Stephan G. MICHA, Urs WYSS
Institut für Phytopathologie, Universität Kiel, 24118 Kiel,
Germany
The efficiency of parasitoids as antagonists of herbivorous insects depends to a large extent on their host finding abilities. The herbivorous hosts and the food plants of the hosts are potential sources of chemical cues needed for parasitoid orientation behaviour. The interactions between plant, herbivourous insect and parasitoid were studied in a tritrophic model system, consisting of oat (Avena saliva), the grain aphid (Sitobion avenae) and the aphid parasitoid (Aphidius uzbekistanicus). Y-tube olfactometer experiments were used to test parasitoid reactions towards volatile emissions of both the plant and herbivore.
The experiments showed that preceding experience of parasitoid females with the plant-host complex is a prerequisite for the olfactoric attraction to plant volatiles. Previous contact to aphids or oats only was not sufficient to evoce this responsiveness. The results indicate the involvement of associative learnig.
Furthermore it could be shown that volatiles of uninfested plants were not attractive to the parasitoids. The emission of attractive volatiles depended on aphid infestation and was restricted to infested leaves. Volatiie emissions of the aphids, including contaminations of the plant by honeydew, exuviae etc., played no role for the attraction of the parasitoids to the infested plants. Therefore, aphid infestation must have induced a characteristic plant odour that confers attractiveness to the parasitoids. These plant volatiles can be termed as herbivor- induced synomones.
Of the aphid-derived stimuli the aphid alarm pheromone was studied in more detail, as parasitoids were attracted towards disturbed aphids. Therefore, the possible function of the aphid alarm pheromone (E)-ß-farnesene (EBF) as a host finding kairomone for A. uzbekistanicus was tested.
Extracts of volatile emissions of undisturbed aphids and of aphids under parasitoid attack were obtained by air entrainment. Extracts of cornicle secretions were gained by disturbing aphids and taking their secretions into solution. Extracts were compared by gas chromatography. Only air entrainments of aphids under attack and solvent extracts of comicle secretions contained the alarm pheromone.
In Y-tube olfactometer bioassays, female A. uzbekistanicus were attracted to aphid groups under attack of parasitoids, presumably by released EBF. High concentrations of synthetic EBF (1.4 µg to 5.7 µg) also attracted A. uzbekistanicus females. Females with oviposition experience reacted more readily to lower concentrations of EBF than females without experience.
As the volatile aphid alarm pheromone attracted A. uzbekistanicus females, it can be concluded that this chemical stimulus acts as a host finding kairomone for this parasitoid species. However, its effect over long distances seems to be limited due to the relatively high concentrations required for reactions.
From the present results it appears that plant-derived emissions induced by aphid attack are the main chemical cues for the host finding by A. uzbekistanicus females.