Scientists with United States Department of Agriculture’s Beneficial Insects Research Unit at Weslaco, Texas, have found that a strain of the fungus Metarhizium anisopliae is deadly to varroa mites.
Varroa mites infest honey bee colonies, sucking blood from the bees and causing weight loss, deformities, diseases and reduced lifespan.
These mites, which can destroy a colony within a few months, now infest honey bee colonies across most of North America.
The honey bee is critical to maintaining natural vegetation, transferring pollen between flowers as it collects pollen and nectar for its hive.
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More than 130 agricultural plants in North America are pollinated by honey bees.
Since 2000, scientists in the USDA’s Agricultural Research Service have been looking for a disease-causing agent or pathogen that can stop varroa mites.
The mite has developed resistance to fluvalinate and coumaphos, the only chemicals now used for control in the United States.
Coumaphos is on the U.S. Environmental Protection Agency’s “hit list” for possible removal from the market. Researchers have looked at various disease agents, tried different dosages and application methods and conducted toxicity tests.
Finally, they selected a strain of the fungus Metarhizium anisopliae that was highly pathogenic to varroa mites.
This potent fungus, which also kills termites, doesn’t harm bees or affect their queen’s production.
To test it, the scientists coated plastic strips with dry fungal spores and placed them inside the hives. Since bees naturally attack anything entering their hives, they tried to chew up the strips, spreading the spores throughout the colony.
In field trials, once the strips were inside the hives, several bees quickly made contact with the spores.
Within five to 10 minutes, all the bees in the hive were exposed to the fungus, and most of the mites on them were dead within three to five days.
The fungus provided excellent control of varroa without impeding colony development or population size.
“We tried to find a pathogen of varroa, and we did it,” said ARS entomologist Walker Jones.
Tests showed that Metarhizium was as effective as fluvalinate, even 42 days after application.
“Commercial beekeepers are very edgy about using fluvalinate and coumaphos and are eager to see this natural control get to market,” Jones said.
Lambert Kanga, former research associate and now chair of the entomology department at Florida A&M University at Tallahassee, collaborates on the project.
“While Metarhizium doesn’t kill as fast as fluvalinate and coumaphos, the result is the same,” Kanga said.
“Metarhizium gets the job done and we won’t have to worry about varroa becoming resistant to the fungus.”
The scientists are now fine-tuning the strategy for transfer to producers.
