Pollen-borne microbes represent an important protein source for larval bees, according to new research.
Newly-hatched blue mason bee larvae feeding on pollen provisions within a hollow reed. Image credit: Shawn Steffan.
Of the more than 20,000 species of bees on Earth, virtually all are widely considered to be strict herbivores, with the rare exception of highly specialized stingless bees that feed exclusively on carrion or fungi.
Adult bees are known to consume large amounts of nectar as a source of carbohydrates, while larval bees consume much more pollen than nectar during their development.
Microbes are naturally occurring in the pollen and feed and multiply within it.
In the process, they increase the pollen’s nutritional value to brood by enriching it with amino acids — the building blocks of protein — that flowering plants alone may not always provide.
The microbes don’t just serve themselves up as critical sources of amino acids, though. They also secrete enzymes that help break down and age raw pollen into a more nutritious and digestible form known as beebread.
“Bees actually require the non-plant proteins of these pollen-borne symbionts to complete their growth and development — which makes them omnivores,” said Dr. Shawn Steffan, a research entomologist with the Vegetable Crops Research Unit of the Agricultural Research Service in Madison, Wisconsin and the Department of Entomology at the University of Wisconsin-Madison.
In the study, the Dr. Steffan and his colleagues from the University of Wisconsin-Madison, Cornell University and Hokkaido University used isotope- and gas chromatography-based methods to calculate the ratio of nitrogen in two types of amino acids (glutamic acid and phenylalanine) in the tissues of adult bees and in beebread.
They chose the method because of its accuracy in determining an organism’s trophic position — where it stands on the proverbial food web of life based on the flow of nutrients and energy from producers to consumers of these resources.
The team’s isotope analysis showed that bee brood’s consumption of both plant and microbial proteins warranted raising the insect’s trophic status from that of a strict herbivore to an omnivore.
The study authors observed the appetite for microbial proteins among brood that spanned 14 species distributed across all major families of social and solitary bees — Melittidae, Apidae and Megachilidae among them.
“The findings underscore the need to examine what effects fungicide use on flowering crops can have on the microbial make up of pollen fed to brood and, in turn, their development,” Dr. Steffan said.
The team’s paper was published in The American Naturalist.
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Shawn A. Steffan et al. 2019. Omnivory in Bees: Elevated Trophic Positions among All Major Bee Families. The American Naturalist 194 (3): 414-421; doi: 10.1086/704281
