Most beekeepers rate Varroa destructor mites as biggest threat to their honey bees (Apis mellifera). Shown here is a honey bee with two Varroa mites, one on the side of the bee’s abdomen near its legs and one atop the abdomen. A new study shows storing honey bee colonies with elevated levels of carbon dioxide reduces levels of Varroa mites, a method that might prove useful in reducing beekeepers’ winter colony losses. (Photo by USGS Bee Inventory and Monitoring Lab via Flickr, public domain)

By Paige Embry

Paige Embry

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

“Varroa” is short for Varroa destructor, an invasive mite that arrived in the U.S. in 1987 and has wrought havoc ever since by weakening honey bees (Apis mellifera), transmitting diseases, and destroying colonies. Beekeepers use an array of methods to try and rid their bees of mites, but the mites keep on coming. Varroa mites aren’t the only scourges honey bees face, but they contribute significantly to the astonishing loss rate of colonies. Since national record-keeping began in 2006-2007, winter colony losses in the U.S. have ranged from a little over 20 percent up to nearly 38 percent. Annual losses climb to more than 45 percent in some years. Think about it: A quarter, a third, half of your stock—your livelihood—gone each year.

Beekeepers don’t like losing bees, but they’ve had to get used to it. Nevertheless, most commercial beekeepers really, really want their colonies to come through the winter with as few losses as possible so they can take their bees to the biggest, best-paying, pollination party in the country—the California almond bloom. Colony rental rates there may run two to four times higher than for other crops, but, because almonds bloom in February, there isn’t time to replace colonies lost during the winter. Decreasing Varroa loads during the winter is one way to lessen colony deaths, and a group of researchers at Washington State University (WSU) is exploring a simple option that may help: hitting the bees and mites with high levels of carbon dioxide. Their findings were published in May in the Journal of Economic Entomology.

Brandon Kingsley Hopkins, Ph.D., assistant research professor at Washington State University and apiary and laboratory manager at the WSU Apiary Program, co-authored a new study published in May in the Journal of Economic Entomology that shows storing honey bee colonies with elevated levels of carbon dioxide reduces levels of Varroa mites, a method that might prove useful in reducing beekeepers’ winter colony losses. (Photo courtesy of Brandon Kingsley Hopkins, Ph.D.)

A subset of beekeepers that live in the northern part of the country store their bees indoors—in climate-controlled buildings—for the winter rather than hauling them south. It protects the bees from killing cold and reduces costs, theft, and wear and tear on the wooden hive boxes. 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 say in the industry is that indoor storages are hotels not hospitals. You get out what you put in.”

The inspiration for the study was a 2011 lab experiment that showed a Varroa mite death 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 real-world winter storage scenario; the bees were in cages at room temperature with the mites sprinkled on before the experiment.  But it was, Hopkins says, “really good work and what inspired us to try this.”

“This” was an experiment closer to real-world cold storage condition. Entire colonies were held at 4 degrees Celsius for 64 days with whatever Varroa mite loads they already carried.  It differed from traditional storage in one significant way: The researchers had to use smaller boxes because the typical size wouldn’t fit in the CO₂ control chambers.

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 loads in each colony from low to high and distributed them so each group had colonies with similar numbers of mites. Mite mortality was greater in the high-CO₂ group (68–78 percent versus 38–50 percent). This study did not see mite deaths like the earlier lab study where nearly half the mites died in two days; however, it did show a steady loss of mites over the entire two-month storage period.

The scientists note that humidity may also have played a role in mite death. Their chambers didn’t allow them to control that factor, and the relative humidity in the high-CO₂ chamber was lower (approximately 60 percent) than in the low-CO₂ chamber (approximately 71 percent). Hopkins says, “At high levels of CO₂, they [mites] can’t control their spiracles—at least, that’s true in insects; they lose control of their spiracle openings, so they lose most of their moisture from their bodies through those spiracle openings. … Lower humidity and higher CO₂ might cause greater mortality in mites.” It’s one more piece to investigate.

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

The scientists are continuing this research with larger control chambers that allow for standard boxes and more colonies. They hope that, eventually, CO₂ might be a way to turn cold storage “hotels” for honey bees into something more like a sanitorium—a safe place to stay with some health benefits thrown in.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.