SB 818 C578 ENT Issued May 17, 1913. Ur S-DEPARTMEND OF ‘AGRICUL TORE, BUREAU OF ENTOMOLOGY—CIRCULAR No. 172. L. O. HOWARD, Entomologist and Chief of Bureau. THE RED SPIDER ON COTTON. BY E. A. McGREGOR, Entomological Assistant. ae WASHINGTON : GOVERNMENT PRINTING OFFICE : 1913 78463°—13 BUREAU OF ENTOMOLOGY. L. O. Howarp, Entomologist and Chief of Bureau. Cc. L. Mariattr, Entomologist and Acting Chief in Absence of Chief. R. S. Crirton, Executive Assistant. W. F. Tastet, Chief Clerk. F. H. CHITTENDEN, in charge of truck crop and stored product insect investigations, A. D. Hopxins, in charge of forest insect investigations. W. D. Hunter, in charge of southern field crop insect investigations. F. M. WesstTER, in charge of cereal and forage insect investigations. A. L. QUAINTANCE, in charge of deciduous fruit insect investigations. E. F. Puinuies, in charge of bee culture. D. M. Roaers, in charge of preventing spread of moths, field work. Roituia P. Currin, in charge of editorial work. MABEL CoxLcorD, in charge of library. SOUTHERN FIELD Crop INSECT INVESTIGATIONS. W. D. HUNTER, in charge. W. D. Pierce, G. D. SmitH, J. D. MircHELL, Harry PINKUS, B. R. Coan, R. W. MoreLAND, A. W. J. Pomeroy, engaged in cotton-boll weevil investigations. F. C. Bisuorr, A. H. JENNINGS, H. P. Woop, W. V. KING, engaged in tick life- history investigations. A. GC. Moraan, G. A. Runner, S. E. Crums, D. C. PARMAN, engaged in tobacco imsect investigations. T. BE. Hottoway, E. R. BarBer, engaged in sugar-cane insect investigations. BE. A. McGrecor, W. A. THOMAS, engaged in red spider and other cotton insect investigations. J. L. Wess, engaged in rice insect investigations. R. A. Cootry, D. L. VAN Dine, A. F. Conran, collaborators. Il pum BLS COPIES of this publication may be procured from the SUPERINTEND- ENT OF DOCUMENTS, Government Printing Office, Washington, D. C., at 5 cents per copy CIRCULAR No. 172. Issued May 17, 1913. United States Department of Agriculture, BUREAU OF ENTOMOLOGY. L. O. HOWARD, Entomologist and Chief of Bureau. THE RED SPIDER ON COTTON. (Tetranychus bimaculatus Harvey.) By E. A. McGrecor, Entomological Assistant. INTRODUCTION, The minute reddish mite commonly known as the red spider appar- ently is becoming one of the serious enemies of the cotton plant in the United States. It is now prevalent throughout the cotton belt, and, especially in the last few seasons, has caused much alarm among the farmers in certain sections. Seasons of excessive drought are favorable to the development of the mite, and at such times the pest increases so rapidly that the damage often becomes severe before its presence is detected. Red-spider infestation is frequently miscalled “rust” by farmers, since infested leaves soon turn deep red on their upper surface. Such leaves, however, if examined underneath, reveal the presence of the red spiders and the inconspicuous webs behind which they are feed- ing and laying their eggs. HISTORY AND DISTRIBUTION, With the exception of an outbreak in Louisiana, reported by Prof. H. A. Morgan in 1893, severe occurrence of the red spider on cotton had not been reported until 1903, at which time complaints of dam- age came from South Carolina and Georgia. In 1904 Mr. E. S. G. 1 This circular is based primarily upon work done at Batesburg, 8S. C., in 1911 and 1912, under the direction of Mr. W. D. Hunter, but also ineludes the results of observations by Messrs. G. A. Runner and H. F. Wilson during the two preceding seasons. It is a revision of and supersedes Circular No. 150. 1 oe THE RED SPIDER ON COTTON. Titus, then of this bureau, found severe infestation in fields about Batesburg, S. C., and the following year he reported severe injury in North Carolina, South Carolina, Georgia, and Alabama. Since then the additional records of Dr. F. H. Chittenden and Messrs. G. P. Weldon, D. T. Fullaway, and others, as well as the writer, have established the presence of this red spider from Maine to Flor- ida and westward to California and in the Hawaiian Islands. With the exception of western Colorado and portions of California no complaints of an alarming nature regarding this pest have come to the writer’s attention other than from ; Y the southeastern portion of the cotton \ i belt. It is here that the red spider ap- \\ yy pears to have found the conditions most , \ y) /) suitable for its development. \\ . iy The red spider was described by Har- WX \ Ms ‘i vey in 1893 as Tetranychus 2-moculatus. \APX ap Harvey considered it quite distinct from tod aN the European species 7’. telarius L. His types were from Orono, Me. In 1907 Prof. H. A. Morgan published observa- tions on the cotton mite, and apparently ) 1 h accepted the determination of the species fi ; \ \ as 7. telarius. In 1900 Mr. Nathan Banks IN /\ \ described the cotton mite under a dis- f/ | i tinct name—TZetranychus gloveri—but fy IN from the study of additional specimens | i has now concluded that the name is | | synonymous with Harvey’s 7. bimacu- ’ latus. Specimens of red spiders on cotton Re, ORS MC gcse from South Carolina have upon two re- chus bimaculatus: Adult fe. cent occasions been determined by Prof. male, Greatly enlarged A. Berlese as the continental species— Tetranychus telarius. As there seems to be considerable doubt on this point, we shall follow Mr. Banks in considering the form with which we are dealing as Tetranychus bimaculatus. DESCRIPTION. The typical female (fig. 1) is 0.50 mm. long by 0.26 mm. wide, broad-oval, widest in front, and the legs are shorter than the body. Its color is usually brick-red. The typical male is 0.27 mm. long by 0.15 mm. wide, oval-wedge shape, narrowed behind, the legs about equaling the length of the body, and its color is usually reddish amber. Individuals of both sexes usually possess on either side of the body a dark spot, caused by the food contents. This spot may vary greatly in color, size, and outline. Similarly, depending upon THE RED SPIDER ON COTTON. 3 the host plant and upon locality, the general color of the red spider is subject to great variation. The eggs are very minute, but in proportion to the mites they are large. They are perfectly round, and when first laid are as clear as water. Each female lays (in the months of June, July, and August) about 50 to 60 eggs, although her brood may in cases exceed 100. Usually about 6 eggs are deposited per day for a period of 9 or 10 days. Less than 3 eggs or more than 10 are rarely deposited in one day in summer weather. During the warmer months the eggs hatch in about 4 days after being laid. The newly hatched red spider, called the larva, is almost round, has six legs, and is nearly colorless. It begins feeding at once, and (in summer time) after two days’ activity it becomes quiet, darkens in color, casts its skin for the first time, and emerges as the primary nymph with an added pair of legs—making eight. The primary nymph becomes larger in size and darker in color, but gives no indication of sex. Feeding continues actively, and at the termination of another two-day period (in summer months) a second molting occurs, which gives rise to the third stage—the secondary nymph. With this last nvmphal stage the first indication of sex appears. As with the preceding stages, two days usually suffice in summer for the completion of this period, at the end of which time the skin is shed for the third time and at last the perfectly developed adult mites appear. At the occurrence of each molt the skin splits in two, crosswise, and the creature crawls out of the two halves. The old cast skins are usually to be seen in abundance among the fibrils of the web. THE ADULT MALES AND FEMALES COMPARED. Concerning the relative abundance of females and males it may be said that there seems to be a predominance of females throughout the summer, but toward the approach of cold weather the occurrence of the sexes becomes more nearly equal. The period of life of the adult female varies from 17 days in midsummer to several months in winter. The male is shorter lived. As before stated, the female is decidedly larger than the male, more rounded behind, and of a much deeper color. She does not move about much, and when she does her motion is rather slow. On the other hand, when not mating, the male is frequently seen moving rapidly about. The body and legs are well beset with bristles, which are somewhat more con- spicuous in the males than in the females. In addition, the legs of the male are longer in proportion to the body than are those of the female. The legs of the first pair are especially long in the male, and are doubtless adapted for clasping. The eyes, consisting each of two orbs, one close behind the other, are situated near the front edge of the body directly over the second pair of legs. 4 THE RED SPIDER ON COTTON. MATING. Almost immediately upon becoming adult, the red spiders mate and begin egg laying. The males seem to recognize unfertilized females with ease. The first eggs are frequently deposited on the same day upon which the transformation occurs from the last nymphal to the adult stage. PARTHENOGENESIS. Experiments conducted with unmated female red spiders clearly prove that they are normally capable of laying eggs, which in turn hatch and develop into mature individuals. No tests, however, have been successfully conducted to determine the sexual fertility of the latter. GENERATIONS. The season of 1911 at Batesburg, S. C., was one of unusual drought and heat, and there were about 17 generations between March 11 and November 5. The time required for a single generation varied from 35 days in March and early April to 10 days throughout most of June, July, and. August, and to 25 days in the greater portion of October and early November. During the same period of 1912 at Batesburg only 16 generations were produced. The 1912 season was late in commencing, was somewhat cooler than that of 1911, and more showers occurred. From April 24 to September 25 there was small variation in the time required for the generations—averaging about 11 days. Two generations developed each in 9 days, which is less than was required for any brood in 1911. Thirty days was re- quired for the October-November generation. The following table presents the duration of each stage of each of the 17 generations for 1911: TaBLe I.—Development of gencrations of the red spider on cotton in 1911. | - | Incuba 7 Nymph | Nymph | Ovipo- | J pee Period covered by generation. | tion ee (1) (2) sition to El period. | P period. | period. | adult. | Days. Days. Days. Days Days. Wiest tis. | Mar. (tocar. 145240 eee ce eee - < | 13 9 6 7 35 aus ‘Aprety tow prone eo eee. 2 6 3 3 3 15 3 one Apr S30 tO May domess ose ee eee eee eee «<3 6 3 3 2 14 | May 14 tonMiay one. a5 ona eee - “1nd 5 3 2 2 12 Sepa \ Maly: 26:tONNEIG 4. 4ee come e eee ene - jas 5 3 2 2 12 G20 S.). June 7 to ume 7 so. esse Sacer eee - - 4.5 255) 2 2 11 (Snes June 1S to sune Zit seen ces ae ee 4 2 2 2 10 Bae: = June 28itowal yates ss eens eee == 3) 4 2 2 2 10 LO ae July '$to July 18: os. snes eee - - <2 4 2.5 2.5 2 il LO wualy 19ito Duly 28e case ee eee. Ie | 4 2 2 2 10 § I Ue duily:29'to Aug i7)4 Soe o os SSE - <= } 4 2 2 2 10 (Le tip 8 to Ate aya eee Fc eae. | ot 4 2 2 ps 10 hie eS AUIS 18 to AUgi29) te ee te eee - | 5 235 2 2.5 12 La ee UIE S0ito Sept. 92-550 eer eee eee eee. - oe | 5 2 2 2 ll eee ac epi. 10 to Sept: 245.2 eee eee sass 52] 5 3.5 3 3.5 15 1G 2s eee Seni25 to Oct. 10s ss eee eee eee . oe 6 3 3 4 16 1 (4 ae OcteHl to NOV. 4328.2 tat see anaes oe 9 6 5 5 25 JAG) RCM S eee a2 DUE So . ee 5.5 sak 2.7 2.8 14 THE RED SPIDER ON COTTON. 5 Table II indicates the duration of, and the period covered by, each of the 16 generations for 1912 as well as for the 1911-12 wintering generation. TasBLe I1.—Development of generations of the red spider on cotton in 1912. eee Period covered by generation. renin bere Period covered by generation. ne Days. Days Als 5s Nov. 5, 1911, to Apr. 23,1912..... i 0 fh eee AE. 310) AIG FLA SS eae oe eee 12 ee ae. ANT 28 COMMinY AD a1. -RED SPIDER ON COTTON. 9 That this pest remains more or less active throughout the winter there can be no doubt. Mr. G. A. Runner found active adults at Batesburg, S. C., on December 21, 1909. Mr. H. F. Wilson observed red spiders feeding in early February at the same locality. The writer found all stages alive on English violets on March 11, and adults as late as December 19 at Batesburg on the same host in 1911. Adults were seen alive on violets at Batesbure on February 23, 1912, follow- ing the coldest winter that section had experienced for many years. I'ic. 5.—An uninfested cotton plant, growing in same field as shown in figure 9, but just beyond the boundary of infestation by the red spider. (Original.) The finding of the active red spiders during the coldest weather is certainly an additional indication that hibernation does not take place in South Carolina. DISPERSION. How do red spiders become established upon cotton? They have no wings and their legs are very minute. Close observations reveal that on the ground they normally travel at the rate of 1 inch per 15 seconds, which, if maintained, would total 480 feet in 2+ hours. Red spiders are doubtless occasionally transferred by dogs, chickens, 10 THE RED SPIDER ON COTTON. other domestic animals, insects, and birds. Strong winds may serve occasionally to transfer them from plant to plant. It is the writer’s firm belief, however, that the chief means of dispersion is the red spider’s own efforts. When once established in a field they may be further distributed by farm hands and by stock while cultivation is bemg carried on. They also spread from plant to plant along the interlacing branches, but traps specially prepared with tanglefoot and placed in the field have proven that individuals commonly crawl from plant to plant by way of the stalk and the ground. Since the red spider apparently uses no instinct or intelligence in finding cotton plants, it follows that the pest must hit upon the cotton stalks entirely by chance. The result of this haphazard manner of migration must necessarily result often in the penetration of the spiders far into the center of fields, thus giving rise to the mistaken impression that they had hibernated at these points. Furthermore, as the likelihood of the discovery of cotton by the spider is doubtless in proportion to the thickness of the “ stand,” it should follow that the thick broadcasting of a narrow border strip along the edge of a field adjoining a source of infestation would serve as a trap crop to.intercept the majority of migrating spiders. This strip should be plowed in as soon as there seems to be danger of a general movement to the main field. (Fora practical test of this idea, see under Prevention, p. 17.) There is an old adage which has come to the writer’s attention several times the past season from the lips of old planters: “* When the pokeweed turns red, look out for the cotton *rust.’” This ex- pression, said to have been employed in antebellum days, is of con- siderable interest, since it contributes evidence tending to prove the long existence of the cotton mite in the South, as well as the function of the pokeweed. TIME OF APPEARANCE AND DISAPPEARANCE. In 1911 the work of the pest on cotton first became noticeable about June 1 at Batesburg. The past season (1912), however, red-spider work was not evident on cotton until about the last of June at that locality. From the answers to a large number of inquiries sent to farmers throughout South Carolina, the average time of first appear- ance in fields the past season is found to have been June 30. The lower (sandy) section of the State averages one-half month earlier in the pest’s arrival than does the upper (clay) section—June 25 marking the arrival time for the former and July 10 for the latter. Although the pest does not become readily noticeable on cotton until some time in June, it really establishes itself considerably earlier. Mites have oceasionally been seen by the 1st of May on seedling plants not over THE RED SPIDER ON COTTON. iik 2 inches high. In both 1911 and 1912 the pest abandoned cotton about the last of August. This makes the period of activity cover about 12 weeks. NATURE OF DAMAGE. The presence of the pest on cotton is first revealed by the appear- ance on the upper surface of the leaf of a blood-red spot. As leaves become badly infested they redden over the entire surface, become I'iG, 6.—a, Cross section of normal cotton leaf; b, cross section of cotton leaf injured by the red spider, The puncture is near lower right-hand corner. Highly magnified. (Author's illustration.) distorted, and drop. Figure 5 shows an uninfested cotton plant for comparison with figure 7, infested. The lower leaves usually are first attacked, but infestation spreads upward until often only the bare stalk and one or two terminal leaves remain. (See fig. 8.) Such plants almost invariably die. The injury to the leaf and the dis- coloration which follows the feeding of the mites are easily under- stood by referring to figure 6, which represents ,(@) the appearance of healthy cotton leaf-tissues and () the condition of the tissue after feeding by the pest. As previously intimated, the worst spots of infestation are either to be found in close proximity to yards with borders of violets or to a clump of pokeweed stalks. Large fields are probably never completely damaged, but smaller fields frequently become wholly affected. A thorough examination of all fields within 1 mile of the center of Leesville, S. C., was made with a view of deter- mining the exact status of red-spider infestation at one specific ey THE RED SPIDER ON COTTON. locality. In all, 99 fields were examined as carefully as possible. The following table presents the results: TABLE IIIl.—Results of an investigation to determine the degree of infestation of cotton by the red spider at Leesville, S. C.. 1912. : ; ‘ Number oe fone Degree of infestation. | of fields ex- Bla amined. : :: amined. Meryfacute: Sos ese 18 18. 2 Very considerable. -_. 20 20.2 Slipht sco s Cea Ts 35 39.70 NONE 22-see- te sees eee 26 26.3 Total. Xess 99 100. 0 Fic. 7.—Cotton plant in an early stage of Fic. 8.—Cotton plant in well ad- infestation by the red spider. Many leaves vanced stage of infestation by the are discolored and some of the lower ones red spider. Nearly all _ leaves, have dropped. (Original.) squares, and bolls have been shed. (Original. ) This occurrence was one of the severest and most general that has at any time come to the writer’s attention. Perhaps the most severe case In this locality (and one of the worst yet seen) was one which had its origin in a large clump of badly infested pokeweed stalks at the edge of town (fig. 9). The pest spread fanlike until it reached in one direction a point 600 feet from the pokeweed. The final affected area, semicircular in shape, comprised 13 acres, and within its confines the occurrence was general. While such a case as this was unusual, 4-acre or 5-acre spots, with 25 to 100 per cent dam- age, are frequently to be seen. THE RED SPIDER ON COTTON. hes NATURAL CONTROLLING AGENCIES. In both the seasons of 1911 and 1912 red-spider occurrence was most severe throughout July and early August. The last two weeks of August in both cases, however, witnessed so great a reduction of the pest that by the end of that month it was hardly noticeable, as a rule. This phenomenon, indeed, happens suddenly, and the agen- cies which work to produce it are unquestionably of great economic value. The ageing and toughening of the leaves at about this time may cause many mites to desert the cotton for other plants, but another factor of probably much greater importance is the increased abundance of several species of insect enemies which seem to gain dominance at just this time. ' Pic, 9.—A severe example of red-spider work in a cotton field. Nearly all plants in the _foreground are in the condition shown in figure 8. The source in this case was certain pokeweed stalks growing in the weed border seen in the upper right-hand corner of illustration. (Original.) CLIMATIC INELUENCES. As before mentioned, climatic conditions exert a marked influence upon the welfare of the pest. During times of little rainfall and high temperature reproduction goes on by leaps and bounds; on the other hand, long, heavy rains work havoc to the red-spider population. In spite of the fact that the red spiders inhabit the underside of the leaves, many are washed off by rains and many more are destroyed by the upward bombardment of sand particles, which may always be seen coating the lower leaves after storms. In fact, it appears true that a few heavy rains, especially if they continue for some time, accomplish as much toward controlling the pest as 14 THE RED SPIDER ON COTTON. anything which can be done by man during the season. From obser- vations made, both early in the season and at the beginning of win- ter, it is doubtless true that the young stages are killed by freezing weather. This naturally prevents any considerable winter increase, and in addition some adults probably perish. The influence exerted upon the red spider by preseasonal condi- tions appears to be even greater than that of the factors operating in summer weather. The winter of 1911-12 was the severest in South Carolina for many years. Hence it might be expected that the fol- lowing season would be a mild one, from the standpoint of injury by the red spider, through the assumed heavy mortality of the pest during the winter. On the contrary, as has been stated before, the 1912 occurrence was the severest on record. The only possible ex- : planation appears to em- brace two suppositions. One is that the adult. red spider is little susceptible to extreme cold. The other is that the insect enemies of the mites suc- cumb much more easily to minimum temperatures than do the mites them- selves. Both of these hy- potheses are sustained by all observations of the Fic. 10.—Arthrocnodax sp., a predaceous enemy of writer. Naturally, then, a the red spider. At left, mature predaceous larva, severe winter is precisely ee Ome, right, adult female, greatly what would most favor the subsequent increase of the red spider through the destruction of its insect enemies. Con- versely, an abnormally mild winter, free from decidedly low tempera- tures, should furnish conditions most favorable for the survival of the repressive species, and the infestation for the following season would be mild. INSECT ENEMIES. Hot weather, although favoring red-spider development, probably encourages even more the increase of insect enemies, of which sev- eral have been observed. Were it not for these inconspicuous friends of the farmer the depredations to his crops by the red spider would unquestionably be more severe. The following are a few of the more important beneficial species observed during the season of 1912 at Batesburg, S. C. THE RED SPIDER ON COTTON. 15 Arthrocnodax sp.’ (fig. 10) —During the season of 1912 a species of Arthrocnodax, a fly belonging to the family Itonide (formerly Cecidomyiide), was first noticed toward the last of June, at which time it was not at all common. It increased very rapidly, how- ever, and by the end of July had become quite abundant. It is the larva (grub stage) of this insect which assists the farmer, and the attack seems to be limited entirely to the red-spider eggs. Coming in contact with an egg, the larva instantly thrusts its piercing organ through the shell and begins to feed. Large larve will devour an egg in from one to two minutes, while the smaller larvee require more time, according to their size. When mature, the larva spins a woolly cocoon 1 mm. in length. The adult fly issues after a brief pupal stage, and the full generation requires little time. This insect is probably the most ef- fective enemy of the cot- ton mite, and to its final superabundance is prob- ably largely due the late summer decimation of the red spider. It has been recorded from the following localities: Allendale, Anderson, Batesburg. Brownsville, Fic. 11.—Triphleps insidiosus, an important enemy of and St. Matthews, S. C.; the red spider : Adult. Much enlarged. (Author’s Chase City, Va.; Macon, an Ga.; and Albertville and Boaz, Ala. It evidently has a wide distri- bution. Triphleps insidiosus Say (fig. 11).—A small anthocorid bug, Triphleps insidiosus Say, was seen as early as May 16. It is hardier than the Arthrocnodax and appears earlier in the season. Through- out July and August it is extremely common and both in the nymphal and adult stages is second only to the itonid as a red spider enemy. Coming upon a red spider, like a flash the adult thrusts its sharp proboscis through the pest’s back and proceeds quietly to siphon out the body contents. The first victim observed was “ drained ” in about five minutes, but each succeeding meal was of shorter duration as the appetite became satisfied. The actions of the nymph (fig. 12) are similar, but the individuals observed were seen only to destroy eggs of the spider. In this operation 1 The determination is by Dr. E. P. Felt. 16 THE RED SPIDER ON COTTON. the proboscis was not inserted far into the ovum, and two minutes sufficed for draining an egg. A species of lace-winged fly (Chrysopa quadripunctata Burm., determined by N. Banks) is abundant at Batesburg throughout most of the summer, and its larva is doubtless very active in reducing the pest. Two species of thrips, Huthrips fuscus Hinds and &. occidentalis Pergande, have been determined this season from cotton. They are commonly found throughout the season about red-spider colonies, and may be instrumental in spider destruction. —§ Scolo- thrips sexmaculatus Pergande has been recorded as an enemy of the red spider by Pergande and by Duffy. Lady-beetle larvee and adults of several species are com- monly seen on infested leaves. These are usually either Coc- cinella 9-notata Ubst. or Hip- podamia convergens Gueér., but a small black species, (Scymnus) Stethorus punctum Lec., is frequently observed. The larger beetles are prob- ably more intent upon cotton aphides, but the last-mentioned | species, although later in ap- pearing and not overnumerous, seems to be more restricted to the the cotton mite than are other species. Fic.12.—Triphleps insidiosus: Nymph. Greatly enlarged. (Author's illustration.) REMEDIAL MEASURES. PREVENTION. From the abundant experience of the past two years the writer has been forced to the conclusion that the eradication of the red spider must be accomplished through preventive efforts rather than repressive, if it is to be economically effected. The location of the mites through the winter and spring, their preference for the culti- vated violet and the pokeweed, and the manner of dispersion of the pest lead to the presentation of the following cultural expedients. Clean culture-—First among preventive measures against the red spider is doubtless that of exterminating the weeds and plants which breed the pest. Pokeweed, Jerusalem-oak weed, Jamestown weed, wild blackberry, and all border weeds and underbrush about fields should be burned or grubbed out during the winter or early spring. THE RED SPIDER ON COTTON. 1k’ This plan has been tested in several instances and has given complete immunity the following season. Too much emphasis can not be placed on the importance of destroying, so far as possible, all weed growth—especially the pokeweed, which should be grubbed out by the roots. Control on violets—As before stated, most cases of infestation in urban localities have their origin in borders of cultivated violets growing in near-by house yards. In several instances violets adjoin- ing fields of past severe annual infestation have been thoroughly sprayed, with the result that no red spiders appeared subsequently in these fields. The objection to this treatment is the failure on the part of the average person to persevere with the spraying until the pests have been entirely exterminated. The most satisfactory pro- cedure in such cases consists in the removal and destruction of the offending violets. Varietal immunity in cotton—From several tests conducted in different fields with numerous standard varieties of cotton, and from the information volunteered by farmers from many portions of South Carolina, data have been accumulated which clearly indicate that certain varieties are susceptible to red-spider infestation, while others exhibit considerable immunity. Careful observations on a considerable number of varieties grown for the purpose showed that Dixie “* Wilt Proof,’ Toole, Peterkin, Broadwell, and Cook suffer most (in the order named) from the attack of the pest, while Hite, Russell, Summerour “ Half and Half,’ and Cleveland showed the greatest immunity of all the varieties investigated. Broadcasted cotton.—An opportunity was accidentally provided of testing the value of thickly broadcasting cotton at the boundary of a field as a trap crop for red spiders. This cotton, intended as a cover crop, intervened between the cotton field proper and a large, heavily infested border of violets—a former abundant source of migration. The broadcasted cotton became infested and was later plowed in. The adjoining field remained free from mites. The suc- cess of this experiment would strongly indicate that the cotton trap crop is a practical cultural expedient to be used in controlling this pest. Spacing —Experiments at Batesburg, S. C., have shown that the red spider commonly travels between plants upon the ground. This shows the futility of spacing as a remedial measure. Time of planting.—There is vet much doubt as to the relative ad- vantages of early and late planting. Extremely early planting nat- urally permits the plants to develop a maximum growth of weed and fruit by the time of serious mite appearance. It is noticeable that plants of considerable size are rarely killed by the pest, nor are well-advanced bolls commonly shed from infestation. On the other hand, several fields about Leesville, S. C., which were planted as 18 THE RED SPIDER ON COTTON. late as June 20 seem to have largely escaped the infestation which was so general at that locality. Late planting, however, is almost universally objectionable to the farmer, ‘since in ordinary seasons it results in a reduction of the yield. Rotation.—In an effort to test the rotational value of other crops. numerous field crops have been planted in or near infested areas. In addition, frequent examinations have been made of a great many garden and vegetable crops in infested localities. Besides cotton, red spiders are known to occur not at all uncommonly upon the fol- lowing field crops: Cowpeas, clover, corn, hops, and watermelon. They are also found frequently on the following garden crops: Peas, beans, onion, tomato, lettuce, okra, turnip, mustard, squash, beet, sweet potato, and strawberry. A really acute infestation on corn (as above recorded) was seen at the height of the past season (1912). Cowpeas are particularly attractive to the pest, and sweet potatoes have been noted to be badly infested. Should an immune crop be found and employed, it is extremely probable that the pest would reinvade the fields upon the return to cotton culture with as great ease and quickness as it has done during any previous season, provid- ing the sources of infestation were yet at hand. Rotation, then, does not promise to contribute toward the solution of the problem. Effects of fertilizers—aA rather elaborate series of tests with fer- tilizers has been conducted in an attempt to determine whether the various applications:assisted cotton to withstand the injurious effects of infestation. Since almost no infestation appeared in these test plats it was impossible to deduce positive conclusions. REPRESSION. We have just discussed cultural measures which may kelp to pre- vent infestation. We will now consider what may be done to combat the pest when it has already gained entrance to a field. The writer believes in prevention rather than cure, in the case of the red spider. When once well established in a cotton field the pest is a most difficult one to wipe out. That it is possible, however, to eradicate the pest from infested fields has been demonstrated be- yond doubt, but in many cases the task is so tedious that only the most determined farmers will undergo the effort necessary to accom- plish the extermination. Removal of infested plants—The experiment has been thoroughly tested of pulling up and destroying the first few plants which show infestation. In such cases the operation was repeated several times. Great care was observed in locating every plant showing the charac- teristic red spots, and these were carefully taken from the field and burned. If infestation has not advanced far, this treatment is usually satisfactory, and a red-spider invasion often may be “ nipped in the bud ” and entirely eradicated. THE RED SPIDER ON COTTON. 19 If infestation has spread until a considerable patch has become involved it is sometimes advisable, in the case of a large field, to plow up all the affected portion in order to save the balance of the field. Such a drastic measure, however, should only be resorted to in ex- treme cases, and the planter concerned must be the judge of its advisability. Occasional observations of instances wherein infestation had abruptly stopped at a much-traveled road suggested the idea of plow- ing a wide swath just outside the boundary of infestation. This was attempted in one case where the occurrence had covered about 2 acres. A 10-foot ring was plowed around the spot, and all stalks, both in the swath and in the inclosed area, were immediately burned. Unfortunately the farmer did not make sure that he was beyond the outermost zone of infestation, and consequently a sufficient number of affected plants remained outside the pulverized barrier to con- tinue, somewhat, the dispersion of the pest. This idea should be given further tests, as it seems there should be great efficacy in the operation, provided the swath is kept stirred frequently. Insecticides—During the season of 1911, 26 spray combinations were thoroughly tested under conditions entirely natural. The field used for this purpose was about 1 acre in extent, and infestation had become both very general and very severe.