INSECTS INJURIOUS TO THE HOP PLANT 



277 



generation. Three generations feed upon the plum, the third 

 becoming winged and flying to its favorite food in the hop-field. 



Throughout "the summer the aphides reproduce partheno- 

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

 rate of one to six, or an average of three 

 per day, under favorable 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 mil- 

 lions. 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 million before the period of 

 blooming or burning; so that the issue 

 from a single stem-mother may, under 

 favoring circumstances, blight hundreds 

 of acres in the course of two or three 

 months." * 



During September a brood of winged females are produced 

 which fly back to the plum-trees, and in the course of a few days 

 give birth to three or more young. These never become winged, 

 but are the true sexual females which lay the winter eggs. The 

 true winged males are developed during the latter part of the 

 season and may be found pairing with the wingless females at 

 that time, these being the only males during the year. 



Remedies. From a knowledge of the above life history 



several methods of treatment have been devised. By spraying 



plum-trees neighboring the hop-yard and infested with aphides 



while they are laying the eggs, during fall or in the spring 



* Riley, The Hop-louse, Insect Life, Vol. I, p. 135. 



FIG. 204. The hop 

 plant-louse, true sex- 

 ual female enlarged. 

 (After Riley, U. S. 

 Dept. Agr.) 



