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Refrigerating honey bees to fight mites, colony collapse

PULLMAN, Wash. – Saving honey bees is easier when varroa mite infestation is reduced. WSU researchers are hoping mid-season hibernation can help in the fight against the mighty mites.

A honey bee stands on a honeycomb with a brown circular varroa mite taking up about half of his back.
A varroa mite seen living on a honey bee. Mites weaken bees’ immune systems, transmit viruses, and siphon off nutrients. Photo by Scott Bauer, USDA Agricultural Research Service.

Varroa mites are pests that weaken bees’ immune systems, transmit viruses and siphon off nutrients. They’re a huge factor in colony collapse around the country.

“Most treatments only kill varroa on adult bees, and are generally only effective for three days,” said Brandon Hopkins, assistant professor of entomology and manager of the WSU bee program. “But a lot of mites live in the brood, which are under a wax cap that treatments can’t touch. Those bees hatch out and are already afflicted.”

Currently, treating for mites requires three treatments over a 21-day period to make sure you treat all the new bees that come out infested with mites.

These treatments are difficult and expensive because beekeepers must treat all their colonies on a specific schedule. It’s very labor intensive to treat thousands of colonies by hand three times at precise timing cycles, Hopkins said.

Cold storage

Bees don’t truly hibernate, but they do change their behavior in winter. Queens stop laying eggs, so no new ‘brood’ is created at that time.

Last August, WSU researchers put 200 honey bee colonies into refrigerated storage. This is a time when bees are still active, but have finished making honey for the season, and there are no crops that require pollination. It’s also when beekeepers normally do a round of mite treatments.

By placing colonies in refrigerators, the queen stops laying new eggs, which stops the production of brood. When the bees come out of refrigeration, there is no ‘capped brood’.

At that point, Hopkins and his team apply a varroa treatment on the adult bees.

The initial results were overwhelmingly positive. Researchers found an average of five mites per 100 bees on the control colonies (not refrigerated) one month after the normal three-cycle mite treatment.

The refrigerated colonies had an average of 0.2 mites per 100 bees one month after the single mite treatment.

“That’s a significant decrease,” Hopkins said. “Refrigeration is expensive, so we need to do more work to prove the cost is worth it for beekeepers, but we’re really excited so far.”

Additionally, the infestation levels varied tremendously from colony to colony in the control samples. That’s because of the difficulty in treating colonies consistently over three cycles. The colonies that had the refrigeration treatment had consistent mite numbers with little variation.

Doubling down

Brandon-Hopkins-WSU asst professor of entomology
Brandon Hopkins in his bee lab.

After hearing about this research, a few beekeepers approached the WSU scientists about doing a similar round of refrigeration in the early spring. Most commercial beekeepers in the U.S. take their colonies to California for almond pollination in February and March. But there’s a time gap between the end of the almond pollination season and the start of pollination season in the northwest.

“Beekeepers generally have two periods of time for mite treatments, before the bees make honey and after,” Hopkins said.

Once bees have mites, the infestation increases during the pollination and honey production months.

“But if they can start with low mite numbers, the bees are healthier during the honey production period,” Hopkins said. “A lot of varroa damage comes while the bees are making honey.”

Calculated risk with 100 colonies

This spring, Belliston Bros., a commercial Idaho beekeeper, donated 100 honey bee colonies to do a refrigeration study just like the one done in August last year.

“It’s a big risk for them,” Hopkins said. “But if it works, beekeepers would have significantly better varroa control while using fewer chemicals. And they’ll have better colony survival during the following pollinating season. It’s a win all-around.”

Nobody really knows how bees will react to being put back into their winter mode in what is normally the middle of their active season, he said. But that’s what science is all about. And if this works, it could be a major and environmentally sound victory in the great varroa mite battle that beekeepers have been waging for decades.

“We’re hopeful,” Hopkins said. “We won’t have results back for several months, but we’re excited we may have a way to help beekeepers keep their colonies strong and stable.”