U.S. bee research gives varroa mites the cold shoulder

Canadian winters benefit beekeepers because it gives a natural break to the growth of mites. | Mike Sturk photo

Producers south of the border are experimenting with using cold storage to 
help break the cycle of varroa infestation

Canadian beekeepers have long benefited from fierce winters to help keep varroa mites under control, an advantage American researchers are hoping to move south.

“We’ve got a really long winter,” said Geoff Wilson, Saskatchewan’s apiculture specialist.

“I know everybody up here curses it and wants to go to California or Texas or wherever to get away from it, but it’s actually good for our bees because it gives a natural break to the growth of the mites.”

Varroa destructor is a parasite that attaches itself to honeybees, weakening or killing them as it feeds. It often spreads viral diseases as well.

Beekeepers south of the border have been experimenting with using cold storage to help break the cycle of varroa infestation.

Entomologist Brandon Hopkins and colleagues at the Washington State University put 200 bee colonies into refrigeration at 4 C, putting the bees into winter mode. The queen stops laying eggs, which means no bee larvae, or brood.

This deprives varroa mites of their favourite strategy, which is to hide out in the brood cells under the food meant for the bee larvae and parasitize them once they start developing. Making matters worse, the worker bees cap off the brood cells with wax, protecting the parasites against mite-killing acaricide treatments applied by beekeepers.

As the bee colonies came out of their artificial winter with only adult bees and minimal capped brood, Hopkins and his team applied acaricide. The results were significant.

A WSU article reported that the researchers found non-refrigerated colonies were afflicted with an average of five varroa mites per 100 bees. The cold-treated colonies had just one mite per 500 bees.

But in the absence of winter, refrigeration is expensive, even if some U.S. beekeepers have been able to take advantage of vacant storage space such as that used for potatoes and apples.

If beekeepers want to create a minimum brood period, they can simply cage the queen, that is, separate her from the workers. Hopkins said this technique is widely used in Europe and is becoming increasingly popular in North America. But it has its limitations.

Each colony must be disassembled, by hand. An experienced beekeeper must locate the queen and safely separate her from the rest of the colony. While this labour-intensive strategy is feasible for hobbyists or European operations that typically run a few hundred colonies, it falls down at North American scales.

“The reason we’ve pursued this as a potential management strategy in the U.S. is simply a fact of scale,” Hopkins said. “Here in the U.S. and in Canada, a commercial beekeeper has probably 2,000 colonies or more. Maybe the average would be closer to four to six thousand.”

Using the chilling strategy, hundreds of colonies could be loaded onto a semi-trailer and driven right into cold storage. Once enough time had passed to minimize the brood in the hives, they could be wheeled out again and treated with acaricide as the colonies once again become active.

Hopkins said chilling the colonies capitalizes on the bees’ natural behaviour. As the days get shorter in late August, bees forage on such late-season crops as buckwheat and mustard, the latter grown as a nematode (worm) control in Washington potato fields.

Just before the bees settle in for winter, the hive produces a special “winterized” generation of worker bees with more fat to help them through the cold period — even a Saskatchewan winter.

“The neat thing is they stay warm all winter,” Wilson said. “They don’t hibernate. They actually stay warm. It will be -40 C outside, and in the centre of the cluster, it’ll be 30 C.”

While colonies can survive extreme cold, it takes a toll on the hive. Workers keep the hive warm by vibrating their wings, which takes a lot of energy.

It’s a task that not all of them survive. Hopkins said Canadian beekeepers have been bringing their colonies inside for decades to help ensure their hives come out of the cold with a healthy population in the spring.

“They’re most metabolically efficient at 4 C,” he said.

Indoor environments are also free of wind and temperature fluctuations.

Wilson said Canadian prairie beekeepers do their main acaricide treatment in the early spring. This is when the bees start becoming active and the queen has not yet begun to lay eggs. It’s also when there are the fewest bees in the colony, and hence fewer individuals that need treatment.

Some beekeepers have begun treating their colonies as early as March, Wilson said, to get their bees an early start on the year.

This past winter has been kind to Saskatchewan bees, being relatively mild. The one significant cold snap came in midwinter, and not in March where low temperatures can cause real damage to the colonies.

As a bee colony increases production, Wilson said the queen can lay from 2,000 to 2,500 eggs per day. The overwintering population of about 20,000 bees quickly grows to 60,000, then to 80,000 individuals in midsummer. As temperatures drop and days grow shorter, the colonies naturally draw down their numbers again, and beekeepers do a “touch up” treatment for varroa mites before winter.

With healthy bee colonies on the Prairies this year, the remaining variables are honey price and weather. Agricultural crops, especially canola, provide a lot of food for bees.

“Current (honey) prices are very strong. They’re much stronger than they’ve been in quite a while,” Wilson said.

“It’s nice to see. Good bees, good prices, hopefully we’ll have a decent crop, and hopefully the beekeepers do well.”

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