INSECTS IXJURIOUS TO THE HOP-PLANT. 273 



winged and at once flying to its favorite food in the hop- 

 field. 



Throughout the summer the lice are produced partheno- 

 genetically, as are almost all plant-lice (see page 136). 

 They " multiply with astonishing rapidity for from five to 

 twelve generations, carrying us in point of time to the 

 hop-picking season.'' '^Each parthenogenetic female is 

 capable of producing on an average one hundred young 



Fig. 155. — AYinter Eggs of the Fig. 156.— The Hop Plant-louse, 



Hop Plant-louse, and sliriv- stem-mother, with enlarged an- 



eled skin of the sexual female t^'umie above. (After Riley, 



which laid them — enlarged. U. S. Dept. Agr.) 

 (After Riley, U. S. Dept. Agr. ) 



(the stem-mother probably being more prolific), at the rate 

 of one to six, or an average of three per day^, under favor- 

 able conditions. Each generation begins to breed about 

 the eighth day after birth, so that the issue from a single 

 individual easily runs up, in the course of the summer, to 

 millions. The number of leaves (700 hills, each with two 

 poles and two vines) to an acre of hops, as grown in the 

 United States, will not, on the average, much exceed a 



