Closely related parasitoids induce different pupation and foraging responses in Drosophila larvae

Few examples exist where parasites manipulate host behaviour not to increase their transmission rate, but their own survival. Here we investigate fitness effects of parasitism by Asobara species in relation to the pupation behaviour of the host, Drosophila melanogaster. We found that Asobara citri parasitized larvae pupate higher in rearing jars compared to unparasitized controls, while A. tabida pupated on or near the medium. No change in pupation site was found for three other species. A follow-up experiment showed a non-random distribution of parasitized and unparasitized pupae over the different jar parts. To test the adaptiveness of these findings, we performed pupal transfer experiments. Optimum pupation sites were found to be different between host individuals; wall individuals survived better than bottom individuals, but bottom individuals did worse at the wall. Two parasitoid species that alter pupation site significantly showed high rates of diapause at their 'preferred' pupation site. For one of them, A. citri, pupation occurred at the optimal site for highest survival (emergence plus diapause). From literature we know that pupation height and foraging activity are genetically positively linked. Therefore, we implement a short assay for rover/sitter behavioural expression by measuring distance travelled during foraging after parasitism. For one out of three species, foraging activity was reduced, suggesting that this species suppresses gene expression in the for pathway and thereby reduces pupation height. The parasitoid species used here, naturally inhabit widely different environments and our results are partly consistent with a role for ecology in shaping the direction of parasite-induced changes to host pupation behaviour. More parasitoids are found on the wall of the rearing jar when they originate from dry climates, while parasitoids from wet climates pupate on the humid bottom.