1. 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. 2. 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). 3. 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. 4. Both flower choice in the laboratory and flower visitation rates in the field are well correlated with nectar accessibility and realized adult survival. 5. 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. 6. 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.

Nectar accessibility and flower-specific hoverfly longevity, choice, visitation and abundanceThe Excel file contains 5 data sets, one for each test described in the related paper: 1. Flower size of selected plants species. 2. Hoverfly survival: survival time of male and female Episyrphus balteatus in relation to the flower species present in cage. 3. Flower choice: time allocation of male and female Episyrphus balteatus to one of four flower species present in cage, translated into selection events. 4. Flower visitation: composition of flower visiting insects per plant species in field margin strips. 5. Hoverfly abundance: number of zoophagous hoverflies (and other flower-visiting insects) in relation to flower abundance and composition of field margin strips. The first sheet provides a more detailed description of the 5 data sets.Data Flowerrange JAE.xlsx