Bumble bees face numerous environmental stressors, including gut-parasite infection and exposure to agricultural fungicides, which can negatively impact colony health. This study evaluates the interactive effects of these stressors on bumble bee (Bombus impatiens) microcolonies, focusing on colony development, worker survival, and parasite infection dynamics. Our aim in evaluating these interactions was to determine if bees would experience synergistic negative health outcomes compared to single stressor exposures. We reared forty queenless bumble bee microcolonies, and treated them with either fungicide-contaminated pollen, inoculation with a gut parasite, both, or neither. Contrary to original expectations, we did not observe significant synergistic interactions between the two stressors, however we found that consumption of fungicide was associated with higher likelihood of gut-parasite infection, and delayed recovery from infection. Fungicide consumption was also connected to smaller workers, and smaller male offspring. We also found that gut-parasite infection was correlated with decreased pollen consumption overall, decreased worker survival, and fewer developed pupae. This study provides insights into the impacts of co-occuring stressors affecting bumble bees and emphasizes the importance of sublethal effects on pollinator health.

Bumble bees (genus Bombus) are critical pollinators—providing the necessary ecological services for food and crop production. In western North America, species-rich bumble bee communities inhabit mountain ranges. However, as climate change increases temperatures, montane populations are restricted to higher elevations and their ability to disperse and maintain genetic diversity decreases as suitable habitat connecting populations decreases. This genetic isolation could lead to the extirpation of local pollinator populations and a loss of genetic diversity for pollinator species. We analyzed the genetic diversity of four broadly sympatric species of bumble bees with differing elevational niches—Bombus flavifrons, B. melanopygus, B. mixtus, and B. sylvicola—across the Rocky and Cascade Mountains of western North America to assess range-wide population genetic structure. We used microsatellite markers to assess genetic differentiation among populations (FST) and performed Bayesian clustering analyses to identify genetic groups within each study species. Further, we investigated if observed genetic differentiation within study species were better explained by isolation by distance (IBD) or isolation by resistance (IBR) by incorporating habitat suitability models (HSMs) into population structure analyses. Although we expected range-wide genetic differentiation for species with more narrow niche requirements, we found evidence of this differentiation for all four study species, with western populations experiencing significant genetic structure relative to inland populations. Additionally, IBR predicted genetic structure better than IBD for B. flavifrons and B. mixtus. Our results suggest that considering habitat connectivity across the geographic range of montane bumble bee species is important for understanding their population structures.

Declines in bumblebee species ranges and abundances are documented across multiple continents and have prompted the need for research to aid species recovery and conservation. The rusty patched bumblebee (Bombus affinis) is the first federally-listed bumblebee species in North America. We conducted a range-wide population genetics study of B. affinis from across all extant conservation units to inform conservation efforts. To understand the species’ vulnerability and help establish recovery targets, we examined population structure, patterns of genetic diversity, and population differentiation. Additionally, we conducted site-level analysis of colony abundance to inform prioritizing areas for conservation, translocation, and other recovery actions. We find substantial evidence of population structuring along an east-to-west gradient. Putative populations show evidence of isolation by distance, high inbreeding coefficients, and a range wide male diploidy rate of ~15%. Our results suggest the Appalachians represents a genetically distinct cluster with high levels of private alleles and substantial differentiation from the rest of the extant range. Site-level analyses suggest low colony abundance estimates for B. affinis compared to similar datasets of stable, co-occurring species. These results lend genetic support to trends from observational studies suggesting B. affinis has undergone a recent decline and exhibits substantial spatial structure. The low colony abundances observed here suggest caution in overinterpreting the stability of populations even where B. affinis is reliably detected interannually. These results help delineate informed management units, provide context for the potential risks of translocation programs, and can help set clear recovery targets for this and other threatened bumblebee species.