and procedures for fumigating quarantined materials. In ig$i he became leader of the division of stored product insect investigations. M. C. Lane is in charge of the truck crop and garden insect investigations laboratory at Walla Walla, Wash. He has been with the Bureau of Entomol- ogy and Plant Quarantine since igij and has been studying the life history and control of wireworms of the Pacific Northwest since IQ20. Besides evolv- ing several cultural control methods for wireworms on irrigated lands, he and his coworkers have evolved methods of ridding soil of wireworms through the use of fumigants and soil insecticides. Fumigating Stored Foodstuffs R. T. Cotton Fumigants can penetrate large bulks of stored foodstuffs and get at insects working far beneath the surface. They work fast and effectively against all stages of insects, even those concealed within kernels of grain. Their volatile nature insures the eventual disappear- ance of poisonous residues from fumi- gated foodstuffs. The cost of fumiga- tion usually is low. The materials are inexpensive. Small dosages suffice. Lit- tle equipment is required to apply them. Fumigation is usually considered a curative measure, but it is essential in most programs for the prevention of insect damage to foodstuffs in stor- age. Fumigants are available to treat any type of foodstuff under almost any circumstance and for a long enough time to kill insects in the places where they are. For those reasons, fumigants are more important in preserving stored foods from insect damage than any other agent or combination of agents. They have some drawbacks, however. Some foodstuffs, notably those rich in oil, may retain obnoxious odors from certain fumigants and others may be adversely affected by repeated fumi- gations or excessive concentrations. The viability of seed may be reduced by some fumigants under certain con- ditions. If the commodities are in good condition, however, some fumigant or other can be relied on to do an efficient job without materially affecting them. Grains, milled cereals, feeds, dried fruits, nut meats, dried meats, cheeses, powdered milk and egg, beans, peas, chickpeas, spices, coffee, and practi- cally all dried foodstuffs can be safely and effectively fumigated. For proper fumigation, one should know the fumigant or fumigants best adapted for treating each type of com- modity under the varied conditions of storage he might encounter and the capabilities and limitations of the more important fumigants. For successful fumigation the insect must be surrounded by the fumi- gant in a concentration heavy enough and for a sufficient time to produce death. Special techniques may be need- ed to tighten enclosures and insure the uniform distribution of the vapors. If enclosures cannot be made tight enough to hold fumigants, foodstuffs can be treated in bins, atmospheric vaults, barges, vacuum chambers, rail- way cars, under tarpaulins, or in indi- vidual packages. With products such as grain in bulk it is possible to fumi- gate successfully large piles stored in loosely constructed buildings because of the ability of the grain to absorb and hold the vapors for considerable periods. The infestation of grain by insects may start in the field or soon after it is placed in storage on the farm. As a preventive measure, it is wise to fumi- gate grain immediately after it is placed in storage in areas where field infesta- tion occurs and within 6 weeks in all other areas. The tendency of bulk grain to absorb fumigants makes it possible to treat grain successfully even though the bin in which it is stored is not airtight. The dosage of fumigant required will vary with the tightness of the bin and with the type of grain. Generally speaking, small grains require smaller dosages than corn since they retain the vapors for longer periods by their greater sorp- tive properties. On the other hand, the smaller size and still greater sorptive properties of grain sorghum obstruct the uniform diffusion of fumigants through the bin when they are ap- plied to the surface, so that larger dosages are required for treating grain sorghum than for any other grain. Low temperatures, layers of moist grain, and the presence of pockets of 345 34^ dockage in bins of grain are factors that adversely affect the performance of grain fumigants. All these factors usually are considered in calculating the required dosages. Many proprietary fumigants on the market differ slightly in composition from the compounds I have listed in the table. They may be used at the dosages recommended for the mixtures that they most closely approximate. For best results in fumigating grain in farm bins, the surface of the grain should be level and at least 6 inches below the top of the side walls of the bin. Because strong winds and high temperatures accelerate the evapora- tion and loss of fumigant, applications should be made in the cool part of the day and when the air is quiet. In ap- plying fumigants, the operator should cover the surface of the grain as uni- formly as possible with a coarse spray. He should treat the grain from the out- side of the bin to avoid exposure to the fumes. For small operations, a bucket pump or a knapsack sprayer can be used, but for larger operations a power sprayer is desirable. A pump with bronze fittings, which will not be affected by carbon tetrachloride or similar chemicals, and one that will pump the chemical directly from the drum is useful. In all operations a plastic-lined hose or one that will re- sist the action of carbon tetrachloride should be used. Washers should also Yearbook of Agriculture 1952 be resistant to this and similar chemi- cals. Fumigation of grain in elevator storage is much simpler than in farm bins, because elevator bins are usually much tighter and the uniform distri- bution of the fumigant is facilitated by its introduction into the grain stream as the grain is transferred from one bin to another. Dosages of fumigants listed in the table, as modified for use in steel bins, can be used for treating grain in steel or concrete elevators. Besides these fumigants, calcium cyanide at io pounds and chloropicrin at 2 pounds per 1,000 bushels of grain can be used. In wooden-crib elevator bins, the dosage should be doubled. These dosages will give an excellent kill of adult insects but will seldom kill all the immature stages of weevils that breed within the kernels. Somewhat heavier dosages therefore should be used if the kill is to be complete. Because many factors affect the effi- ciency of grain fumigants, the results are not always predictable. Grain that is high in moisture content, is cold, contains a lot of dockage, or has stood for a long time without turning is dif- ficult to fumigate and may require much heavier dosages than normally are used. Under average conditions the fumi- gant can best be applied to the grain Fumigants and Dosages for the Treatment of Grain Stored in Wooden Farm Bins 1 Dosage per i ,000 bushels Small grains except Fumigant sorghums Sorghums Corn Gallons Gallons Gallons Carbon tetrachloride 5 8 6 Carbon tetrachloride: 4 parts + carbon disulfide 1 part 2 3 8 6 1 part + ethylene dichloride 3 parts 2 6 10 6 19 parts 4- ethylene dibromide 1 part 3 8 6 1 In steel bins the dosages may be reduced 50 percent for small grains and about 20 percent for corn and grain sorghum. 2 The addition of 5 percent by volume of ethylene dibromide improves the kill of immature stages of insects in grain. Fumigating Stored Foodstuffs stream while the bin is being rilled. Special applicators designed to feed the fumigant into the grain stream at the desired rate are used for chloropicrin or calcium cyanide. Other fumigants are poured into the grain stream at regular intervals by hand or may be applied with an automatic applicator adjusted to operate continuously when the grain is running. When grain can- not be turned, the fumigants other than chloropicrin or calcium cyanide can be applied by spraying the entire dosage uniformly over the top layer. If grain temperatures are above 8o° F., the vapors will penetrate the mass of grain to the bottom of the bin. For control of surface infestation by the Indian-meal moth or the almond moth, the various bin openings (venti- lators, manhole covers, loading chutes) should be closed and sealed and a fumi- gant applied as a fine spray or vapor. The aim is to retain the fumigant at the top of the bin rather than have it sink down through the mass of the grain. Chloropicrin alone can be applied by means of a garden sprayer to the space above the grain in closed-top bins at the rate of 1.5 to 2 pounds per 1,000 cubic feet of space above the grain. Mixtures of 80 percent methyl bromide and 20 percent chloropicrin, or 80 per- cent methyl bromide and 20 percent ethylene dibromide likewise can be ap- plied at the rate of 1 .5 pounds per 1 ,000 cubic feet of space above the grain. Some grain-elevator bins in Europe and North Africa are equipped for cir- culating a gas within the bins during and immediately after its introduction. Although the method has not been adopted for elevator bins in the United States, it has been successfully used in the fumigation of grain in steel tanks of 350,000-bushel capacity in Texas. Blowers introduce the fumigant into the top of the bin and pull it down through the grain and out through ducts to the blower again so that it can be recirculated. This method gives a uniform distribution of the fumigant within 30 minutes, and the fumigant can be removed and replaced with 347 fresh air after the fumigation. Methyl bromide can be used successfully by this method at dosages so low that fumigation costs are extremely reason- able. For grain sorghum a dosage of 3 pounds of methyl bromide per 1,000 bushels of grain gives excellent results. For the temporary storage of large stocks of surplus grain, Quonset huts, airplane hangars, barracks, and warehouses of all kinds are used. The grain is usually stored in a pile on the floor and seldom completely fills the structure. The buildings usually are not tight, and the problems of storage are complicated. Piles of grain in such storages can be successfully treated with fumigants sprayed over the surface of the pile, even though the buildings are not tight. The grain mass holds the fumigant so well that excellent kills can be ob- tained. During cool weather, insect colonies tend to bunch together near the center of such piles. Such infesta- tions are eliminated by spot applica- tions of fumigants. The area of infestation can be determined by tak- ing probe samples. The fumigant should be applied directly over the in- fested area, so that it covers a few feet beyond the limits of the infestation. In both corn and wheat, dosages of 4 to 5 gallons of 4 parts of carbon tetrachloride and 1 of carbon disulfide per 1 ,000 bushels have given excellent results in spot treatments or in the fumigation of the entire pile. The fumigant can be applied with a power sprayer that delivers the liquid rapidly as a coarse spray. Many Quon- sets have roof hatches, through which the fumigant can be applied; if not, the operators can enter the building and spray the fumigant uniformly over the pile, starting from the rear and working towards the exit. Adequate hose and a pump capable of throwing a stream about 75 feet at the rate of 100 gallons a minute should be used. During manufacture and process- ing and subsequent storage in ware- 348 houses, dried foods are exposed to in- festation by insects that become estab- lished in the machinery or in various parts of the mill, manufacturing plant, or warehouse. A planned program of fumigation can do much to prevent such infestation. In former years a general fumiga- tion once or twice a year with hydro- cyanic acid, methyl bromide, or chloro- picrin was relied on to keep premises free from insects. Modern demands for food entirely free from insect infesta- tion have caused the adoption in many plants of a biweekly program of local fumigation, whereby individual mill- ing units or food-handling machines are fumigated and the fumigated stock removed by heavy-duty vacuum clean- ers. In some mills the biweekly appli- cation of local fumigants takes the place of a general fumigation. Local fumigants, if regularly used, will main- tain a low insect population in milling machinery but cannot be expected to destroy infestations in all parts of the plant, so that an occasional general fumigation is helpful. General fumigants are usually in- troduced into the open space of the building, but sometimes are also intro- duced through piping systems directly into the machinery. Local fumigants may be applied by hand by pouring them into the individ- ual milling units or machines. Liquid fumigants may also be applied with permanently installed dispensers. Port- able fumigant dispensing tanks are used to force the fumigant into fabric tubes installed permanently inside con- veyors or other units. A fully automatic system dispenses the fumigant in vapor form from a central supply connected by tubing to individual milling units. Chloropicrin, hydrocyanic acid, and mixtures of carbon tetrachloride with ethylene dibromide, ethylene dichlor- ide or other chemicals are used ex- tensively as local fumigants. For the fumigation of warehouses filled with grain, feed, flour, or other dried foodstuffs, methyl bromide or mixtures of methyl bromide with Yearbook of Agriculture 1952 chloropicrin or ethylene dibromide have been found most effective. Per- fect penetration of large stacks of bagged materials can be obtained with dosages of 1.5 pounds per 1,000 cubic feet of space. To obtain uniform distri- bution of the fumigant and to prevent stratification of the vapors near the floor, electric fans should be operated for 1 hour after release of the gas. The use of tarpaulins or gas- proofed fabrics in the fumigation of stored foodstuffs sometimes may be more convenient than fumigating in large, partly filled warehouses or in atmospheric vaults. The tarpaulin, which takes the place of the fumiga- tion chamber, is portable and occupies little space when not in use. The free air space is reduced to a minimum and aeration is facilitated by the complete removal of the tarpaulin from the stack of commodities after fumigation. The products to be fumigated are generally stacked on a concrete floor and cov- ered completely by the tarpaulin, the edges of which are weighted down carefully to prevent leakage of gas around the base. Provision is made for an air dome at the top by using two sacks placed edgewise about 4 feet apart. The air dome will provide free air space to permit diffusion of the gas. A rubberized fabric or a light duck material coated with ethyl cellulose usually is used. Any fumigant suitable for the treatment of bulk commodities in atmospheric vaults or warehouses can be used to treat foodstuffs under tarpaulins. Atmospheric vaults are useful for the fumigation of foodstuffs when warehouses are not tight enough for efficient fumigation or when small lots — incoming raw materials, re- turned goods, used bags, and out-going products of all kinds — need treatment. Many different materials can be used to construct atmospheric vaults, but a metal vault or one with a metal lining is most efficient. Tubing and spray nozzles for introducing volatile fumigants and fans for circulating the Fumigating Stored Foodstuffs fumigant or exhausting the vapors are necessary. The actual process of fumigation is simple. The commodity is loaded into the vault by hand or run in on trucks or skids. The door is closed and the fumigant introduced. At the end of the fumigation, the exhaust fan is turned on and allowed to run until the vapors, not absorbed by the fumigated com- modity, have been removed. While the vault is being unloaded, the exhaust fan should be kept running. Sometimes auxiliary fans may be needed to supply fresh air for the workmen unless they wear gas masks. Methyl bromide and mixtures of methyl bromide with chloropicrin or ethylene dibromide are most efficient for use in treating dried foodstuffs in atmospheric vaults, although hydro- cyanic acid, chloropicrin, and many other fumigants can be used. Dosages depend on the commodity to be fumi- gated, the quantity involved, and the fumigant. Fumigation by vacuum consists of placing the commodity in a gas-tight steel chamber, removing the air, and replacing it with a gas lethal to insects. By this method a more rapid penetra- tion of commodities by the gas is ob- tained than in atmospheric fumigation, and insects are reached and killed faster than in an atmospheric vault. The removal of a large part of the oxygen from the chamber makes the insects more susceptible to fumigants. The length of exposure ranges from i to 3 hours, compared to 10 to 24 hours under atmospheric conditions — an im- portant factor in industries where speed is essential in handling food- stuffs. Vacuum fumigation has several other advantages. At the end of a fumigation, the removal of the fumi- gant from the treated commodities can be speeded up by a process known as air washing. It consists of drawing a vacuum of 27 inches or more and breaking it with air. There is little danger that workmen will enter a vault undergoing fumigation, and the dan- 349 ger from breathing the vapors during the unloading of a vault is lessened. It is advantageous to draw as high a vacuum as possible and to hold the vacuum throughout the exposure. By circulating the gas in the tank for 15 minutes after it is introduced, the dis- tribution of the fumigant will be aided greatly, and much less fumigant will be needed to effect a kill than if the gas is not circulated. The fumigants usually employed in vacuum vaults are methyl bromide, hy- drocyanic acid, and a 1-9 mixture of ethylene oxide and carbon dioxide. Dosages vary with the fumigant, the commodity, and the length of the ex- posure. The shorter the exposure, the larger the amount of the fumigant required. The fumigation of individual pack- ages of foodstuffs is practiced in some food industries, but is expensive be- cause comparatively large quantities of fumigant must be used. In this method, the individual packages trav- eling along a belt pass under an appli- cator, which automatically injects a certain amount of fumigant into each one. The packages are then sealed. Each package is its own fumigation chamber. The method was first used extensively in the United States to treat packages of dried fruit, for which ethyl formate, methyl formate, and iso- propyl formate have been used. Other products so fumigated are dried soup- stocks, rice, dog biscuits, popcorn, and such. Besides the formates, acrylonitrile in admixture with carbon tetrachloride has been used for individual packages. R. T. Cotton has been an entomolo- gist in the Bureau of Entomology and Plant Quarantine since igig and has been in charge of field research on the control of the insect pests and stored- grain and milled-cereal products since IQ34- He has specialized on fumigation and other methods of controlling the insect pests of stored foodstuffs. Dr. Cotton holds degrees from Cornel' University and George Washington University. 97