by Jim MacInnis

Nearly one-third of the food we eat is a result of pollination by insects, so the widespread disappearance of wild bee populations has been triggering alarm bells around the world. Concern for the insect’s demise has been heightened further because scientists have been unable to determine the cause of its decline. A recent University of Toronto study, however, has provided some possible answers, suggesting that an intestinal parasite common in bees raised commercially is at least partly to blame.

Researchers studied Crithidia bombi, an often fatal parasite that can be passed between bees through feces and foraging on contaminated flowers. C. bombi has ravaged Europe’s bumblebees and was introduced to North America in the 1970s. The authors of the study, Michael C. Otterstatter and James D. Thomson, examined nine sites in southern Ontario over the course of two years and discovered that up to 75 percent of bees in wild colonies adjacent to industrial greenhouses known to contain infected commercial bees had contracted C. bombi. (Because commercial bee populations are provided a constant source of sugar-rich nectar, they are less affected by the parasite than wild bees are. Wild bees must continually forage for nectar and are usually on the brink of starvation. Their weakened condition makes them more susceptible to the parasite’s ill effects.)

Conversely, the prevalence of infection appears to decline the greater the distance between a colony and a greenhouse. The intensity of infection – measured by the number of pathogen cells in an infected bee’s gut tract – also declines with distance.

Otterstatter and Thomson suspect that infected commercial bees escape from their “workplaces” through vents and pollinate flowers that bees from nearby wild colonies visit. Infected wild bees return to their hive, where the infection can spread through the colony.

The researchers found that, during three months of initial contact between segregated populations, commercial bees infected up to 20 percent of wild bumblebees within two kilometres of the greenhouse. After this three-month period, however, the parasite explodes in the population, infecting anywhere from 35 to 100 percent of the members of the colony. This “travelling wave of disease” moves away from the greenhouse at the speed of two kilometres a week.

The authors suggest that minor modifications to greenhouses could eliminate infection. Even a measure as simple as putting mesh screens in the venting systems would keep nearby colonies safe, not to mention save greenhouse operations the cost of losing countless expensive commercial pollinators.

“My hope,” says Otterstatter, “is that these findings will motivate improved management of domestic bees through, for example, greater attention to their parasite loads and their overlap with wild species.”