Cavity-nesting Hymenoptera prefer standing over laying deadwood for nest construction in forests

Cavity-nesting Hymenoptera play key roles as pollinators and predators in forest ecosystems, yet their nesting site selection in forests remains poorly understood. We investigated how tree diversity, deadwood position, and ant exclusion influence nesting activity of solitary bees and wasps in 64 experimental forest plots spanning a gradient of 1- 16 tree species. Each plot received three deadwood blocks with pre-drilled cavities – one laying on the ground, one standing with ant exclusion, and one standing without ant exclusion – offering cavities of varying diameters. After a full year of exposure in the field, 29 host species from 1104 individuals, and 12 natural enemy species from 123 individuals emerged from the deadwood. Bees and wasps strongly preferred standing over laying deadwood for nest construction, while ant exclusion had no effect. Tree richness promoted deadwood availability and the diversity of cavity-nesting communities. This highlights the potential of tree diversity to increase laying and standing deadwood and subsequently to enhance the diversity of cavity-nesting arthropods and their natural enemies in forests.