Semi-natural habitat, ecotones and landscape-scale management enhance the occupancy, richness and fitness of cavity-nesting Hymenoptera

Context Cavity-nesting Hymenoptera play key roles in pollination and biological control, yet their populations are threatened by habitat loss and agricultural intensification. Understanding how landscape-scale conservation and habitat heterogeneity affect these insects is crucial for effective biodiversity management. Objectives We assessed the effects of landscape-scale conservation management, semi-natural habitat cover, forest cover and ecotones on the abundance, species richness, parasitism and fitness of cavity-nesting Hymenoptera using trap-nests in an agriculturally dominated landscape. Methods In the Günz Valley of southern Germany, we used trap-nests at 16 extensively managed meadows, located within and outside designated conservation core areas (CAs). We analyzed how landscape variables influenced Hymenoptera occupancy, wild bee and wasp richness, Osmia bicornis reproduction, and Melittobia parasitism. Results Results demonstrated a positive impact of increased semi-natural cover and decreased forest cover on Hymenoptera occupancy and reproductive output in isolated conservation patches (non-core areas; non-CAs), while no significant effect of surrounding habitat was observed in high quality landscapes dominated by extensively managed meadow patches and nature protected areas (CAs). Wasps exhibited higher species richness in CAs, likely due to greater resource availability in larger, extensively managed landscapes. Additionally, semi-natural cover enhanced the reproductive success of the red mason bee, Osmia bicornis , a key indicator species, while edge density positively influenced brood survival, highlighting the importance of ecotones for cavity-nesting Hymenoptera. Parasitism rates by Melittobia wasps were higher in non-CAs, possibly due to a honeypot effect. Conclusions Our findings emphasize the valuable role of semi-natural habitat in supporting cavity-nesting Hymenoptera, particularly in intensively managed agricultural landscapes. They also highlight the need for targeted conservation strategies that improve habitat connectivity and resource availability to promote biodiversity.