Room for rent: how habitat and materials influence the use of insect nesting boxes

The decline of pollinator populations and associated ecosystem services has gained media coverage in recent years. Insect nesting boxes, sometimes called bee houses or insect hotels, are marketed as a way to support wild, native pollinator populations by providing additional nesting habitats, especially in urban contexts, but their actual value as such is controversial. This study aims at documenting the use of nesting boxes by cavity-nesting hymenopterans in urban habitats and assessing preferences in terms of materials and cavity diameter. We compare colonization rate, parasitism rate, and species assemblages between three habitats (green alleyways, community gardens and the Montréal Botanical Garden) and two materials (log and reed). Sixty nesting boxes were built using reeds and logs presenting narrow (3–5 mm wide) and wide (5–8 mm wide) cavities. 660 bees and wasps were collected during emergence in spring 2021, representing 14 families and 28 confirmed species, including six new records for the province of Québec. Species number was higher in the Botanical Garden. We found that native species were more diverse, but less abundant than introduced species. Material and cavity diameter were drivers of colonization rate and species richness, but not habitat. Colonization rate and species richness was higher in logs and in narrow cavities. Half of the nesting boxes were parasitized, but there was no difference between habitats in parasitism rate. Our results show that nesting boxes are used by species with complementary ecological roles.