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Title: To bee or not to bee: Larval bees require pollen-borne microbes to survive

Author
item DHARAMPAL, PRARTHANA - University Of Wisconsin
item CARLSON, CAITLIN - University Of Wisconsin
item CURRIE, CAMERON - University Of Wisconsin
item Steffan, Shawn

Submitted to: Proceedings of the Royal Society. B. Biological Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2019
Publication Date: 6/12/2019
Citation: Dharampal, P., Carlson, C., Currie, C., Steffan, S.A. 2019. To bee or not to bee: Larval bees require pollen-borne microbes to survive. Proceedings of the Royal Society B. 286(1904). https://doi.org/10.1098/rspb.2018.2894.
DOI: https://doi.org/10.1098/rspb.2018.2894

Interpretive Summary: Conservation of crop pollinators is an important goal of much agricultural research, especially in fruit crop systems such as cranberries. By demonstrating that solitary (non-social) bee larvae require external symbiotic microbes to pre-digest their pollen-provisions, we reveal the existence of tightly-linked, external bee-microbe symbioses. To the extent that such microbes were eliminated via sterilization, the larval bees in this study endured increasing mortality rates. This finding should underscore the need to consider the microbial symbionts of bees when developing strategies for pollinator conservation. US fruit crop growers will benefit from a better understanding of how best to conserve native pollinators.

Technical Abstract: Teeming within hive-stored pollen are diverse communities of symbiotic microbes. A variety of functions have been attributed to these microbes, most important of which is the fermentation of raw pollen into a nutrient-rich, well-preserved resource for larval bees. Microbes, themselves, may also represent a major dietary resource for direct larval consumption. Despite their importance in sustaining bee health, empirical evidence linking pollen-borne microbes to larval health is currently lacking. Here, we examined the effects of microbe-deficient diets on the fitness of larval mason bees. In a series of diet manipulations, microbe-rich maternally-collected pollen provisions were replaced with increasing fractions of sterilized microbe-deficient pollen, prior to larval consumption. We found that larvae feeding on increasingly sterile diets had significant adverse effects on larval growth rates, biomass, and survivorship. Trophic biomarker analysis revealed that larval bees derived a substantial amount of nutrition from microbial prey. When deprived of microbes, the predominantly pollen-eating larvae showed marked decline in health. We conclude that pollen-associated microbes are central to bee health, not only as nutritional mutualists, but also as a major dietary component. In an era of global bee decline, the conservation of such vital bee-microbe symbioses represents an important facet in pollinator protection strategies.