18 THE HOP APHIS IN THE PACIFIC REGION. 



alternate host, and part being wingless and depositing young (sixth 

 generation) upon the hop, producing the male aphides which fly to 

 and copulate with the young deposited on the alternate host by the 

 winged individuals of the fifth generation. These fertilized females 

 deposit the winter eggs, which, hatching in the following spring, 

 produce the viviparous insects for that season. 



Table IV gives one series of dates for the life cycle. This series 

 will vary according to the date of the emergence of the first generation 

 from the egg in the spring. The variation in the date of hatching of 

 the winter eggs causes an overlapping of generations, so that these 

 are not in the least distinct. 



Table IV. — Life cycle of the hop aphis as observed at Perkins, Cal., in 1912. 



Emergence from eggs ■. June 3 



Appearance of second generation, winged June 17 



Appearance of third generation (first on hops) June 30 



Appearance of fourth generation (second on hops) July 11 



Appearance of fifth generation (third on hops) July 22 



Appearance of sixth generation (fourth on hops) Aug. 2 



Appearance of seventh generation (fifth on hops), part winged Aug. 15 



Appearance of eighth generation (sixth on hops), males Aug. 25 



Appearance of eighth generation, sexual females, on plum Aug. 25 



Deposition of eggs Sept. 14 



In the report of the Department of Agriculture for 1888, Prof. C. V. 

 Riley gives the following data upon the life history, which vary 

 somewhat from the data given in this bulletin : 



Three parthenogenic generations are produced upon Prunus, the third being winged. 

 * * * A. number of parthenogenic generations are produced upon the hop until 

 in autumn, and particularly during the month of September winged females are again 

 produced. 



This account is also at variance with the writer's own observations 

 in that no winged forms are noted during the summer. 



THEORY REGARDING SUMMER MIGRANTS. 



The presence of winged forms of the hop aphis throughout some 

 seasons and the absence of such forms during 1912, both in the 

 laboratory and in the field, except at the end of the fifth generation, 

 are explained as follows: The eggs of the hop aphis have been ob- 

 served to hatch individually during a period of one month and five 

 days, April 5 to May 10. The winged forms were observed beginning 

 to migrate from the prune May 24, and migration was not completed 

 until June 20. Thus until the 20th of June migrants from the 

 alternate host (the plum or hop) were present in the hopyards. 

 Beginning the life cycle with the first insects that migrated, the 

 seventh or winged generation on the hop would be mature July 19. 

 These insects, finding some tender hop leaves upon which to settle, 





