Scientists have been struggling to understand the recent decline in various bee populations in North America. For example, a virus brought from Australia has been implicated in massive honeybee deaths last year.

Canadian researchers studied another type of bee, the bumblebee, near two large greenhouse operations in southern Ontario where commercially reared pollination bees are used in the growing of crops such as tomatoes, bell peppers and cucumbers.

The researchers first observed that the commercial bumblebees regularly flew in and out of vents in the sides of the greenhouses, escaping from the facilities.

The researchers then devised a mathematical model to predict how disease might spread from this "spillover" of runaway commercial bees to their wild cousins.

The model predicted a relatively slow build-up of infection in nearby wild bumblebee populations over weeks or months culminating in a burst of transmission generating an epidemic wave that could affect nearly all of wild bees exposed.

The model also predicted a drop-off in infection rates as you get further from the greenhouses.

GREENHOUSE BUMBLEBEE PARASITES

The researchers then sampled wild bumblebee populations around the greenhouses, catching bees in butterfly nets, holding them in vials and taking them back to a laboratory to screen for pathogens, including testing their feces.

The patterns that had been predicted by their mathematical model were borne out by studying the wild bees, they said.

Most of the parasites in the wild bumblebees were found to be at normal levels except for one intestinal parasite known as Crithidia bombi that is common in commercial bee colonies but typically absent in wild bumblebees.

The researchers found that up to half of wild bumblebees near the greenhouses were infected with this parasite.

"All of the different species of bumblebees that we sampled around greenhouses showed the same pattern: really high levels of infection near greenhouses and then declining levels of infection as you moved out," said Michael Otterstatter of the University of Toronto, one of the researchers.

"It was quite obvious that this was coming from the greenhouses and it was a general adverse effect on the bumblebees," Otterstatter added in a telephone interview.

He said the parasite weakens and often kills bees. The "spillover" of disease from commercial colonies may be a factor in the decline of bee populations in North America, he added.

The study, published in the Public Library of Science journal PLoS ONE, can be read at http://www.plosone.org/doi/pone.0002771.

(Editing by Maggie Fox and Sandra Maler )