THE GRAPE PHYLLOXERA I1ST CALIFORNIA. 33 



The rate of egg deposition is usually indicated by a sharp rise shortly 

 after commencement, followed by a gradual decline. During the 

 period of egg laying the adult feeds, and after the last egg is laid 

 may live for as long as three weeks. 



Incubation naturally is influenced by temperature, and the dura- 

 tion of the incubation period may vary from five days in July to 

 over a month in December. Very few eggs are laid in December, but 

 in March and April, when many eggs are deposited, the maximum 

 period of incubation is 27 days. 



The larvae mature in midsummer in about 15 days, and in April 

 and November in about 34 days, and the hibernant generation de- 

 velops in about 180 days. The winged forms mature more slowly 

 than do the wingless individuals, since the fourth or nymphal instar 

 is noticeably extended beyond that of the corresponding wingless 

 stage. 



In the late fall a few individuals intermediate in structure be- 

 tween the nymphs and radicicoles are found. These are called 

 " nymphicals " or intermediates and, so far as is known, they de- 

 posit the same type of eggs as the radicicoles, although they are 

 not prolific. From egg deposition to the molting of the final skin, 

 the period covered by the sexes, which develop from eggs of two 

 sizes laid by the winged forms, was about 12 clays in confinement. 



All stages of the phylloxera molt four times, and the first instar 

 is always the longest (the adult instar excepted) . 



HIBERNATION. 



The phenomenon of hibernation. — Throughout autumn and early 

 winter an ever-increasing percentage of newly hatched radicicole lar- 

 vae, instead of increasing in size and maturing normally, remain 

 as very small brown phylloxerae (PL IX, d, p. 64). As winter pro- 

 gresses, the mature individuals die, leaving only the small brown 

 larvae and a few unhatched eggs. As soon as these late eggs hatch, 

 the larvae settle down, becoming brown like the others. These small 

 larvae are the hibernants, and as such they remain throughout the 

 dormant period. Occasionally phylloxerae that have passed one or 

 two molts hibernate. This type is quite unusual, and probably con- 

 sists of individuals that have reached a certain stage of development 

 and are unable, through lack of nourishment, to mature, most of them 

 dying before spring. 



Hibernant larvae occur on all kinds of vines — on viniferae and 



on American varieties and hybrids. While this form of phylloxera 



occurs more or less sparingly on American resistant vines (Vitis 



rip aria, V. rupestris, V. berlandieri, etc.) and on some American 



1900°— 21 3 



