ISTew York Agricultural Experiment Station. 431 



Table IV. — Activities of Hibernating Brood of Pear Pstlla and Conditions 

 of Trees During the Spring of 1910. 



commenced within twenty-four hours after emergence. It appeared 

 from these observations that the movements of the psyllas depended 

 entirely on heat influences. In 1910 the mean temperature at the 

 time psylla " flies " became active was 46° F. and the insects spread 

 rapidly over the trees. In 1911 the " flies " emerged at a mean 

 temperature of 52° F. During the following week the warmth 

 decreased to 29° F. and most of the " flies " returned to the shelter 



