Could carbon dioxide be a new tool against Varroa mites?

By Paige Embry

Ask almost any beekeeper in the United States about the biggest threat to their honeybees, and they’ll give you a one-word answer: “Varroa.” A few years ago, I asked Sue Cobey, a beekeeper and geneticist at Washington State University, what the three greatest threats were to honeybees. Her reaction? “Varroa. Varroa. Varroa.”

“Varroa” is short for Varroa destructoran invasive mite that arrived in the US in 1987 and has been wreaking havoc ever since by weakening honeybees (Apis mellifera), transmit diseases and destroy colonies. Beekeepers use a variety of methods to try and rid their bees of mites, but the mites keep coming. Varroa mites aren’t the only pests honeybees face, but they contribute significantly to the amazing colony loss rate. Since national registration began in 2006-2007, winter colony losses in the US have ranged from just over 20 percent to nearly 38 percent. Annual losses amount to more than 45 percent in some years. Think about it: a quarter, a third, half of your stock – your livelihood – goes away every year.

Beekeepers don’t like to lose bees, but they have had to get used to it. Nevertheless, most commercial beekeepers really want their colonies to get through the winter with as little loss as possible so they can take their bees to the biggest, highest paying pollinator feast in the country – the California almond bloom. Rents for colonies there can be two to four times higher than for other crops, but because almonds bloom in February, there is no time to replace colonies lost during the winter. Decreasing Varroa Loading during the winter is one way to reduce colony mortality, and a group of researchers from Washington State University (WSU) is investigating a simple option that could help: Hitting the bees and mites with high levels of carbon dioxide. Their findings were published in May in the Journal of Economic Entomology.

Some of the beekeepers living in the north of the country keep their bees indoors – in climate-controlled buildings – for the winter instead of dragging them south. It protects the bees from killing cold and reduces the cost, theft and wear and tear of the wooden hives. It does not protect them from Varroa mites, however. One of the researchers on the new study, Brandon Kingsley Hopkins, Ph.D., says, “What we’re saying in the industry is that indoor storage is hotels, not hospitals. You get out what you put in.”

The inspiration for the study was a 2011 lab experiment that showed that Varroa mite mortality rate of 46 percent in two days when the bees and mites were exposed to elevated carbon dioxide (CO₂) levels. That experiment was far from a realistic winter storage scenario; the bees were in cages at room temperature with the mites sprinkled on them before the experiment. But it was, Hopkins says, “really good work and what inspired us to try this.”

“This” was an experiment closer to the real state of cold storage. Entire colonies were kept at 4 degrees Celsius for 64 days with whatever Varroa mite loads they were already carrying. It differed from traditional storage in one important way: The researchers had to use smaller boxes because the typical size wouldn’t fit the CO.₂ control rooms.

For the experiment, two groups were tested with eight colonies each: a low CO₂ (0.12 percent) group and a high CO₂ (8.5 percent) group. The scientists ranked the Varroa loaded into each colony from low to high and divided them so that each group had colonies with a similar number of mites. The mite mortality was greater in the high CO₂ group (68-78 percent versus 38-50 percent). This study saw no mite deaths like the earlier lab study where nearly half of the mites died in two days; however, it showed a steady loss of mites over the entire two-month storage period.

The scientists note that humidity may also have played a role in the death of mites. In their rooms they couldn’t control that factor, and the relative humidity in the high CO₂ room was lower (about 60 percent) than in the low-carbon₂ room (about 71 percent). Hopkins says, “At high CO. levels₂she [mites] can’t control their spiracles – at least that’s true with insects; they lose control of their spiracle openings, so they lose most of their fluids from their bodies through those spiracle openings. … Lower humidity and higher CO₂ could lead to greater mites mortality.” It’s still a lot to explore.

Also three of the eight colonies in the CO .-rich₂ group died, but none in the low CO₂ group. The researchers aren’t sure why. Carbon dioxide levels alone should not be a problem because, the authors write, “preliminary trials have shown that colonies survived for extended periods of time at 10 percent levels.” Nor could all three colony losses be attributed to Varroa taxes, which were equalized between the two groups. Hopkins notes that, with only 16 colonies in total, “that’s a relatively small experimental number, and so we couldn’t really rule out what might have caused those deaths.”

The scientists are continuing this research with larger control rooms that allow for standard boxes and more colonies. They hope that CO. in the end₂ could be a way to turn cold storage “hotels” for honeybees into something more like a sanatorium – a safe place to stay with a number of health benefits.

Paige Embry is a freelance science writer based in Seattle and author of Our native bees: North America’s endangered pollinators and the struggle to save them. Website: www.paigeembry.com.

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