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A new strain of Metarhizium brunneum, a common fungus found in soils around the world, could provide a chemical-free method for eradicating Varroa destructor, the devastating ectoparasite of honeybees. Unlike other strains of Metarhizium, the new strain can survive in the warm environments common in honeybee hives, which typically have a temperature of around 35 degrees Celsius (95 degrees Fahrenheit).

Varroa destructor is an ectoparasite of honeybees widely considered to be the primary driver of declining honeybee health in recent decades.

Although a variety of interacting factors including pathogens, pesticides, and nutritional stress have contributed to declining honeybee health, these parasites are the most commonly reported cause of colony loss for commercial beekeepers and is considered the single greatest threat to apiculture world-wide.

Varroa feed on adult and immature hone bees by puncturing the exoskeleton with sharp mouthparts and consuming bee tissues through extra-oral digestion.

Feeding by Varroa weakens bees, reduces worker life-span and foraging capability, and vectors some of the most destructive honeybee viruses.

If left untreated, Varroa infected colonies have an expected lifespan of 1-3 years. Additionally, Varroa infected bees are more likely to drift to neighboring colonies, introducing Varroa and associated viruses to uninfected colonies.

This is especially problematic for bees in commercial pollination settings where thousands of hives from different beekeepers and locations are crowded seasonally into orchards and agricultural fields.

Currently, beekeepers are largely reliant on chemical acaricides to control Varroa despite the dangers that these chemicals pose to bees and the ongoing issues with chemical resistance in the mites.

“We’ve known that Metarhizium could kill mites, but it was expensive and didn’t last long because the fungi died in the hive heat,” said Professor Steve Sheppard, a researcher in the Department of Entomology at Washington State University.

“Our team used directed evolution to develop a strain that survives at the higher temperatures. Plus, we took fungal spores from dead mites, selecting for virulence against Varroa.”

When Metarhizium spores land on a Varroa mite, they germinate, drill into the mite, and proliferate, killing it from the inside out. Bees have high immunity against the spores, making it a safe option for beekeepers.

In the study, the researchers screened more than 27,000 Varroa mites for levels of infection to get the new strain.

“It’s providing a real one-two punch, using two different fungi to help bees fight Varroa,” said Paul Stamets, co-owner and founder of Fungi Perfecti.

“The extracts help bee immune systems reduce virus counts while Metarhizium is a potentially great mite biocontrol agent.”

The next step is to seek approval from the Environmental Protection Agency to use Metarhizium on hives used in agriculture.

“We hope in 10 years that, rather than chemical miticides, Metarhizium is widely used to control Varroa mites,” Professor Sheppard said.

“And that the mite problem for beekeepers has been significantly reduced.”

The team’s results were published in the journal Scientific Reports.

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J.O. Han et al. 2021. Directed evolution of Metarhizium fungus improves its biocontrol efficacy against Varroa mites in honey bee colonies. Sci Rep 11, 10582; doi: 10.1038/s41598-021-89811-2

Photo - Varroa mite on a bee. The picture was taken with an electron microscope © United States Department of Agriculture

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