135 



Fig. 34. Phorodon humttli, eggs and shriveled 

 skin of female which laid them — enlarged 

 (original). 



sexual females (Fig. 32). Somewhat later, on the Hop, the true winged 

 male (Fig. 33), and the only male of the whole series, is developed, and 

 these males also congregate upon the Plum, on the leaves of which toward 



the end of the season they may be 

 found pairing with the wingless fe- 

 males, which stock the twigs with 

 the winter eggs (Fig. 34). Such, 

 briefly, is the life history. Twelve 

 generations may be produced dur- 

 ing the year, but there is great ir- 

 regularity in the develoi)ment of 

 these generations and the return 

 migrant from the Hop is produced 

 at the the end of the season whether 

 from individuals of the fourth or 

 fifth generation, or of the twelfth. As I have remarked elsewhere* 

 ''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 trillions. The number of leaves (seven hun- 

 dred 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 hun- 

 dreds of acres in the course of two or three months. 



" While meteorological conditions may materially affect the increase 

 and power for injury of the species, these are far more truly predeter- 

 mined and influenced by its natural enemies, many of which have been 

 studied and will be described. 



" The slight colorational differences, as also the structural differences, 

 including the variation in the tubercles or cornicles on head and basal 

 joints of antenute, whether upon Plum or Hop, are peculiarities of brood 

 and have no specific importance whatever. 



" The exact knowledge thus gained simplifies the protection of the 

 hop plant from Phorodon attack. Preventive measures should consist 

 in destroying the insect on Plum in early spring where the cultivation 

 of this fruit is desired, and the extermination of the wild trees in the 

 woods wherever the hop interest is paramount ; also in avoiding the in- 

 troduction of the pest into new hop countries in the egg state upon 

 plum cuttings or scions. Direct treatment is simplified by the fact that 

 the careful grower is independent of slovenly neighbors, infection from 

 one hop yard to another not taking i)lace." 

 The bearing of these facts will probably best be brought home to 



i * Paper read before the British Association, Manchester, September 2, 1887. 



