18 FARMERS’ BULLETIN 640. 
wheat always enjoys the greatest freedom from the fly in Maryland. 
Practically the same corresponding delay in wheat sowing in the 
fall should be followed to the southward. So far these dates are only 
approximate, and serve to show in a general way about the time when 
the fall brood of the fly will have largely disappeared over the wheat 
belt east of the Mississippi River. As the larger part of the fall brood 
appears and is gone within a week, it is possible for a farmer so to 
time his seeding as to avoid it, and this is by far the most practical 
and effective preventive measure that can be applied. 
CROP ROTATION. 
Aside from the general benefits to be derived from crop rotation, 
this practice compels the Hessian fly, when it emerges from the 
Fic. 15.— Merisus destructor, a parasite of the Hessian fly. Much enlarged. (Original.) 
_ stubble in the fall (or spring in the Northwest), to travel a greater or 
less distance to reach young wheat plants. 
If, during this season of migration, storms or heavy winds occur, 
these frail creatures will be driven about or beaten down until a com- 
paratively small number survive to reach their destination. On the 
other hand, if they are not obliged to leave the field where they 
emerge, this mortality will be vastly lessened. 
BURNING STUBBLE. 
Burning the stubble is the most efficient measure of all, as fire will 
reach and destroy not only the Hessian fly but all other insects in- 
festing the stubble, including the joint-worm. Unfortunately it can 
not be generally carried out. Over a large portion of the Middle 
West timothy and clover are sown, either with the wheat or during 
late winter or early spring, and therefore stubble-ground can not be 
burned over. Fields not followed by grass or clover can usually be 
burned over if the grain is cut rather high at harvest and a mower is 
