In my thesis I investigated the foraging behaviour of the wasp Hyposoter horticola, an egg-larval parasitoid of the Glanville fritillary butterfly Melitaea cinxia, in the Åland islands in Finland. The particularity of this system is that the wasp is resource limited and faces strong intraspecific competition. ---------- I first focused on behaviour at an individual scale. In a series of experiments I tested how H. horticola s host searching behaviour was affected by developmental timing of both the parasitoid and the host, and direct intraspecific competition among foraging females. I found that the wasps visit host egg clusters before the hosts are susceptible to parasitism, presumably to cope with the limited time availability of the hosts. As the unparasitized hosts matured their value increased, competition became more frequent, and the wasps foraged more actively. Competition can also affect the parasitoid at earlier stages in its life. As larvae inside the hosts, the immature H. horticola suffered from competition due to superparasitism. Combining behavioural experiments in the laboratory and genetic analyses of sibship, I found that adult H. horticola deposit a chemical marking after oviposition that deters conspecifics from parasitizing a previously exploited host cluster. This protects parasitized host clusters from further exploitation. I found that the effectiveness of the deterrent persisted under natural conditions, where individual host egg clusters were each primarily parasitized by a single female H. horticola. Even when several females parasitized a cluster, the great majority of the offspring were full-siblings and the parasitism rate did not increase above the average 1/3 observed throughout the population. Considering that H. horticola is resource limited and faces intraspecific competition when foraging for hosts, it is surprising that only they parasitize a fraction of the hosts in each host egg cluster. After testing several physiological and evolutionary hypotheses for what might lead to this sub-maximal rate of host exploitation, I concluded that optimal foraging with avoidance of superparasitism was the most plausible explanation, as long as the search time between host clusters was low. ------ Then, I worked at a larger scale than individual behaviour. In the Åland islands, the butterfly host lives as a classic metapopulation with a high extinction rate of local populations. Due to strong competition, almost all the M. cinxia egg clusters in the population are found and parasitized by H. horticola. This suggests that the wasps must be good dispersers, which could influence the spatial genetic structure of the parasitoid population. I used DNA microsatellite markers and analysed H. horticola individuals sampled from over the entire population. My results indicate that, contrary to theory that higher trophic level species are more affected by habitat fragmentation than the species upon which they depend, the H. horticola population was less strongly genetically structured than the metapopulation of its butterfly host. It seems that H. horticola s dispersal ability allows it to compensate for the fragmented distribution of its host and not suffer from the metapopulation dynamics of the host local populations.
Overall, the results of my thesis show that interactions between H. horticola and its host M. cinxia are strongly affected by competition among the adult female wasps. Intraspecific competition has an important role from an evolutionary perspective. Hyposoter horticola s deterrent marking behaviour has evolved in response to competition and the risk of superparasitism faced by immature offspring. Avoidance of superparasitism to limit competition is also the fundamental mechanism that controls H. horticola s optimal foraging strategy. And intraspecific competition modifies individual female host searching behaviour, increasing their foraging activity. -------- Interactions within a multitrophic system are complex and predictions concerning host-parasitoid interactions are difficult to generalise. However, as in this system, competition is factor that should receive more attention in empirical and theoretical studies of host-parasitoid interactions.Ei saatavill
