Commercial beekeepers entered 2025 with hives that should have been ready for almonds. Yet, many watched their colonies dwindle week after week.

Reports from across the United States pointed to deep bee losses and a puzzle that demanded answers fast.

In late winter, federal teams pulled samples and tallied the damage.

A USDA press account put the average losses above 60 percent since the prior summer, about 1.7 million colonies, with an estimated cost of $600 million.

Bee colonies collapse under parasite

A USDA preprint traced the crash to a sharp rise in viruses in honey bees, coupled with widespread resistance in the parasitic mite that spreads them.

The team screened pooled samples from living colonies and individual sick bees, then checked the mites for resistance markers.

The culprit was Varroa destructor, an external parasitic mite that feeds on bees and spreads viruses among them. When mite levels surge, virus transmission accelerates inside a colony, and the workforce ages out faster than it can be replaced.

In the lab, inocula taken from dying bees killed healthy bees even at extreme dilutions, aligning with field observations of rapid collapse.

The authors concluded that viruses were a likely end stage in the failure sequence. Other pressures probably primed colonies to collapse.

Viruses pushing hives over the edge

Two pathogens stood out: deformed wing virus and acute bee paralysis virus. Both are single-stranded RNA viruses, and both can shorten adult lifespans and cripple brood production when they run unchecked.

Lead researcher Zachary S. Lamas is with the USDA Agricultural Research Service, Bee Research Laboratory (ARS).

“Miticide resistance was found in all collected Varroa, underscoring the urgent need for new control strategies for this parasite,” he said.

When treatments stop working

For years, many operations leaned on amitraz, a synthetic acaricide used to knock back mites. A miticide is a pesticide formulated to kill mites, and rotating products is one way to slow the development of resistance in the target pest.

An outside study linked a mutation in Varroa mites to their growing ability to survive amitraz treatments in the United States.

That mutation turns up when selection pressure favors mites that can survive a treatment, which leaves beekeepers with fewer reliable tools.

Bee parasites spread viral threats

Honey bees are a managed pollinator, and their viruses do not always stay in one place. There is evidence that deformed wing virus (DWV) can replicate in bumble bees under certain conditions, even though natural transmission routes can be limited.

Spillover risk rises around dense apiaries where mites and viruses are abundant – an issue that matters because wild pollinators add resilience to landscapes that rely on many different insects to carry pollen.

That risk is especially relevant in large-scale pollination events. California’s almond bloom is the biggest managed pollination effort in the world.

Each year, about 2.4 million hives are required, with more than 1.5 million trucked in from across the country.

“Our nation’s food supply thrives and is sustained, by the work of our pollinators,” said Acting ARS Administrator Joon Park. And food production leans on bees for far more than just nuts.

The mite behind the spiral

Varroa mites move quickly among adult workers and feed on developing brood. Even low numbers of this bee parasite can seed a virus outbreak that escalates as colonies ramp up.

Unchecked mite growth can push a colony past a tipping point, where older workers die faster than new workers emerge. At that stage, the population curve bends downward and the colony falters.

Pressure from many directions

Pesticide exposure, poor forage, heat, and transport stress can all weaken a colony’s ability to handle infection.

Those stressors do not act alone, and their combined effects show up in shorter lifespans and poorer brood patterns.

“While viruses are a likely end-stage cause of colony death, these results do not rule out the importance of other long-known challenges to honey bees,” said ARS Research Leader Dr. Judy Chen. She emphasized that viruses likely act as the last step in a chain of problems. 

Early action on bee parasites saves colonies

Operations benefit from measuring mite loads on a schedule and treating before thresholds are crossed. Rotating active ingredients and incorporating non chemical tools like brood breaks can slow resistance and reduce virus pressure.

Nutrition still matters. Access to diverse pollen can support immune function in workers and help colonies recover from routine stress.

Researchers are screening bees and mites from new loss events to monitor resistance and viral combinations. The search is also on for new chemistries and non chemical strategies that can slot into real world workflows.

Better early warning would help. If beekeepers can catch mite numbers before winter, they can prevent the late season spiral that takes colonies down when they are least able to rebuild.

The study is published in bioRxiv.

Image: Varroa destructor recto verso, by Gilles San Martin, via Wikimedia Commons, CC BY-SA 3.0 license.

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