Contacts:
Marla Spivak, University of Minnesota Entomology Department,
(612) 624-4798
Deane Morrison, News Service, (612) 624-2346
'Hygienic' Bees Resist Mites
Since its introduction to the United States in 1987, the huge Varroa mite has marched through the American honeybee population like Sherman through Georgia. But a University of Minnesota entomologist has found that a line of "hygienic" bees can defend their hives against the mite and also against two common honeybee diseases, a finding that spells hope in the face of a 100 percent Varroa infestation rate in hives nationwide. Marla Spivak describes the hygienic bees' defenses in field experiments in the current issue of the journal Apidologie, and in a talk to the American Bee Research Conference Friday (Jan. 10) in Memphis, she will describe the bees' success against the mite in commercial hives.
"The hygienic trait occurs naturally in all honeybees, so the behavior can be found and selected from any race or line of bees," Spivak said. "Hygienic bees are also gentle and good honey producers." Last winter, half the hives in the country succumbed to Varroa mites, other pests, cold weather or a combination of factors. The usual mortality is about 10 to 20 percent, Spivak said.
Breeding hygienic bees holds potential for controlling a severe pest without resorting to risky and sometimes ineffective chemical treatments. Here's how the Varroa mites operate and how the hygienic bees foil them. The adult (female) Varroa mite rides around on bees, sucking its bodily juices. Big enough to easily show up in pictures of infested bees, the mite hops off when she is ready to lay eggs. She seeks out a cell where a bee larva is preparing to pupate and gets in. Soon the worker bees seal the cell with wax to let the pupa complete its development, and the mite feeds off the pupa and lays eggs.
When the eggs hatch, the brother and sister mites mate; the males die, and the females leave the cell with the pupa when it hatches as an adult bee. The females will lay the next generation of mite eggs in new cells. Hygienic bees, however, can detect the mite in sealed pupal cells. They respond by throwing out the pupae, mites and all. But just how well do they perform the task?
To answer the question, Spivak began by screening bee colonies at the University of Minnesota for hygienic behavior by freezing 25-square-centimeter sections of honeycomb to kill the pupae inside. Colonies that removed the dead pupae within 48 hours were deemed hygienic.
Spivak raised queens from hygienic colonies and artificially inseminated them with semen from drones of separate hygienic colonies. She then tested hives of the resulting offspring for their degree of hygienic behavior by slipping mites into some pupal cells, resealing the cells and watching the bees' response. She also tested nonhygienic colonies. In experiments in 1994, she found that hygienic bees removed 70 percent of infested pupae while nonhygienic bees removed only 15 percent. In 1995, there was no difference in the abilities of hygienic and nonhygienic bees, but in 1996, the hygienic bees repeated their 1994 performance.
What happened in 1995? For one thing, Spivak upped the infestation that year by introducing two mites to some pupal cells rather than one per cell as in 1994.
"Infestations were very high, so maybe the bees were habituated to the odor of infected pupae," said Spivak, who thinks that smell may be the key to hygienic bees' ability to detect infested pupae. "We're repeating the experiment in 1997 to see if hygienic bees exhibit this behavior three years out of four."
In her talk Friday, Spivak will describe the performance of hygienic bees in commercial apiaries. In that study, hygienic queens from Spivak's stocks and top-quality nonhygienic (commercial) queens from beekeepers' stocks were allowed to mate naturally with local drones in Texas.
The queens were then taken to Wisconsin, where they produced hives from hygienic queens and from commercial queens. The hives were distributed among four apiaries, with each getting hygienic and commercial colonies in close to equal proportions. Spivak checked the hives for the presence of chalkbrood, a fungal disease of bee larvae; American foulbrood, a very contagious bacterial disease; and Varroa mites. In all four apiaries, the commercial bees had significantly more chalkbrood and foulbrood. In three of the four apiaries, the commercial bees had significantly more Varroa mites. In one apiary, the hygienic bees had more mites.
Overall, a very good record, said Spivak, who plans future investigations into the hygienic bees' abilities as well as more field tests in commercial hives. "I'm trying to determine the cues bees use to identify mites," she said. In preliminary tests, one of her graduate students found that hygienic bees could discriminate the odor of chalkbrood pupae from that of live pupae much better than could nonhygienic bees.
This summer they will test bees' ability to distinguish the odor of Varroa-infested pupae from the odor of normal pupae. In 1995, the U.S. honey industry produced $135 million worth of honey from 2, 647,000 hives, according to Tom Kruchten, agricultural statistician with the U.S. Department of Agriculture.
News releases also on WWW at http://www.umn.edu/urelate/news.html
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