PESTS OF THE HOP CROP. 121 



abundance of plant lice during a rainy spring is of com- 

 mon occurrence. Should the rains continue they in- 

 crease beyond measure, but with the first stopping of 

 the rain their natural enemies become active, reproduce 

 with wonderful rapidity and destroy the lice by the 

 wholesale. Then, too, when we have, as we occasion- 

 ally do in late June or early July, a day or so when the 

 temperature*runs high up into the go's, the lice may be 

 killed off by the wholesale by the heat alone. The 

 writer remembers a case in the city of Washington, 

 where in a single day every plant louse of countless 

 millions upon the box-elder shade trees was killed by 

 a temperature of 101. At the same time hop plant lice 



FIG. 60. HOP PLANT LOUSE AND EGGS. 

 Showing shriveled skin of female. Greatly enlarged. (From Insect Life.) 



which were under observation upon the grounds of the 

 Department of Agriculture in the course of experi- 

 mental work were also destroyed by the heat. It may 

 be incidentally remarked that hot, sunny weather is 

 prejudicial to the increase of this insect in the hop 

 plantations of England. The natural enemies of 

 the lice consist of the slimy maggots of several 

 species of Syrphus flies, of the active and vora- 

 cious larvae of the lace-winged flies, of the lady- 

 bird beetles, of the internal feeding larvae of 

 an entire sub-family of parasitic flies known as 

 Aphidiinae, and of several species of little parasitic 

 hymenopterous insects, which, curiously enough, be- 



