Effect of interference and exploitative competition on associative learning by a parasitoid wasp: a mechanism for ideal free distribution?

Abstract

Competition among foraging individuals of the same species occurs for several resources including, but not limited to, food, mates, nesting sites and, in parasitoid wasps, hosts. Individuals should therefore adapt their resource exploitation decisions accordingly and learning ability has been shown to be used in intraspecific competition situations to optimize fitness. We investigated the effect of interference and exploitative competition on aversive learning of a parasitoid wasp Venturia canescens (Hymenoptera: Ichneumonidae) as a model. We studied the effect of the presence of one to five competitors and of prior competitors, that is, the presence of parasitized hosts, on olfactory learning of wasps based on a Pavlovian conditioning system. We found that the odour of peppermint, which was previously positively associated with the reward of an oviposition, was rejected by females that experienced this odour while exploiting a patch of unparasitized hosts in the presence of conspecific females. Unexpectedly, the number of competitors in cases of interference did not increase the level of aversion to the peppermint odour. Thus conditioned stimuli previously encountered during the exploitation of a patch of parasitized hosts were rejected when females oviposited in the presence of unparasitized hosts. Our results suggest that intraspecific competition, by interference and by exploitation, acts as motivation for females to avoid stimuli related to competition; this may explain how an ideal free distribution can be reached by learning processes.