VARIABILITY IN GRAPE PHYLLOXERA PREFERENCE AND PERFORMANCE ON CANYON GRAPE (VITIS-ARIZONICA)

We tested the deme-formation hypothesis experimentally with four popul ations of leaf-galling grape phylloxera, Daktulosphaira vitifoliae, an d its host, canyon grape (Vitis arizonica). An experiment designed to examine preference and performance showed that phylloxera populations did not significantly prefer their original host relative to other hos ts in the percent available leaves galled. There were significant herb ivore population effects (P<0.01), host effects (P<0.001), and populat ion x host interaction effects (P<0.001). Herbivore populations had di fferent colonizing abilities (performance, as measured in the mean num ber of galls per leaf) on an individual host (P<0.001), but there was no host effect. Host genotype significantly affected phylloxera perfor mance, measured as survivorship (P<0.01), but a phylloxera population did not necessarily have higher survivorship on its original host. Dif ferences in fecundity, another measurement of performance, were due to intrinsic differences among herbivore populations (P<0.05), and not r elated to host genotype. There was no correlation between distance fro m a phylloxera population in the field and a host's susceptibility to attack. There was a significant positive relationship between levels o f infestation on a clone in the field and its susceptibility to coloni zation experimentally (P<0.05), suggesting inherent differences in hos t resistance and susceptibility. These results did not support the dem e-formation hypothesis. III a second experiment, host clone x water tr eatment interactions affected phylloxera survivorship (P<0.05) and fec undity (P<0.05). We conclude that host genotype x environment interact ions may prevent sessile, parthenogenetic herbivores from locally adap ting to individual host genotypes.