American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Omnivory in bees: Elevated trophic positions among all major bee families”

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Shawn A. Steffan, Prarthana S. Dharampal, Bryan N. Danforth, Hannah R. Gaines-Day, Yuko Takizawa, and Yoshito Chikaraishi (Sep 2019)

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Bees are omnivores, mainly because as larvae, they eat lots of ‘microbial meat’

Bees eat microbial meat

A bumble bee alighting on a flower.<br />(Credit: Shawn A. Steffan)
A bumble bee alighting on a flower.
(Credit: Shawn A. Steffan)

Bees are widely thought to derive all protein directly from floral resources. Recent findings suggest this is largely untrue. It appears that larval bees feed extensively on pollen-borne prey, as well as on the pollen, itself. These prey are the microbes that are suffused throughout a fermenting pollen-provision. Because the microbes are actively consuming the pollen, these herbivorous organisms represent ‘microbial meat’ within the pollen-provision. When a larval bee consumes aged pollen, the bee is consuming both microbial and plant biomass, assimilating the amino acids of microbial prey as well as those of the plant material—analogous to eating bacon bits in a salad. The degree to which a bee assimilates microbe-derived amino acids can be measured empirically as the trophic position of the bee (the more meat consumed, the higher the trophic position). Importantly, gut microbiota do not elevate the hosting animal’s trophic position. To assess the pervasiveness of bee omnivory, the trophic positions of bees representing the six major bee families on Earth were examined. Adult bees were collected over the course of two years, from the cranberry marshlands of Wisconsin to the forests of New York. There was consistent, significant evidence of elevated trophic positions among all the bees in the study (54 specimens across 14 species, 12 genera), suggesting that most bees, if not all, are omnivorous. Such reliance on microbial nutrition also suggests that pollen-borne microbes represent true symbionts for bees; thus, to conserve bee fauna, their microbial symbionts will require attention, too.


Larval bees (<i>Osmia</i>, Megachilidae) feeding on microbe-colonized pollen masses. A) The black arrow indicates the distinctly “U-shaped” bee larva, situated atop its pollen-provision within a hollow reed. B) Older megachilid larvae feeding on aged pollen masses. The white arrow indicates a highly fermented pollen-microbe complex.<br />(Credit: Shawn A. Steffan)
Larval bees (Osmia, Megachilidae) feeding on microbe-colonized pollen masses. A) The black arrow indicates the distinctly “U-shaped” bee larva, situated atop its pollen-provision within a hollow reed. B) Older megachilid larvae feeding on aged pollen masses. The white arrow indicates a highly fermented pollen-microbe complex.
(Credit: Shawn A. Steffan)

Abstract

As pollen- and nectar-foragers, bees have long been considered strictly herbivorous. Their pollen-provisions, however, are host to abundant microbial communities, which feed on the pollen before/while it is consumed by bee larvae. In the process, microbes convert pollen into a complex of plant and microbial components. Since microbes are analogous to metazoan consumers within trophic hierarchies, the pollen-eating microbes are, functionally, herbivores. When bee larvae consume a microbe-rich pollen complex, they ingest proteins from plant and microbial sources, thus should register as omnivores on the trophic “ladder.” We tested this hypothesis by examining the isotopic compositions of amino acids extracted from native bees collected in North America over multiple years. We measured bee trophic position across the six major bee families. Our findings indicate that bee trophic identity was consistently and significantly higher than that of strict herbivores, providing the first evidence that omnivory is ubiquitous among bee fauna. Such omnivory suggests that pollen-borne microbes represent an important protein source for larval bees, which introduces new questions as to the link between floral fungicide residues and bee development.