48 
BULLETIN 903, TJ. S. DEPARTMENT OF AGRICULTURE. 
of eggs laid on rotting tuberosities. The eggs have a considerable 
resistance to water at ordinary temperatures and may also liatch 
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 
June 13-Sept.6 
June 13-Aug.l9... 
June 5-Aug. 18 
July 7-Aug. 20 
Aug.9-Sept.2 
Aug.18-0ct.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 
21 
II 
do 
Laboratory shelf. . . 
Cellar 
10.8 
9.8 
10.2 
II 
III 
Laboratory shelf. . . 
Cellar 
8.5 
11.7 
IV 
IV 
Laboratory shelf. . . 
Cellar 
7.2 
13.3 
i 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 • ays, respectively. The period cov- 
ered was from June 19 to Octobtr 3, but the great majority of the 
total of 55 eggs were laid during June. A small series of 27 sixth- 
