Supplemental nesting habitat increases bee abundance in apple orchards

Abstract

1. Identifying the resources that limit bee populations is essential for both bee conservation and pollination management in agroecosystems. Land-use change typically leads to decreased habitat availability for wild pollinators, including loss of nesting habitat, which is an essential but often-overlooked resource for wild bees. Cavity-nesting bees, such as many Osmia spp. (Hymenoptera: Megachilidae), occupy holes in wood or reeds to build their nests; due to their nesting habits, they are frequently scarce in agricultural settings. Nevertheless, under the right circumstances, these bees can be ideal pollinators of apple and other orchard crops. Artificial nesting structures (“bee hotels” or “trap-nests”) are often used to study cavity-nesting bees and have been proposed as tools for bee conservation. 

2. To evaluate the effects of additional nesting habitat on the local abundance of pollinators, we selected 24 sites in apple orchards in eastern Canada in 2021 and 2022. Each site comprised two study plots: one in which we installed artificial nesting structures for cavity-nesting bees, and one without added nesting habitat (control). 

3. Pollinator surveys were conducted in both plot types to measure pollinator visits to apple blossoms and, after apple bloom, to flowers in the undergrowth and/or in shrubs. Numbers and size of developing fruit were also recorded. Osmia bees and overall pollinator numbers were both significantly higher in the treatment with trap-nests (respectively, 33% and 22% higher during the 2021 growing season, and 113% and 30% higher during apple bloom across the two years of the study); however, there was no consistent difference in apple size or fruit set between the two treatments. Thus, trap-nests locally increased numbers of cavity-nesting bees as well as total pollinator numbers, but they had little effect on apple yield, likely because apple production was not pollinator limited in the years of this study. 

4. Synthesis and applications: These findings suggest that bee populations in apple orchards can be limited by nesting resources, such that addition of constructed nesting habitat could be an effective means of increasing pollinator numbers in for orchards, and potentially, supporting bee conservation.