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Riikka Kaartinen 2005: The significance of odours and learning in the host location behaviour of the parasitoid wasp Hyposoter horticola (Gravenhorst). MSc-Thesis.

The endangered Glanville fritillary butterfly Melitaea cinxia is parasitized by hymenopteran parasitoid Hyposoter horticola (Gravenhorts). The host population is extremely scattered in the Åland islands, occupying dry meadow patches in pastures, rocky areas and roadsides. The parasitoid wasp is large in size, long lived and reaches its host over practically its whole distribution range in the Åland islands. It parasitizes a constant fraction of about one third of each host egg clusters. The host is susceptible to parasitism only during a very short time just before the first instar larvae come out of the egg shell. Yet H. horticola is able to efficiently parasitize them. The probable explanation is that the parasitoid repeatedly visits host egg clusters before they are ready to be parasitized, but the mechanism how it is able to find the hosts over and over again is not clear.

In this project the use of wasp produced marking odour, innate egg odour and spatial learning were studied as possible mechanisms for host location. The study was conducted by putting out host eggs at different phases of their development and also moving already present egg clusters to different locations inside the host subpopulation. Also adult parasitoid behaviour on the host egg cluster was observed. The results suggest that the main mechanism that H. horticola uses to find host eggs repeatedly is spatial learning at the level of local host population, and once on plant it probably finds the eggs according to their short-distance odour or spatial memory, or both. This is the first parasitoid species known to be able to learn future resources and the first case reported where parasitoid is using spatial learning to repeatedly visit hosts.