54 



BULLETIN 903, IT. S. 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 viniferse, 

 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 grape phylloxera under cellar conditions, during 1911 and 1912, Walnut 

 Creek, Calif. 





Num- 

 ber of 

 indi- 

 vid- 

 uals. 



Incubation period. 



Num- 

 ber of 

 indi- 

 vid- 

 uals. 



Developmental 

 period. 



Num- 

 ber of 

 indi- 

 vid- 

 uals. 



Generation cycle. 



Aver- 

 age 

 tem- 



Generation. 



Maxi- 

 mum. 



Mini- 

 mum. 



Aver- 

 age. 



Maxi- 

 mum. 



Mini- 

 mum. 



Aver- 

 age. 



Maxi- 

 mum. 



Mini- 

 mum. 



Aver- 

 age. 



pera- 

 ture 

 during 

 period 

 of de- 

 velop- 

 ment. 



1 



49 

 58 

 21 

 10 



Days. 

 17 

 13 

 12 

 18 



Days. 

 10 



8 

 8 



7 



Days. 

 13.6 

 10.2 

 10.1 

 13.3 



1S1 



352 



30 



8 



13 



18 



Days. 

 48 

 61 

 41 

 24 

 208 

 45 



Days. 

 13 

 16 

 16 

 17 

 125 

 14 



Days. 

 29'. 6 

 31.7 

 26.6 

 21.9 



183.0 

 27.6 



49 



58 



21 



8 



Days. 

 56 

 74 

 52 

 38 



Days. 

 26 



Days. 

 40'. 5 



"F. 



63 



2 



25 42.2 



64 



3 



26 

 24 



37.2 

 34.6 



64^- 



4 



64 



4i. . 





5-9 2 . . 



3 



12 



11 



11.3 



3 



41 



35 



37.3 









1 Hibernating individuals, maturing in 1912. 



2 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 phylloxerse 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 



