476 THE SMALL GRAINS 



of this parasite, which was greatly reduced in numbers 

 by the severe winter of 1912-13. The parasite has been 

 introduced into the state of Washington, where the Hes- 

 sian fly was destructive, and the former has become since 

 not only common but in some cases abundant, though it 

 could not be found previous to this introduction. 



521. Effect of climate on the Hessian fly. The im- 

 munity of late-sown wheat from attacks of the fly is not 

 due to firost, but to the fact that by the time severe frosts 

 usually occur most of the flies have gone. The greatest 

 climatic effect upon the fly is that caused by heat and 

 drought, especially the two combined. In the South the 

 two generations are widely separated by the long summer. 

 Also emergence from the flaxseed is much delayed by dry 

 weather. Humidity has greater influence in the devel- 

 opment and distribution of the fly than either altitude or 

 latitude. Migration of the fly westward from the me- 

 ridian of 100 and eastward from the Cascade Mountains is 

 no doubt prevented by heat and drought. 



522. Preventive measures. Any effective methods of 

 combating the fly, it appears, must aim at the elimina- 

 tion of the pest from young wheat in the fall. Probably 

 the most important of such methods is late seeding, or 

 seeding after the second brood has gone. Experiments 

 in this line have been made to a sufficient extent probably 

 to justify the conclusion that in years of normal rainfall 

 wheat may be sown in northern Michigan without danger 

 from Hessian fly attack from September 1 to September 

 15 ; in southern Michigan to central Ohio from Septem- 

 ber 15 to September 25; in southern Ohio and Indiana 

 and northern Kentucky from September 25 to October 10 ; 

 in southern Kentucky and Virginia and northern Ten- 

 nessee from October 10 to October 20; in Georgia and 



