PESTS OF THE HOP CROP. 119 



The rate at which they are produced is extraordi- 

 nary. A female in the prime of life will give birth to 

 several young each 24 hours. Each of these, in the 

 course of eight days, becomes full grown and begins 

 giving birth to young. Each female may live in the 

 active, prolific stage for several weeks, so that a given 

 individual may have living offspring to the fourth or 

 even fifth generation before the end of her life. From 

 this it results that from a comparatively small number 

 of original migrants a large hop yard may be com- 

 pletely overrun with lice in a few weeks, under the most 

 favorable circumstances. Were it not for the activity 

 of the natural enemies of the lice, there would appar- 

 ently be no hope of ever saving a crop. In September 

 all the lice on the hop again acquire wings, whether 

 they are of the fifth or the twelfth generation. We may 

 have then ten wingless generations, and we always 

 have two winged generations. 



The first individuals to acquire wings in the au- 

 tumn are always females, and these leave the hop yards 

 and fly back to the neighboring plum trees. The la- 

 ter individuals of this generation, and frequently, if 

 not usually, all of the individuals of an additional gen- 

 eration on the hop, are true males, the male thus mak- 

 ing its appearance for the first and only time in the 

 annual life round of the species. By the time they have 

 developed, however, the first issuing females will have 

 settled upon the plum trees and will have given birth 

 (parthenogenetically, as before) to a generation of 

 wingless individuals which comprise the true females, 

 not the virgin females as before, but the true females 

 which must be fertilized by the males. So that, by the 

 time the winged males have developed upon the hop 

 crop and fly back to the plum, we have this generation 

 of wingless, sexual, or true females awaiting them. Im- 

 pregnation then takes place, the males die, and these 

 wingless, sexual females give birth to the winter eggs, 



