Performance of native grape phylloxera on host plants within and among terrestrial islands in Arizona, USA

In Arizona, USA, the canyon grape, Vitis arizonica Englemann, and grape phy lloxera (Daktulosphaira vitifoliae Fitch, Homoptera, Phylloxeridae) are dis tributed among mountain ranges that are surrounded by expanses of desert la cking Vitis habitat, thus forming a system of terrestrial islands. Both her bivore and host populations may have diverged genetically among mountain ra nges under the influence of restricted gene flow and variable selection amo ng sites. Herbivore adaptation to local hosts would be expected to ensue, w ith the potential to promote divergence, both in traits under selection and by further reducing the probability of interisland colonization. To test t he hypothesis that phylloxera are adapted to local hosts, demographic compo nents of fitness of field-collected native grape phylloxera were measured i n the greenhouse on vines of V. arizonica that were categorized as either n atal, neighboring, and or isolated hosts. There was no evidence for greater adaptation to natal or neighboring hosts but there were significant intera ctions between herbivore and host treatments in one experiment. There was g enetic variation for gall formation among six clones tested. Though a failu re to detect local adaptation could have resulted from low statistical powe r, benign experimental conditions, or inadequate genetic variation, the div ergence of isolated grape populations is suggested to have been insufficien t to promote local adaptation in grape phylloxera at the level of isolated mountain ranges. It is further suggested that, within populations, adaptati on to individual host plants could be forestalled by selection for 'general purpose genotypes' through wind-borne displacement of colonizers into the unpredictable environment of a heterogeneous array of hosts. In addition, s hort-term extinction/colonization dynamics could promote gene flow such tha t time is insufficient for adaptive mutations or gene combination to arise.