THE GRAPE PHYLLOXERA IN CALIFORNIA. 



71 



age of 61.75° F. this period was 24.75 days, under 65° F. it was 

 almost 22 days, arid under 68° F. it was lowered to 20.3 days 

 (individual on unthrifty root disregarded). Under the lower tem- 

 peratures all the instars are correspondingly longer than under the 

 highest midsummer temperature, but the first instar is proportion- 

 ately less lengthened than are those following it, a phenomenon 

 that becomes quite apparent in the case of the hibernants, provided 

 their first instar be considered in a restricted sense to cover only that 

 period between the time when they commence feeding in spring and 

 the date of the first molt. The hibernant feeds for two and one-half 

 weeks before and for three weeks after its first molt, while in mid- 

 summer the larva feeds for 13 days before and for 8 days after its 

 first molt before it matures. 



MAXIMUM AND MINIMUM GENERATIONS YEARLY. 



In 1911 overwintered adult radicicoles matured at the end of 

 April, throughout May and June, and as late as July 7. Eggs of 

 the first generation were deposited from the end of April until 

 October 1. From the earliest eggs there followed seven complete 

 generations from hibernant to hibernant inside of the one year. No 

 observations were taken of the hatching of the late eggs deposited 

 by late first-generation phylloxerse in September, but in the light of 

 contemporary observations on individuals of later generations there 

 is no doubt that a certain percentage of these late eggs would have 

 hatched into hibernants, thus giving a minimum of one generation 

 per annum. In 1915, taking advantage of a hibernant which ma- 

 tured exceptionally early in the spring, it was possible to secure 

 eight complete generations within the year. Table XXIII records 

 the development of these generations. 



Table XXIII. — Maximum series of generations of radicicoles of the grape 

 phylloxera, reared under cellar conditions, Walnut Creek, Calif., 1915. 



Generation No. 



Date of egg 

 deposition. 



Date of egg 

 hatching. 



Date insect 

 matured. 



Genera- 

 tion 

 cycle. 



Temper- 

 ature 

 (average). 



li 



—,1914 



Feb. 26,1915 

 Apr. 27,1915 

 June 7, 1915 

 July 5, 1915 

 July 28,1915 

 Aug. 23,1915 

 Sept. 25, 1915 

 Oct. 27,1915 



—,1914 



Mar. 19,1915 

 May 11,1915 

 June 16,1915 

 July 14,1915 

 Aug. 4, 1915 

 Aug. 31,1915 

 Oct. 7, 1915 

 Nov. 10, 1915 



Feb. 26,1915 

 Apr. 22,1915 

 June 5, 1915 

 July 4, 1915 

 July 28,1915 

 Aug. 23,1915 

 Sept. 23, 1915 

 Oct. 27,1915 

 —,1916 



Days. 



°F. 



2 



55 

 39 

 27 

 23 

 26 

 31 

 32 



58.25 



3 



61.20 



4 



64 50 



5 



69 50 



6 



68 50 



7 



67 00 



8 



62 50 



9 1 











1 Hibernant. 



In this experiment the food supplied to the phylloxerse was, as 

 far as one could judge, of equal quality and very nourishing. The 

 influence of temperature is noticeable. 



