48 



BULLETIN 903, U. S. DEPARTMENT OF AGRICULTURE. 



of eggs laid on rotting tuberosities. The eggs have a considerable 

 resistance to water at ordinary temperatures and may also hatch 

 under water. Many, probably 25 per cent, of those that are laid on 

 rotting tuberosities fail to hatch. They seem to be so impregnated 

 with dampness and influenced by the rotting root tissues surrounding 

 them that they turn dark brown prematurely and finally collapse 

 after the embryo dies. It must be considered also that very slight 

 pressure applied to the eggshell may rupture it and kill the em- 

 bryo. 



INCUBATION PERIOD. 



The first incubation record at Walnut Creek took place during 

 April, 1909. Between April 9 and April 26, 24 eggs were observed 

 in the laboratory with the results shown in Table V : 



Table V. — Incubation period of the eggs of the grape phylloxera, Walnut 



Creek, Calif., 1909. 



Days. 



Average incubation stage 13. 8 



Maximum incubation stage 15 



Minimum incubation stage 12 



No temperature records were taken. The eggs were presumably 



deposited by overwintered adults. During 1911 and 1912 a large 



series of incubation records was obtained. Table VI gives incubation 



records for each generation during 1911. 



Table VI. — Incubation records of the eggs of the grape phylloxera at Walnut 



Creek, Calif., 1911. 



Genera- 

 tion. 



Environment. 



Dates of period of 

 incubation. 



Average 

 tempera- 

 ture. 



Number 



of eggs 



laid. 



Incubation period. 



Maxi- 

 mum. 



Mini- 

 mum. 



Aver- 

 age. 



1 1 



Cellar 



Apr. 28- May 18 



June 4-Aug. 19 



Junel3-Sept.6.... 



June 13-Aug. 19... 



June 5- Aug. 18 



July 7-Aug. 20 

 Aug. 9-Sept. 2 

 Aug.18-Oct.26.... 



61 

 64 

 ( 3 ) 



64.5 



( 3 ) 

 64.6 



( 3 ) 

 64 



49 

 889 

 412 



1,797 



235 



969 



551 



10 



Days. 

 17 

 15 

 13 



14 

 11 

 14 

 10 

 18 



Days. 



10 

 8 



7 



7 

 6 

 7 

 6 

 7 



Days. 

 13.6 



21 



.do 



10.8 



21 

 II 



Laboratory shelf. . . 

 Cellar 



9.8 

 10.2 



II 

 III 



Laboratory shelf. . . 

 Cellar... . 



8.5 

 11.7 



IV 



IV 



Laboratory shelf. . . 

 Cellar 



7.2 

 13.3 









1 Eggs deposited by overwintered adults. 



2 Later series of eggs deposited by overwintered adults. 



3 Temperature at least 5° higher than that in cellar at corresponding dates. 



From Table VI it will be seen that the influence of temperature 

 was very considerable. The records of 1912 are much more scanty 

 and bear out the observations of 1911. Under an average tempera- 

 ture of 70° F. the egg stage in 1912 averaged 8.9 days, with a maxi- 

 mum and minimum of 10 and 7 days, respectively. The period cov- 

 ered was from June 19 to October 3, but the great majority of the 

 total of 55 eggs were laid during June. A small series of 27 sixth- 



