Although the foraging ecology of bumblebees (Bombus spp.) has been studied extensively, the overwintering ecology of this taxon is poorly understood. In order to understand bumblebee population dynamics and conservation, we need to understand resource needs throughout the life cycle. In the fall of 2019, we observed queen B. impatiens excavating overwintering sites near their natal nests, providing an opportunity to explore the overwintering ecology of this species. Here, we evaluate the micro-habitat selection and survival of queen bumblebees overwintering in areas surrounding three B. impatiens colonies.
We characterized the vegetative cover surrounding each of 300 potential overwintering sites near nests as well as 200 random points around each nest. We also measured the distance from overwintering sites and random points to the natal nest site, the nearest tree, and the nearest woody debris. We confirmed that queen bumble bees were overwintering at potential sites by digging up 105 overwintering sites in late November. For each overwintering queen recovered, we measured weight and IT span of the queen, and the depth at which the queen was buried. Queens were then reburied: In the spring of 2020, we will plan to estimate the survival of these overwintering queens.
Results/Conclusions
We recovered queens at over 85% of potential overwintering sites at 2 nests, and at 61% of potential sites at the other nest. On average, the queens we encountered overwintered only 1.58 meters from nest site entrances. Overwintering sites also tended to be closer to trees when compared to random points. In general, overwintering sites were surrounded by less bare ground and less woody debris than random points. There was considerable variation in the depth at which queens overwintered in the soil: some queens chose to overwinter only a few centimeters beneath the leaf litter, while other queens overwintered as much as 10.5 centimeters below the ground surface. However, there was no relationship between the depth at which the queen overwintered and the size of the queen, the distance to the nearest tree, or the distance to the natal nest site. This research demonstrates the feasibility of monitoring overwintering queens in nature. It also shows that bumblebees prefer different habitat features (e.g., trees) for nesting than for foraging (flowers).