494 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



and then making a deep furrow in it across which the chinch- 

 bugs cannot pass. The furrow must be kept in good condition 

 by frequent cleaning or dragging. In wet weather it will be 

 necessary to resort to a barrier made by a narrow line of coal- 

 tar or some similar substance placed across the path of the 

 invaders. Such barriers require constant care and attention 

 during the ten days or two weeks that the bugs are migrating. 

 Sometimes when the chinch bugs have gained an entrance into 

 a cornfield those that have massed on the first few rows of 

 corn can be destroyed by a blast torch or by spraying with 

 kerosene emulsion. 



The Hessian-fly (M ayetiola destructor) . Probably about 10 

 per cent, of the wheat crop is destroyed annually by the Hessian- 

 fly and in some regions the loss 

 may reach 40 or 50 per cent, in bad 

 years. The adult is a very small 

 blackish fly with a pair of delicate 

 wings that are provided with very 

 few veins. The eggs are usually 

 laid on the leaves, and the larvae 

 make their way toward the base of 

 the stem where, protected by the 

 leaf sheath, they begin sucking the 



A. ,T -s a iuice from the plant. The pupar- 



FIG. 233. The Hessian-fly, J . , . , 



Mayetiola destructor, adult mm in which the pupal stage is 

 male. (Much enlarged; after passed looks so like a flax seed that 



the pupal stage is usually referred to 



as the " flax-seed stage." There may be one to four generations 

 a year, according to the region and climatic conditions, but the 

 early spring and late fall broods are most destructive. The 

 flies that issue late in the fall lay their eggs on the young win- 

 ter wheat, and the pupal, or flax-seed, stage is reached before 

 cold weather comes. The adults issue from these pupae early 

 the next spring. 



The best method of control is to refrain from planting the 

 wheat in the fall until the flies have laid their eggs elsewhere. 

 Only a close study of the fly and the weather conditions that 

 control its time of issuing in the fall will enable one to 



