Grassland set‐asides provide nesting habitat for bumble bees in agroecosystems

Wild bee declines threaten pollination services in agricultural landscapes. The availability of adequate nesting habitat, especially within intensively farmed regions, has been identified as an area where more research is critically needed. We assessed the potential for grassland set‐asides (GLSAs), an agricultural diversification practice that temporarily takes fields out of production to cover crop them with grasses and forbs, to provide high‐quality nesting habitat for wild bumble bees ( Bombus spp.). During the spring of 2023, we surveyed nest‐searching queen abundance across multiple agricultural land use types, including GLSAs, in an intensively managed agricultural landscape. Then, during the summer, we mapped worker distributions and performed microsatellite analysis of tarsal DNA to estimate colony abundance across these land use types. Native nest‐searching queens were observed more frequently in GLSAs, relative to other land types (while accounting for their overall frequency in the landscape), with queen densities in GLSAs comparable to unmanaged grassy areas and significantly higher than in crop fields, pastures or roadsides. The area of GLSAs in a landscape was also a strong predictor of native bumble bee worker abundance and supported a higher estimated nest abundance for the species Bombus mixtus , indicating that the positive effects of GLSAs on bumble bees extend beyond the nest‐site selection phase. Synthesis and applications . Our study demonstrates that GLSAs can provide nesting habitat for bumble bees within intensively managed agroecosystems. To maximize their conservation value and to avoid creating ecological traps, land managers should consider incorporating GLSAs into farming landscapes for the full duration of the colony cycle (i.e. spring queen emergence through late‐summer gyne and male production). Moreover, farmers and communities should incorporate knowledge about alternative nesting sites, for example by staggering GLSA schedules at the landscape level, to mitigate the loss of habitat when GLSAs are returned to production.