Effects of extra floral and floral nectar of Centaurea cyanus on the parasitoid wasp Microplitis mediator: Olfactory attractiveness and parasitization rates

Summary

Nectar is an important food source for adult parasitoids and can increase their longevity and fecundity and hence their parasitization rate. Both floral and extrafloral nectar are used as food sources by parasitoids. While floralnectar exploitation by parasitoids has been extensive lystudied, little is known on how parasitoid slocate extrafloral nectar,nor whether the availability ofextrafloral nectar increases parasitization of pests in the field. We conducted aY-tube olfactomete rexperiment to determine if the parasitoid Microplitis mediator utilizes olfactory cues tolocate the extrafloral nectar of Centaurea cyanus. Inaddition, weperformed asemi-fieldexperiment to investigate whether M.mediator are differentially attracted by C.cyanus offering nonectar,only extrafloral nectar or extrafloral and floral nectar and whether this translates indifferential parasitization rates of the cabbage moth, Mamestr abrassicae,inclose vicinity to the plants. Female M.mediator showed aninnate attraction toolfactory cues of open flowers but not toextrafloral nectar of C.cyanus. Under our experimental conditions M.mediator showed higher parasitization rates of M.brassicae larvae in the treatment with both floral nectar and extrafloral nectar compared to the treatment with only extrafloral nectar. Our results indicate that M.mediator may require the floral signals of open flowers to actually locate and exploit the plant’s extrafloral nectaries. This isrelevant from anapplied point because itdemonstrates that accessible (extra)floral nectar sources may be insufficiently exploited ifthey are not attractive to parasitoids. However,this can be resolved when the accessible nectar isassociated with attractive floral signals.