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