June, ’23 
PARKER: HESSIAN FLY 
299 
FIVE YEARS OF HESSIAN FLY STUDIES IN OHIO 
By T. H. Parks, Ohio State University 
Abstract 
Fall plant infestation counts have been made duriny five years in fields and plats 
of known dates of sowing. Actual safe sowing dates as they occurred for the same 
points in northern counties, varied widely during this time. This was apparently 
due to the difference in intensity of the brood and to meteorological influences. 
Permanent safe seeding dates are now established, but the entomologists, through 
surveys, locate the counties where the most intense fall emergence will occur. They, 
through the county extension service, devote their efforts toward guiding these 
counties past such an emergency. 
Parasitism has been high in all flaxseeds passing the summer months above ground. 
Submerged flaxseeds gave rise to the most of the fall emergence during these years. 
Parasitism of submerged flaxseeds is limited largely to Platygaster hiemalis Forbes, 
which deposits its eggs in the fall in the eggs of the host. This parasite has been effi¬ 
cient in killing a high percent of the host larvae developing from eggs deposited during 
the first wave of emergence of adults, but does not control the host. It is absent 
in the fields during the visitation of the stragglers of the brood which, in Ohio, is the 
portion that perpetuates the insect in fields sown neai the proper seeding dates. 
In the fall of 1919 Ohio experienced the worst outbreak of hessian fly 
in its history. This outbreak was not unexpected since the entomologists 
had forecasted it by means of the wheat insect survey. Warnings 
against early seeding were announced but were not heeded by probably 
twenty percent of the farmers. Moreover the fly-free dates in northern 
counties were afterward found to be ten days later than expected. This 
resulted in widespread damage during the following summer. In 1920 
general observation of the seeding dates was carried out by the farmers 
but the results were complicated by the summer hessian fly puparia 
dividing into three well defined groups in giving up their adult flies. 
The majority of the puparia gave up their adults as usual during the 
latter half of September. Another wave of emergence appeared almost 
simultaneously over all of the western half of Ohio from October 12th 
to 18th and during this period of time egg-laying took place on all wheat 
above ground. A third group, a much smaller one, remained in the 
summer puparia in the old stubble until the period of normal emergence 
of the spring brood. This peculiar behavior of the hessian fly in 1920 
resulted in practically all wheat in northwestern Ohio becoming infested 
in October. The larvae grew rapidly throughout November and by 
December 10th were beginning to change to puparia. This transforma¬ 
tion continued throughout December and January though many of the 
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puparia were undersized. These gave up their flies during April^and 
May and at the same time as the puparia resulting from the normal 
