Some Minnesota honeybees disappeared this fall. Blame mites and a warm autumn.

Kirk Nelson walked outside a week ago to check on the honeybees he’d been tending for about six years, and got a stark surprise.

One of the two colonies in his backyard in Orono had buzzed out of its hive — the queen, workers and drones alike — leaving nothing but honey.

“We had been out in the hive two weeks before that and it looked like it was doing great, and all of a sudden they were just gone,” Nelson said.

Nelson isn’t the only Minnesota beekeeper who’s missing their prized honey makers. He said he spoke with two others who had lost hives in the same way. And similar reports have come in to the University of Minnesota’s Bee Lab, according to Marla Spivak, a professor in the department of entomology.

Asked what the cause might be, Rebecca Winkels, a beekeeper with the West Metro Bee Club, guessed the pervasive varroa mite.

“[My bees] did the same few years back,” she wrote in an email. “Mites were crazy this fall.”

Varroa mites are present in every honeybee hive across the United States, Spivak said. The parasites suck bees’ blood and spread viruses from bee to bee. Spivak agreed that mites and the viruses they spread were the likely culprit in Nelson’s case. In late fall, if viruses are rampant, bees often fly away suddenly when they are about to die, she added.

Mites are a well-known problem to beekeepers, who cited the parasites as the biggest single impact on bee health in a survey conducted by the Bee Lab. Nelson said he had treated his hives this September.

An additional factor made the mites worse this year, however: A warm fall has allowed queen bees to keep laying eggs long past their usual time, Spivak said. If the brood keeps growing after a mite treatment, the mites will keep reproducing too — and the beekeeper might not know it’s happening unless they check. Apiarists should always check after a mite treatment to make sure it has worked, Spivak added.

“We don’t have any way of knowing how many people this is happening to,” Spivak said, because Minnesota has no beekeeper registration or tracking program. The Bee Lab does offer classes for beekeepers, however, and keepers can send questions to beelab@umn.edu.

The mite infestations do not qualify as “colony collapse disorder,” a phenomenon first reported in 2006 where hives would similarly disappear but leave their queens behind. In those situations, bees disappeared in the summer. Scientists still don’t know the exact cause of that problem, Spivak said. Annually, 30% to 40% of U.S. colonies die every winter, mostly from mites and viruses.

The Washington Post recently reported that the number of American honeybee colonies had surged, with the USDA reporting 3.8 million hives nationally in 2022. Much of that growth, however, came from Texas, which has changed its tax code to apply lower property tax rates to land where bees are raised.

Nelson said he and his niece, who tend the bees together, don’t plan to give up their backyard operation. Their original hive is still going strong and feeding on honey the other bees left behind. They’ll likely start a new hive next spring, he said.