THE GRAPE PHYLLOXERA IN CALIFORNIA. 81 



Under average conditions the proportion on the larger roots was 

 between 5 and 10 per cent. Kegarding the American vines of non- 

 resistant type, a considerable diversity in the production of nymphs 

 has been observed. On some, like Moore's Early, this production 

 may be proportionately very large, while on others, like Isabella and 

 Catawba, it may be smaller than on viniferse, as occurred in the ex- 

 periments in caged and potted vines. Vines like Agawam, Lenoir, 

 and Delaware, vinifera crosses, bore about the same proportion of 

 nymphs as the viniferse, but among the labrusca types (Isabella, 

 Moore's Early, Concord, Champion) there was considerable variation. 



On resistant vines, the nymphs are developed on the nodosities, but 

 on viniferse and American vines of nonresistant type they are also 

 produced on other portions of the root system. On phylloxerated 

 viniferse, the most abundant production of nymphs occurs on the 

 fleshy and fibrous surface rootlets frequently observable in the 

 vineyard. These rootlets are sent out in May and June, and often 

 become grossly infested with phylloxera? in June and July. Toward 

 the end of July, they decay or dry out, and after that nymphs are 

 produced only on the larger roots and on nodosities deeper in the soil. 

 On the larger roots relatively few nymphs are produced before August 

 or after September. 



Among viniferse the more vigorous vines produce the greater pro- 

 portionate numbers of winged forms. Badly stunted vines showing 

 several years of phylloxeration produce comparatively few, while 

 the recently attacked vines around the periphery of " spots " produce 

 large quantities. Viniferse vines in pots produce great numbers the 

 first year of infestation, but if the soil is unchanged in the second 

 and third years, as the vines become weakened, they produce fewer 

 winged forms. 



As far as has been observed, all varieties of viniferse produce the 

 same proportion of migrants. 



It has been observed frequently that a humid environment stimu- 

 lates the production of migrants and a dry one precludes it. This has 

 been especially noticeable in the cases of young vines in pots and of 

 the severed roots kept under cellar conditions. The late appear- 

 ance of the migrants in the experimental vineyard in 1913 as com- 

 pared with those of 1914 and 1915 was perhaps due to lack of moisture 

 in the soil in summer. The spring of 1913 was exceptionally dry, 

 and the ground became very dry by June, whereas in the two years 

 following, moisture was conserved in the top soil until July. The 

 total migrant development of 1913, however, although at first re- 

 tarded, was finally just about as large as those of the succeeding 

 years. To hold the severed pieces of roots, glass jars and dishes were 

 used in the cellar, and it was found that in the summer and fall 

 1900°— 21 6 



