THE GEAPE PHYLLOXERA IN CALIFORNIA. 



55 



of individuals lived from 90 to 105 days on a particularly innutri- 

 tious piece of root without maturing. 



During 1913 two series of further experiments were undertaken. 

 One series was reared in the cellar and the other in an electric in- 

 cubator, the latter under somewhat higher temperatures. Genera- 

 tions were followed from May to October. Table XI summarizes 

 these observations. 



Tarle XI, 



-Development of ra.dicicoT.es of the grape phylloxera. Walnut Creek, 

 Calif., 1913. 



Environment. 



Genera- 

 tion. 



Number 

 of indi- 

 viduals. 



Average 

 period of 

 growth. 



Average 

 tempera- 

 ture. 



Cellar 



1 

 2 

 1 

 2 

 3 



44 

 10 

 15 



37.25 

 32.40 

 35.60 



°F. 

 65.1 



Do 



69.8 



Incubator 



65.6 



Do 



24 1 28.80 

 3 



71.1 



Do 



70.3 













From this table it is noticeable that temperature exerted consider- 

 able influence on the growing period of the aphids, and that warmth 

 accelerated their development. In a series of generations reared in 

 1915 on very nutritious food, recorded under the heading " Maximum 

 and minimum generations yearly " (p. 71) , this temperature influence 

 is very apparent. The greater constant warmth in the incubator in- 

 duced the aphids to remain active later in the fall, after those in the 

 cellar had hibernated. In comparing the 1913 series with those of 

 1911, it was found that the aphids of the former developed more 

 slowly than did those of the latter, and this notwithstanding the fact 

 that both the series of 1913 enjoyed higher temperatures than did the 

 cellar series of 1911. The roots supplied in 1913 were of much poorer 

 quality than were those supplied in 1911. 



Develop?nent on living roots, 1913-1915. — During 1913, 1914, and 

 1915 the habits and development of the radicicoles were observed on 

 living roots of vines growing in cages (Pis. V; VI, fig. 1; VII) kept 

 in a trench where the temperature approximated that obtaining 

 beneath the surface of the soil. As far as the temperature was con- 

 cerned, the monthly averages ranged less than did those obtaining 

 about 2 feet below the soil surface, but the daily fluctuations were 

 considerably in excess of those in the soil. In the cages the roots were 

 subjected to an average daily fluctuation of about 3° F. in summer 

 and about 2° F. in winter. Two feet beneath the surface, the tempera- 

 ture never fluctuated more than 1° in any given day. As far as could 

 be observed, this temperature fluctuation had little effect on the 

 growth of phylloxerse, except that it seemed to cause the nodosities to 

 decay more rapidly than they would normally. Occasionally it was 

 noted that some nodosities would dry up quickly after the cage had 



