Variation in performance on two grape cultivars within and among populations of grape Phylloxera from wild and cultivated habitats

When an indigenous insect becomes a pest, comparisons of performance of pest and non-pest populations on crop plants and of genetic variation in that performance may provide insight into the evolution of pest populations. To measure such genetic variation, 8-15 clones of the grape phylloxera (Daktulosphaira vitifoliae Fitch) were collected from wild grapevines in each of 3 geographically isolated sites (populations) and from commercial vineyards in northern California. A complete life table was made for clonal replicates from populations collected from wild grapevines on each of two commercial grape cultivars, the susceptible Vitis vinifera (L.) cultivar Cabernet Sauvignon, and the phylloxera-resistant rootstock ‘AxR# 1'. Variation in mean performance on these two hosts was partitioned among clones within collection sites and among sites. Performance measures included an individual analog to the intrinsic rate of increase (r), age at first oviposition, fecundity in the first ten days of reproduction, total fecundity, and longevity. The overall performance of phylloxera from the wild grapevines on the resistant cultivar AxR# 1 was greater than or equal to that on the susceptible cultivar Cabernet Sauvignon. There was significant variation among clones within populations from wild grapes in the rate of increase on ‘AxR# 1' and marginally significant clonal variation in some of the component parameters. There was no significant variation among clones within populations on ‘Cabernet Sauvignon' and no significant differences between populations on either crop in any trait. In a second experiment we compared the relative performance of 15-17 clones from wild grapevines and from commercial vineyards when reared on ‘Cabernet Sauvignon' and ‘AxR# 1'. Phylloxera from commercial vineyards had much higher overall performance on ‘Cabernet Sauvignon' than did phylloxera from the wild grapevines. Phylloxera from the commercial vineyard also had higher performance on ‘Cabernet Sauvignon' than on ‘AxR' l' but the performance of the phylloxera from wild and commercial grapes did not differ on ‘AxR# 1'. Our results show that there is genetic variation in traits related to performance on a resistant root-stock within these indigenous non-pest populations of phylloxera, but not among them. The pattern of performance of pest and non-pest populations on two commercial cultivars suggests that current levels of phylloxera performance on crop cultivars are the result of adaptation to those cultivars which has occurred while phylloxera has been associated with viticulture. Implications of these results for understanding the recent adaptation of phylloxera to ‘AxR# 1' in California are also discussed.