Ecology of the gall-forming aphid, Slavum wertheimae, on Pistacia atlantica: Population dynamics and differential herbivory

The aphid Slavum wertheimae HRL (Homoptera; Pemphigidae, Fordinae) is monoecious on its host tree, Pistacia atlantica (Anacardiaceae). S. wertheimae induces large, conspicuous, coral-shaped 'cauliflower' galls and is very abundant on some trees, while rare or absent on others nearby. It is the on species among Israeli Fordinae which does not alternate between host plants. We investigated the changes in gall size, and in clone size of their aphid inhabitants, from 1994 to 1996, in order to understand the causes of differential herbivory. Gall abundance on male and female trees was similar but galls on male trees were larger and contained twice as many aphids as those on female trees, suggesting sex-dependent resource allocation in the host plant. Trees heavily galled in 1994 were recolonized in 1995 and 1996, while other trees nearby remained uncolonized. Similarly, within galled trees, shoots on branches carrying old galls were more likely to be freshly galled than shoots on previously ungalled branches. Alate sexuparae were trapped on ungalled P. atlantica trees in smaller numbers than on galled trees (none were trapped on non-host trees). Differential herbivory in S. wertheimae may be explained in part by the behavior of the alates, which may not wander far from their natal trees. Budburst (but not leaf abscission) times were correlated with gall abundance on the same trees.