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Christian Anton 2001: Host selection and performance of the parasitoid Cotesia melitaearum  in a tritrophic context. MSc-Thesis.

Most phytophagous insects are not strictly monophagous, but instead exploit at least a few related plant species. Plant secondary metabolites, nitrogen content and the phenology and distribution of the host plant can have a significant effect not only on the herbivore, but also on the behaviour, development and survival of the herbivore's enemies, such as parasitoids.

In this thesis, I investigated the influence of two host plant species of a butterfly host on four steps of successful parasitism of a parasitoid wasp: host habitat location, host location, host acceptance, and host suitability. While the first three steps can be considered as aspects of host selection, host suitability specifically refers to the quality of a host as a resource for the parasitoid. As a study system caterpillars of Melitaea cinxia were used. These larvae feed on Plantago lanceolata and Veronica spicata . Larvae of this butterfly are parasitised by the braconid wasp Cotesia melitaearum . An arena set-up was used to investigate which odours C. melitaearum utilizes in host habitat location. Experiments showed that female parasioids are attracted to herbivore-damaged host plants and frass from larvae and third instar-larvae. Artificially damaged host plants of both species did not elicit a response. Female parasitoids were more attracted to frass and herbivore-damaged Veronica leaves compared to herbivore-damaged Plantago and frass from larvae. During the host location on the host plant, wasps spent more time on V. spicata than P. lanceolata . The third step in successful parasitism, host acceptance, was not influenced by the host plant species. However, there was a significant difference in host plant suitability between P. lanceolata and V. spicata for both the caterpillar and the parasitoid. Pupal weight of M. cinxia and adult weight of C. melitaearum was higher for individuals reared from P. lanceolata . Thus, host plant suitability for the host and the parasitoid is positively correlated.

This study shows that host plant species affects different steps of successful parasitism of C. melitaearum . However, host selection and host suitability were not positively correlated. Herbivore-damaged V. spicata plants may emit a strong cue which is more attractive than herbivore-damaged P. lanceolata . Moreover, the recently discovered aggregated distribution of V. spicata might enhance the attractiveness of V. spicata habitat patches. The combination of detectability and efficient foraging behaviour in V. spicata habitats, due to clumped aggregation of caterpillar groups, may outweigh the lower adult weight of wasps from caterpillars feeding on V. spicata .