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

Abstract

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 floral nectar exploitation by parasitoids has been extensively studied, little is known on how parasitoids locate extrafloral nectar, nor whether the availability of extrafloral nectar increases parasitization of pests in the field. We conducted a Y-tube olfactometer experiment to determine if the parasitoid Microplitis mediator utilizes olfactory cues to locate the extrafloral nectar of Centaurea cyanus. In addition, we performed a semi-field experiment to investigate whether M. mediator are differentially attracted by C. cyanus offering no nectar, only extrafloral nectar or extrafloral and floral nectar and whether this translates in differential parasitization rates of the cabbage moth, Mamestra brassicae, in close vicinity to the plants. Female M. mediator showed an innate attraction to olfactory cues of open flowers but not to extrafloral 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 is relevant from an applied point because it demonstrates that accessible (extra)floral nectar sources may be insufficiently exploited if they are not attractive to parasitoids. However, this can be resolved when the accessible nectar is associated with attractive floral signals.