Common eastern bumblebees (Bombus impatiens) are more likely to catch a diarrhea-inducing gut parasite from purple coneflowers, black-eyed Susans, and other similarly shaped flora than other flowers, researchers report in the July Ecology. Because parasites and diseases contribute to bee decline, the finding could help researchers create seed mixes that are more bee-friendly and inform gardeners’ and land managers’ decisions about which flower types to plant.
The parasite (Crithidia bombi) is transmitted when the insects accidentally ingest contaminated bee feces, which “tends to make the bees dopey and lethargic,” says Rebecca Irwin, a community and evolutionary ecologist at North Carolina State University in Raleigh. “It isn’t the number one bee killer out there,” but bees sickened with it can struggle with foraging.
In laboratory experiments involving caged bees and 16 plant species, Irwin and her colleagues studied how different floral attributes affected the transmission of the gut parasite. They focused on three factors of transmission: the amount of poop landing on flowers when bees fly and forage, how long the parasite survives on the plants, and how easily the parasite is transmitted to new bees. Multiplied together, these three factors show the overall transmission rate.
Compared with plants with long, narrow flowers like phlox and Bluebeards, short, wide flowers had more feces land on them and transmitted the parasite more easily to the pollinators, increasing the overall parasite transmission rate for these flowers. However, parasite survival times were reduced during these blooms. This is probably due to the open floral shapes’ increased exposure to ultraviolet light, speeding the drying out of parasite-laden “fecal droplets,” Irwin says.
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