Contrasting olfactory responses of two egg parasitoids to buckwheat floral scent are reflected in field parasitism rates

Conservation biological control programs advocate the planting of flower strips alongside crops to improve the survival and effectiveness of parasitoids. Ideally, the provided food plants are highly attractive and benefit the targeted biocontrol agents without promoting pests or intraguild competition. Previous laboratory studies showed that Trissolcus basalis, an egg parasitoid of the stink bug Nezara viridula, is highly attracted to floral odors of buckwheat and that its nectar increases the wasp’s fecundity. In the field, T. basalis competes with the co-occurring parasitoid Ooencyrtus telenomicida for host eggs. Therefore, in the present study, we explored whether O. telenomicida shows similar attraction to buckwheat volatiles. We then carried out a 2-year field experiment to assess whether the laboratory-observed olfactory behaviors of both species are reflected in changed parasitism rates of stink bug eggs. Parasitism by naturally occurring egg parasitoids was measured in tomato plots with and without a margin of flowering buckwheat. Our results showed that, unlike T. basalis, O. telenomicida was repelled by the odor of buckwheat flowers. In both years, T. basalis found more egg batches and parasitized more stinkbug eggs when flower margins were present. Egg parasitism correlated positively with proximity to the buckwheat margin. In accordance with our prediction, egg parasitism by O. telenomicida occurred almost exclusively in the control plots and for a shorter period during the season. We conclude that buckwheat strips can influence intraguild competition and hypothesize that the effect was mediated by floral volatiles.