The ventral (bottom) view of Varroa destructor shows its legs (I), mouth and feeding parts, which are collectively called the gnathosoma (III), and numerous hairs called setae (IV), while the dorsal (top) view shows its dorsal shield (II). (Photos by Noble Noble, Ph.D., University of Florida)

By Leslie Mertz, Ph.D.

Leslie Mertz, Ph.D.

Varroa mites have been the bane of commercial and hobby beekeepers for decades. The mites latch onto the bodies of honey bees and hitch a ride back to the hive, where they infest both adults and immature bees, reproduce on developing larvae and pupae, and, along the way, transmit deadly viruses that can lead to the collapse of the colony.

Originally native to Asia and a parasite on Asian honey bees (Apis cerana), the mites began parasitizing honey bees (Apis mellifera) sometime near the middle of the 20th century and swiftly expanded around the world. For instance, the mites arrived in the United States in 1987 and, within a couple decades, spread into virtually every colony throughout the country. Today, the mites are considered the world’s most devastating pest of honey bees and cause huge losses in both honey production and pollination services.

For several reasons, these 2 millimeter-wide, oval-shaped parasites have eluded suppression efforts, says Cameron J. Jack, Ph.D., of the Honey Bee Research and Extension Laboratory at the University of Florida. But all is not lost. He and fellow researcher James D. Ellis, Ph.D., published an article in the open-access Journal of Insect Science in September outlining current management options for Varroa mites and describing those on the horizon. The article is part of a special collection featuring research in the United States on fundamental and applied aspects of honey bee biology, published in collaboration with the American Association of Professional Apiculturists.

Why Mites are Mighty

Cameron J. Jack, Ph.D.

One of the reasons that Varroa mites, and specifically the species aptly known by the scientific name of Varroa destructor, are difficult to manage is that honey bees are social insects, Jack says. The bees not only live tightly packed in the hive, but they also sometimes stop in at neighboring hives, dropping off and picking up mites as they move around. In addition, the brood cells in the hive provide a great place for the mites to reproduce. A female mite enters a brood cell and lays both male and female eggs on the bee larva. As soon as they mature, the newborn mites mate and the females emerge from the cell ready to repeat the process.

Another issue is that Varroa mites in temperate regions reach their peak in population size in fall, months after the bee colony has peaked, so bee numbers are on the decline when mite numbers are ramping up, Jack says. As a result, the ratio of mites to bees rises as the year progresses, which puts more and more stress on the honey bees and, in turn, makes them even more susceptible to the viruses that the mites introduce.

Cameron J. Jack, Ph.D., prepares to monitor a honey bee colony’s Varroa mite population by sampling honey bee adults. Photo courtesy of UF/IFAS Honey Bee Laboratory. (Photo courtesy of Cameron J. Jack, Ph.D.)

The control side has a couple of hurdles too. Beekeepers often use chemical pesticides, called acaricides, to fight the mites. Some of them can work well, but unfortunately none will exterminate all the mites, he says. Some have genetic differences that make them resistant to one chemical or another, so an acaricide will exterminate the mites susceptible to it but leave the resistant ones behind to flourish.

To get past the resistance challenge, experts encourage beekeepers to alternate between different acaricides, but “the major corporations that develop these chemical treatments don’t see a big enough market to yield a good return on investments, so they’re not investing heavily in developing them. Consequently, there is not a lot of effective choices for beekeepers,” Jack says.

Another option for beekeepers is to monitor mite numbers carefully, so they can choose the appropriate type of control, including which synthetic or natural acaricides to use and when to apply them and whether they can try other methods. “We preach to beekeepers until we’re blue in the face about how important monitoring is, but, in a commercial setting, monitoring techniques take time, and time is money,” Jack says. (He and Ellis point out several monitoring techniques and many non-chemical methods in their paper.) “Monitoring is a lot to ask of commercial beekeepers, but I feel like it’s very critical to really manage their bee colonies well, especially since the alternative is basically creating these super-mites resistant to all chemical treatments,” he says.

This chart is an aid to help beekeepers select the most appropriate Varroa mite treatments. A critical part of the chart is determination of mite-bee infestation ratios, which requires monitoring of the hive. (Available organic and synthetic chemical treatments vary, so this chart does not include them in the options.) (Image originally published in Jack and Ellis 2021, Journal of Insect Science)

Now What?

Despite the obstacles, Jack remains optimistic that solutions are on the way. A few of these include:

• Mite-resistant honey bees. Some resistant stocks keep Varroa mite populations low, while others are designed to survive even heavy mite infestations. While some work very well, Jack says, they are expensive and often have trade-offs, such as decreased honey production. “At this point, these stocks haven’t caught on much in the U.S., but it is an option we try to promote because it’s chemical-free,” he says.
• Biocontrol. Some projects use fungi, bacteria, or predatory mites to battle Varroa mites and have shown promise in the lab, but they have not yet demonstrated effectiveness in the hive, Jack says.
• Virus management. Control measures that focus on the viruses that Varroa mites transmit rather than mites themselves could also help. “If there’s something like a medicine to help bees outlast these high viral loads, I think that would be really beneficial and accepted well in beekeeping community,” he says.
• Lab colonies of Varroa mites for study. “The biology of Varroa is so specific and so closely tied to the honey bee colony—they only reproduce inside the honey bee brood cells—it’s very difficult to culture mites in the lab, but a lab-reared colony is going to be really crucial in helping us to understand these mites better,” Jack says.
• New chemical treatments. Many research groups, including Jack’s, are finding promising active ingredients, but convincing chemical companies to invest in developing treatments will be an uphill battle. “Still,” he says, “if we had a few more options available, then it might be easier to convince beekeepers to rotate their treatments rather than just using one thing, so we can limit resistance.”

Jack says Varroa management has been a slowly progressing field, with a lot of growing pains, but he is hopeful that solutions will be found. “With honey bee biology being so interesting and fascinating, I think the bees may very well adapt and handle Varroa now that they are consistently part of their existence,” he says. “And I think beekeepers will do the same and figure out ways to manage this terrible parasite. I am hopeful that we will get there.”

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.