70 



BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 



All the individuals utilized in this experiment were reared on 

 severed pieces of roots in a petri dish under cellar conditions. In- 

 dividuals 19 and 20 lived on a much poorer root than the others, and 

 thus is explained the relatively slow growth (34 days) of the one 

 and the early hibernation of the other. Individuals 4 and 14 moved 

 away after their first and second molts, respectively. It will be 

 observed from the summary that the average period of the first instar 

 (13.3 days) is considerably longer than is that of the three succeed- 

 ing instars combined (7.0 days). The comparative periods of the 

 instars are about constant; that is, an individual with a short first 

 instar will have short succeeding instars and one with a long first 

 instar will have long succeeding instars. 



The records of Table XX were made in midsummer at a tempera- 

 ture of 68° F. In the soil at such a time of the year the temperature 

 is higher and the development of the phylloxera more rapid, while 

 in spring and late fall the development is correspondingly slower. 

 The developmental period of the hibernated larvae varies greatty, 

 not so much from temperature as from other causes. There is an 

 average period of two and one-half weeks from the commencement 

 of feeding to the shedding of the first skin, and after that an average 

 period of three weeks between the casting of the first skin and the 

 shedding of the fourth, the second, third, and fourth instars occupy- 

 ing an average space of a week each. As summer progresses the de- 

 velopment of the radicicoles becomes accelerated, as may be observed 

 from Table XXII. 



Table XXII.- 



-Molting records of radicicoles of the grape phylloxera, March to 

 July, 1915, Walnut Creek, Calif. 



Indi- 

 vid- 

 ual 

 No. 



Date egg 

 hatched. 



Date of 

 first 

 molt. 



First 

 instar. 



Date of 

 second 

 molt. 



Sec- 

 ond 

 instar. 



Date of 

 third 

 molt. 



Third 



instar. 



Date of 

 fourth 

 molt. 



Fourth 

 instar. 



Total 



grow- 

 ing 

 period. 



Aver- 

 age 

 tem- 

 pera- 

 ture. 



Gen- 

 era- 

 tion. 



1 



2 



3 



4. 



Mar. 19 

 ...do.... 

 ...do.... 

 . -do...- 



Apr. 3 

 Apr. 7 

 Apr. 8 

 Apr. 10 

 May 27 

 ...do.... 

 June 7 

 June 27 

 June 30 

 July 3 



Days. 

 15 

 19 

 20 

 22 

 16 

 16 

 15 

 11 

 14 

 17 



Apr. 10 

 ...do.... 

 Apr. 13 

 Apr. 17 

 May 30 

 ...do.... 

 June 10 

 June 30 

 July 2 

 July 6 



Days. 

 7 

 3 

 5 

 7 

 3 

 3 

 3 

 3 

 2 

 3 



Apr. 16 

 ...do.... 

 Apr. 18 

 Apr. 21 

 June 1 

 ...do.... 

 June 13 

 July 1 

 July 4 

 July 9 



Days. 

 6 

 6 

 5 

 4 

 2 

 2 

 3 

 1 

 2 

 3 



Apr. 22 

 ...do.... 

 Apr. 23 

 Apr. 25 

 June 5 

 ...do.... 

 June 16 

 July 4 

 July 9 

 July 10 



Days. 

 6 

 6 

 5 

 4 

 4 

 4 

 3 

 3 

 5 

 1 



Dai/s. 

 34 

 34 

 35 

 37 

 25 

 25 

 24 

 18 

 23 

 24 



°F. 



58.25 



58.25 



58.25 



58.25 



61 



61 



63 



65 



65 



65 



A. 

 A. 

 A. 

 A. 



5. 



6 



7 



8. 

 9.. 



May 11 

 ...do.... 

 May 23 

 June 16 

 ...do.. 



B. 

 B. 

 B. 



c. 

 c 



10.... 



...do.... 



C. 



The individuals enumerated in Table XXII were reared under 

 cellar conditions on equally succulent pieces of severed roots. Table 

 XXII, both by itself and taken in conjunction with Tables XX and 

 XXI, indicates the influence of temperature upon the development 

 of the radicicole under equal food conditions. Under a temperature 

 of 58.25° F. the period of growth averaged 35 days, under an aver- 



