THE GRAPE PHYLLOXERA IX CALIFORNIA. 63 



For the first generation, eggs deposited by adult hibernants were 

 secured from a Zinfandel vineyard, and thereafter only eggs de- 

 posited in the cages and of known generations were used in the inocu- 

 lations. 



The average growing periods of the summer generations of wing- 

 less aphids varied from 34.5 to 18.25 clays, but in all except two gen- 

 erations this period ranged between 18.25 and 24.2(1 days. Individ- 

 uals varied between 36 and 10 days. The winged forms developed 

 more slowly than the wingless, nine individuals averaging 314 days- 

 The hibernants developed in an average of 6f months.- 



Eggs were placed for the most part on roots never before infested, 

 and tuberosities usually followed rapidly after the hatching of the 

 larvae. Xodosities were formed upon side rootlets. The main roots 

 were all between one-sixth and one-third of an inch in diameter. 



It was found that about 10 per cent of the larvae remained on the 

 exposed portions of the roots, the rest finding their way to the other 

 portions. In spring a large percentage and in summer and autumn 

 a smaller percentage of larva? settled close beside the eggshells from 

 which they had issued. In spring the laiwae did not display a tend- 

 ency to roam, but in summer and autumn they wandered consid- 

 erably, especially if the root had begun to decay or was drying too 

 rapidly. Similar conditions occur in vineyards, and it is in summer 

 and autumn that the typical wandering larvae are found. 



Excluding the winged migrants and the hibernated individuals, 

 the summary of the growing period of all the phylloxeras developing 

 on livinof roots during the vears 1913. 1911. and 1915 is recorded in 

 Table XIV. 



Table XIV. — summarii of Table XIII. 



Number of individuals : 114 



Average period of growth days — 22. '15 



Maximum period of growth do 36 



Minimum period of growth do 10 



Taking into consideration the individuals removed before they 

 attained their full development, the average growing period is to be 

 estimated at about 25 days. The cellar experiments with severed 

 pieces of roots in 1911 and 1912 combined yielded an average of 

 30.57 days, and the experiments in the cellar and incubator combined 

 in 1913 averaged 31.16 clays. The cellar temperatures of 1911 and 

 1912 averaged about 14 c F. lower than the combined cage tem- 

 peratures for the period 1913-1915 for the months from May to 

 October, inclusive. The cellar temperatures for 1913 averaged about 

 H° lower than the incubator temperatures for 1913 and about -| 

 higher than the cage temperatures for that year. 



