Study Suggests Modern Beekeeping Gives Varroa Mites Ideal Conditions to Spread

Study Suggests Modern Beekeeping Gives Varroa Mites Ideal Conditions to Spread

As the managed honey bee industry continues to grapple with significant annual colony losses, the Varroa destructor mite is emerging as the leading culprit. And, it turns out, the very nature of modern beekeeping may be giving the parasite the exact conditions it needs to spread nearly beyond control.


In an article recently published in the Entomological Society of America’s Environmental Entomology, researchers argue that the Varroa mite has “co-opted” several honeybee behaviors to its own benefit, allowing it to disperse widely even though the mite itself is not a highly mobile insect. The mite’s ability to hitchhike on wandering bees, the infections it transmits to bees, and the density of colonies in managed beekeeping settings, make for a deadly combination.

“Beekeepers need to rethink Varroa control and treat Varroa as a migratory pest,” says Gloria DeGrandi-Hoffman, Ph.D., research leader and location coordinator at the USDA-Agricultural Research Service’s Carl Hayden Bee Research Center in Tucson, AZ, and lead author of the research.

In the wild, bee colonies tend to survive despite Varroa infestations, and colonies are usually located far enough apart to prevent mites from hitching rides to other colonies on foraging bees. Wild bee colonies’ natural habit of periodically swarming — when the colony grows large enough that a portion of its bees splinter off to create a new colony elsewhere — also serves as a mechanism for thinning out the density of mite infestations and their associated pathogens. In managed honeybee settings, though, these dynamics are disrupted, DeGrandi-Hoffman says. Colonies are kept in close proximity, and swarming is prevented.

DeGrandi-Hoffman, USDA-ARS colleague Henry Graham, and Fabiana Ahumada of AgScience Consulting, conducted an 11-month study of 120 honeybee colonies in one commercial bee operation, comparing those treated with mite-targeting insecticide (miticide) in the spring and fall with those treated only in the fall, and they found no significant difference in the results: more than half of the colonies were lost across the board. This aligns with what has been seen by beekeepers and researchers alike in recent years: Varroa populations continue to grow even after being treated with effective miticides. But why? The answer may be in its dispersal mechanisms.

The researchers also conducted mathematical simulations of Varroa mite population dynamics to examine the effects of both migration of foragers between colonies and swarming. When bees can wander into other colonies — either to rob them of their honey or because they’ve simply lost their way — Varroa populations across colonies climb. Likewise, prohibiting colonies from splintering periodically via swarming also leads mite populations to rise.

In the wild, DeGrandi-Hoffman and her colleagues note, driving a colony to collapse is against Varroa mites’ own interest; if the colony dies, the mites die with it. But in commercial beekeeping settings, increasing infestation of a colony activates the dispersal mechanisms the mites need to spread. Weakened foragers are more likely to wander to other colonies, and weakened colonies are more likely to see foragers from healthy colonies visit to rob them of honey. In both cases, mites can hitch a ride from one colony to another.

It all adds up to a critical point for managed honeybee industry. The researchers cite the need for new integrated pest management strategies to treat Varroa destructor as a migratory pest, as well as for further research into the specifics of Varroa dispersal.

“Colony losses in the U.S. are at unsustainable levels for commercial beekeepers. These beekeepers supply colonies for the pollination of crops that represent one-third of U.S. agriculture and are essential components of heart healthy and cancer-prevention diets,” says DeGrandi-Hoffman. “This research provides evidence that the tried and true ways of controlling Varroa are no longer feasible and that new methods that are designed for control of a migratory pest are required.”

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Mike says:

I have been saying this for several years. We are creating the perfect environments to “raise varroa mites” contradictory to mother nature.

mike bispham says:

Sorry, its rubbish. Commercial bees have no resistance because resistance is not just not systematically bred in, but positively bred out. Ferals derive resistance from natural selection, and an live at high densities without needing to swarm. Breeding in the resistant alleles is the whole answer. Maximising output using systematic treatments, regardless of their effect of each new generation (and on wild bees) is the whole problem when it comes to varroa. In all areas of organic husbandry a single principle rules, and directs all management: make the new _only_ from the best. Beekeepers are astonishingly ignorant of this principle. So, even more astonishingly, are the great majority of the academics who pontificate on the problem.

Mark Regan says:

Sorry Mike but your one sided view is short sighted. There isnt simply a single answer, if it was simply about commercial bees not being as aggressive as feral bees, the answer would be abandon our current lines and adopt onky feral bees. But even wild caught swarms face the same problems once introduced into a commercial beekeeping operation.
We must take a wide view of the issue, look at beeleeping operations as much as bee behavior and see what works and what doesn’t, then employ as many positive techniques as possible. To brush off a study because you’ve made up your mind is short sighted.