Varroa desturctor. The name evokes evil entities and comic book mayhem. But there is nothing funny about the Varroa honey bee mite.
The tiny beast — an inadvertent stowaway on bees smuggled into the U.S. sometime before 1987 — now infests honey bee colonies across most of North America and is responsible for widespread destruction of hives. The mites, which feast on the blood of immature bees as they develop in their wax-capped brood cells, cause weight loss in the adult bee together with deformities, disease and reduced lifespan. Untreated, an entire honey bee colony can be wiped out within two years or less.
Steve Sheppard, professor of entomology and holder of the Roy Thurber Endowed Chair, has been battling Varroa mites on the Palouse since coming to WSU in 1996. Using a number of innovative practices, Sheppard and his associates have discovered successful ways to contain the mites, and for the first time — this fall of 2005 — the hives will not require active treatment for the infestation.
Integrated pest management
Historically, bee mite treatments have ranged from peppermint oil to heated “mite zappers” to chemical pesticides such as fluvalinate and coumaphos (now on the EPA watch list of chemicals that may be banned). The chemicals work for a time, but Varroa mites quickly develop resistance to them. A better answer, according to Sheppard, is to use IPM, or integrated pest management to try to keep mite populations below damaging levels rather than trying to eliminate them completely.
“We have been working with a combination of techniques,” Sheppard said, “such as monitoring the mite populations in the bee colonies; employing various cultural methods; using genetically resistant bee stocks; and the cautious application of pesticides.”
Sheppard explained that cultural methods are those that take advantage of bee and mite behavioral traits. For example, it is known that queen bees measure the size of the comb cells with their antennae to decide whether to lay a male or female egg. Beekeepers, therefore, can add special combs to each hive that will only produce drone — or male — bees. Varroa mites preferentially reproduce on drone brood and thereby become concentrated in that comb. Once the cells are full, they are put into a freezer to kill the mites.
“This is known as drone brood trapping — a practice that is only economical for small beekeepers as it is too labor intensive for commercial operations,” he said.
In their search for a less toxic chemical to kill mites, Sheppard, together with graduate student Jamie Strange, investigated the possibility of using a sugar ester compound called sucrose octanoate. Their studies showed that it was effective in killing Varroa mites at concentrations safe for honey bees. The biopesticide was originally used to combat aphids, but through the efforts of Sheppard and Strange, it is now sold as Sucrocide and officially labeled for use on Varroa mites.
Breeding the strong
Over the past five years, Sheppard and his colleagues have also discovered answers in the genetics of their honey bee breeding program. During his research in Brazil, he worked with Africanized honey bees that have a natural resistance to Varroa mites. These bees, descendents of an African subspecies introduced to South America in the 1950s, now range from Argentina to the southern US. Though mites exist in their hives, they do not kill the colonies, so don’t require treatment.
Inspired by these findings, Sheppard diversified his WSU breeding stock by buying queens from commercial producers across the U.S and implementing a rigorous selection program — in search of mite resistant blood lines. “Now, every year we choose our best breeder queens based on a criteria such as high honey production, gentleness, disease resistance, suppression of mite reproduction and “hygienic behavior”. Bees with “good” hygiene are somehow able to detect infected larvae and then tear open the cell and clean it out. This behavior has been related to disease resistance as well,” he said.
Judiciously monitoring the various aspects of integrated pest management, Sheppard and Strange noticed that the mite populations had fallen below the economic threshold levels this September. “We’ll have to wait to see if these results are truly related to our program, but it seems we may be close to achieving our ultimate goal, which is to use chemicals as a last resort,” said Sheppard.
Crops depend on beekeepers
Apis mellifera, the common honey bee, is not native to North America. The insects were transported here by early European settlers with a taste for honey that resulted in the establishment of large populations of feral bees. These wild bees helped pollinate most of our backyard gardens and agricultural crops over the years, but today, their numbers have decreased dramatically due to infestations of the Varroa mite as well as widespread pesticide use.
This leaves the fate of about one third of the human diet (which is pollinated by bees) in the hands — and hives — of migratory beekeepers, according to Sheppard. An entire industry thrives on transporting bees around the country, typically beginning with the California almond crop in February. Later in the spring, they relocate to Washington to pollinate the apple and cherry blossoms. From there, the hives are moved to Montana and the Dakotas for the honey crops.
“There are about 2.3 million managed honey bee colonies in the U.S.,” said Sheppard, “and a million of these alone are needed to pollinate the California almond flowers. This past year, there weren’t enough bees to do that.” Reportedly, bees had to be flown in from Australia to finish the job.
Yet there are new developments on the horizon. In addition to the strides being made in genetic selection and cultural methods, a recent breakthrough has been reported by USDA-ARS researchers in Texas and Florida. A fungus was discovered named Metarhizium anisopliae that proves lethal to Varroa mites and harmless to honey bees, according to a recent edition of the Capital Press. In a microscopic version of “what goes around, comes around,” the fungi feed on mite blood, eventually killing them with little chance of the mites ever becoming resistant to it.
This is good news for the nation’s beekeepers who produced 181 million pounds of honey in 2003, according to the National Honey Board. It is also promising for the 130 WSU bee colonies.
“We’re waiting to see if the fungus works out for commercial use,” said Strange. “In combination with other practices, it could be very useful,” and not only for research purposes. Between 2,000 to 4,000 pounds of honey are harvested from WSU hives each fall. Strange said the honey is primarily of three types: blackberry — from research colonies in Puyallup; black locust and sweet clover — from colonies in Pullman. All three types are combined in bulk and sold as wild flower honey which is available for sale at both Ferdinand’s and the WSU Visitor Center. Profits from the sales help support WSU honey bee research.