Circular No. 106. 
 
 ' 
 
 I nited States Department of Agriculture, 
 
 BUREAU OF" KNTOMOI.OGY. 
 
 u HI 
 
 . 
 
 itomologUt iin.l 
 
 
 BTB \\\ norm 
 
 miml, Rile) \^ j£ 
 
 Till: WHEAT 
 
 i ■ 
 
 Bj I U U BBSTl R, 
 
 ■/, Plant : 
 ind 
 i ii c I Rebvbs, 
 •eal and I 
 
 wheal strawworm (fig. I sustains about the same relations to 
 winter-wheat culture west of the Mississippi River 
 thai the joint worm (Isosoma tritici Fitch ' does 
 
 to the Cultivation Of this cereal east of this river. 
 
 Both, when excessively abundant, occasion losses 
 
 Taiying from slight to total A wheat stem at- 
 tacked by the joint worm may produce grain of a 
 more or less inferior quality and less of it ; but the 
 spring attack of the wheat Btrawworm (fig. 5) is 
 fatal to the plant affected, as no grain at all is pro- 
 duced; and while the second generation of the 
 >;inie has a less disastrous effect in the field, it 
 nevertheless reduces the grade and weight of the 
 in. 
 
 In the Ohio Valley and south of Pennsylvania 
 the ranges of these two insects overlap (fig. _' 
 both species are often to be found in the same 
 
 held, the wheat strawworm, however, being less 
 
 abundant and doing usually but slighl injury, 
 while the joint worm occasionally becomes a seri- 
 ous pest. 
 
 uing the last two yean Mr Reeves ha 
 in the investigation of this peal in the State of Was! 
 in cooperation with the agricultural experiment station of 
 thai State. With the exception of the - 
 relative to the l>..|i.i\i.>r of the peel in the Northwest, for 
 which exclusive credit i- to i the ji 
 
 senior author i- reeponsihle for the circular 
 ► See Circuit Bureau i ' Tnu 
 
 culture. I I 
 
 B1572- Cir. 
 
 : 
 
 fmitmmttjot Agri- 
 
Wesl of the Mississippi River, throughout the winter-wheat grow- 
 ing territory, the jointworm, if it occurs at all, is never destructive. 
 The wheat strawworm, on the other hand, appears to he generally 
 distributed, at times committing very serious depredations which 
 have, at least in some cases, been charged to the Hessian fly, as shown 
 by the fact that some of these ravages have occurred hi sections where 
 the Hessian fly is not known to exist. Besides, while the Hessian fly is 
 manageable with more or less difficulty, the wheat strawworm is 
 
 MJ 
 
 
 
 
 
 
 II r~\* 
 
 A 
 
 * 
 
 9 
 
 •7 i 
 
 
 
 
 
 
 \ 
 
 
 
 ■ 
 
 
 y» ( 
 
 \ • 
 V • 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 • 
 
 
 
 • 
 
 ••*••' 1 
 
 
 1 • J r- 
 
 
 r\j^i» 
 
 
 
 
 
 • ; 
 
 
 
 
 
 
 \\ 
 
 
 
 
 ^t^\ 
 
 
 
 
 
 
 S. \ * ASOSOHW Tf?/r/C/ fitch. V, I 
 
 
 Fig.2. Present known distribution <>f Isoaoma grandU and Tsosoma triti<i in the United Slates. 
 
 (Original, i 
 
 one of the easiest of all insect pests to control by entirely practical 
 measures. Indeed, it need not be allowed to become a pest at all. 
 
 DESCRIPTION'S AND SEASONAL DEVELOPMENT. 
 
 There are two generations of the insect annually, the adults of the 
 first generation (fig. 3) differing considerably in appearance from 
 those of the second (fig. 4). To the farmer they will all look like 
 minute or large, shining black ants, with or without wings, their legs 
 more or less banded with yellow, and having red eyes. Individuals 
 of the first generation emerge in April from the outstanding straws 
 and stubble. They are very small, most of them are females, and 
 many are wingless. The females deposit their eggs in the young wheat 
 plants, the stems of which at this time extend but little above the 
 surface of the ground. The egg is placed in or just below the embry- 
 onic wheat head (see fig. (i) and the larva or worm works within the 
 [Cir. 106] 
 
3 
 
 Btem, HsnalK causing a slight enlargement. When the worm is full 
 grown it will be found in the crown of the plant, having eaten out 
 i,n,l totally destroyed the embryi "I. its 
 
 bodj occupy ing i he ca^ itj t hua formed. 
 
 The females !'iL r . 3 I which deposit these e 
 being small and frequentlj wingless, are in no 
 v.;,\ fitted for traveling long distances. The 
 larva or worm of a verj light stra^ 
 
 color, indeed almosl white, with brown jaws, the 
 form of the bod} being as shown in the illustra- 
 tion. These worms develop \er\ rapidrj and, 
 iis they feed on the most nutritious pari of the 
 plant, they become robust and larger than those 
 found iii the mature straM in late summer. In 
 Ma\ the larva become fullgrown (fig. 5) and pass 
 at .nice through a short pupal 9tage. (See fig. 7. 1 
 The pupa' are at first the same color as the larvse, 
 but later change to a shining jet black. En a fVw 
 days the fully developed insects gna\* circular 
 holes through the wall- of the stem and make their waj out. These 
 adults (fig. 1 1 are much larger and more robust than the individuals of 
 
 ■lull ul 
 mm of f'Oionnj 
 il.) 
 
 Fu.. I 
 
 the lir-t generation and arc provided with fully developed, serviceable 
 wings. That they make good use "1* their wings, and scatter them- 
 Belves aboul over fields adjacent to their place of development, ia 
 
 Kir. 106] 
 
shown by their occurrence in fields of grasses (in the stems of which 
 they do not breed) situated considerable distances from wheat fields. 
 
 In ovipositing, the females of this generation select the largest and 
 most vigorous-growing stems in which to place their eggs. 
 
 The adults of the second generation deposit their eggs from early 
 May, in Texas, up to the middle of June, in northern Indiana, or 
 about the time the wheat is heading. Their aim at this time is to 
 place the eggs singly in the growing stem just above the youngest 
 and most succulent joints, which are not so covered by the enfolding 
 leaf sheaths as to be inaccessible to them (see fig. 8). Thus it is that 
 the stage of advancement in the growth of the wheat stems at the 
 time of oviposition of the summer generation of 
 females determines whether the larvae will be well 
 upward in the straw, and therefore removed after 
 harvest, or lower down and consequently left in 
 the field in the stubble. 
 
 The method of oviposition and the point where 
 the egg is usually inserted are shown in figure 8. 
 The shape of the egg after being placed in the stem 
 is also shown in the same figure. The larva forms 
 no gall, nor does it harden the stem within which 
 it develops. There is normally but one larva in 
 each joint; but if several eggs have been placed 
 between joints and produce larva' there will be 
 one in the center of the stem just above the joint 
 and others in the walls just under the internal 
 wall covering or inner epidermis. These larva 1 in 
 the walls of the straw do not, as a rule, kill the 
 stem, but their effeet is to shrink kernels, curtail- 
 ing the yield by reducing the weight. The larvae 
 develop rapidly and reach their full growth before 
 the straw has hardened. By October, in the Mid- 
 dle West, though earlier in the South, they pass 
 into the pupal stage, in winch, as a rule, they remain until early 
 spring, whereupon they develop to adults and gnaw their way out. 
 
 Fir,. .">, — Larva of the 
 wheat striuvworm (Iso- 
 soma grande): e. An- 
 tenna; /, jaw. Line at 
 right indicates natural 
 length. (After Riley.) 
 
 HISTORY OF THE SPECIES. 
 
 In June, 1880, Mr. J. K. P. 
 wheat straws containing larvae 
 Government Entomologist, w 
 straw in his field was infested 
 was inclined to fall before the 
 these straws that the types of 
 October, and later on in 1881, 
 
 [Cir. 106] 
 
 Wallace, of Andersonville, Tenn., sent 
 of this species to Dr. C. V. Riley, then 
 ith the statement that nearly every 
 by similar larva 1 and as a consequence 
 grain had fully ripened. It was from 
 Isosoma tritici Riley were secured. In 
 Mr. J. G. Barlow, of Cadet, Mo., sent 
 
Doctor Riley man) infested straws, from which adults of the spring 
 form were afterwards reared. At Carbondale, III., in June, 1882, 
 Prof. (i. II. French found the species infesting wheal fields, in Bome 
 cases '. , :: per cent of the Btraws being affected and from one to three 
 larvffl being found in «-:i«-l i Btrav. 
 
 September L6, [882, straws containing pups and an occasional 
 larva were received bj Doctor Rile) from Mr. J V Starner, Dayton, 
 Columbia County, Wash. On October I, 1883, the senior author 
 received infested Btraws from 
 Stockton, Cal., through Professor 
 French, and from these he was 
 alilt< to rear the spring form of 
 adults during the following Febru- 
 ary . On Ma) 8, 1884, In- found 
 adults abroad in a wheal field near 
 Bloomington, 111. On Ma) 9 
 oviposition was observed, larva? 
 were found in the wheat plants 
 28, and a single pupa was 
 found on the following day, all in 
 the same field of growing wheat. 
 Straw taken from thi-~ same field 
 early in the following July pro- 
 duced the spring form nf this 
 species the following spring 
 April, 1885. 
 
 \ Oxford. Ind.. dune 6, L884, 
 the senior author found large 
 w inged adults of the summei form 
 ovipositing in the stems of grow- 
 ing wheat, and in a small devel- 
 oped -tem like those observed in 
 May, about Bloomington, III., a 
 living pupa Was found. Straws 
 from the Oxford field gave pre- 
 cisely the same form the following April as did the -traw from 
 Bloomington. The larger form, observed at Oxford dune <> and 
 
 later. WAS also collected in fields nhout Bloomington a few days 
 earlier. Larva 1 of this larger form were found in wheat held- in 
 southern Illinois in May, and the adults were observed in the same 
 tion of country in late May and early June by Prof. 11. Garman, 
 at that time assistant to Dr. S. A. Forbes.* Doctor Riley de- 
 
 « here 
 big jwsition of egg 
 
 ■ Prairie Farmer, Jurj 8, 1882 
 
 6 Fourteenth Rep. State Km. 111.. L885 
 
 [Clr. i<».;| 
 
scribed this larger form as a new species, giving it the name grande. a 
 During the years 1884 and 1885, however, the senior author 
 reared Riley's Isosoma tritici from straws in which only his /. 
 grande had oviposited and his /. grande from plants to which only 
 /. tritici had access. As this last name had been applied by Doctor 
 
 Fitch many years earlier to an- 
 other insect, the jointworni, the 
 name grande must necessarily be 
 given to both forms. 
 
 BUREAU NOTES AND OBSERVA- 
 TIONS BY ASSISTANT 8. 
 
 Strangely enough, one of the 
 earliest reports of tins species 
 came from Mr. J. A. Starner, 
 Dayton, Columbia County, Wash. 
 
 Infested straws containing a few 
 larva 1 , but mostly pupae, were 
 received September 16, 1882, 
 showing that even at that early 
 date it was sufficiently abundant 
 to attract the attention of fa lin- 
 ers. Mr. D. W. Coquillett found 
 it at Anaheim and Atwater, Cal., 
 in 1SS5, while Mr. Albert Koe- 
 bele found it during the same 
 year at Folsom, Cal. It was 
 also sent to the Department by 
 Mr. J. F. Donkin, from Grayson, 
 Cal., during the same year. In 
 1885 Prof. F. H. Snow reported 
 it as doing serious injury in 
 McPherson, Morris, Osborne. 
 Ottawa, and Saline counties, 
 Ivans., and mentioned it as a 
 new pest, 6 and in 1S91 it was 
 prevalent in central and west- 
 ern Kansas.'' In 1886 Mr. Coquillett found it at Los Angeles, Cal. 
 During August, 1890, infested straws from Washington State were 
 received at the Department from Moses Bull, Pullman; J. W. Jessup, 
 Rosalia; G. W. Dunn, Tekoa ; and Milton Evans, Walla Walla. 
 
 a Bul. Brooklyn Km. Soc., KM. pp. II L2. 
 b Monthly Rep. Kans. Bd. Agr., June, L885. 
 c Loc. (it ., Feb., 1892. 
 [Clr. L06] 
 
 Fig. 7. — Isosoma grande: Pupa of summer form in 
 young wheat. I < Original,) 
 
Although there were no reports of damage in the spring, injur} was 
 
 Bufficientrj marked to attract the attention of farmers, and f i 
 
 these one frequently hears, even up to the present time, descriptions 
 and reminiscences of what Beema to have 1 mm-h the first serious out- 
 break of the pesl in the United States. 
 
 The senior author found the species abundant but not destructive 
 ni Princeton, Ind., in 1887 and again in 1902. June 27, 1893, int 
 straws were sent to the Department of Agriculture bj Mr. E. J. 
 Woodville, [ndiantown, Va. In 1905 the junior author found the 
 species infesting growing wheat about Conway, Ark., April 7. the 
 larvae being apparentlj full grown; and at Petty, Tex., April I l. he 
 found larvae and pupaa present in great numbers, in some cases ">o 
 per cciii of tli<' young wheal plants being infested He also found 
 
 half - grown larvae ;it Concordia, 
 
 -., April 21 . During the same 
 year the senior aul bor found Ian as 
 in young wheat at Lexington, N. C, 
 April 12; and at Statesville, V C . 
 on t hr follow ing day, he found not 
 only larvae in the young plants, l>ut 
 
 femalesof the spring form in tli t 
 
 of ovipositing. At Charlotte, N. C, 
 April l I, larvae and pupae were found 
 abundantly in the yoimg w heat . ( )n 
 the imIi of the same month young 
 larvae were found in young wheat 
 plants at Dublin, Va. Mr. W. J. 
 Phillips found the species quite nu- 
 merous in young wheat plants at 
 Nashville, Tenn., April 20, and 
 studied the oviposition of bothforms 
 at Richmond, Ind., from May L'ti to 
 June _'7. June 1 6 t he Bureau received infested 31 raws from Mr. 1 1 . W. 
 Joy, Hays, Cans. On May 2 I of the following year. Mr. Phillips found 
 the species abundanl at Geneva, Ind.. where the females of the summer 
 form were jusl beginning to emerge from the young wheat; and the 
 senior author observed it again at Charlotte, V ( '.. May 9, with indica- 
 tions that it had done considerable injury to young wheat. Mr. 
 Phillips observed it during 1907 at Kingfisher, Okla., and at Beloit, 
 Elans., on April 15; at Lenora, Kans.,May20; and at Kearney, Nebr., 
 Ma\ 24. In all cases it was more or less abundant, dune 1 of the 
 same year Mr. ('. \. Ainslie found adult- of the spring form at 
 Wellington, Kan-. ; from April :>0 to May 7 he found the adult females 
 of this form abroad in the fields at Sterling, Kan-. ; and on May '_'•'• he 
 found, them ovipositing in wheat ai Hay-. Kan-., and other points 
 
 [fir. 106] 
 
 " 
 of summer form 'It: a. 
 
 Won In 
 
 I, AftiT 
 
8 
 
 farther west. He found them ovipositing in durum wheat at Oakley, 
 Kans., June 3, and on the following day also in durum wheat at 
 Colby, Kans. Again, June 8, he found them abroad in wheat fields 
 at Manhattan, Kans. Females of the summer form were observed by 
 Mr. E. O. G. Kelly ovipositing at Caldwell, Kans., May 14, 1908. 
 
 May 26 to 28, 1908, Mr. Ainslie found both sexes, the females ovi- 
 positing, near Chambersburg, Pa. This is the farthest east that we 
 have any record of the existence of the species, the most eastern 
 point of occurrence previously known being at Andover, in extreme 
 northeastern Ohio, where the senior author found the larvae hi wheat 
 straw, August, 1904. 
 
 As will be observed from the foregoing, the adult female emerges 
 early in spring and deposits her eggs in the young wheat plants before 
 the latter have made much growth. By the time wheat is beginning 
 to show the heads within the enfolding sheath leaves, adults of the 
 
 Fig. 9.—Mu,pelmus aUynii: Male, much en- 
 larged. (From Riley.) 
 
 Fig. 10. — Eapelmus aUynii: Female, much enlarged. 
 I From Riley 
 
 second generation are abroad and depositing in the upper steins; the 
 larva 3 from these pass the winter as pupae and emerge as adults early 
 the following spring, exact dates of course varying with the latitude 
 and other influences. 
 
 NATURAL ENEMIES. 
 
 Probably the most efficient enemy of this species is a small, slender, 
 four-winged fly (figs. 9, 10), with a somewhat brilliant metallic body 
 and yellow legs. It has a very slight resemblance to an Isosoma, and, 
 indeed, was described by French as Isosoma aUynii, but it is now 
 known as Ewpelmus aUynii. A somewhat similar insect with metallic 
 body and yellow abdomen, SemioteUus isosomatis Riley (fig. 11), is 
 very efficient in destroying the larva 3 in the straw. Ilomoporus 
 (SemioteUus) chalddiphagus Walsh (fig. 12), and beyond a doubt 
 other chalcidids, are also instrumental in holding it in check. These 
 
 [Clr. L06] 
 
parasites are all the more efficient as the} are double-brooded, 
 developing in late summer and at once ovipositing in other larvae. 
 
 In 1908, al Wellington, Bums., Mr. K. O. G. Kelly, beginning April 
 10 with the larvae in the st ram . followed the insect up to June I, wit- 
 nessing oviposition in both the spring and summer forms. Through 
 the remainder of June and most of Jurj he was absent, and on his 
 ret urn he was unable to find the larvae in the straw, although stubble 
 nil 1 1 si raws perforated \\ ii li holes made bj the escaping parasites were 
 found everywhere in the fields. On account of the abundance of such 
 evidence in the fields, Mr. Kelly was convinced that the pest had been 
 largely overcome in that locality by E/updmus allynii. At Pratt, 
 Kan-.. November 17, a few larvae were found in wheal straws, with 
 indications that parasites had been present to a limited extent, and 
 practically the same*situation was found to occur at Sedgwick, Kans., 
 the following day. Quite significant, however, was the fact that in 
 the vicinit} of Prat t, on Sept em Iter 16, Mr. T. 1). Urbahns found in an 
 
 Pie. i 
 
 I 
 
 old wheal field not a single living fsosoma larva, while 85 per cent of 
 the straws gave unmistakable evidence by the holes in them that 
 these larva' had been present, hut were destroyed by parasites. 
 
 Quite in accord with this it will be remarked that the junior author, 
 in bis discussion of the outbreak of the pesl in Washington Mate, in 
 1908, does not mention parasites at all not because he overlooked 
 the matter, hut because he reared only a single individual parasite, 
 probably Semiotettus isosomatis Riley, ami there was no evidence "I 
 the presence of parasites in any numbers in the section of country 
 visited by him. This will perhaps throw some tighl upon the prime 
 causes of this outbreak. Besides, it opened the w,i\ for an.inti 
 duction into Washington State of the parasites affecting the pest in 
 the Bast. It i- as yet to., soon, however, to expect definite results 
 from these introductions. 
 
 In Indiana and the Middle West when the wheat i- harvested the 
 
 straw is frequently, and. in fact, almost invariably, cm <,ir between 
 
 Kir 
 
10 
 
 joints, thus leaving the larvae, if there are such in the straws at that 
 point, exposed to attack from predaceous insects. The larvae of a 
 small, slender, black and yellow carabid beetle (Leptotrachelus dor- 
 salis Fab.) crawls up the stalks, descends into the stubble, and devours 
 t he Isosoma larva?, but unfortunately its taste is such that it docs not 
 confine itself strictly to Isosoma but devours parasite as well as host. 
 The mite Pediculoides (Hetero'pus) ventricosus (fig. 13) is also an 
 enemy, gaining access to the larvae precisely as do the beetle larvae 
 previously mentioned. 
 
 PREVENTIVE MEASURES. 
 
 There are no remedial measures, since it is impossible to alleviate 
 or influence in any way the effect of larvae in the infested wheat plants. 
 As it is the larger and more vigorous plants that the females select for 
 egg-laying purposes, no amount of enriching the 
 soil will affect the numbers of the pest or lessen their 
 influence. Preventive measures are, however, sim- 
 ple, practicable, and effective. Indeed, the wheat- 
 straw worm is one of the very easiest of all insects 
 to control. The diminutive size of the female that 
 emerges in early spring renders extended migrations 
 fatal, since during that period, when the weather is, 
 of all seasons, the most erratic and liable to sudden 
 and extreme changes, it tries to avoid the frequent 
 storms by seeking protection on the underside of the 
 leaves of the growing plants. But, most fortunate 
 of all for the farmer, large numbers, if not indeed 
 much the greater proportion, of these females have 
 no wings and can only make their way from place 
 to place by crawling. 
 The advantage of all of this to the farmer is shown by the fact that, 
 where the preceding crop has been other than wheat, the injury to the 
 young wheat plants in spring is confined to the margins, and especially 
 where such margins border on other fields that have been devoted to 
 wheat for a number of consecutive years. Mr. Reeves, as will be ob- 
 served further on in this publication, has been able to measure the 
 relative diminution of attack from the borders of the field inward 
 toward the center. Similar observations had previously been made 
 in other sections of the country. One of the earliest notes made on 
 this species by the senior author was in June, 1884, in a field of wheat 
 sown the previous autumn, among corn, the land bordering to the 
 south on a stubble field. The following spring there were many 
 wheal plants injured along the south margin of the field, but none 
 could be found along the northern border. With the appearance of 
 
 [Cir. 106] 
 
 Fig. Vl.—Pcdiculoidfs 
 venlricosus Newp., a 
 mite which destroys 
 the larva. Much 
 enlarged. (After 
 
 Marl 
 
II 
 
 the seconil generation, composed of large, vigorous females with well 
 developed wings, the diffusion would become comparatively general, 
 and, therefore, the second year, If the held were to be continued in 
 wheat, would find the infestation more nearly uniform throughout. 
 
 li is significant, t"", thai the pesl becomes much more destructive 
 
 the longer w heal is groM n cont inuousl) on the sa ground. As early 
 
 as 1882 Prof. <i. II. French observed thai while 93 per cenl of the 
 straws were infested in a field where the same grain had been grown 
 the year before, in a field where clover had been grown the previous 
 year only aboul 5 per cenl of the straws were infested. At Petty, 
 Tex., April I I. 1905, the junior author found a field of wheal thai had 
 been in this grain for two consecutive years very seriously injured. 
 There were spot- several yards square where the ground was bare, 
 and surrounding these were an 'a- where half of the living plants were 
 infested. At Sawyer, Kan-., in an old field, Mr. Urban ns found that 
 
 per cenl of the straws gave unmistakable evidence of having been 
 in IV- ted. All of this points conclusively to the efficiency of a rot at ion 
 of crops that will eliminate the grow ing of v. heat two years in succes* 
 si.m on the same land. When- for any reason this can nol be dour, 
 tlit* measure nexl in efficiency will be the burning of the stubble and 
 outstanding straw during the fall or winter. 
 
 RELATIONS OF THE WHEAT STRAWWORM TO WHEAT PRODUCTION IN 
 
 THE NORTHWEST 
 
 By Geo I R 
 
 The wheat -t raw worm, by reason of h- relation to both the climate 
 and the farm method- of the mosl productive portion of the Pacific 
 Northwest, is capable of great injury to wheat growers. Though 
 it- greatest inroad- are made upon spring wheat, it is winter and 
 volunteer wheal which render those inroad- possible. It- work is 
 of such nature that it passes unobserved, mile— an attack of unusual 
 erity causes the farmer to scrutinize his fields more closely than 
 he is accustomed to do. The constant small losses occasioned by 
 this insect and the possible destruction of the crop over considerable 
 area- may be prevented by such measures as every good farmer 
 
 employs to maintain the fertility of his -oil. 9ecure a good - I bed, 
 
 retain moisture, and eradicate weeds. 
 
 The strawworm may be found wherever wheat is grown in the 
 Columbia basin, thai great and fertile plain of varying topography 
 and climate which i- inclosed by the Okanagan Highlands on the 
 north, the Rocky Mountain- on the east, tin 1 Blue Mountain- on the 
 south, and the Cascade Mountains on the west; bul it is not equally 
 destructive over all thai territory. The central and western parts 
 of this basin, except iii the (ioldcndale and Horse Heaven districts, 
 
 [fir 106] 
 
12 
 
 are arid, producing little or no wheat, and a strip cast of this, known 
 as the dry-farming region, where the scanty rainfall makes it nec- 
 essary to summer-fallow the land every second year, is by that 
 means protected from serious injury; but the eastern zone, a strip 
 200 miles long, included between the dry -farming area and the upper 
 limit of wheat culture at the elevation of 3,000 feet, affords ideal 
 conditions for the production of small grains and also for the prop- 
 agation of this insect. The climate is characterized by mild winters 
 suitable for the growing of winter wheat — cold, wet spring weather, 
 such as is desirable for all small grains, and hot, dry summers per- 
 fectly adapted to the maturing and harvesting of the crop. The 
 soil is such that after more than thirty years of almost continuous 
 production it still yields 30 to 40 bushels of wheat per acre. These 
 conditions encourage the seeding of wheat year after year upon the 
 same ground, and such a practice, together with the presence of 
 winter or late volunteer wheat, supplies the requirements for the 
 unhindered multiplication of the strawworm. Owing to the drought 
 of summer the volunteer wheat usually does not germinate until 
 about the same time as the fall-sown wheat, and the significance of 
 this fact appears when we come to study the insect's life history. 
 
 This Isosoma passes the winter in the larval (fig. 1) or pupal 
 (fig. 7) stage and reaches maturity early in April. It gnaws its 
 way out of the stubble in which it has hibernated and deposits its 
 eggs in the heart of the young plants of winter or volunteer wheat 
 (fig. 6), which must be near the place of emergence, since this early 
 generation is largely wingless (fig. 3), and its effective range as 
 measured by the spread into an adjoining field is not much more 
 than 12 feet. The food plants of the early generation may be sup- 
 plied by winter wheat or by volunteer wheat not too far advanced 
 to attract the insect. The oviposition is finished before any spring 
 wheat is up, so that that crop suffers no attack from the first genera- 
 tion. The winter and volunteer wheats lend an essential aid to the 
 perpetuation of the species. 
 
 The larva (fig. 5), when it hatches from the e^, is surrounded by 
 the most delicate tissues of the plant and feeds upon them, destroy- 
 ing the young head and thus excavating a residence to accommodate 
 its rapidly increasing size. The outer, tougher layers, which it 
 does not attack, gradually become fleshy, stiff, and brittle, forming 
 a hollow, bulblike swelling at the base of the plant. This enlarge- 
 ment is discernible from the outside of the infested plant, and its 
 presence may be verified by squeezing the crown of the plant be- 
 tween the lingers, whereupon the bull) at first firmly resists pressure 
 and then breaks open, disclosing the larva. The infested plant 
 suffers a change in external form, which causes it to resemble wheat 
 
 [Cir. 10C] 
 
13 
 
 injured l>\ the Hessian fly and by Meromyza, but it may usually 
 be distinguished from the former by the curly and slightU flesh} 
 form of the blades; from the latter l>\ the absence of the brown, 
 threadlike central Bhoot, as well as the form "f the blades; and from 
 l>"th 1>\ the presence of the crisp l>ull> at the crown. The lower 
 blades are broad, dark bluish green, curl} . and slightly fleshy, because 
 they arise between the rool and the point of injury and thus receive 
 the Bap that is being diverted l>\ the larva from the main growing 
 point of tin' plant. The plant, when attacked thus earl) at the 
 heart, remains low, does not produce any head, and perishes when 
 the lower blades of the plants naturally die, about the middle of 
 June. At about the Bame time the adults of the BecoriH genera- 
 tion (fig. H begin to emerge from these plants and deposit eggs 
 upon the wheat. These insects, as were those of the first generation, 
 are nearly all females, and the} also reproduce withoul the assist- 
 ance of males, Imi they differ from the earlier form in being lai 
 and in possessing wings, which enable them to attack fields at a 
 distance from their origin. Thej prefer the younger plants, and as 
 the Bpring wheat is small at this time, while the winter wheat is old 
 and tough, the Bummer attack is concentrated upon Bpring wheat. 
 if there be any in the vicinity. Thej attack it in much the Bame 
 way that the spring generation employs upon the young winter 
 wheat, and with the Bame result; the smaller plants simulate a 
 luxuriant growth, but remain short and soon die: the larger ones 
 may produce heads, a few inches above ground, but such heads 
 contain no kernels. Thirty acre- of spring wheat near ( lolfax, Wash., 
 were completely ruined in this manner in the summer of 1 
 
 Since spring wheat is not so extensively grown as formerly in this 
 region, the summer adults are often compelled to oviposit in winter 
 wheat, and in those cases the plant i- le— severely injured, because 
 it is old and tough; hut the larva of the insect and the resulting 
 adult take the same form and size w hether they develop in the tender 
 Bpring wheat nr in the more mature winter wheat. The succulence 
 of the former does not cause an increase in the size of the larva' which 
 feed upon it. nor does it produce a larger proportion of winL r <'d indi- 
 viduals among the adults. On the contrary, it seems that austere 
 and unfavorable circumstances tend to increase the numher of winged 
 adults, as a large percentage of those reared from stubble collected 
 at Goldendale, St. Germain, and Waterville, Wash., in the arid dis- 
 trict, have wings. The egg is placed in a tender joint near the In 
 (fig. 8) and the larva hatches and gnaws the inner surface of the 
 stalk, too late to prevent the formation of the head, hut early enough 
 to shorten it and hinder the filling of the kernels. To ascertain the 
 amount of this injury 1,452 heads taken from an apparently unin- 
 
 [Cir. 106] 
 
14 
 
 jured field of winter wheat near Pullman, Wash., were dried and 
 weighed. Thirty-nine per cent of these were afterwards found to 
 have grown upon stems damaged by the strawworm, and the average 
 weight of the J leads from these damaged stems was 22 per cent below 
 the average of healthy heads, although no difference in size was 
 noticeable until the heads were placed side by side, when it was found 
 that most of the larger heads were those from uninjured plants, 
 while most of the shorter ones were from infested plants. Between 
 heads of the same size there was a difference of 7 per cent, which 
 could not be detected except by weighing. 
 
 Both the work of the spring brood in fall wheat and that of the 
 summer brood in spring and winter wheat are so carried on that the 
 owner does not know r of Ins loss unless it amounts to a very large per- 
 centage of the crop, and even then he may attribute it to some other 
 cause. The presence of a large number of unusually rank and appar- 
 ently sturdy plants among the winter wheat does not seem alarming, 
 but the reverse ; and there is no other indication of work of the spring 
 brood. Later, when these infested plants begin to die they are con- 
 cealed by the yellowing blades of the healthy plants, so that even the 
 trained entomologist can hardly find them. The same is true of the 
 spring wheat which is attacked by the summer brood, while winter 
 wheat plants attacked by the summer brood are, as we have just 
 shown, so little altered in appearance that the presence of the insect 
 can be detected only by weighing the heads or splitting the stems to 
 look within for the larvae. 
 
 Many of the larvae remain in the stubble if the grain is cut with a 
 harvester, and most of them if it is cut with a header. They begin 
 to pupate during the latter half of July, but many remain in the larval 
 condition until May of the following year. 
 
 The life history just related shows that strawworm injury is in- 
 duced by growing crops of winter wheat repeatedly upon the same 
 "round; by leaving volunteer plants among the spring wheat; by 
 allowing summer fallow to grow foul with volunteer wheat, even at 
 a distance from wheat fields ; and by growing spring wheat near winter 
 wheat. All of these cases occur at times in the wheat country wliich 
 we are considering. The essential conditions in each of these cases 
 are, first, the presence of infested stubble, plowed or unplowed, to 
 furnish the early adults; second, a growth of winter or volunteer 
 wheat early in the spring to receive the eggs of the first generation; 
 third, a crop of winter or, preferably, spring wheat to receive the eggs 
 of the second brood. The remedy lies in avoiding these conditions 
 by rotation of crops, clean early summer fallowing, and the aban- 
 donment of spring-wheat culture. Fall plowing ol the stubble does 
 not prevent the insects from coming out and attacking wheat in the 
 [Cir. 106] 
 
1. 
 
 spring. Burning the stubble i- impracticable, because the joi 
 uliicji contain the larvte remain sappj during the fall and will not 
 burn. Rotation, to be effective, must b • pi inned with r< ference to 
 the work of the insect, since two adjoining fields which are in wheal 
 in alternate years have between them a trip 2 rods wide which alwi 
 furnishes ideal conditions for its pread. The stubble in one field 
 always furnishes insects to attack the growing wheat in the otl 
 and while the damage done !»\ the wingless first generation U limited 
 to the l-rod strip next the fence, the winged Becond generation 
 spread- at \\ ill i hrough the field. 
 
 There is uo reason whj an insect, so readily held in check merer) 
 h\ careful farm methods, should cause constant loss and an occasional 
 menace to the wheat country of the Northwest, bul there is no doubt 
 thai it does so. 
 
 Approved : 
 
 .1 kMES Wn BON, 
 
 Secretary of Agricutiun . 
 
 \Y 1SH1NGTON, 1). ( .. April JO, 19 
 
 100J 
 
 o 
 
UNIVERSITY OF FLORIDA 
 
 3 1262 09216 5140