98 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 5 



THE TIME WHEN WHEAT SHOULD BE SOWN TO ESCAPE 

 THE FALL BROOD OF HESSIAN FLY 



By Thomas J. Headlee, Ph.D., Head of the Division of Entomology and Zoology 

 in the Kansas Stale Agricultural College and Experiment Station 



This is a phase of the Hessian fly problem which has received more 

 or less attention from every student of that subject, and I should not 

 presume to take the time of this association were it not for the fact 

 that I believe it still to be an open question, and one in need of funda- 

 mental study. 



There can be no doubt that the seasonal periodicity so characteristic 

 of animals and plants generally is exhibited in both the Hessian fly 

 and its host plants — that there is a period of time in the fall during 

 which, under normal conditions of food supply, the emerging flies 

 have the best possible opportunity to perpetuate their kind and that 

 there is likewise a period during which wheat placed in the soil stands 

 the best chance to produce the maximum yield. This period may be 

 designated as the normal time of fall-brood fly emergence and the 

 normal time for wheat sowing respectively. The problem of deter- 

 mining when wheat should be sown to escape the fall brood of fly 

 involves the explanation of the relationship existing between the 

 normal period of fly emergence and the normal period of wheat sowing. 

 Fortunately, experimental tests of the former have been made and 

 recorded in several parts of the country and of the latter in the state 

 of Ohio. One sowing experiment at Columbus^ continued for nine 

 years gave a normal wheat sowing period of about five and one-half 

 weeks, beginning September 6 and extending to October 15. Another 

 sowing experiment at Wooster,^ continued also for nine years, gave a 

 period of four weeks, beginning August 31 and ending September 29. 

 The investigations of Webster^ have shown that the end of the injurious 

 fall brood of fly is reached by September 25 at Columbus and Septem- 

 ber 20 at Wooster. Thus the normal period of fly emergence is seen 

 to close one to three weeks before the normal period of wheat sowing 

 comes to an end. These normal periods for parasite and host vary in 

 time of occurrence with the latitude and with altitude. This variation 

 is, of course, due to the changes in the climate characteristic of different 

 latitudes and altitudes. To go deeper, we may say that within the 

 limited area covered by the work of Webster and Hopkins, tempera- 

 ture is, in the light of our present knowledge, the only climatic factor 

 sufficiently variable to bring about such large differences. 



iBul. 136, Ohio Expt. Sta., p. 13, 1902. 



2 Bui. 231, Ohio Expt. Sta., p. 6, 1911. 



= Webster, F. M., Bui. 107, Ohio Expt. Sta., p. 275, 1899. 



