PESTS OF THE HOP CROP. 117 



the vines are generally pulled up and removed from 

 the fields before they are killed by the heavy frosts of 

 late autumn. The life history of the insect has ac- 

 commodated itself wonderfully to this cultural practice 

 and the lice acquire wings and leave the plant at just 

 the proper time for the preservation of the species. 



As before stated, the general relations of the insect 

 to the two plants plum and hop were practically de- 

 termined years ago in England, but it was only after 

 the observations of 1887 made by the force of the Divi- 

 sion of Entomology of the United States Department 

 of Agriculture, and especially by Messrs. T. Pergande 

 and W. B. Alwood, under the direction of the late Dr. 

 C. V. Riley, that the full life round of the insect was 

 known and the exact periods of development and of 

 life upon either species of plant. These observations, 

 which were carried out with the utmost care, and the 

 results and methods of which form a model for similar 

 work, have been recorded in Government publications, 

 notably in the annual report of the United States De- 

 partment of Agriculture for 1888, and in Insect Life, 

 Volume I; also in Circular No. 2, Second Series of the 

 Division of Entomology. 



Life History Briefly, it may be stated that the first 

 plant lice in the spring hatch from winter eggs on the 

 twigs of plum trees in the vicinity of hop yards. This 

 first generation of lice is composed of wingless indi- 

 viduals which give birth to living young. These young 

 settle upon the buds and young leaves of the plum tree, 

 and after a few days give birth to other young. The 

 second generation, like the first, is wingless, but the 

 third acquires wings. There are no males among these 

 lice, and the phenomenon of reproduction without the 

 intervention of the male (termed parthenogenesis) is 

 one of the wonderful features of insect life. By the time 

 this winged generation makes its appearance, the hop 

 plants in the yards have made a good start and the lice 



