402 



REPORT OF STATE BOARD OP HORTICULTURE. 



females which fly back to the plum tree and give birth to the true sexual 

 females (Fig. 4), which never acquire wings and never leave the plum tree. 



Fig. 3— The Hop Plant-louse, third generation on plum- the generation which 

 flies to the hop— enlarged ; head below at right — still more enlarged. 



By the time this generation has matured, which involves but a few days, 

 varying according to the temperature, belated winged individuals, which 



are the true males (Fig. 5), fly in from the hop 

 fields. These fertilize the wingless true females 

 upon the plum leaves and these soon thereafter lay 

 the winter-eggs. Thus there is but one generation 

 of sexed individuals produced and this at the close 

 of the life-round — the females wingless on plum 

 trees ; the males winged on hops. All intervening 

 generations are composed of virgin females only 

 (parthenogenetic). This is the invariable round of 

 the insect's life. 



REMEDIES. 



From the life-history just given three important 

 facts are obtained : (1) It will pay to make a pre- 

 ventive application of some one of the mixtures 

 mentioned further on, with apparatus there de- 

 scribed, to all plum trees in the neighborhood of 

 hop yards, either (a) in the spring before the appearance of the first winged 

 generation and its consequent migration to hop, or (h) in the fall after hop- 

 picking and after the lice have once more returned to the plum and are 

 making their preparations for the laying of winter eggs. The latter time 

 will, perhaps, be preferable, for the reason that in the fall the plum trees 

 will be less susceptible to the action of the washes and a stronger solution 



Fig. 4 — The Hop Plant- 

 louse, true, sexual female 

 — enlarged. 



