CEREAL PESTS INSECTS 483 



and distort the culms causing them to break on thrashing, 

 and decrease the yield by preventing or checking the 

 development of the spikes. 



The ivheat joint-worm (Isosoma tritici, Fitch) is a true 

 gall insect, producing swellings or enlargements in the 

 walls of the wheat culms. The galls are usually at or 

 near the joint, more commonly the second joint, but may 

 occur near any one. 



The adult insect is a minute black, 4-winged fly, f to less than 

 i inch long, and resembles its own parasites and the parasites of 

 the Hessian fly. The galls are in groups of three or four. Inside 

 the galls are the larvae, yellowish white with brownish-tipped mouth 

 parts. The species is single-brooded. 



FIG. 163. Winged adult of the spring grain louse. 



The barley joint- worm (Isosoma hordei, Harris) possesses 

 habits similar to those just described, but occurs on 

 barley. 



The wheat straiv-worm (Isosoma grande, Riley) is closely 

 allied to the joint- worm, but is distinguished by its habit 

 of living free in the hollow culms of the wheat, and pro- 

 ducing no gall. It also winters in the culm in the pupal 

 stage instead of the larval stage, and has two broods. 



The adults of the two broods are quite dissimilar in appearance. 

 Those coming from the over-wintering pupae are rather minute and 



