Parasitic Wasps on Butterfly Expedition

Parasitic wasps or parasitoids are a large group of insects, whose larvae live in or other host insects and kill the latter after finishing their development. The main task of the female parasitoid is to find the right host, i.e. oviposition site, for her offspring. The host foraging behavior of parasitoids is intensively studied with the main goal to enhance their effectiveness as biological control agents of crops against insect pests. A tremendous variety of host location strategies are known, in which chemical cues, i.e. infochemicals, play an important role. This thesis focuses on interactions and infochemicals transferred between parasitoids of eggs and larvae of the large cabbage white ( Pieris brassicae) and its host plant Brussels sprouts plants. A fascinating strategy by Trichogramma egg parasitoids was shown and adds a new dimension to our understanding of host-parasitoid associations: the minute wasps hitch a ride on the female butterfly to the oviposition site and subsequently attack the butterfly eggs after being deposited. To detect the mated female butterfly the egg parasitoids spy on a chemical host cue, a so-called anti-aphrodisiac, transferred during mating to the female. Furthermore the impact of infochemicals of the host plant, on parasitoids was demonstrated. Butterfly eggs deposited on Brussels sprouts plants were shown to induce chemical modifications of the leaf surface detected by the Trichogramma wasps and arresting them in the vicinity of the eggs. Again the butterfly anti-aphrodisiac plays a key role, this time in interactions between the host eggs, the host plant and the egg parasitoids. Feeding caterpillars of the large cabbage white induce Brussels sprouts plants to emit a modified volatile bouquet of chemicals detected by Cotesia larval parasitoids. Here, it was demonstrated that the induced plant volatile production decreases after a successful recruitment of the Cotesia wasps, with an advantage for both the parasitoids and the plant itself. The knowledge assembled in this thesis provides additional information to both fundamental and applied research and may help to improve a targeted application of parasitoids as biological control agents of crop pests.