Much of the world’s crop production depends on pollination services provided by wild bees. Yet agriculture modifies landscapes and soils in ways that could alter habitat suitability for these pollinators. Females of ground-nesting bee species (which make up about ¾ of all bees) excavate underground tunnels leading to cells in which they lay eggs on top of a food reserve. Although ground-nesting is a common trait and believed to be ancestral in bees, little is known about the nesting requirements and preferred soil characteristics of ground-nesters. In this study, we asked what soil variables influence the abundance and diversity of ground-nesting bees in agroecosystems around Ottawa, Canada. We used pan traps to sample the whole bee community at 131 total plots on 15 and 30 farms throughout the summers of 2018 and 2019, respectively. Bees were identified to genus and sorted into ground-nesters and above-ground nesters, and soil properties (texture, hardness, slope and vegetation cover) were measured at each plot over the two years of study.
Results/Conclusions
We collected a total of 11,878 individuals across 37 genera over the two years. Of these, 72% were ground-nesting bees, 24% above-ground nesters (including honey bees) and 4% parasitic. The most abundant bee taxa were Peponapis pruinosa—the squash bee—followed closely by Lasioglossum spp. We found a significant negative association between silt percentage and bee abundance. Among the other predictors, soil compaction and percent bare ground were both significantly positively associated with bee abundance; slope had no detectable effect. Although previous studies have found ground-nesting bees to be associated with sandy soils, our results suggest that silt can be equally or more important. Because of the wide variation in body size and morphology among ground-nesting bee species, we expect interspecific variation in the soil characteristics that are preferred for nest site location. Further analysis is needed at the species level to better understand nesting preferences of individual bee taxa. Understanding the factors that influence nest-site suitability for ground-nesting bees in agricultural landscapes will enable us to more effectively promote their populations and foster crop pollination.