LIFE HISTORY. 



17 



average number of young (3.3 per day) over an average period of 

 19 days, would give birth to 63 aphides. 



At this rate of reproduction, provided that none of the aphides 

 were destroyed before they had lived an average life, one winged 

 aphis which settles on the hop in the spring would at the end 

 of the fifth generation be the parent of 4,068,989,826 living aphides. 

 These aphides would weigh 2,152 pounds. From these figures the 

 very sudden and extensive infestations by this insect are readily 

 explained. 



THE FALL MIGRANTS. 



The nymphs of the fall migrants (fig. 6) became winged in the breed- 

 ing cages at Perkins, Cal. , on Au- 

 gust 26 and in the field August 

 28. Migrants were observed 

 upon plum at Independence, 

 Oreg., September 22, 1912. 

 Young were being deposited 

 there and upon the next visit 

 to the locality, October 16, large 

 numbers of male aphides were 

 observed copulating with the 

 sexual females that had been 

 deposited by the migrants. 

 Many eggs were also present at 

 this time. 



THE WINTER EGG. 



The winter egg, when first 

 deposited, is a shiny-green ob- 

 ject, ovate in shape, and a little 

 smaller than the head of a pin 

 (f mm. in length) . Soon, how- 

 ever, it turns dark green and 

 then black and appears as a shiny-black point (PI. I, iig. 2) on the 

 branch of the alternate host plant. 



The eggs are usually deposited close around the buds or on the rough 

 leaf scars, but may sometimes be found upon the smooth parts of 

 the twigs. 



THE LIFE CYCLE. 



The entire life cycle where the writer's observations were made is 



as follows: Two generations occur on the alternate host, the second 



one being winged. Five and six generations occur on the hop, a 



part of the fifth becoming winged and depositing young upon the 



74956°— Bull. 111—13—3 



Fig. 6. — The hop aphis: Nymph, showing wing-pads. 

 Greatly enlarged. (Original.) 



