Neonic pesticide reduces bees' ability to cleanse deadly mite, research shows

Honey bees exposed to a common pesticide are less likely to groom themselves and remove a parasitic mite, which can lead to their premature deaths, a new study from University of Guelph researchers shows.

The study, published in the Nature journal Scientific Reports, is the first to evaluate the impact neonicotinoids have on the grooming behaviour of bees. It looked at how the combination of clothianidin, a commonly used neonicotinoid pesticide, and Varroa mites can lead to early death in bees and, potentially, a bee colony collapsing.

Varroa mites kill bees slowly, the researchers said. They feed off the body fat and blood of bees, and also can transmit deformed wing virus. To protect themselves, honey bees groom aggressively, and this gets rid of the mites.

Using the university's own bees, researchers exposed bees to clothianidin.

The researchers used three dose levels of clothianidin. All the doses were low enough to be considered "sublethal," meaning it could affect the bees but not kill them and, researchers noted, they were not more than what bees would encounter in pesticide-treated farmers' fields.

The bees did not reduce the amount of grooming they did when exposed to clothianidin alone.

But when Varroa mites were introduced to the bees, researchers noted that fewer of them continued their aggressive grooming.

As well, the researchers found bees exposed to the lowest dose were more likely to contract deformed wing virus — an effect not seen at higher doses.

"These results showed a complex and non-additive interaction between these two stressors," environmental sciences professor Ernesto Guzman said in a release about the findings.

"This study highlights the importance of reducing stressors that bees are exposed to, to reduce the risk of disease and consequently colony mortality."

Health Canada reevaluates three neonics

The researchers said this is an important study now that Health Canada has placed new limits on the use of three key neonicotinoids.

Earlier this month, Health Canada announced it has completed a reevaluation of three neonicotinoid pesticides: clothianidin, imidacloprid and thiamethoxam.

"The scientific assessments show varying effects on bees and other pollinators from exposure to each of these pesticides," the health agency said in a release April 11.

"To protect bees and other pollinators, Health Canada … will be cancelling some uses of these pesticides, and changing other conditions of use such as restricting the timing of application."

Continued use on canola seeds and greenhouse vegetables are not expected to pose "unacceptable risks" to bees and other pollinators, Health Canada said.

For clothianidin, Health Canada has cancelled its use on orchard trees and strawberries, and for application on municipal, industrial and residential turf sites. It's also reducing to once per season the number of times it can be applied to the leaves of gourd vegetables such as cucumbers, pumpkins and squash.