Episyrphus balteatus

Abstract: Understanding the consequences of declining diversity and abundance of pollinators for crops and floral biodiversity is a major challenge for current conservation ecology. However, most studies on this issue focus on bees, while other invertebrate taxa are largely ignored. We investigated the pollination efficiency of the globally abundant hover fly Episyrphus balteatus on the common crop, oilseed rape (Brassica napus). The study was conducted over a period of 2 consecutive years by means of enclosure experiments at an agricultural site located in Central Hesse (Germany). E. balteatus significantly increased both seed set and yield. This effect was very constant in the 2 years, despite considerable interannual differences in total seed numbers and seed mass. It highlights the important role of hover flies as pollinators of arable crops under varying environmental conditions. In contrast to bees, the effect of E. balteatus was lower at high pollinator densities than at low pollinator densities. This suggests adverse effects of density-dependent factors on pollination efficiency at high densities. Thus, models ignoring the modulating effect of biotic interactions by generally assuming a simple positive relationship between pollinator density and pollination efficiency might not apply to a vital component of the pollinator community.

Abstract: Summary In modern agricultural landscapes, many organisms providing ecosystem services such as pollination and natural pest control are likely constrained by shortage of nectar and/or pollen required for adult nutrition. More and more flower-rich field margin strips and other habitats are created to eliminate these constraints. For most target organisms, however, it is not well known which (types of) flowers are effective in providing suitable pollen and nectar. We studied the suitability of a wide range of flowers as a food source for zoophagous hoverflies (hoverflies with predatory larvae) at five different levels: nectar accessibility (based on flower morphology), realized adult fitness, flower choice (both based on cage experiments), flower visitation and hoverfly abundance in mixed vegetation (both based on field observations). Realized survival of Episyrphus balteatus is related to effective flower depth by a sigmoid function. The critical flower depth is 1·6 mm, which is less than the proboscis size of the hoverfly. For Asteraceae, the critical floret depth is even less than 1·0 mm, which – in contrast to common knowledge – rules out most species within this family. Both flower choice in the laboratory and flower visitation rates in the field are well correlated with nectar accessibility and realized adult survival. In mixed floral vegetation, the number of zoophagous hoverflies is highly correlated with the abundance of only those flowers that have accessible nectar for these hoverflies. Synthesis and applications. This comparative study demonstrates that nectar (and not pollen) accessibility is the main driver determining flower resource suitability, flower choice and abundance of zoophagous hoverflies in arable field margins. The study identifies the limited range of plant species that can effectively support these beneficial insects. Preserving the right flowers in and around agricultural fields could enhance local populations and the pest control and pollination services they provide.

Abstract: Adults of the Syrphinae subfamily display no strong flower preferences but exploit pollen and nectar produced by native plants having large inflorescences and flat corollae (e.g. Apiaceae, Asteraceae. Ranunculaceae and Rosaceae). Seven foraging guilds are defined according to the dietary patterns of hoverflies, reflecting mainly a sequential exploitation of flowers at different times of the year and in different habitats. The majority of species live in forests where they form highly diversified communities. Few Syrphinae colonize successfully open and anthropogenic habitats, such as field margins and fallow areas. Episyrphus balteatus, Melanostoma mellinum, Eupeodes corollae, Sphaerophoria scripta and Platycheirus spp. are dominant in the communities of Syrphinae from open habitats, all over western Europe. These species are highly polyphagous and characterized by elongated mouthparts as well as a long and slender body. They have access to pollen and nectar in flowers with small and tubular corollae. It is suggested that their polyphagy is an important asset for colonizing open and ephemeral habitats.