August, ’24] 
481 
DRAKE, FENTON AND BUTCHER: HESSIAN FLY 
flaxseed count has been the only constant and reliable factor to use in predicting 
accurately the actual fly-free-date. One hundred or more flaxseeds were collected at 
each station each day (average about 1,000 per day at Monona county station) from 
old stubble dug up at random in heavily infested fields. These flaxseeds were then 
classified as parasitized, empty and viable. The viable flaxseeds were then dissected 
to determine the per cent of larvae, of pupae and of parasitized larvae. Hence, the 
flaxseed count gave the daily rate of transformation from the larva to the pupa and 
to the adult state, as well as the rate of approach of the fly-free-date. When this 
count yields very few pupae and only a small per cent of larvae in contrast to a large 
number of empty flaxseeds, it is proof that the actual fly-free-date is at hand and 
that the few remaining pupae will immediately issue as flies, lay their eggs and 
perish before wheat drilled at this time will come up. Thus it is possible to predict 
the fly-free-date so that farmers can begin drilling wheat six or seven days before the 
end of the egg laying period of the fall brood of flies. The records of the Iowa Ob¬ 
servation Stations for 1922 and 1923 are given in the form of a summary. 
On account of the importance of the wheat crop to man, probably 
no insect pest has received more attention or has ever had more said or 
written about it and its depredations than the Hessian fly, 1 the principal 
field enemy of the young-growing wheat plant. For many decades it 
has been a well-known fact that there was a safe date (short period) to 
sow winter wheat and also, at the same time, to produce a maximum 
yield. The solution of the problem of predicting the fly-free-date 
has been a perplexing one for years. 
Seeding late to avoid Hessian fly damage was recommended by 
many of the earliest writers. This general recommendation was followed 
by the “fixed” fly-free-date method. The latter method, based largely 
on altitude, longitude, latitude and an average of previous safe-sowing 
records, proved satisfactory in “average” or “normal” years. Fre¬ 
quently, however, farmers who scrupulously followed these “approved” 
or “fixed” dates suffered heavy losses. While the theory of late plant¬ 
ing is good, the fallacy of setting a fixed or static safe-date is evident 
when it is known that the period of the emergence of either the spring or 
the fall brood varies from year to year with climatic conditions. 
In spite of seeding wheat on the proposed safe dates, the Hessian 
fly has been a serious pest in the Mississippi Valley during the past few 
years. These outbreaks brought out forcibly the fact that the “fixed” 
seeding dates are unreliable and that each year the seeding date should 
be correlated with the season and with the fall brood of the flies. As a 
result, several changes have been made in methods for determining the 
fly-free-date. Field studies carried on in Ohio by Gossard and Parks 
disclosed the fact that the fly-free-date varies considerably from year to 
1 Phytophaga destructor Say; family Cecidomyiidae; order Diptera. 
