In an effort to stem a massive bee die-off, government scientists have developed a population of honeybees that can root out a main culprit in the epidemic — a parasite that feeds on pupae in nests and spreads viruses within hives.
U.S. Department of Agriculture (USDA) scientists hope the population of Varroa mite-detecting honeybees could potentially improve the health of the overall honeybee population.
Domestic honeybee stocks have been waning since 2004 because of a mysterious illness scientists call colony collapse disorder, which causes adult bees to forsake their broods. During the winter of 2007, the disorder wiped out around 1 million colonies in North America.
In addition to stemming the tide of honeybee die-offs, the USDA hopes that the effort to eliminate the varroa mite threat could buoy the $8 to $14 billion food industry that relies on commercial honeybee pollination.
"Varroa is still considered the No. 1 pest of honeybees worldwide," said Jeffrey Harris, an entomologist with the Honeybee Breeding, Genetics and Physiology Research Unit in Baton Rouge, La., which is part of the USDA's Agricultural Research Service.
Mites resistant to pesticides
For more than 20 years, varroa mites have not only decimated millions of honeybee colonies in North America; they've also grown resistant to pesticides.
The tiny parasites invade a honeybee colony and feed off honeybee hemolymph, which is the mixture of blood and fluids inside the insect's body. Varroa mites especially target immature pupae in capped broods, or covered hive cells, stunting their growth and causing wing and leg deformities. The pests are also vectors for virus. Within two years, varroa mites can destroy an entire colony.
But some bees have a low-frequency genetic trait termed varroa-sensitive hygiene (VSH) that enables them to better locate and remove varroa mites from hives. These bees team up to open the covered brood cells and remove the mite-damaged pupae and any accompanying varroa mites from the hive.
"We do think they're much better at smelling the odors associated with varroa-infected cells," Harris said.
After identifying and isolating that genetic trait, Harris and other USDA entomologists have now developed a population of honeybees with a high expression of the VSH. In a field trial of 40 colonies, those with the highest levels of VSH showed significantly lower mite infestation and bee mortality than the control groups. The study results were published in the Journal of Apicultural Research.
Not meant for mass reproduction
Harris emphasizes that the VSH honeybees aren't meant for mass reproduction as a pure stock, since that would result in excessive inbreeding, with single queens mating with up to 20 drone bees.
"By outcrossing them and breeding VSH queens with non-VSH bees, it guarantees that there will be mongrels and mutts out there to keep the genetic diversity up," Harris told Discovery News.
Although Dr. Keith Delaplane, director of the Honeybee Program at the University of Georgia, agrees that varroa mites are one of the top contributors to Colony Collapse Disorder, he has reservations about the long-term potential of breeding for the VSH trait.
"These specific characteristics tend to erode pretty quickly unless the beekeeper is applying stiff selection to the colonies," Delaplane said.
The VSH trait expresses infrequently in the wild and some stocks, such as Russian honeybee, have developed some mite-resistance naturally over time.
Delaplane thinks it's important weigh the pros and cons of selectively breeding for traits versus promoting more "hybrid vigor" by allowing queens to mate of their own accord.
"Natural selection doesn't necessarily favor traits we think it ought to favor," he notes.
From a commercial standpoint, VSH bees have so far met the USDA's expectations of thwarting varroa mite infestations and fulfilling their crucial roles as pollinators.
"As far as performance, they do just as well as the other lines in terms of making honey and pollinating," said Tom Glenn, professional beekeeper and owner of Glenn Apiaries in Fallbrook, Calif.
In 2001, Glenn began artificially inseminating pure VSH queens from the USDA Honeybee Research Unit. To limit inbreeding, he impregnates the queen with semen from around 200 drones, rather than the 10 or 20 she would normally mate with. He doesn't breed bees that are too closely related.
Considering the reproductive and hygienic success he's witnessed with the VSH colonies, Glenn is optimistic that the honeybee industry is on the right track toward recovery from the varroa pest and Colony Collapse Disorder.
"Actually, (the VSH bees) have been doing so well that in 2002 we stopped treating for the mites altogether," Glenn said.