Natural selection on a tropical system: Gall-size distribution on Waltheria indica (Malvaceae)

Abstract

The optimal foraging theory has provided a successful explanation of the complex aspects involved in resource utilization and selection by interacting species. Gall-inducing insects and their natural enemies have played an important role in generating testable evolutionary hypotheses. One scenario is provided by the galling insect Anisodiplosis waltheriae (Diptera: Cecidomyiidae), its natural enemies and host plant, Waltheria indica (Malvaceae). Here, we present our results of field studies concerning gall size and propose a foraging model that incorporates optimization of attack by natural enemies on size-dependent galls as the principal force driving natural selection. From our analysis we can conclude that unidentified enemies attacked the smallest size classes of the galls of A. waltheriae, while predators preferentially attacked medium diameter galls and parasitoids preferentially attacked the largest galls. The preference of different types of enemies results in an unexpected selection effect detected in post-selection gall-size distribution. Further, our model was able to characterize resource partitioning among the interacting natural enemies of the galling insect. This study may provide the fuel to stimulate more studies on the dynamic nature of how natural enemies select hosts based on gall size.