WALNUT APHIDES IF 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 tliat 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. 
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 
60 
9. . 
82 
16 
84 
23.. 
100 
Total. .. 
2,081 
1,229 
69 
61.6 
