Varroa mites are a constant threat to managed honey bees (Apis mellifera), but the most commonly used pesticide used against the mites appears to have negative effects on reproductive patterns of honey bee queens (shown here at center). A new study explores these effects and offers beekeepers insight on balancing mite management and hive health. (Photo by Jessica Louque, Smithers Viscient, Bugwood.org)

By Leslie Mertz, Ph.D.

One of the biggest challenges facing honey bees worldwide is a destructive foe: Varroa destructor mites. Varroa mites are tiny, eight-legged creatures that not only attack and feed on both adult and developing honey bees (Apis mellifera), but also transmit deadly viruses. To combat the Varroa mites, beekeepers use mite pesticides, or miticides. While that does keep the mites at bay, the miticides have unintended effects on honey bee queens, most notably in their mating habits, according to a new study published last week in the Annals of the Entomological Society of America. Fortunately, beekeepers can take a few precautions to protect their hives from ravaging mites while also keeping the queens healthy.

Leslie Mertz, Ph.D.

“Varroa mite infestation is the number-one problem in apiculture still today, and if the number of these mites per colony goes above a certain threshold (roughly between a 3–5 percent infestation rate) and the beekeeper doesn’t do anything, it’s very likely that colony will collapse and die within a year or maybe two at the most,” says study co-author Juliana Rangel, Ph.D., associate professor of entomology at Texas A&M University in College Station. Most beekeepers keep Varroa at bay by using mite-fighting chemicals collectively called miticides. “Over the past few years, however, we in our lab as well as people in other labs have noticed that there are some sub-lethal, non-target effects of these chemicals on the very honey bees that we actually want to protect,” she says.

The Mating Problem

The current study investigated whether differences existed between mated queens that had been developed from brood cells containing one of four treatments: 1) amitraz, which is the most widely used anti-Varroa miticide today; 2) a combination of two other miticides (tau-fluvalinate and coumaphos) that were popular before amitraz; 3) a combination of two commonly applied agrochemicals (chlorothalonil and chlorpyrifos); or 4) no pesticides. For all miticides and agrochemicals, the concentrations in the experiments were field-relevant, or typical of those seen in honey bee hives kept by commercial beekeepers.

The researchers, led by doctoral student Elizabeth Walsh (now graduated), were especially interested in how many drones mate with an individual queen. In honey bees, the queen mates with multiple drones during a series of mating flights that occur shortly after she becomes an adult. She then stores the collected sperm in an internal receptacle (a spermatheca), and doles it out over her one- to three-year lifespan to fertilize her eggs and produce new young for the colony.

Researcher Elizabeth Walsh holds up a frame containing an active honey bee hive. (Photo by Rong Ma)

A honey bee queen will mate with multiple drones (a drone is shown here) during a series of mating flights that occur shortly after she becomes an adult. A new study by researchers at Texas A&M University showed that this “mating frequency” increased among queens that had been exposed to amitraz during development. (Photo by Elizabeth Walsh)

Entomologist Juliana Rangel, Ph.D., and her research group at Texas A&M University College Station found that a widely used pesticide—the miticide amitraz—used to protect honey bees from mite infestation had an effect on mating among the queens. She is now continuing her studies of the effects of this and other pesticides on queen and drone honey bees. (Photo courtesy of Juliana Rangel, Ph.D.)

To determine the number of drone partners, also known as the queen’s mating frequency, the researchers ran a form of paternity test on the queen’s young, specifically the pupae. They were surprised to find that the amitraz-raised queens exhibited “as much as 40 percent higher effective mating frequency as compared to pesticide-free and other pesticide-reared queens,” says Walsh, who is now a postdoctoral research fellow in the laboratory of Stephen Pernal, Ph.D., at the Beaverlodge Research Farm, a part of Agriculture and Agri-Food Canada.

An increased mating frequency may sound like a benefit, but it likely isn’t, Rangel says. “There is an average mating frequency that is probably the evolutionary stable strategy, which means that honey bees evolved this standard mating frequency that allows them to cope well against disease, be productive, and still maintain colony cohesion. This higher frequency is not normal, so it’s telling us that something in their reproductive physiology is probably somehow compromised,” she says, adding, “This is very troubling because amitraz is the number-one miticide that is currently used by beekeepers.”

Walsh adds, “These results demonstrate the importance of limiting pesticide use to control Varroa mites in beekeeping operations, as we can see that beekeeper-applied miticides can negatively affect honey bee queen health.”

Tips for Beekeepers

Based on the results of this study and other work Rangel and her group have done, she offers several recommendations to beekeepers to keep their hives healthy:

• Replace the comb every other year because many miticides and agrochemicals—either applied or inadvertently gathered by foraging bees and brought to the nest—can linger and accumulate in wax combs, potentially harming honey bees.
  

  Rangel and her research group take a break in the field. Back row (left to right): Kyle Harrison, Alex Martinez, Elizabeth Walsh, E.T. Ash, and Rong Ma; kneeling: Michael Wong and Juliana Rangel, Ph.D. (Photo courtesy of Juliana Rangel, Ph.D.)

• When sourcing wax to start a new comb, choose suppliers that either “use no chemical treatments, or at least not very harsh miticides to control Varroa,” and that also filter their reused wax so it is as clean as possible.
• Employ integrated pest and pollinator management (IPPM) strategies, including regularly measuring Varroa levels, and, if needed, apply the recommended dosage of amitraz or any other chemical treatment no more than once a year, but alternate it with other Varroa treatments so that amitraz remains effective (and the mites don’t develop resistance to it).
• Talk to the local agricultural community about the timing of pesticide applications as a way to limit hives’ exposure to agrochemicals, many of which are also known to have deleterious effects on honey bees.

Rangel’s group is planning more studies focusing on the impact of miticides and agrochemicals on honey bees, and particularly reproductive females. She says, “Continuing to explore what makes a good queen is a very important question to apiculture.”

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