Nectar accessibility determines fitness, flower choice and abundance of hoverflies that provide natural pest control

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 th...