Bumblebees sniff and taste microbial compounds in flower nectar

A foraging bumblebee faces a veritable buffet of options when deciding which flowers to visit for a snack. Cues such as flower shape, size, color, and scent all play a part in attracting a bee and enticing it to feed on a particular flower’s nectar. At the American Chemical Society national meeting in San Diego on Sunday, researchers reported that chemical cues produced by microbes that live on flowers also influence a bumblebee’s feeding decisions.

Microbes colonize up to 70% of flowers, but scientists know little about the role they play in attracting or repelling pollinators. In addition to taking pollen from one flower to another, bees can bring microbes such as bacteria and fungi with them. Colonies of microbes feed on the abundant sugars in flower nectar, producing a variety of metabolites—both volatile compounds that waft through the air and nonvolatile ones that stay dissolved in the nectar. Robert N. Schaeffer, a biologist at Utah State University, and his colleagues wanted to see if bumblebees (Bombus impatiens) could sense these compounds, learn from them, and use that experience to make informed decisions about which sources of nectar to feed on. He presented the work during a talk in the Division of Agrochemicals.

The researchers prepared a synthetic nectar and spiked it with either a bacterium (Asaia astilbes) or a yeast (Metschnikowia reukaufii) commonly found on flowers. After allowing the microbes to incubate in the nectar for 4 days, the researchers placed the sugary liquids in separate arms of a Y-shaped tube and allowed bumblebees that had been starved for 6 h to fly to whichever one they preferred. The bees spent more time in the arm with the bacteria-inoculated nectar, showing that they were more attracted to the volatile compounds produced by A. astilbes. But when the researchers placed bees in individual chambers and gave the insects just one nectar to feed on, the bumblebees ate more of the yeast-inoculated nectar (Biol. Lett. 2019, DOI: 10.1098/rsbl.2019.0132).

A. astilbes produces nearly twice as much 2-ethyl-1-hexanol than M. reukaufii, so that volatile compound may play an important role in attracting the bees, Schaeffer said. But the bacterium also produces a lot of acetic acid that stays dissolved in the nectar instead of volatilizing. So once bees get a taste of the resulting sour nectar, they may decide not to feed on it.

Schaeffer wonders if one day farmers could use microbes or their compounds to make specific flowers more attractive to pollinators, but scientists have yet to explore that approach.

Nurhayat Tabanca of the US Department of Agriculture’s Agricultural Research Service in Miami, who co-organized the symposium and was not involved in the work, praised the study and said that it shows the important role of microbes in the chemical ecology of plants and their pollinators. “We’re searching deeper to understand these cues from microbes and how the insects respond,” she said.