PS 43-85 - Room for rent: ecological preferences of insects using nesting boxes in urban habitats

Background/Question/Methods

The decline of pollinator populations and associated ecosystem services has gained media coverage in recent years. In response, many citizens are looking for domestic solutions to help pollinators. Insect nesting boxes, sometimes called bee houses or insect hotels, are human-made structures designed to be used as nesting sites by bees and other arthropods. They are marketed as a way to support pollinator populations by providing additional nesting habitats, especially in urban contexts. The efficiency of nesting boxes in enhancing bee diversity have rarely been assessed. Parameters influencing cavity attractiveness are not well known either. The goal of this project is compare their use by insects in different habitats and with different materials. 60 nesting boxes were built using reed and logs presenting small (3-5 mm wide) and large ( 5-8 mm wide) holes. They were installed in Montreal, in urban gardens (n=20), green alleys (n=20) and in the Botanical Garden (n=20), between June and November 2020. Emerging insects were collected in spring 2021.

Results/Conclusions

676 bees and wasps were collected, distributed in 14 families and 31 species. About half of species and specimens overall were predatory solitary wasps, mostly Crabronidae and Pemphredonidae. Mean diversity was higher in green alleys than in community gardens and the Botanical garden, although more species were found in the Botanical garden. Parasitism rate was higher in green alleys. Logs were associated with a higher abundance and species diversity than reed. Species composition differed between small and large cavities, but not diversity. Higher diversity was reached with a mix of small and large cavities.

Our results show that nesting boxes are used by insects playing complementary ecological roles. While they are marketed as habitats for pollinators, they are also used by predators and parasites. Most predators found in our study were aphid wasps that likely act as biocontrol agents in gardens. Logs with holes should be preferred to reed stems when making nesting boxes. Carving holes with different diameters is worth the effort to foster a higher insect diversity.