Abstract
Teeming within pollen provisions are diverse communities of symbiotic microbes, which provide a variety of benefits to bees. Microbes themselves may represent a major dietary resource for developing bee larvae. Despite their apparent importance in sustaining bee health, evidence linking pollen-borne microbes to larval health is currently lacking. 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 provisions before being fed to developing larvae. Convergent findings from amino acid and fatty acid trophic biomarker analyses revealed that larvae derived a substantial amount of nutrition from microbial prey and occupied a significantly higher trophic position than that of strict herbivores. Larvae feeding on increasingly sterile diets experienced significant adverse effects on growth rates, biomass and survivorship. When completely deprived of pollen-borne microbes, larvae consistently exhibited marked decline in fitness. We conclude that microbes associated with aged pollen provisions 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 bee–microbe interactions may represent an important facet of pollinator protection strategies.
Highlights
Bees are the predominant group of insect pollinators on the Earth
Bees are thought to derive a wide range of benefits from non-pathogenic microbes present within the larval pollen provisions
Our findings offer strong support for this hypothesis, revealing that larval bees are quite dependent on microbes as nutritional, external symbionts
Summary
Bees are the predominant group of insect pollinators on the Earth. Widespread declines in managed and wild bee populations have major implications for global food security and ecosystem stability [1,2]. As nutritional mutualists and as prey items, these microbes represent both symbionts and prey for bees—the pollen-borne microbes appear to be cultured by bees much in the same way that leaf-cutter ants cultivate symbiotic fungi [49,50] These microbial communities may strongly influence larval development in solitary bees that do not receive extensive brood care (e.g. trophallaxis, social tending and incremental feeding), and rely entirely on the microbes within the maternal provisions [51]. In order to better conserve solitary bee populations, a more refined understanding of the trophic function of their pollen-borne symbionts is needed To this end, we use a combined approach of diet manipulations and advanced trophic biomarker-based assays to investigate the effects of excluding pollen-borne microbes on the health of solitary bee larvae. Through these multiple lines of inquiry, we converge on the contribution and impact of microbes as a dietary subsidy for larval bees
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