The suitability of field margin flowers as food source for zoophagous hoverflies

Hoverflies with zoophagous larvae are among the most common natural enemies of pests in Dutch arable fields. Their effectiveness is partly limited by the availability of nectar and pollen for the adults. In this study we examined the suitability of flowers of ca. 30 plant species as food source for the common Episyrphus balteatus, both with choice tests and with non-choice survival tests. Many common field margin flowers appear to be unsuitable, as they do not allow the hoverflies to survive up to their reproductive age. The results can well be explained by flower morphology: only flowers with nectar available at a depth of less than 2 mm are suitable. Choice tests indicate that the hoverflies mainly select flowers with accessible nectar. Moreover, field studies showed that field margins with a higher proportion of flowers with accessible nectar attract higher numbers of zoophagous hoverflies. These results stress the importance of laboratory bioassays for selecting the right plants for functional field margins.