CEREAL PESTS INSECTS 475 



the whole plant is drooping, and the tillers spread out and 

 cover the ground. The infested plant is without stem, the 

 leaves are broader and usually shorter, deep bluish green 

 in color, and resembling those of oats. The plant stands 

 more erect, and is a mass of short overgrown leaves that 

 are usually killed by the first frost. Later the infested 

 plants change to yellow and then brown. In summer the 

 stems of both spring and winter wheat infested break over 

 before harvest, and the plants are said to be " straw fallen." 



There is no " fly proof " wheat. All wheats in the 

 young stage are affected by the fly. By the time the 

 attack of the second brood, however, the plants have 

 reached a stage where there is apparently a considerable 

 difference in the resistance of varieties. The ranker 

 growing varieties with strong stiff straw are least affected. 

 Tillering is important, in order that new tillers may pos- 

 sibly be sent out from the old roots of plants killed by the 

 fly, which tillers should be able to withstand the winter. 

 Durum wheats, in the later stages of growth, are little or 

 not at all affected by the fly. In many cases common 

 wheat plants mixed in the same field with durum wheat 

 have been found badly infested, while the durum wheat 

 plants were free. 



520. Natural enemies of the Hessian fly. There 

 is no doubt that parasites take a most important part in 

 the control of the Hessian fly. A considerable number of 

 such parasites is known to occur in this country, among 

 the most important of which is a black species (Polygnotus 

 hiemalis, Forbes), with dark brown legs, banded with 

 yellow, and yellow feet. As many as forty larvae of this 

 parasite have been found in a single flaxseed. It is be- 

 lieved by Webster (1915) that the serious ravages of the 

 Hessian fly in 1914 were possible because of the absence 



