University of Guelph researchers find that insecticide affects honeybees’ ability to rid themselves of deadly varoa mites
A University of Guelph study is the first to discover the serious impact of the neonicotinoid insecticide called clothianidin on a honeybee’s ability to groom and rid itself of deadly varroa mites.
Neonicotinoids are the most commonly used insecticides in Canada. They are sprayed in fruit and vegetable production, which are resources bees frequently use for pollen.
On the Prairies and in Ontario, it is more commonly used as a seed coating treatment on canola, corn and soybeans, a practice deemed more bee friendly.
However, results from the Guelph study are startling. Varroa mites feed off the body fat of honeybees, slowly killing them. Mites have contributed to 85 percent of colony collapses. In addition, the varroa mites can transmit a condition called deformed wing virus (DWV).
Bees defend themselves from the mites by vigorously grooming. The recent study has shown that exposure to clothianidin causes their self-grooming behaviour to decline.
“As social organisms, honeybees have developed mechanisms to defend themselves against pathogens, such as hygienic and grooming behaviours,” said Nuria Morfin-Ramirez, a PhD student in the university’s School of Environmental Sciences and lead author of the study.
The research team used bees from colonies of the Buckfast strain at the university’s Honey Bee Research Centre. The queens were mated under controlled conditions to guarantee purity and uniformity of the genotype. The bees had not been previously exposed to pesticides. The varroa mites were obtained from highly infested colonies. The bees were exposed to low doses of clothianidin either on its own or in combination with the insects.
“Considering that clothianidin is a neurotoxin, its effects on behaviour was not surprising,” said Morfin-Ramirez. “However, it is important to consider that the doses that the bees were exposed to were low (sublethal doses), but even at these doses we were able to see an effect. It was surprising to see that there is indeed an interaction between the two stressors. However, the interaction seems to be complex.”
According to the team’s published report, “a parasite and sub-lethal doses of a neonicotinoid have some shared as well as unique negative impacts on the honeybee brain, like cell damage, proliferation, and senescence (biological aging), that are consistent with aspects of neurodegeneration.”
The study concluded that sublethal, chronic exposure to clothianidin similar to that expected in agricultural field conditions can more negatively impact the self-grooming behaviour of honeybees when combined with the varroa mite than applied alone.
Morfin-Ramirez said the study demonstrates how grooming behaviour is dependent on neural responses and, with clothianidin as a neurotoxin, its effects on the central nervous system and the brain could well be related.
What surprised the team was that the research showed that both deformed wing virus levels and the proportion of bees that self-groomed were affected by the lowest dose of clothianidin without exposure to the varroa mite.
“The fact that DWV increased only by the lowest dose of clothianidin could be related to an immunosuppressive effect of low doses of neonicotinoid insecticides,” she said. “This confirms that the impact of neonicotinoid insecticides in honey bees is complex, but at certain doses could affect immune responses, which is the ability of the bees to defend themselves against pathogens, such as DWV.”
In addition to grooming, the researchers are also studying the effects of neonicotinoids and varroa mites on bee hygiene and foraging behaviours.
“Bees are exposed to a number of stressors, including pathogens and chemicals used in agriculture,” she said. “Also, bees are exposed to stressors related to unpredictable weather conditions, nutritional stress and intense management. All these factors contribute to affect honeybee health and increase the chances of mortality. Thus, studies on the effects of multiple factors should be conducted to have a better understanding on how the stressors interact and affect honeybees.”
The research comes at a time when Health Canada has completed its re-evaluations of the neonicotinoid pesticides clothianidin, imidacloprid and thiamethoxam. To protect bees and other pollinators, the agency announced in April this year that it will be cancelling some uses of these pesticides and changing other conditions of use, to be implemented over a two- to three-year period.
The report was published in the Nature journal Scientific Reports.