SMR—this honey of a trait protects bees from deadly mites

Agricultural Research, May, 2004 by Erin Peabody

We all wish for good genes, especially those that guard us against serious illness and health problems. Having the right genes makes a difference--even in the world of honey bees.

Two entomologists with the Agricultural Research Service would agree. They've recently unleashed a trait in honey bees that offers defense against the parasitic Varroa mite. Known as SMR, for suppressed mite reproduction, the trait protects the important pollinators by stopping the mites from reproducing.

Honey bees--key deliverers of pollen grains--are needed in the production of $15 billion worth of U.S. crops. This includes a staggering list of fruits, vegetables, and other valuable crops.

Aware of the need for healthy honey bee populations, John R. Harbo and Jeffrey W. Harris have spent almost a decade working to produce bees with built-in mite protection. Both are entomologists in ARS's Honey Bee Breeding, Genetics, and Physiology Research Unit in Baton Rouge, Louisiana.

"Finding natural resistance has been our goal ever since the mite moved into the global population of honey bees," says Harbo.

And for good reason. Vurroa destructor is the number-one threat to honey bees worldwide. The mites attach themselves to adult bees, often causing blood loss and spreading viral infections. Perhaps most apparent is the mites' impact on immature bees. Varroa invade brood cells where the soft larvae and pupae develop, leading to undersized adults and malformations like misshapen wings.

"If left untreated, mites will cause a beekeeper to lose a beehive and honey crop in 2 years or less," says Harbo.

The standard treatments for Varroa mites are chemical-based miticides, such as fluvalinate and coumaphos. But miticides are not long-term solutions, and mite populations are becoming resistant to them.

Of Mites and Men

"It seemed like a pretty hopeless situation," says Harbo of their initial thoughts on finding mite resistance in bees. "In nature you usually find a few individuals that pop up with resistance. We weren't finding any."

Despite the bleak scenario, the two researchers started chiseling away at the 20-year-old Varroa problem. Their first move? To try to isolate a factor responsible for controlling mite populations in bee colonies. Honey bees sometimes engage in activities that fend off mites, including hygienic behavior, where bees uncap and purge the contents of diseased brood cells. Some exhibit grooming behaviors, such as adult bees biting mites from their bodies or from the bodies of other bees.

But by the end of the studies, another characteristic appeared even more salient--the condition of nonreproducing mites. These are mites that enter brood cells to reproduce--but don't, for one reason or another. Most notable of these are female mites that don't lay any eggs. Harbo and Harris believed a trait of the honey bee was causing this to happen.

By measuring mite reproduction in a group of genetically related bee colonies, they determined that the trait did have a genetic component in the honey bee. But because environmental factors, especially heat, can also affect mite reproduction, the researchers wanted to find out how much of the affected mite reproduction should be attributed to the environment and how much to genetics. When their research uncovered an encouraging heritability level of 44 percent, Harbo and Harris began selectively breeding for the SMR trait.

Beekeepers Benefit

Under a cooperative research and development agreement that ended last year, ARS provided the SMR trait to Glenn Apiaries, a commercial queen honey bee producer in Fallbrook, California. Under the agreement, Glenn Apiaries propagated and sold pure SMR breeder queens to other U.S. breeders and to beekeepers who wish to insert mite resistance into their bee populations.

"The goal is to provide beekeepers with some immediate relief in their battle with mites," says Harbo.

Selective breeding can bring mite reproduction to zero in worker brood. And because the trait is additive, less-selective breeding still produces results. For instance, if an SMR queen is mated to a male bee, or drone, without the SMR trait, the colony will still benefit from an intermediate level of around 50 percent resistance to Varroa.

The trait, which may be controlled by only two genes, can be bred into any population of bees. So, beekeepers can add desired mite resistance while maintaining genetic diversity and other good qualities in their colonies.

And what are beekeepers saying about SMR? "Beekeepers are happy that we're finding genetic resistance to Varroa, but most are cautious--and they need to be when it comes to controlling Varroa. Some are unsure whether they can completely eliminate dependence on chemical mite treatments. But those who monitor their mite populations will be able to determine the effectiveness of their SMR bees," says Harbo.

Another Pair of Genes Coming Soon

Harbo and Harris are studying a second trait in bees that offers mite resistance. They've been measuring the potential trait--dubbed "percentage of mites in brood" or P-MIB--for as long as they've been working on SMR, but they've just begun evaluating it in breeding studies. Mites reproduce only in brood cells, so selecting for low PMIB would diminish the rate of mite reproduction.