Dichotomy of male and female responses to hoverfly-driven cues and floral competition in the parasitoid wasp Aphidius ervi Haliday

Abstract

The aim of this study was to test the hypothesis that floral visitation and foraging by the hoverfly Episyrphus balteatus De Geer would have no effect on the attraction to, or use of, flowers by the parasitoid wasp Aphidius ervi Haliday. Results demonstrated that in two-way choice tests, air-streams emanating from live Fagopyrum esculentum Moench (buckwheat) flowers were significantly more attractive than clean air to both male and female wasps, by more than one and two orders of magnitude, respectively. When air-streams from flowers presented alone were compared to those from flowers presented with E. balteatus, or exposed to E. balteatus prior to use, no preferences were detected. Nevertheless, air that had passed through chambers containing only live E. balteatus was significantly repellent to female wasps, with individuals spending 18 times less in these air-streams than clean air. Conversely, air passed over E. balteatus was attractive to male A. ervi, with wasps spending more than six times longer in this vs. clean air. We argue that hoverfly floral visitation/use should have minimal impact on floral attractiveness to A. ervi based on these results, and that for male wasps greater benefits might be gained by responding positively to hoverfly volatiles in the absence of floral cues.In a separate experiment, a consistent trend for reduced fitness of female A. ervi on caged F. esculentum was observed under increasing competitive pressure from E. balteatus. High densities of hoverflies significantly reduced wasp longevity by more than 25%, though lower densities had no significant effect. Conversely, male A. ervi appeared to survive for longer where higher densities of hoverflies were present, though results were not as clear-cut statistically. We argue that antagonistic interactions seen between hoverflies and female A. ervi were the result of behavioural interference and would only be observed in the field under conditions of extreme competitive pressure.