WALNUT APHIDES IN CALIFORNIA. 



13 



as July, while on trees attacked by few lice these forms will not occur 

 until September or October. It therefore seems that the more 

 heavily a tree is infested the earlier will the sexed forms be produced. 

 A glance at Table IV demonstrates that the first oviparous form 

 of the life-cycle material became adult September 2, and was a 

 member of the eighth generation. In the field the first oviparous 

 form was observed in 1911 on July 7, and in 1912 on July 9. Both of 

 these occurred on early-leafing trees and probably belonged to the fifth 

 or sixth generation. As the growth of the aphis colony may be said to 

 have reached its zenith about the middle of July, it is at the time of 

 its greatest abundance that the sexed individuals begin to appear. 

 And, indeed, for the welfare of the species they appear none too 

 soon, for it is in July and August that the hordes of natural enemies 

 work tremendous havoc, frequently cleaning up a bad infestation on 

 a tree within three weeks. Those sexual females that have gone to 

 the trunk and limbs to deposit their eggs very often escape destruc- 

 tion while the others remaining on the foliage are devoured. Until 

 the middle of August the sexual forms are comparatively rare and 

 comprise less than 5 per cent of the whole, but after that time they 

 become more and more abundant. The sexed females always greatly 

 outnumber the males. Table V indicates the advance and decline 

 of the sexual forms in the late summer and fall. The data were ob- 

 tained by weekly visits to badly infested trees, during which the 

 lice on a certain number (50) of leaflets picked out at random were 

 counted. It must be borne in mind that as the fall advances more 

 and more sexual females repair to the limbs for the purpose of depos- 

 iting their eggs, and that therefore in the case of the later counts a 

 really greater proportion of these were present on the trees than 

 would appear from the records. 



Table V. 



-Comparative numbers of sexual and asexual forms of Chromaphisjuglandicola 

 observed on the foliage at different dates, San Jose, Cal., 1911. 





Number of— 



Percent- 

 age of— 



Date of collection. 



Vivipa- 

 rous 

 forms. 



Ovipa- 

 rous 

 forms. 



Males. 



Vivipa- 

 rous 



forms. 



September 15 



241 

 258 

 203 

 171 

 166 

 207 

 172 

 420 

 233 

 10 



247 



355 



143 



66 



74 



117 



111 



80 



36 







4 

 5 

 6 

 9 



14 

 9 

 4 



11 

 7 

 



49 



23 



42 



30 



58 



October 7 



70 



14 . . 



65 



24 



62 



November 2 



60 



9 



82 



16 



84 



23 



100 







Total 



2,081 



1,229 



69 





Average 



61.6 













