Microbes make the meal: Oligolectic bees require microbes within their host pollen to thrive

Authors: DHARAMPAL, PRARTHANA - University Of Wisconsin; HETHERINGTON, MATTHEW - University Of Wisconsin; Steffan, Shawn

Submitted to: Ecological Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2020
Publication Date: 8/12/2020
Citation: Dharampal, P.S., Hetherington, M., Steffan, S.A. 2020. Microbes make the meal: Oligolectic bees require microbes within their host pollen to thrive. Ecological Entomology. 45(6):1418-1427. https://doi.org/10.1111/een.12926.
DOI: https://doi.org/10.1111/een.12926

Interpretive Summary: MAJOR FINDINGS: Microbial colonization of pollen-provisions is critical to the development of immature bees. IMPORTANCE OF RESULTS: Bees are critical pollinators of US crops, and thus a more refined understanding of their microbial symbionts is extremely important for conservation efforts. WHO WILL BENEFIT: US fruit crop growers SPECIFIC BENEFITS OF FINDINGS: US cranberry growers will know how/why they should try to conserve their native pollinator populations.

Technical Abstract: The availability of nutritionally adequate pollen is essential to maintaining healthy populations of wild solitary bees. Specialist bees, which rely on a narrow range of host plants, are especially sensitive to the availability of nutritionally appropriate host pollen. In addition to pollen and nectar, the provisions also support an external community of symbiotic microbes. These pollen-borne microbes serve as both nutritional mutualists and direct prey, mediating larval access to the nutrients locked within an aging fermenting pollen mass. This mixture of pollen, nectar, and microbes serve as the single source of nutrients for the developing larvae. Here, we investigated the relative importance of pollen type (i.e., pollen nutritive value), and pollen-borne microbes in shaping larval health. Using a diet manipulation experiment, we reared larvae of a pollen specialist on host- and non-host pollen in presence, and absence of pollen-borne microbes. Our findings showed that the interaction between pollen type and pollen-borne microbes had a significant impact on larval fitness. For both host- and non-host pollen, presence of pollen-borne microbes resulted in significantly healthier larvae. In contrast, pollen type predicted larval performance only when pollen-borne microbes were also available. Nutrient analysis revealed that across both microbe-rich and microbe-deficient diets, non-host pollen represented a higher quality diet compared to host pollen. However, as long as microbes were present, larvae performed better on the lower quality host pollen than on non-host pollen. Interestingly, consuming host pollen devoid of microbes caused significant decline in larval health because the absence of microbes not only limited access to the pollen nutrient pool, but also deprived the larvae of key microbial prey. Larval performance on both microbe-rich and microbe-deficient pollen was comparable probably due to the easier accessibility and higher nutrient content within non-host pollen. Therefore, pollen identity only appeared to predict larval health to the extent that the associated pollen-borne microbes were also present. Taken together, these results underscore the fundamental impact of pollen borne microbes in shaping the nutritional adequacy of larval diet.