Hormaphis hamamelidis and gall size: a test of the plant vigor hypothesis

We tested the Plant Vigor Hypothesis by determining the distribution of gal ls formed on leaves of witch hazel, Hamamelis virginiana, by the aphid Horm aphis hamamelidis, and by determining various factors that affect the fecun dity of the gall-forming fundatrices. We also studied the role of the funda trix in host plant manipulation. While the mean number of galls per leaf wa s low. galls had an aggregated distribution among leaves. Among trees, the average number of galls per leaf was not related to the mean leaf size, con trary to the preference prediction of the Plant Vigor Hypothesis. While fun datrices preferred the distal leaves of buds, which grew more than the prox imal leaves, being on distal leaves conferred no increase in fecundity for fundatrices. contrary to predictions of the Plant Vigor Hypothesis. Gall si ze was the factor that explained the largest proportion of variation in fun datrix fecundity, fundatrix size explained somewhat less of the variation. Also. gall position on the leaf, number of aphid galls on the leaf, and on which leaf of the bud the gall was located all played small, statistically significant roles in explaining fundatrix fecundity, but their effects were variable between experiments. Removal of fundatrices shortly after galls h ad enclosed them limited the growth of galls, indicating the role of the fu ndatrices in Pall growth. We compare and contrast this system versus other gall-forming insects, as well as discuss the adaptive significance of the a phid manipulation of the host plant. Much of the data contradict prediction s of the Plant Vigor Hypothesis, and we discuss how gall size, as a measure of plant growth caused by insect manipulation, explains the observed patte rns.