A parasitic Varroa mite is attached to a European honey bee. Varroa mites, which spread diseases, have become widely immune to the most common insecticide used against them. (Courtesy USDA)

Scientists believe massive honey bee die-offs were caused by alarmingly high levels of viral infections from parasitic Varroa mites — the tiny arachnids had genetic resistance to the most common treatment used against them.

The USDA Agricultural Research Service conducted a study that analyzed samples from six major commercial beekeeping operations affected by colony losses.

Samples were collected in February as bees were being staged for California’s almond season.

“While viruses are a likely end-stage cause of colony death, other stressors such as nutritional stress and agrochemicals may have also played significant roles,” stated a manuscript submitted by USDA-ARS researchers to a scientific journal.

“Now that they’ve released the analysis, beekeepers can better understand what happened and take action,” said Matt Mulica, facilitator of the Honey Bee Health Coalition.

“We understand more analysis is occurring and we look forward to learning more about the other potential causes of these significant colony losses,” he added, in a news release from the organization.

About the study

Researchers found high levels of deformed wing virus and acute bee paralysis virus in pooled colony samples or in individual bees shaking and other signs of morbidity.

Experimental inoculation further confirmed these viruses as the likely cause of mortality in collapsing colonies.

All Varroa mites collected from affected colonies tested positive for a genetic marker linked to resistance to amitraz, the most commonly used miticide in commercial beekeeping.

New treatments needed

The study’s findings underscores the urgent need for new tools and integrated pest management strategies.

“This is a critical moment for honey bee health,” said Patty Sundberg, president of the American Beekeeping Federation.

“We continue relying on a single line of defense against Varroa. These mites are evolving quickly and our management tools must evolve just as fast,” she added, in the news release.

Help may be on the way.

The Environmental Protection Agency has proposed registering a new type of insecticide to control Varroa mites, which are originally from Asia.

Massachusetts-based GreenLight Biosciences developed the active ingredient vadescana for an insecticide that interferes with Varroa mite reproduction.

Pouches containing the product would be placed directly into hives. The USDA has said that the new insecticide could replace more toxic treatments.

Important pollinator

Nationwide, more than 60% of commercial hives have been lost since summer 2024, representing a total of 1.7 million colonies and an estimated financial impact of $600 million, according to the USDA.

Honey bees pollinate a wide variety of crops and more than ⅓ of U.S. produce.

The value of crops that require bee pollination is estimated at more than $20 billion annually in the U.S. and $387 billion globally.

“Our nation’s food supply thrives, and is sustained, by the work of our pollinators,” said Joon Park, acting ARS administrator.

“USDA scientists continue to research major stressors and new parasite treatment strategies, which will help reduce the agricultural challenge presented by the Varroa mites in honey bee colonies,” Park added, in a USDA news release.