Local habitat, not landscape diversity, drives richness and reproduction of cavity-nesting bees and wasps in agricultural landscapes

Studying insect decline in human-dominated landscapes, especially in intensively managed agricultural areas, requires understanding its drivers. Studies of cavity-nesting bees and wasps with trap nests provide information on the number of locally reproducing species and their potential offspring, and how this relates to local resources and landscape diversity. We investigated insect communities across all major habitat types in agricultural landscapes in Germany using a grid-based design to evenly and proportionally cover the major habitats of our landscapes. The landscape plots covered a gradient from complex landscapes with a variety of habitats to simple landscapes dominated by arable land. At the local scale, we found that bee species richness was highest in grasslands and traditional orchards and lowest in forests, mirroring patterns of local floral resources. However, unlike wasp richness, bee richness was unaffected by local hedgerow cover. Wasp species richness was highest in forests, followed by orchards and lowest in grasslands and arable land. Brood cell abundance of the dominant bee species (Osmia bicornis, O. cornuta) was highest in orchards and lowest in forests, whereas the dominant wasp species (Ancistrocerus nigricornis, Trypoxylon figulus) showed the opposite pattern. Neither group was affected by hedgerows. At the landscape scale, landscape diversity neither affected species richness nor the number of brood cells of bees and wasps. Our results highlight the importance of local habitat diversity in agricultural landscapes for supporting diverse cavity-nesting bee and wasp communities, as bees benefit from flower-rich grasslands and orchards, while wasps rely on forest resources. To counteract insect decline, conservation efforts should target both seminatural and managed habitats.