54 
BULLETIN 903, U. 
DEPARTMENT OF AGRICULTURE. 
slowly, and as a rule the larger and more fleshy the lesion the more 
rapid is the growth of the insect thereon. On resistant vines the 
newly hatched larva rarely fastens on any place except the apex 
of the rootlet or on a nodosity already formed. On American non- 
resistant vines the larva? settle in the main as they do on vinifera?, 
but on some varieties a decided preference is given to the growing 
rootlets over the larger roots. 
During the years 1911 and 1912 experiments were conducted to 
determine the growth and development of radicicoles under cellar 
conditions. Table X summarizes these observations. 
Table X. — Summarized records of Incubation and development of the radicicole 
of the arape phylloxera under cellar conditions, during 1911 and 1912, Walnut 
Creel:. Calif. 
Incubation period. 
Num- 
ber of 
indi- 
vid- 
uals. 
Developmental 
period. 
Num- 
ber of 
indi- 
vid- 
uals. 
Generation cycle. 
Aver- 
age 
tem- 
Generation. 
berof 1 
indi- 
££ ' Maori- 1 Mini- 
iiaib. muirL mum _ 
Aver- 
age. 
1 
Maxi- Mini- Aver- 
mum. mum.i age. 
! 1 
Maxi- 
mum. 
! 
Mini- Aver- 
mum. age. 
pera- 
ture 
during 
period 
of de- 
velop- 
ment. 
1 
Da ?. 
49 17 
58 13 
21 j 12 
10 j IS 
1 
Days. 
10 
8 
8 
Da us. 
13.6 
10.2 
10.1 
13.3 
Days. 
1S1 48 
352 61 
30 1 41 
8 24 
13 1 20S 
Davs. 
13 
16 
16 
17 
125 
14 
Days. 
29' 6 
31.7 
26.6 
21.9 
183.0 
27.6 
49 
53 
21 
8 
Day?. Davs. Davs. 
56 j 26 40'. 5 
74 25 42.2 
52 j 26 37.2 
3S i 24 34.6 
°F. 
63 
2 
64 
3 
4 
41 
&4§ 
64 
5-9* 
3 12 
11 11.3 
IS 
45 
3 41 1 35 1 37.3 1 
1 
1 Hibernating individuals, maturing in 1912. 
- In 1912 the records extend from Mar. 20 to July 22. 
A summary of the observations made on the growth and develop- 
ment of the radicicole on severed root cuttings in the cellar in 1911 
and 1912 may be given in brief. The great variation existing in 
the growth of individuals under the same temperatures, and even on 
a given piece of root, is resultant entirely from the condition of the 
food. An aphid living on a callus formation or tuberous lesion 
develops more rapidly than one living on the normal surface of 
the same piece of root. Individuals living on vigorous roots de- 
velop more rapidly than those on decayed or dying roots. Oc- 
casionally a decaying root will send out very fleshy lesions, and 
these, while they remain fresh, provide ample nourishment for the 
aphids and enable them to grow quickly. After a root reaches a 
certain point in decay or dryness the phylloxera? can no longer de- 
velop on it and must seek better food or perish. 
The growing period of the aphids recorded in Table X ranged 
from 13 to 61 days, hibernating individuals excluded. The grand 
average, hibernants not considered, was 30.57 days, practically one 
month. That the maximum period may be prolonged is evidenced 
from an observation made in the summer of 1912. in which a series 
