LIFE HISTORY. 



13 



reproduction of the winged migrants of both the spring and the fall 

 generations. The stem-mothers are very prolific, as shown in Table 

 III, one aphis being capable of populating several leaves in a very 

 short time. 



NUMBER OF GENERATIONS ON ALTERNATE HOST. 



Mr. Franz Remisch, of Saaz, Bohemia, who observed the emergence 

 from the winter eggs, obtained two generations on plum. The writer 

 did not observe the emergence of the first generation, but during the 

 spring only two generations were found on prune, the second one 

 being winged. In Bulletin 160 of the California experiment station 

 Prof. W. T. Clarke reports the 

 appearance of winged aphides 

 14 days after the first wingless 

 insects were observed. This 

 would be sufficient time for 

 but one generation to mature, 

 and it is very probable that 

 there are only two generations 

 upon the prune in the Pacific 

 region. 



MIGRATING FORMS. 



The winged or migrating 

 aphides, except for the pres- 

 ence of two pairs of relatively 

 large, delicate wings, some dark 

 spots on the thorax, and a 

 slightly more slender body 

 (fig. 3 and PI. I, fig. 1) differ 

 fit tie from the wingless form. 

 They appeared at Perkins, 

 Cal., May 15 and were present 

 there and at Santa Rosa until June 15. Five belated individuals 

 were observed at Perkins the latter part of July, but the migrations 

 had taken place by the 20th of June. 



MIGRATORY ACTIVITIES. 



These winged aphides, which mature on the plum, are the first 

 migrants. They are weak fliers, but when aided by a light wind 

 may travel some distance. Their progeny, the wingless viviparous 

 females, which are the most common forms found upon the hopvines 

 throughout the season, are incapable of migration from one vine to 

 another except where two vines come into contact so that the aphis 



Fig. 2. — The hop aphis (Phorodon humuli): Windless 

 viviparous female. Greatly enlarged. (Original.) 



