Reduced abundance and earlier collection of bumble bee workers under intensive cultivation of a mass‐flowering prairie crop

Abstract

One of the most commonly seeded crops in Canada is canola, a cultivar of oilseed rape (Brassica napus). As a mass‐flowering crop grown intensively throughout the Canadian Prairies, canola has the potential to influence pollinator success across tens of thousands of square kilometers of cropland. Bumble bees (Bombus sp.) are efficient pollinators of many types of native and crop plants. We measured the influence of this mass‐flowering crop on the abundance and phenology of bumble bees, and on another species of social bee (a sweat bee; Halictus rubicundus), by continuously deploying traps at different levels of canola cultivation intensity, spanning the start and end of canola bloom. Queen bumble bees were more abundant in areas with more canola cover, indicating that this crop is attractive to queens. However, bumble bee workers were significantly fewer in these locations later in the season, suggesting reduced colony success. The median collection dates of workers of three bumble bee species were earlier near canola fields, suggesting a dynamic response of colonies to the increased floral resources. Different species experienced this shift to different extents. The sweat bee was not affected by canola cultivation intensity. Our findings suggest that mass‐flowering crops such as canola are attractive to bumble bee queens and therefore may lead to higher rates of colony establishment, but also that colonies established near this crop may be less successful. We propose that the effect on bumble bees can be mitigated by spacing the crop more evenly with respect to alternate floral resources.

Wetland cover in agricultural landscapes is positively associated with bumblebee abundance

“…We know from previous work that canola cover is positively associated with B. borealis , B. rufocinctus and B. ternarius queen abundance in the early season, but that worker abundance decreased with increasing canola cover after flowering (Galpern et al, 2017). B. rufocinctus abundance has a strong positive relationship with higher‐than‐average crop area in the canola bloom period (Figure 4d), suggesting a heavy reliance on that mass flowering event.…”

“…Site selection and sampling location were based on criteria developed for each study, each of which investigates how land cover, landscape composition or distance from certain covers might influence pollinators. Across all years, 583 traps were placed in canola fields, 1738 were in vegetated ditch/verge areas alongside low‐traffic roads in agricultural areas (Galpern et al, 2017, 2021), 274 were in native vegetation (Purvis, Vickruck, et al, 2020), 228 were alongside or within 100 m of pothole wetlands and 162 were in wheat fields (Vickruck et al, 2019) (Figure 1).…”

“…The pollinator dataset was collected between 2015 and 2018. Analyses of subsets of these data have appeared in four publications that address different research questions at smaller geographic extents (Galpern et al, 2017(Galpern et al, , 2021Purvis, Vickruck, et al, 2020;Vickruck et al, 2019), whereas other data within this dataset have not been published before. Bees were trapped using blue vanes and coloured cups (12 oz.…”