Identifying Mechanisms behind Interacting Stressors on Wild Bees to Inform Pollinator-Friendly Agricultural Landscapes

View the project final report

Commodities

• Animals: bees

Practices

• Crop Production: pollination, pollinator habitat, pollinator health
• Education and Training: display, extension, on-farm/ranch research, youth education
• Natural Resources/Environment: biodiversity, habitat enhancement, wildlife

Sustainable agriculture depends on healthy wild pollinator populations, especially as managed bees continue to face colony losses. Native bee declines are due to several factors, including the interacting stressors of reduced floral diversity and emergent pathogens. However, the impacts of this interaction on pollinator communities are poorly understood, particularly for non-model, solitary bees that are critical pollinators of many crops. Previous work on social bee species found that diet and gut microbial communities separately affect bee pathogens, but the relationships between gut microbes, diet, and pathogen resistance have not been researched in combination. I propose to investigate the interactions between diet, the gut microbiome, and pathogen resistance in social and solitary bee species. I will first assess natural gut microbial communities along a gradient of floral diversity at farms. I will then experimentally test how diets impact gut microbial communities and immune response in two ecologically and economically important generalist species, Bombus impatiens and Megachile rotundata. In addition to improving our understanding of how diet diversity impacts bee pathogen resistance via the gut microbiome, this study will also test mechanisms by which diets inhibit pathogens. For example, previous work has found that sunflower pollen reduced bumble bee pathogens. I will include sunflowers in this work to understand how the gut microbiome might mediate this effect. By assessing relationships in both the lab and on farm, my projects will inform growers about optimal native plant communities to support diverse and healthy bee populations.

Objective 1: I will document variation in gut microbiomes of multiple bee species along a floral diversity gradient in Amherst, Massachusetts. I have selected six farms that range in floral composition from highly diverse (>10 co-flowering species) to sunflower-only plantings. I will collect solitary leaf-cutter bees (Megachile spp.) and bumble bee workers (Bombus spp.) at each site and sequence their gut microbiomes. I will assess patterns and differences between solitary and social taxa and between landscapes. If bees within the same site host similar communities of bacteria or patterns of microbial diversity, this would suggest that the floral community has an important role influencing the gut microbiome. Alternatively, we may find that host genus or sociality plays a more important role than landscape in structuring gut microbial communities. This study may also allow us to identify specific bacterial taxa and community patterns driven by sunflower pollen in an agricultural context, shedding light on the underlying mechanism of sunflower’s medicinal effect.

Hypotheses: Gut microbiome diversity will increase with floral diversity, but gut microbiomes of Megachile species will reflect site differences more than Bombus species because solitary bees acquire gut microbes primarily from their environment, while social bees acquire microbes primarily from their nest [22].

Objective 2: To complement findings from my field survey, I will experimentally test how diet diversity and quality affect the gut microbiomes and immune responses of two bee species, Bombus impatiens and Megachile rotundata. If diet diversity increases immune response, then bees would be more susceptible to pathogens in low diversity habitats. On the other hand, if diet diversity reduces immune response, then bees in less diverse habitats may have increased performance in the presence of pathogens. However, an active immune response is energetically costly in the absence of pathogens, requiring increased food intake and decreasing bee performance. Thus, floral diversity and pathogens could synergistically affect bee performance.

Hypotheses: Gut microbiome diversity and immune response will increase with diet diversity in both species, but B. impatiens will have lower gut microbial diversity across treatments than M. rotundata due to coevolution with a small group of bacterial taxa. Furthermore, M. rotundata will have overall higher immune responses than B. impatiens because solitary insects tend to exhibit higher physiological immune responses than social insects due to the lack of hygienic behaviors.