18 THE HOP APHIS IX 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 IT 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 I 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 i, 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 1SSS, 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 REGAEDIXG SUMMER MIGRAXTS. 
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, 
