| Drosophila melanogaster larvae (Diptera: Drosophilidae) parasitized by the solitary koinobiontic endoparasitoid Leptopilina boulardi (Hymenoptera: Eucoilidae) alter their behaviour, burrowing more deeply into their food medium. I experimentally verified this behaviour (previously described only anecdotally), and attempted to explain its ultimate causation, based on (1) a tradeoff between better nutrition (and thus faster growth) at the medium surface, against increased safety from predators in the interior; and (2) the possibility that the optimal balance of growth and risk is different for host and parasitoid. I considered all logically possible explanations--that the behaviour evolved because it increases the fitness of the parasitoid (host manipulation), that it evolved because it increases the fitness of the host (host defense) or that it is not adaptive to either host or parasitoid. I found no plausible hypotheses for host defense, but developed and tested two hypotheses for host manipulation. I first hypothesized that parasites suffer less from a reduction in nutrition than do hosts, because existing host tissue represents a sufficient food supply for parasitoid development. My second hypothesis was that controlling host nutrition acts as a method for the parasitoid to control the timing of host development. Neither hypothesis was supported experimentally. I concluded by exploring the growth-risk tradeoff theoretically, using a state-dependent model. |
