Size does matter: variation in herbivory between and within plants and the plant vigor hypothesis

The plant stress and plant vigor hypotheses (PVH) are two of the most widely recognized hypothesis invoked to explain differential distribution of insect herbivores among their host plants. In both cases, the emphasis is on bottom-up processes (i.e. host-plant quality), but a recent meta-analytical review of the literature has shown that the plant stress hypothesis might have limited support among insect herbivores. In this study, we conducted a meta-analysis of the effects of plant vigor on insect herbivore abundance and survivorship by reviewing 71 published articles that explicitly tested the PVH and enabled 161 independent comparisons. Z-transform was used as the metric to standardize the results of all independent comparisons. Our quantitative results have shown that Hymenoptera (sawflies) was the most abundant group in the reviewed studies, representing 28.1% of the independent comparisons, followed by Diptera (25.1%) and Homoptera (22.6%). Amongst all the guilds studied, gall-formers were the most representative group (68.0%), whereas leaf-miners and stem-borers were underrepresented (less than 4.0% of the available comparisons). Insect herbivores were significantly more abundant on more vigorous plants (E++ = 0.6432, CI = 0.7558-0.7280), but no significant effect was detected on herbivore survivorship. When herbivores were categorized into feeding guilds, effects of plant vigor on herbivore abundance were stronger for sap-suckers, leaf-miners and gall-formers. Our results have shown a strong herbivore preference for more vigorous plants, although our results do not support a preference-performance linkage.