Careful Honeybee Breeding Combats Tracheal Mite Pests

January 20, 1998

COLUMBUS, Ohio -- A study at Ohio State University has shown that selective breeding helps honeybees develop resistance to tracheal mites, pests that beekeepers normally control with insecticide.

The research indicates that with a combination of selective breeding and other natural controls, beekeepers may maintain healthy hives without relying on chemical controls.

Before Ohio State began selectively breeding for mite resistance, about half of the bees at the university had tracheal mites. As of spring 1997, the number had dropped to nearly undetectable levels, without the use of chemicals.

“The problem with treating bee hives with chemicals is that the honey could become contaminated,” said Susan Cobey, apiarist at Ohio State. “The mites eventually develop a resistance to the chemicals anyway. We are trying to develop more natural techniques for controlling tracheal mites that will be practical for commercial beekeepers.”

Cobey explained that tracheal mites are but one of the problems facing honeybees in North America: cold winters, wet springs, and pesticides have all combined to lower the bee population in recent years. In fact, wild populations of bees in the United States are nearly extinct.

Beekeepers currently manage about 3 million colonies of honeybees in the United States, and they have suffered losses as well, although not as severe. Estimates of managed bee colony losses to mites vary from state to state.

In a recent issue of the American Bee Journal, Cobey explained how beekeepers could control tracheal mites effectively over the long term -- by breeding bees that carry the genetic traits that help them combat the mites.

Cobey said that such genetic resistance occurs naturally. Bees in Europe, where scientists first discovered tracheal mites in the 1920s, have long since developed a resistance to them. When the mites first migrated to the United States on European honeybees in the 1980s, they decimated entire bee populations here.

Tracheal mites attach themselves to the inside of a bee’s breathing tubes and feed on the bee’s blood. The bees that survive are usually so weak that they can’t maintain their hive.

“Infested bees become very lethargic,” said Cobey. “In really severe cases, the bees end up crawling in the grass because they’ve lost their ability to fly. They stop foraging and feeding their young so the population dwindles. They also develop secondary infections caused by bacteria or fungi. In that condition, they can’t defend their hive from invaders. The mites continue to spread because healthy bees from other hives invade these weakened colonies to steal honey, and they carry tracheal mites back with them.”

Since the 1980s, bees in the United States have begun to develop their own resistance -- slowly. Cobey said the process normally takes decades.

“What we’re trying to do is speed up the process of natural selection by breeding the bees that seem least affected by the mites,” said Cobey.

Cobey oversees about 200 hives on the Ohio State campus. As in the commercial beekeeping industry, each hive consists of a stack of wooden boxes that she and the other researchers expand as the population increases, and occasionally dismantle to study the interior conditions.

Since 1991, Cobey and her colleagues have been maintaining a special breed of bee they call the New World Carniolan (NWC). Through artificial insemination, they have been breeding queens and drones that appear to show mite resistance and high honey production. Twice a year, the researchers dissect a random sample of bees and search their tracheas for mites.

Cobey said that the selective breeding program is responsible for cutting the mite infestation in the NWC population from about 50 percent to 1 percent in 6 years.

Rather than figure out exactly how the bees develop resistance -- which Cobey says would be too lengthy a task -- she and the other researchers are simply selecting and breeding the bees that seem to prosper the most overall despite the presence of the mites and other environmental conditions. As a result, the Ohio State bees are hearty in general.

“Our strategy seems to be working, judging from this past spring,” said Cobey. “We had a cold wet spring, and many bees died in the general population, but our population did tremendously well.”

Cobey called for beekeepers and their suppliers to put more effort into maintaining breeding stock, like the farm industry does. She said that suppliers of queens and drones don’t control breeding enough to assure the quality of their bees.

“You can ask for the pedigrees of chickens, pigs, or cows before you buy them, but you can’t ask for a bee pedigree,” said Cobey.

But beekeepers can’t easily control who their bees mate with. Queen bees mate with up to 20 different drones in a season. They mate in mid-flight, sometimes with drones from neighboring hives.

“Artificial insemination is the best way to guarantee the quality of the bees,” said Cobey, “but it’s a fairly technical laboratory procedure, which industry has been slow to adopt.”
As a result, most beekeepers just let their bees mate freely, and any beneficial genetic traits that their bees initially possess become lost in succeeding generations.

Cobey wants to organize queen producers on a national level to promote bee breeding. She offers a summer class in bee breeding to encourage beekeepers to establish their own breeding programs. Cobey warned that bee breeding requires patience and a long term commitment because results aren’t always immediately visible.

The California State Beekeepers Association sponsored the work, along with the California Almond Board.

Commercial farmers of products like almonds and alfalfa rent bees to pollinate their crops. Cobey said that honeybee pollination supplies us with over one third of the food we eat.

Cobey founded the NWC project in 1982 when she was a commercial apiarist. Ohio State University is now expanding the NWC project to make breeder stock available to industry.

Ohio State University

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