June, ’23 
PARKER: HESSIAN FLY 
303 
Parasitism 
Each year since 1918 we have observed parasitism increase rapidly 
during July and August among all hessian fly puparia located above 
ground. By September 1st it is usually very difficult to find unpara¬ 
sitized hessian fly in the puparia above ground. It seems that external 
feeding parasites are able to take care of almost all of these insects which 
do not become covered with soil after larval maturity in June. 
The question is raised as to whether or not external feeding hessian 
fly parasites, which of necessity must attack the host above ground, 
will ever become the controlling factor in suppressing an outbreak. 
Under Ohio conditions the writer has found during September a large 
percent of the flaxseeds beneath the ground. In Kansas the larger 
number of flaxseeds are buried in the soil by September. Such observa¬ 
tions would lead us to believe that burning stubble even though it 
could be done, would be harmful rather than beneficial. By this means 
one would destroy practically all of the flaxseeds which either contain 
parasites or hessian fly larvae which make possible rapid parasite de¬ 
velopment. 
We have two species of Platygaster which are known to oviposit in 
the egg of hessian fly and emerge from the puparia. Platygaster hie- 
malis Forbes, the fall emerging species, is the most common in Ohio. 
These parasites are capable of taking care of hessian fly even though 
the host becomes subsequently buried and inaccessible to most of the 
external feeding species. However, our recent studies reveal that 
Platygaster hiemalis, which is known to emerge only in the fall, is 
present only during the early emergence wave of hessian fly. It was 
entirely absent from the fields during the mid-October emergence of 1920. 
At Sandusky, Ohio, in 1921, Platygaster hiemalis adults which had been 
emerging freely during September in the hessian fly cages, ceased emerg¬ 
ing September 29th. Hessian fly emergence and egg-laying were quite 
heavy on September 30th, and continued in reduced numbers until 
October 17th. Four years analyses of hessian fly puparia show that 
this parasite is abundant in the puparia infesting early sowed wheat 
which we discourage but is usually absent or very scarce during the period 
of visitation of stragglers of the main brood of hessian fly adults appear¬ 
ing late in September. Since it is stragglers of this brood which are 
liable to perpetuate the species in most seasons, by late sowing we are 
unconsciously working against these egg-infesting parasites perhaps to 
even greater extent than their host. 
