Can we fine-tune tallgrass prairie restoration and management with controlled burns for bumble bee conservation?

In the Upper Midwestern USA, tallgrass prairie restoration requires intentional, controlled burning (i.e., prescribed fire) to maintain plant abundance and diversity. Although restoration efforts often target plant communities, they may also affect bumble bees through improved food and nesting resources and habitat connectivity. However, management activities such as controlled burns may negatively impact bees directly through mortality. In this study, we combined field observations and population genetics to evaluate how tallgrass prairie restoration and management with controlled burns impact bumble bees at across life stages (workers, nests, queens). Between 2021 and 2024, we surveyed bumble bee workers and queens and nest abundance at 105 unique site-year combinations in South Central Wisconsin at restored tallgrass prairies that varied in burn frequency: from never burned to 0 –13 years since last burn. We found evidence to suggest that the impacts of controlled burns vary depending on the life stage of interest. Specifically, summer worker populations are more abundant in years immediately following a burn but their relative abundance declines with more time since last fire. However, in the growing season immediately following a spring burn there are few to no nests present relative to sites that were not burned the same year. To complement information on bumble bee worker and nest abundance, genetic analyses are being used to estimate the number of colonies via genetic relatedness, and to quantify important population attributes for conservation such as genetic diversity and gene flow and how they vary among sites with different burn histories.