DIFFERENCES IN HOST UTILIZATION BY POPULATIONS OF NORTH-AMERICAN GRAPE PHYLLOXERA (HOMOPTERA, PHYLLOXERIDAE)

Grape phylloxera, Daktulosphaira vitifolia (Fitch) collected in New York from Vitis riparia (Michaux) showed lower levels of attachment to 'Cabernet Sauvignon' in root bioassays than did phylloxera collected in Califomia from Vitis vinifera (L.). Differences found using root bioassays were confirmed by an experiment using potted plants. After rearing phylloxera from New York in the laboratory for several generations on roots of 'Cabernet Sauvignon', no differences in attachment levels between the Califomia and field-collected New York populations were found. However, Califomia and New York populations differed in development and fecundity on 'Cabernet Sauvignon', even after being reared for several generations on 'Cabernet Sauvignon'. Our tests showed differences in attachment levels on 'AxR#1' between phylloxera collected from two different grape species in New York (V. riparia, V. labruscana Bailey 'Concord') and phylloxem from V. vinifera in Califomia. We observed an egg-producing phylloxera from V. riparia on '5C Teleki. Comparisons made between phylloxera populations collected from a single Vitis species within a relatively small area in New York showed significant differences in attachment to 'Cabernet Sauvignon'. Our findings suggest the need to evaluate rootstocks against highly variable native populations of phylloxem. Collections made throughout the summer indicated that large seasonal differences did not occur in the propensity of leaf-form phylloxera to attach to the root pieces. The sensitivity of bioassays to laboratory conditions requires that comparisons between populations be made using simultaneous bioassays.