Benchmarking nesting aids for cavity-nesting bees and wasps

In urban areas, the diversity and abundance of cavity-nesting Hymenoptera may be restricted due to scarce nesting resources. Artificial nesting sites (nesting aids) are being installed to compensate for this shortage in a growing number of private gardens and public greenspaces to support Hymenoptera (especially bee) diversity. Various nesting aids are commercially available, but their effectiveness has so far not been investigated empirically. We compared a low-budget commercial nesting aid with a customized version based on scientific evidence. Commercial models comprised bamboo and coniferous wood cavities with fixed short lengths and little variation in diameter, whereas customized models comprised hardwood, reed and bamboo cavities with varying lengths and diameters. Both models were exposed pairwise in private gardens over one season and nesting Hymenoptera species identified. The commercial nesting aids were less well occupied, hosted fewer brood cells and had lower species diversity. Hardwood showed the highest rate of occupancy but reed cavities hosted the highest species diversity due to diverse cavity diameter and length combinations. Cavities with diameters between four and eight mm were occupied most often. Regardless of material, cavities with smooth entrances were strongly preferred. Nesting aids designed in accordance with our findings may thus support high and diverse populations of cavity-nesting Hymenoptera in anthropogenically transformed habitats such as urban areas.