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Researchers from the USDA’s Agricultural Research Service (ARS) and the University of California, Davis, are helping beekeepers protect their colonies from destructive varroa mites. In a new study, the researchers investigate the effectiveness of combining a widely used mite-killing pesticide with an agent that inhibits the mites’ ability to tolerate the pesticide. 

Honeybees play a crucial role in U.S. agriculture, with the value of crops that require pollination estimated at more than $20 billion annually in the United States. Varroa mites, also known as Varroa destructor, are a force to be reckoned with in the honeybee world. The parasitic mite causes bodily harm and spreads deadly viruses that have led to major colony losses across the country. 

The pesticide amitraz is a preferred method to control varroa mite populations. It is highly toxic to varroa mites but safe for honeybees when used as instructed. However, a previous ARS  found that varroa mites are becoming increasingly resistant to amitraz due to a genetic mutation, so beekeepers now seek more effective methods for controlling mite populations. 

In their new , the authors found that combining an inhibiting compound with amitraz increases the pesticide’s toxicity and was effective against even amitraz-resistant mites. 

“This compound inhibits a naturally occurring process that prevents certain chemicals, like pesticides, from accumulating inside cells,” said co-leading author Julia Fine, a research entomologist at the U.S. Department of Agriculture’s  in Davis, Calif. “If a chemical toxicant can’t reach a high enough concentration in a cell, it won’t have a toxic effect in the organism. Previously, we didn’t know if this process was part of how varroa tolerate amitraz exposure.”

These findings open a promising new line of research that may lead to the development of novel synergists that can be used to control varroa mites in combination with amitraz or other miticides. 

“Our study shows that we can increase amitraz’s killing power by 50% when used in combination with another compound — a synergist — that weakens the mite but doesn’t cause harm to bees,” said co-leading author , an assistant professor in the �鶹��ý Environmental Toxicology department. “This research aims to provide beekeepers with practical tools and strategies that they can implement to effectively control varroa mites, especially since these mites become increasingly resistant to our limited arsenal of bee-safe pesticides like amitraz.”

Increasing the efficacy of amitraz treatments, especially the initial application, may help beekeepers save time and money. 

“Better amitraz formulations can decrease the need for additional treatments, lower the selection pressure on the mite population, and decrease the economic burden on beekeepers as they protect their colonies,” said Fine. 

Fine noted that the inhibitor used in the research is not specific to varroa. It can also diminish the ability of honeybees to tolerate pesticide exposures.

“Now that we know this process is important to amitraz tolerance in varroa, the next step is to develop synergists that specifically inhibit this process in varroa without affecting honeybees,” said Fine.

The research was conducted in collaboration with the ARS  in Beltsville, Md., and ARS  in Baton Rouge, La. This research was supported by a Honey Bee Health Grant through the North American Pollinator Protection Campaign and the Pollinator Partnership to Nicklisch.