USDA identifies cause of recent mass honey bee collapse

By Christine Won

The mass honey bee colony collapse earlier this year, called "the worst bee die-off in U.S. history" by honey bee research organization Project Apis m., was caused by virus-infected, miticide-resistant parasitic mites.

Researchers from the U.S. Department of Agriculture’s (USDA) Agricultural Research Service’s (ARS) Bee Research Laboratory found "high levels of deformed wing virus A and B and acute bee paralysis in all recently USDA-sampled bees," according to a June 2 announcement . "These viruses are responsible for recent honey bee colony collapses and losses across the U.S."

The findings, which have not been peer reviewed, were made available online June 1 on preprint server BioRxiv.

"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," the researchers wrote.

U.S. commercial beekeepers in January began reporting mass honey bee die-offs, and in February, ARS researchers collected colony and bee samples from California and other Western states prior to almond pollination to analyze parasites and pathogens.

"Viruses were indicated in both pooled samples from surviving colonies, and in individual bees showing behavioral morbidities," the USDA announcement stated.

ARS researchers screened Varroa mites—vectors for these viruses—from collapsed colonies. They found signs of resistance to amitraz, a miticide widely used by beekeepers, in "virtually all" collections, "underscoring the need for new parasitic treatment strategies," according to the agency.

The colony collapses culminated in a loss of 1.7 million colonies, representing over 60% of commercial beekeeping colonies since the prior summer, and an estimated financial impact of $600 million, according to the USDA.

That was the highest loss ever recorded in the industry, said Dr. Britteny Kyle, who gave two honey bee sessions at the AVMA Convention 2025 in July in Washington, D.C.

However, honey bee colonies are not actually declining in numbers because they are a managed species, said Dr. Kyle, a PhD candidate in epidemiology with a collaborative specialization in One Health at the University of Guelph Ontario Veterinary College.

“I want to emphasize that honey bees will not disappear, because we control the population—we breed them, and we can replace our losses,” Dr. Kyle said during her presentation on the importance of honey bees from a One Health perspective. “But the fact is, honey health isn’t very good right now.”

Honey bee health is declining, she said. In addition to threats from pests and pathogens such as Varroa mites and American foulbrood, honey bees are dealing with climate and land use changes as well.

That means food will get more expensive, she said, meaning there will be a cost burden on society.

Veterinarians have an obligation to ensure the health and welfare of this vital managed agricultural species, she added.

"We can apply a veterinary approach to learning honey bee health. Honey bees are not really different from the vast array of species that veterinarians traditionally work with," Dr. Kyle told AVMA News. "As veterinarians we possess a unique skill set that can bring value to beekeepers and have a tremendous impact on the bees and the apiary industry."

The USDA says that honey bees (Apis mellifera), as primary managed pollinators, are "an integral component of agriculture, providing key pollination services for a wide variety of crops and over one-third of U.S. produce."

ARS scientists continue to research major stressors and new parasite treatment strategies. Acting ARS Administrator Joon Park said in the agency’s announcement about the honey bee collapse that this work will help reduce the agricultural challenge presented by the Varroa mites in honey bee colonies.

"Our nation’s food supply thrives, and is sustained, by the work of our pollinators," he said.

A version of this story appears in the November 2025 print issue of JAVMA.