The contribution of floral resources and honeydew to the performance of predatory hoverflies (Diptera: Syrphidae)

Abstract

Hoverflies with predatory larval stages are important natural enemies of aphids and other pests in field crops. Many adult hoverflies are actively visiting and feeding on flowers, indicating that pollen and nectar are important resources for them. The scarcity of suitable flowers in intensified agriculture is therefore likely to limit their effectiveness as biological control agents. In this study on Episyrphus balteatus we quantify how permanent and temporal availability of pollen and nectar affects adult survival and reproduction, and assess to what extent aphid honeydew can substitute floral resources.Our results show that honeydew of cabbage aphids (Brevicoryne brassicae) strongly enhances hoverfly survival in the absence of floral resources. Less to be expected, when floral resources are available, honeydew supplementation still generates additional survival.A sucrose solution (as mimic for nectar) strongly increases adult longevity relative to water only, similar to honeydew. Flowers of buckwheat (Fagopyrum esculentum), permanently providing both nectar and pollen, further increases adult longevity and also supports egg production.Feeding on flowers of buckwheat during a 6days pre-ovipositional period allows adult females to produce eggs for a week thereafter, and to survive for another 2weeks, indicating that they can store and benefit from these nutritional resources for a substantial time. This suggests that the hoverflies only occasionally need to commute between aphid patches for oviposition (e.g., in the crop) and floral patches for feeding (e.g., at the border of the field). Honeydew, when locally at sufficiently high levels, may be used by the hoverflies to restore their energy demands, thereby postponing the need to search for floral resources.Cage experiments show that when suitable flowers are present the hoverflies can strongly suppress the growth of cabbage aphid colonies on Brussels sprouts.