The Tenants of Apple Orchards: Cavity-Nesting Structures Increase Bee Visitation to Flowers

Background/Question/Methods

Identifying the resources that limit bee populations is essential for pollination management in agroecosystems and bee conservation. Land-use change can lead to decreased availability of floral and nesting resources for wild pollinators, though the latter are often overlooked as a potentially limiting factor for bee populations. Cavity-nesting bees, such as Osmia spp., occupy holes in wood or reeds to build their nests. Artificial nesting structures are used to study cavity-nesting bees and have been posited as solutions for bee conservation. Given that many Osmia spp. exhibit early spring phenology and affinity for fruit orchards, they are ideal pollinators of apple. To evaluate the effects of additional nesting habitat on the local abundance of Osmia spp. and of bees, artificial nesting structures were installed at 24 sites in apple orchards around Ottawa, Canada, in spring 2021. Each site had two treatments: one without (control) and one with additional nesting structures. Transect walks were conducted three times during the Osmia spp. flight season to measure bee visitation (including Osmia spp. visitation) to apple blooms and subsequently, to flowers in the undergrowth and/or in shrubs. In each site and for both treatments, number of apple buds and developing fruit were recorded.

Results/Conclusions

Poisson-distributed mixed models show that Osmia spp. numbers and overall bee visitation were both significantly higher in treatments with nest boxes, especially during apple bloom. However, a paired t-test showed no difference in fruit set (apple count/bud count) between the two treatments, suggesting that apple yield was not pollinator-limited in the year of our study. This study reveals that nest boxes seem to increase Osmia spp. abundance as well as overall bee abundance but has no apparent effect on apple yield. The increased numbers of non-Osmia bees observed at flowers in the nest-box treatment may have been caused by exploitative or interference competition between Osmia and non-Osmia bees, although this hypothesis remains to be tested. Our study reveals that nest boxes can locally increase Osmia spp. abundance as well as overall flower visitation by bees in apple orchards. Future study will reveal whether this effect can be maintained over multiple years. Our results suggest that bee populations are not only limited by floral resources by also by nesting resources, which has important implications for orchard management practices and bee conservation policy.