INSECTS INJURIOUS TO SMALL GRAINS 



127 



25th to November loth. The exact time will also depend upon 

 altitude as well as latitude. 



A rotation of the wheat crop compels the flies when they emerge 

 from the stubble to travel in search of the young wheat plants. 

 Should storms or heavy winds occur, the frail little flies will be de- 

 stroyed in large numbers, whereas if they found wheat immedi- 

 ately available the mortality would be small. 



Inasmuch as most of the spring brood remain in the stubble 

 in the flaxseed stage after harvest, if the fields be then burned 

 over, large numbers will be destroyed. This may be done by cut- 



FIG. 93. Hessian fly: a, egg, greatly enlarged; b, section of wheat-leaf 

 showing eggs as usually deposited less enlarged; c, larva; d, pupa 

 taken from puparium or "flaxseed" e; c, d, e, much enlarged. (After 

 Webster and Marlatt, U. S. Dept. Agr.) 



ting the grain rather high at harvest, and then mowing the weeds 

 and grass and allowing them to dry a few days before burning. 

 Unfortunately this practice is often impossible, owing to the prac- 

 tice of seeding wheat land to grass and clover. 



As early volunteer plants always become badly infested and the 

 pupse wintering on them give rise to a spring brood which attacks 

 the main crop, all volunteer plants should be destroyed by plowing 

 or disking before the larvae have matured. This principle has some- 

 times been utilized in the form of a trap crop, strips of wheat being 

 sown early so as to attract the flies and then being plowed under 



