A widely used agricultural neonicotinoid (pesticide) – also referred to as “neonic” –  called clothianidin causes honey bees to become more susceptible to deadly varroa mites, according to a University of Guelph, Canada study. The reason behind this is clothianidin’s debilitating effect on bees’ self-grooming behavior, which typically deters infestations of parasitic mites. These findings again highlight the need for industry action to reduce the use of pesticides, or find alternatives to curb insect infestations. The study was published in the Nature journal Scientific Reports.

The research comes as Health Canada places new limits on the use of three key neonicotinoids – clothianidin, imidacloprid and thiamethoxam – while deciding whether to impose a full phase-out of the chemicals, which are commonly coated on canola and corn seeds, or sprayed on crops and trees. Just earlier this month, the General Court of the EU backed a partial ban of the three chemical substances in agricultural practices that was introduced in 2013.

Varroa mites are contributing to colony collapses and have been associated with more than 85 percent of colony losses, according to the researchers. This study of bees is the first to evaluate the specific impact of clothianidin on the self-grooming of bees. Without this behavior, bees become exposed to mites and their viruses, which can quickly kill. 

“When bee colonies began to collapse years ago, it became clear there wasn’t just one factor involved, so we were interested in whether there was an interaction between two of the main stressors that affect bees: varroa mites and a neurotoxic insecticide, clothianidin,”  says lead author Nuria Morfin Ramirez, who completed the research along with Prof. Ernesto Guzman, School of Environmental Sciences, as part of her Ph.D.

The mites kill bees by slowly feeding off their body fat and hemolymph (blood), and transmitting a virus called deformed wing virus (DWV). To test the hypothesis that pesticide exposure was inhibiting bees’ ability to aggressively brush off the mites, the research team used bees from University of Guelph’s Honey Bee Research Centre and exposed them to a widely used neonic clothianidin. The bees were then either left alone or exposed to varroa mites.

The team experimented with three doses of clothianidin, all similar to what the bees would experience while feeding on flower nectar of neonic-treated crop fields, but each low enough to be considered sublethal.

“What we found was a complicated interaction between the mite and the pesticide that decreased the proportion of bees that groomed intensively and affected genes associated with neurodegenerative processes,” says Dr. Morfin.

Bees exposed to medium level doses of the neonic showed no changes in grooming behavior. When they were introduced to varroa mites, however, the proportion of bees that groomed intensively was 1.4 times lower compared to the bees exposed to clothianidin alone.

When exposed to the lowest dose of the pesticide, the proportion of bees that groomed significantly dropped. The lowest dose was also linked to an increased level of deformed wing virus – an effect not seen at the higher doses.

“These results showed a complex and non-additive interaction between these two stressors,” says Dr. Guzman. “This study highlights the importance of reducing stressors that bees are exposed to, to reduce the risk of disease and consequently colony mortality.”