1. Bumble bees are important pollinators that face threats from multiple sources, including agrochemical application. Declining bumble bee populations have been linked to fungicide application, which could directly affect the fungi often found in the stored food and gastrointestinal (GI) tract of healthy bumble bees.2. We test the hypothesis that fungicides impact bee health by disrupting bee-fungi interactions. We examined the interactive effects of the fungicide propiconazole and fungal supplementation on the survival, reproduction and microbiome composition of microcolonies (queenless colonies) using two species, Bombus vosnesenskii and B. impatiens.3. We found that in B. vosnesenskii, fungicide exposure decreased survival, while fungal supplementation mitigated fungicide effects. For B. impatiens, fungicide application had no effect, but fungal supplementation improved survival and offspring production.4. Fungicides reduced fungal abundance in B. vosnesenskii microcolonies, but not in B. impatiens, where instead fungal addition decreased fungal abundance (ITS copy number). Fungal composition varied between treatments but differently between bee species. In B. impatiens, fungal addition increased microbiome diversity. In B. vosnesenskii, the abundance of the pathogen Ascosphaera was negatively associated with survival, while the yeast Zygosaccharomyces was positively associated with survival. 5. Our results highlight that bumble bee species differ in response to fungicides and in the nature of bee-fungi associations. Fungicides can alter bee-fungi interactions with consequences for bee survival and reproduction, and exploring the mechanisms of such interactions, including interactions among fungi in the bee GI tract, may offer insights into bumble bee biology and conservation strategies.

As social insects, honey bees (Apis mellifera) rely on the coordinated performance of various behaviors to ensure that the needs of the colony are met. One of the most critical of these behaviors is the feeding and care of egg laying honey bee queens by non-fecund female worker attendants. These behaviors are crucial to honey bee reproduction and are known to be elicited by the queen’s pheromone blend. The degree to which workers respond to this blend can vary depending on their physiological status, but little is known regarding the impacts of developmental exposure to agrochemicals on this behavior. This work investigated how exposing workers during larval development to chronic sublethal doses of insect growth disruptors affected their development time, weight, longevity, and queen pheromone responsiveness as adult worker honey bees. Exposure to the juvenile hormone analog pyriproxyfen consistently shortened the duration of pupation, and pyriproxyfen and diflubenzuron inconsistently reduced the survivorship of adult bees. Finally, pyriproxyfen and methoxyfenozide treated bees were found to be less responsive to queen pheromone relative to other treatment groups. Here, we describe these results and discuss their possible physiological underpinnings as well as their potential impacts on honey bee reproduction and colony performance.

Bumble bees are important pollinators that face threats from multiple sources, including agrochemical application. Declining bumble bee populations have been linked to fungicide application, which could directly affect the fungi often found in the stored food and GI tract of healthy bumble bees. Here, we test the hypothesis that fungicides impact bee health by disrupting bee-fungi interactions. We examine the interactive effects of the fungicide propiconazole and fungal supplementation on the survival, reproduction, and microbiome composition of microcolonies (queenless colonies) using two species, Bombus vosnesenskii and B. impatiens. We found that both bee species benefitted from fungi, but were differentially affected by fungicides. In B. vosnesenskii, fungicide exposure decreased survival while fungal supplementation mitigated fungicide effects. For B. impatiens, fungicide application had no effect, but fungal supplementation improved survival and offspring production. Fungicides reduced fungal abundance in B. vosnesenskii microcolonies, but not in B. impatiens, where instead fungal addition decreased fungal abundance. In B. vosnesenskii, the abundance of the pathogen Ascosphaera was negatively associated with survival, while the yeast Zygosaccharomyces was positively associated with survival. Our results highlight species-specific differences in response to fungicides and the nature of bee-fungi associations, and caution the use of results obtained using one species to predict responses of other species. These results demonstrate that fungicides can alter bee-fungi interactions with consequences for bee survival and reproduction, and suggest that exploring the mechanisms of such interactions, including interactions among fungi in the bee GI tract, may offer insights into bumble bee biology and conservation strategies.