ay Pe is 7 i { i H wa i 1 r AD er no] i 1 i i i i ; i en ee iy i t in 7 iL 1 60 I "et a *y SB 818 C3576 ENT CIRCULAR No. 93. Issued August 22, 1907. United States Department of Agriculture, BUREAU OF ENTOMOLOGY. L. O. HOWARD, Entomologist and Chief of Bureau. THE SPRING GRAIN-APHIS OR SO-CALLED “ GREEN BUG.” (Toxoptera graminum Rond.) By F. M. WEBSTER, In Charge of Cereal and TForage-Plant Insect Investigations. The early history in America of the spring grain-aphis (figs. 1, 2 b } ee p £ which has come to be generally known as the “ green bug,” was pub- lished in Circular No. 85 of this Bureau, and need not be reproduced here. Suffice it to say that it is an imported species, long known to be destructive to growing grain in Europe, but not known in this country fo} (==! a , “ prior to 1882, and not as a destructive insect until 1890. The litera- Fic. 1.—The spring grain-aphis or “green bug” (Toxroptera graminum) : a, wingless fe- male; b, larva; c, pupa. Much enlarged (from Pergande). ture relating to this, the third and perhaps most destructive outbreak of the pest, is so misleading that this publication seems necessary in order to prevent misapprehension among farmers, and to afford them all possible helpful information in advance of future similar out- breaks. 4631—No. 93—07——1 9 DISTRIBUTION IN THE UNITED STATES. The insect is usually common, and was found, during the present year, from Colorado and New Mexico to the Atlantic coast, approxi- mately covering the area south of latitude 41° and east of longi- tude 105°. Within this territory its area of destructive abundance, as well as the severity of its attack during any year, will be regulated by two factors: First, the presence of young growing wheat, vats, barley, or rye; and, second, weather conditions favorably to its rapid increase and unfavorable for the development of its natural enemies. It will breed freely in the fields from an altitude of less than 20 feet above sea level on the Atlantic coast to an elevation of nearly 5,000 feet in Colorado, and from extreme northeastern North Dakota and Vic. 2.—The spring grain-aphis er “green bug” (Toxoptera graminum) : a, winged mi- grant; b, antenna of same; a, much enlarged; b, highly magnified. (Irom Pergande.) northwestern Minnesota, southern Wisconsin, northern Indiana, and southern Pennsylvania southward to extreme southern South Caro- lina and southern Texas. Yet, with normal weather conditions during the first four or five months of the year, these facts signify absolutely nothing, for this insect will not be abundant enough to attract any attention by its injuries. THE RELATION OF TEMPERATURE TO DESTRUCTIVE OUTBREAKS. In order to make the relation between temperature and destructive outbreaks of the “ green bug” perfectly clear it will be necessary to restate some of the facts given in Circular No. 85. This insect, as with other closely allied species of aphides, repro- duces in two ways. As cool weather approaches in autumn there occur in greater or less numbers both males and females, the latter [Cir. 93] 3 depositing eggs, and it is in the egg state only that, under normal weather conditions as to temperature, the “green bug” passes the winter, and it is from these eggs that it originates in the spring. But from spring to fall there are neither eggs nor males; all are females, and these give birth to living young in a series of genera- tions. With the normal cold of early winter these females gradu- ally disappear and only the winter eggs remain; but if the winter temperature is mild, and the temperature of the following spring abnormally cold, the summer method of reproduction continues throughout the winter and during spring. The “ green bug” will breed freely in temperatures ranging from 100° F. down to below 32°. As the young mature in eight days and themselves begin to give birth to young, it will be seen that an excep- (a) ¥ Fic. 3.—Lysiphlebus tritici, principal parasite of the spring grain-aphis or ‘‘ green bug:” Adult female and antenna of male. Highly magnified (original). tionally mild winter followed by an abnormally cold spring offers the best possible conditions for the excessive increase of the pest, which would ordinarily begin breeding only in spring, and from the eggs. With excessive reproduction and the destruction or aging of its food plants, this insect develops a corresponding abundance of winged migrating females, which are the means of the spread northward or outward from original centers. The “ green bug” in normal years—that is, when its breeding be gins in spring—is effectively held in check by its natural enemies, and notably by a minute, black, wasplike insect, Lysiphlebus tritici Ashm.) (fig. 3) that deposits eggs singly in the “ green bugs,” the grub hatching from the egg feeding internally on the bug and de- [Cir. 93] A = stroying it (see fig. 4). Other natural enemies are the larve of cer- tain predaceous flies, and the larve and adults of lady-beetles. The little wasplike parasite first mentioned, however, is the one that keeps the “green bug” down in normal years, and in years when the latter is most abundant finally overcomes it, as was the case this year in Kansas, North Carolina, and other States in the more northern part of the range of the pest. Unfortunately this parasitic wasp—as with the other beneficial insects—is active only while the temperature is above 56° F., or at least 25° above that at which the “ green bug” breeds freely; and herein is the whole secret of the irregular disastrous outbreaks of the “oreen bug” in grain fields. As accounting for the outbreak this vear (1907), the “ green bug ” had had a whole winter and the follow- ing late spring in which to breed and multiply unmolested, and it accomplished its principal damage, as in Texas and southern Okla- homa, before it was warm enough for the parasite to increase sufficiently to over- come it. As further illustrative of the important bearing of weather conditions, it is found that in the case of the three important outbreaks of this insect, namely, for the vears 1890, 1901, and 1907, the temperature for the first five months of each of these years, Including the latter part of winter and spring, was above the normal for the winter months and below the normal for the spring months; in other words, warm Fic. 4.—Wingless female of ‘‘green bug,’’ containing larva of the : ; parasite Lysiphlebustritici. Much winters and cold, late springs. Mae pa The little parasitic wasp which is so useful in the control of this pest is native to this country, widely distributed, and every year does its work with the “ green bug ” and with other aphides. It is always present in grain fields, as shown by its appearance every year, to war on these pests whenever the weather conditions make its breeding and multiplication possible, and its rate of breeding is so rapid (there being a generation about every ten days) that with a week or two of favorable weather it gains control over its host insects and destroys them. ALTERNATIVE OR SUMMER FOOD PLANTS. In 1890 the writer found that orchard grass (Dactylis glomerata) was a common food plant of the * green bug,” but so far this has not been found as especially attractive to the pest during summer. although Mr. Paul Hayhurst has found it occurring sparingly on this [Cir. 93] 5) erass about Washington, D. C., up to the time of publication of this circular. Mr. E, O. G. Kelly and the writer found it quite common on the underside of the lower leaves of corn at Sharpsburg, Md., July 9, and Messrs. Kelly and Hayhurst also found it on corn near Washington, D. C. Mr. W. J. Phillips found it breeding on Hor- deum pusillum in Oklahoma, and the same food habit was cbserved in Kansas by the writer, by Mr. Ainslie, and by Prof. E. A. Popenoe. Both Mr. Ainslie and Mr. Phillips observed it breeding on marsh foxtail (Alopecurus geniculatus) in Oklahoma, but observations on all three of these grasses were made in March, April, and May. Mr. Ainslie found it breeding on blue grass (Poa pratensis) at Welling- ton, Kans., and Prof. C. P. Gillette mentions this insect as infesting this grass in Colorado. Strangely enough, and happily so for our investigations, a quite serious outbreak of the * green bug ” is at pres- ent (July 25, 1907) in progress on the grounds of this Department at Washington, and chiefly on blue grass, though the insect is also breeding on crab grass (Syutherisma sanguinalis). This outbreak on the grounds of the Department of Agriculture is notable in some respects, in that the continued close cutting of the blue grass has supplied a continual fresh, tender growth as food for the “ green bug,” thus preventing the development of winged females to escape; and especially is it notable in view of the total lack of Lysiphlebus tritici, the most important of the natural enemies of this aphis. The only natural enemy found in this case feeding upon the “ oreen bug” was the diminutive black and yellow lady-beetle (/7y- peraspis undulata), not previously known in connection with this pest. When this outbreak became known to Mr. E. M. Byrnes, superin- tendent of Experimental Gardens and Grounds, he at once had the entire infested block sprayed with a solution of one-half gill of rose leaf nicotine to each gallon of weak soapsuds. The application was, however, ineffective. Four days later a strip through this plat was thoroughly saturated with a strong solution of barnyard manure, inade by soaking the manure in water. While there was no evidence that this killed any of the “ green bugs,” after nine days the pest was notably less on this area than where the application of manure solution was not made. A series of experiments was then undertaken under the writer's direction by Mr. E. O. G. Kelly, as follows: Tobacco dust was applied at rates of 4, $, and 1 pound to each 100 square feet, but after over a week had elapsed from the date of appli- cation no effect was to be observed and no dead insects were found. Kerosene emulsion was applied at 8 and 10 per cent strengths. There was practically no difference in the effect of these two strengths, [Cir. 93] 6 and st the end of nine days no “ green bugs ” were to be found on the areas so treated. Also there was no perceivable injury to the grass. Whale-oil soap solutions, varying in strength from one-fourth of a pound to 2 pounds of soap to each 5 gallons of water, were applied to similar areas. In this case the stronger solution injured the grass slightly, but not permanently; in the*case of the lesser strengths there was no injury whatever. The effect on the * green bug ” was the same in every case. They were not only literally exterminated over the areas treated, but the applications seemed to protect from a reinfesta- tion. In case of even the weakest solution an examination five days after the application was made revealed the “ green bugs ” in myriads and breeding freely on the untreated space, while but 8 inches away and on the treated area living bugs were scarcely to be found, though the dead were to be observed almost as abundantly as were the living on the space untreated. It must be remembered, however, that these experiments were carried out in grass kept. closely cropped by frequent use of the lawn mower, and the results obtained in no way reflect upon similar experiments carried out by Messrs. Ainshe and Phillips in the grain fields of Oklahoma. INVASION OF 1907. ee A better appreciation of the interrelation of the “ green bug ” and its principal parasitic enemy can be conveyed by giving a chrono- logical statement of our investigations of the very disastrous inva- sion of the * green bug ~ durmg the winter and spring of 1907. The first rumors of injuries by this pest came to us early in Janu- ary from east-central Texas, where the * green bugs ” were reported to Mr. W. D. Hunter, in charge of cotton boll weevil investigations of this Bureau, as attacking fall oats. During this month in Texas, east of a line drawn from near Gainesville through Abilene and San Antonio to Galveston, the temperature was 9° above the normal. Within this area was a smaller one, the boundaries of which may be indicated by a line drawn from Texarkana to Fort Worth, Waco, and Joaquin. Over this latter area the temperature for the same month was 12° above the normal, and within this area the pest began its work of destruction. Also, judging from data received later, the pest began to breed rapidly in fall-sown oats in southern South Carolina, where the tem- perature was from 6° to 9° above the normal. During February all over the region west of the Mississippi River and the Great Lakes the temperature was above the normal, and in the Carolinas it was only shghtly below. During this month much damage seems to have been done in Texas, and there is every [Cir. 93] 7 probability that the pest was breeding freely in the Carolinas, though it had not yet been reported from the latter States. Up to this time the outbreak in Texas was being investigated by Mr. E. C. Sanborn, an agent of this Bureau detailed to the Texas Agricultural Experiment Station. The first report of the insect outside of Texas was under date of March 6, from Mr. C. H. Drake, of Summers, Ark., who reported that “a small green bug” had destroyed the wheat in spots in the fields in his locality. This letter, received March 11, led to the prompt dispatching of Mr. C. N. Ainshe for the West, with full instructions to investigate the out- break thoroughly, to experiment with measures for destroying the insects in the fields, especially over the spots where they seemed to be most abundant, and to determine what could be accomplished in checking the ravages of the pest by the early introduction of natural enemies into the infested fields. Arrangements were perfected with Mr. W. D. Hunter to ship to Arkansas living parasites (Lysiphlebus tritici), which were then abundant in Texas, and the same train by which Mr. Ainshe reached Summers brought several boxes of the parasites. These parasites were promptly placed in the infested fields and liberated March 18. But Mr. Ainslie found that both the lady-beetles and the little wasplike parasites (Lys/phlebus tritict) were already present at Summers and near-by points in greater num- bers than could possibly have been introduced. The latter were simply everywhere, running about over the young grain plants and placing their eggs in the bodies of the “ green bugs.” Clearly the importation of parasites would be useless under these conditions, and Mr. Ainslie, hoping to find a more favorable field for the introduction of parasites or direct experimental work with rem- edies, proceeded, on March 23, for various points in Oklahoma. At Chandler and Guthrie, March 23 to 25, the pest was very abundant, but the conditions at these points were again even less favorable than in Arkansas for the introduction of parasites. For example, on one blade containing about 150 of the “ green bugs ~ Mr. Ainslie counted 25 that were parasitized. He then proceeded to Kingfisher (March 26) and here found the parasites apparently less plentiful than at the other points visited, which were to the eastward. Arrangements had been made with Mr. Hunter to furnish parasites from Texas fields when these were called for, and in response to a telegraphic request he dispatched a quantity of material collected by his assistants in Texas and in southern Oklahoma. When this was received and the parasites liberated in the infested fields, native para- sites were already issuing in great numbers. In other words, the parasite was already beginning unaided its active work of control. [Cir. 93] 8 By March 27 the * green bugs” developed winged adults in great numbers, and these seemed to drift northward. Mr. Ainsle was instructed to look for a region to the northward where the “ green bug ” was just starting, and he made his first stop, March 30, at Wellington, Sumner County, Kans., and found the con- ditions there such as to offer a favorable field for experimentation with parasites. By April 1 the fields about Wellington were generally though sparsely infested with the “ green bug,” represented in many cases by winged females that had seemingly migrated to these fields and were giving birth to their young. Lady-beetles were common, but none of the Lysiphlebus was observed. After a couple of days spent in the vicinity of Wellington, Mr. Ainslie returned to Kingfisher to secure parasites for introduction into southern Kansas, but in the meantime severe weather, accompanied by heavy frosts, had pre- railed, and the parasites formerly abundant at Kingfisher had become exceedingly scarce. By the 5th, however, he began again to find very many parasitized “ green bugs,” and by the 7th the parasites themselves began to appear again.