Knowledge of the form and magnitude of selection on insect herbivore traits at the level of individual plants is needed to understand the interaction of selection and gene flow within host-affiliated insect populations when individual plants equate to differing selective regimes. We analyzed phenotypic selection on the trait gall size for a host-specific gall forming insect, Belonocnema treatae (Hymenoptera:Cynipidae) at both the level of the individual host plant, (Quercus fusiformis) and the population level (across host plants) in each of two years. Linear and nonlinear selection and the fitness function relating gall size to the probability of survivorship in the absence of natural enemies were estimated for each level and year.
Phenotypic selection on gall size imposed by the host plant was observed in 19 of the 22 gall-former subpopulations examined. At the population level, both linear (directional) and nonlinear (stabilizing) selection were evident each year. However, population-level estimates of selection masked the significant heterogeneity in the form, magnitude, and direction of selection evident among individual plants each year. Heterogeneity in selection among gall-former subpopulations resident on individual host plants is emphasized by our findings that selection varied from directional to stabilizing among plants and the majority of selection gradients estimated for individual plants did not fall within the 95% CIs of the population-level estimates. Our evidence demonstrates that the subpopulation of gall formers resident on an individual plant experiences host plant-mediated selection on the trait gall size that is specific to the individual plant.