Geoica wertheimae induces spherical galls on its primary host, Pistacia palaestina (Anacardiaceae). We studied the temporal changes in gall size and aphid clone size, as well as gall distribution and abundance on marked trees during two consecutive years. The density of galls (per shoot and per leaf) was low during the study period, and gall distribution was clumped. Gall abundance varied greatly among trees, but gall abundance and tree budburst phenology were uncorrelated. Galls increased eight-fold in volume during the season, in parallel with the increase in aphid clone size, from one individual to several hundred aphids per gall. The trigger for the induction of the alate morph in the galls in late summer seems to be an abrupt change in aphid density within the galls, which occurs when aphid reproductive rate exceeds the rate of change in gall internal surface area.
Two species of Lepidopterous larvae were present in about one-third of the galls. They occasionally destroyed the aphid clone, but many aphids often remained alive. However, the volume of parasitized galls was significantly smaller than that of unparasitized galls, illustrating the dependence of gall size on aphid clone size.