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Bee Mystery Solved?

by Patrick J. Kiger

Scientists are worried these days about drastic declines in the population of pollinating bees, which poses a growing threat to our food supply, since as much as a third of our diet comes from plants pollinated by insects. The cause of colony collapse disorder, in which entire colonies abandon their hives, remains a mystery, though some have sought to link the phenomenon to genetic manipulation of crops. A new study, however, suggests another possible explanation: compelling evidence that commercially raised bees are spreading disease to their wild counterparts.

The study, published in the online journal PLoS ONE by Michael Otterstatter and James Thomson of the University of Toronto's department of ecology and evolutionary biology, investigates the occurrence of disease in wild bumble bees in close proximity to industrial bee-keeping operations in southern Ontario. The scientists used a combination of lab experiments and mathematical modeling to study the spread of disease to wild populations, and to predict its extent and severity.

As Science Daily reports, Otterstatter and Thomson found that commercial bumble bees (the genus Bombus) often carry a highly contagious pathogen, Crithidia bombi, and that these bees regularly escape from greenhouses and interact with wild bees at flowers. Near greenhouses, the rates of infection were startling: up to one-half of wild bumble bees were infected with C. bombi, whereas no bees harbored this pathogen at sites away from greenhouses.

Otterstatter and Thomson’s mathematical model predicts that within three months, escapees from commercial hives can infect up to 20 percent of the population of wild bumble bees within 1.24 miles (two kilometers) of the greenhouse. But over time, a "travelling wave of disease" can suddenly form and sweep outward at a rate of two kilometers each week, infecting from 35 percent to 100 percent of the wild population.

"Given the available evidence, it is likely that pathogen spillover from commercial bees is contributing to the ongoing decline of wild Bombus in North America," the scientists write.

Otterstatter and Thomson say that improved management of commercial bees to reduce disease and overlap with wild populations could reduce or eliminate such pathogen spillover.