94 



INSECTS INJURIOUS TO STAPLE CHOPS. 



Two sj^ecies are commonly injurious, the AVheat Joint- 

 worm {Isoso)ua tritici Fitch) and another species of the 

 same genus, more popuhirly knoAvn as the Wheat Straw- 

 worm {hosoma grande Riley). The adults of /. tritici are 

 small hlack flies from an eighth to three -sixteenths of an 

 inch in length, and with wings expanding about one fourth 

 of an inch. The larvae are yellowish-white with the tips 

 of the jaws brown, of about the same length as the fly, and 

 of the form shown in the figure. 



Life-Mdory. — The larvae of /. grande are much the 

 same; but while the former species has but a single brood 

 each season, this is double-brooded. The summer brood 



Fig. 55. — a, Wheat-straw affected by Joint- worm; b, adult as seen 

 from above. (After Riley.) 



is similar to that of tritici, but the spring brood is peculiar 

 in that the females are much smaller and almost wingless, 

 so that the pest is spread only by the later brood. The 

 larv* of tritici hibernate over winter in the wheat-stubble, 

 coming to maturity in June, and the next brood feeds 

 upon volunteer wheat and the fall planting. /. grande, 

 however, passes the winter in the pupal state, also in the 



