Widespread, Destructive, and Stubborn Mites Target Honeybees
May 18, 2023
Beekeepers in the United States lost a staggering 39% of hives from April 2021 to April 2022, according to data from Bee Informed Partnership. A study published in January by Penn State University found this devastating loss has been partially linked to the presence of parasitic mites.
Across the country, varroa mites threaten the health and well-being of beehives by parasitizing and crippling honeybees. Once a varroa mite infestation reaches a hive, it can bring viral diseases, cause deformities, and impair flight performance.
In Rhode Island, varroa mite infections are able to be examined at a local level.
Stephen H. Burke, president of the Rhode Island Beekeepers Association, said he experiences an annual loss of 20% to 30% of hives from varroa mite infestation. When asked about the impact of mite infestation compared to other hive threats such as disease, “Nothing else even compares,” he said.
Varroa mites originate from the eastern honeybee, Apis cerana, and have spread to the United States through illegal importing by a commercial beekeeper, according to the American Bee Journal. Since the discovery of varroa mites in the United States in September 1987, the effects have been catastrophic.
When infesting a hive, adult female mites enter honeybee brood cells and will lay two to five eggs within the capped cell. Once the varroa eggs hatch, the larvae will feed on the larval bees and break through the cell to continue spreading.
One viral disease brought in by varroa mites is deformed wing virus. This disease is vectored by varroa mites and affects bees by, as the name suggests, deforming their wings and causing paralysis, among other harmful effects.
Steven R. Alm, a professor of entomology at the University of Rhode Island, has a focus in apiary studies and works with local hives near the university’s Kingston campus. Varroa mites actively affect hives that Alm works with, he said.
“We have about 20 hives here at East Farm,” Alm said. “The varroa mite is a problem for us.”
Alm said mite-infested bees may come from packages bought from California or southern states. Each package includes a queen and nurse, forager, drone, and guard bees. Each bee has a different responsibility within the hive, but all bees have the potential to carry varroa mites.
“We’re probably bringing them in every year, every season,” Alm said. “It takes [mites] the season to build up to the point where they can actually kill a hive or transmit so many diseases that [the bees] die from viruses.”
Alm described the process of hive transportation through commercial beekeeping: people pay commercial beekeepers to truck honeybees across the nation’s highways to pollinate large fields, whether it be fruit orchards or crop fields.
The process of hive transportation allows for bees to be close and compact while traveling countrywide, increasing the risk of spreading disease and parasites. The space is not completely compromised, however, as bees traveling at high speeds tend to stay within their hives, Burke said.
There are a few different ways to measure the level of mites that are threatening hives, such as the powdered sugar method, the ethanol/isopropanol method, and the tray method.
Alm described how the mite levels in the hive can be found by placing a half cup of bees (about 300) into a mason jar with a couple tablespoons of powdered sugar and a mesh lid. Shaking the jar will cause the mites to fall off the honeybees when they come into contact with the sugar, as they “hate to get anything on their feet,” Alm said. After the mites drop off the bees, they can be counted while the bees snack on the remaining powdered sugar.
The issue with this method is that mites could possibly evade the sugar and stay latched on. Additionally, shaking the jar heavily is harmful for the bees’ health. If the queen is scooped up as a part of the powdered sugar method, there is a risk in harming or even, according to Burke, killing the queen.
Replacing the powdered sugar with a small amount of ethanol or isopropanol alcohol will kill all the bees that were scooped up and used for measurement, but it offers a more accurate count of mites.
If a varroa infestation is found, there are treatment options available. The Environmental Protection Agency provides a list of approved pesticides, including formic acid pads, oxalic acid, hop beta acid resin, Apiguard, and Apivar.
Formic acid pads come in a plastic pouch that is vented to release formic acid in vapor form. This treatment option is less favored than oxalic acid because of its ability to harm the bees. Formic acid pads work OK, according to Cynthia Holt of Little Rhody Beekeeping. “You also run the risk of harming your queen, but you can use it with honey supers. It’s an organic method.”
The acid is derived either from carbon monoxide undergoing a chemical reaction or from ants as a form of protection against predators.
“The package of formic acid says that it may kill your queen and it may kill 1,500 bees,” Alm said. “They warn you.”
Oxalic acid, the treatment plan favored by Holt, can be administered through a vaporizer or through a dribble method. Holt will use it typically around winter solstice, when the bees are clustered to stay warm and there is no brood in the brood chamber, the location where the eggs, larvae, and pupae develop toward the bottom of the hive. Holt mixes oxalic acid with a syrup solution and dribbles the mixture over the hives.
When oxalic acid goes through sublimation or vaporization, it changes into a gaseous state that can be pumped through the hive using an oxalic acid wand. Holt said it can be hooked up to drill batteries to be used in a portable fashion.
This treatment plan is effective, but it poses a risk to humans. “You have to wear a respirator,” Holt said. “You can basically permanently damage your lungs.”
Alm described another treatment plan that uses fewer chemical ingredients. It involves placing a hot plate under the hive that reaches 104 degrees Fahrenheit for a short period of time. The heat will kill nearby mites; however, the heat is strong enough to also kill a few bees. It is not as effective as acid.
“Everything’s got a bit of a cost to it,” Alm said.
Beekeepers, such as Burke, check hives more often in the summer, about once a week or once every two weeks. Holt emphasized the importance of getting involved with her hives for practice and experience, stating “it’s a daily thing.”
After treatments, mite levels in the hives drop, but beekeepers don’t quite know why. “We’re not entirely sure how or why any of them are effective at controlling mites,” Burke said. “Nobody’s really sure why they work.”
The EPA states that “any Varroa mite population has the potential to become resistant to acaricides” or pesticides used to kill them.
The federal agency advises beekeepers to rotate treatment plans often to avoid the mites gaining any sort of resistance. However, as mites gain resistance to treatment, it is theorized that some species of bees in certain locations may be becoming mite-resistant.