Odour-Mediated Avoidance of Competition in Drosophila parasitoids: The Ghost of Competition

Abstract

A simple foraging model is used to investigate patch selection of Leptopilina heterotoma, a parasitoid of Drosophila larvae, in the field. The model assumes that patches only vary with respect to host species present and encounter rates of parasitoids and hosts. It predicts that L. heterotoma should visit patches consisting of decaying stinkhorn mushrooms. This prediction is insensitive to values of model parameters and assumptions. However, field data show that L. heterotoma females rarely visit stinkhorn patches. This leads us to consider predation risk and interspecific competition on all patch types in the field. Predation was never observed, and generalist predators did not prey on our parasitoids. We hypothesise that L. heterotoma females avoid visiting stinkhorn patches because another, closely related, parasitoid species (Leptopilina clavipes) is often present on these patches. Subsequent experiments show that L. heterotoma females can recognise odours emanating from patches where adult L. clavipes females are present, and that they avoid visiting these patches. We show that L. clavipes is the superior competitor. Hence, by refraining to visit patches with L. clavipes, L. heterotoma females avoid strong interspecific competition. When interspecific competition is included in the model, predictions closely match field observations, indicating that avoidance of competition is an important factor for patch selection by L. heterotoma.