Bret Adee is one of the largest beekeepers in the United States, overseeing around 2 billion bees across 55,000 hives. Beekeeping has been in his family since the 1930s, and each year he transports truckloads of bees from South Dakota across the country to pollinate crops such as almonds, onions, watermelons and cucumbers.
Last December, while his bees were wintering in California, temperatures dropped sharply. The bees clustered on top of their hives in an attempt to stay warm.
“Every time I went out to the beehive there were less and less,” says Adee. “Then a week later, there’d be more dead ones to pick up … every week there is attrition, just continually going down.”
Adee ultimately lost 75% of his bees. “It’s almost depressingly sad,” he says. “If we have a similar situation this year – I sure hope we don’t – then we’re in a death spiral.”
It became the largest recorded die-off of honeybees in US history, with beekeepers on average losing 60% of their colonies – a blow estimated to have a cost of $600m (£440 million).
Scientists have been racing to uncover the cause. Now, early findings are emerging. A research paper by the US Department of Agriculture (USDA), although not yet peer-reviewed, suggests that nearly all colonies had contracted a viral infection spread by parasitic mites – mites which appear to have developed resistance to the main chemical used to control them.
Varroa mites – comparable in scale to a dinner plate on a human – crawl between worker bees and spread disease. On their own, they typically do little harm unless infections are already present, at which point they can rapidly transmit viruses through the hive.
While varroa mites predominantly affect honeybees rather than wild bees, the viral outbreaks among honeybees often spill over into wild pollinator populations, with potentially severe implications for biodiversity.
All the beekeepers involved in the USDA study had used amitraz, a widely employed pesticide designed to combat mites. However, the research found that all tested mites were resistant to it. After decades of extensive use, amitraz appears to have lost its effectiveness. This, researchers warn, highlights “an urgent need for new control strategies” against the parasite.
According to the USDA team, recent surveys show varroa mite populations have reached worrying levels. Hundreds of samples were collected from dead and living hives at 113 sites across the country.
“When mites become uncontrolled, virulent viruses are more likely to take over,” researchers say.
Since the 1980s, varroa mites globally have developed resistance to at least four leading miticides – chemical treatments specifically developed to control mites – creating major challenges for beekeepers globally.
“When mites become uncontrolled, virulent viruses are more likely to take over,” researchers say.
“It was only a matter of time before widespread resistance to amitraz, the only remaining effective synthetic chemical, would develop,” he says.
However, the presence of amitraz-resistant mites does not appear to fully explain last year’s record losses. A range of pressures is thought to be contributing to the repeated collapses of US bee colonies. These include climate change, pesticide exposure, and a reduction in available food as monoculture farming becomes more widespread. Today, many American beekeepers routinely expect annual colony losses of 30% or more.
These combined pressures are also devastating for wild pollinators and native bee species. Managed honeybee colonies – closely monitored and recorded – may be acting as an early warning system for broader declines among insect populations.
Paul Hetherington, of the charity Buglife, says honeybees are in effect “a farmed animal as opposed to wild bees, but they will be suffering from the same stresses as their wild cousins, in particular loss of good habitat, climate stress, chemical stress, light.
Adee says: “We had mites for 20 years, and we never had over 3% losses.” He believes there is a “combination of things” that makes the bees more stressed and the mites more deadly.
Neonicotinoid insecticides are one known contributor. These chemicals, still used in the US, interfere with bees’ nervous systems, leading to paralysis and death. Some scientists have warned of neonicotinoids causing another “silent spring” – referencing Rachel Carson’s 1962 landmark book on the effects of DDT on birds.
Professor Dave Goulson, a biologist at the University of Sussex, noted that the USDA study did not establish a direct link between viral load and colony strength.
“Almost all bee colonies have these viruses, but they only do significant harm when the colony is stressed.”
He suggested that high levels of viral infection could be a symptom of poor bee health rather than the root cause.
Due to staffing cuts within the USDA, researchers were unable to carry out pesticide analysis in the hives and instead asked bee experts at Cornell University to carry out the research. Results are still pending.
There is mounting concern that continued honeybee losses could threaten food security, as these pollinators are responsible for over 100 commercial crops across North America. Reports of further losses have already emerged this year – even before the California almond blossom season, which represents the world’s largest pollination event and relies on 70% of the US honeybee population.
Danielle Downey, director of the nonprofit beekeeping research organisation Project Apis m., which conducted the die-off survey, says: “If you like to eat, you need healthy bees to pollinate crops. Beekeepers try to rebuild each year but they are pushed to the brink as losses and input costs keep increasing.
“If beekeepers fail, there is no backup plan for the pollination services they provide in US food production,” she says.
Meanwhile, beekeepers are approaching breaking point. When Adee was a child, he recalls being disheartened if more than 5% of hives were lost. Today, an annual loss of 30% is considered normal.
“It’s absolutely insane that that’s an acceptable loss in a livestock industry,” he says.
Following the devastating losses of the past year, Adee – like many others – was unable to replenish his hives this season.
“I’m just watching every nickel and dime right now, because I don’t want to get rid of men that have helped me manage these bees for years.”
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