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Fugitives spread bumblebee diseases

Pathogens hitchhike on commercial bees that escape from greenhouses

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5:55pm, July 23, 2008
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Bumblebees sneak out from work, too, and fugitives buzzing away from the job at commercial greenhouses could be spreading diseases to wild bumblebees and contributing to pollinator declines.

Greenhouse growers bring in the bumblebees for tomatoes and other crops that need what’s called “buzz pollination,” a strong vibration that shakes loose the pollen. Honeybees don’t give the buzz, but bumblebees do.

Some of the commercial bumblebees escape their greenhouses and forage among the flowers outdoors, mingling with local wild bees. Observations and a new model of how bee diseases spread now support the idea that these fugitives are bringing pathogens to the bumblebee species living in the wild, Michael Otterstatter and James Thomson, both of the University of Toronto in Canada, report in the July PLoS ONE.

“This paper makes a convincing case that pathogen spillover could be contributing to declines” of North America’s bumblebees, says Rachael Winfree of Rutgers University in New Brunswick, N.J., who studies the effects of habitat on wild pollinators.

Bumblebees are important pollinators, Winfree says. She has studied native bumblebees pollinating fields of watermelons. Of the 50 native bee species that visited, a single bumblebee species, Bombus impatiens, did more than half of the work. Yet researchers worry over signs that bumblebee populations are shrinking in North America.

The scenario of infectious bumblebees grew out of Otterstatter’s earlier survey of bumblebee health. Bees buzzing in the neighborhood of big commercial greenhouses were more likely to have three out of four major bee parasites than were bees far from the operations, he found.

The most dramatic case was the gut parasite Crithidia bombi. It only showed up in bumblebees near commercial greenhouses, not in bees outside the neighborhood, he said.

Infection slows down a bee worker and impairs her botany. Infected bees lag behind healthy ones in learning the most rewarding flower species to visit and the most efficient techniques for collecting pollen or nectar. Infected queen bumblebees fail to start new colonies in the spring.

That’s the parasite Otterstatter and Thomson modeled for the new study of pathogen spillover. Tests in the lab clarified aspects of pathogen spread, such as how much infective material a sick bumblebee leaves on a flower. These results, combined with the bee literature, were the basis for a mathematical model of how the C. bombi parasite spreads.

The unfortunate state of bumblebee health allowed testing of the model at Leamington and Exeter in Canada, two centers of greenhouse agriculture. As the model predicted, infection rates in the neighborhood rose as wild bees and fugitives mingled during the summer.

The model also predicted that the slow rise of infection rates could suddenly peak in an epidemic wave, bringing disease to all outdoor bumblebees in the neighborhood. That didn’t happen around the Canadian greenhouses, he says, but likely only because the wild bumblebee season was too short. Wild colonies shut down in fall and the queens go into hibernation for the winter, when the models predicted the epidemics would have started.

In warmer climates with longer seasons for bees, epidemic waves certainly could form, Otterstatter says. In fact they could already have started, he says, but not been documented.

“I do think that this is of concern, especially since bumblebee colonies are shipped from one area to another,” says Diana Cox-Foster, an entomologist at PennStateUniversity in University Park.

"One thing that is remarkable is that the greenhouses containing bumblebees sound so porous," says Jay Evans, of the U.S. Department of Agriculture's Bee Research Lab in Beltsville, Md. "Hopefully this work will at least spur growers to patch the gaps in their greenhouses and monitor them for wayward bees."

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