Varroa Mites and VSH Honey Bees

Dennis Ring, Wilson, Gary K.  |  8/30/2011 12:09:07 AM

Varroa mites first made their presence felt in the United States during the 1980s and presented beekeepers with a tremendous problem. The mites feed on both adults and developing honey bee larva. The feeding may kill the honey bee, deform the bee or may have no visible effects. Although they look healthy, honey bees parasitized by only one mite may have their life expectancy reduced by 50 percent. Feeding on the blood and body fluids may cause the honey bees to be smaller than normal, be deformed or compromise their immune system. Weakened honey bees suffer secondary viral infections than can kill bees. Mites have been shown to transmit viruses, stimulate viruses, and carry viruses.
 
Scientists with the US Department of Agriculture have discovered a low-frequency genetic trait, varroa-sensitive hygiene (VSH), which enables the honey bees to locate and remove mites from hives. The word "hygienic," as it is commonly used, denotes cleanliness. Hygienic behavior in honey bees is a specific response to diseased and parasitized brood. Hygienic bees detect, uncap and remove diseased brood from the combs.

The hygienic behavior was first noted as a natural defense against American foul brood. It is not known what chemical or scent triggers this response. Some work has been done to screen for hygienic behavior using liquid nitrogen to freeze-kill the brood and then measuring the time it takes the bees to react to the damaged brood and begin removing the dead larva. It is thought that these bees are better able to smell the odors associated with varroa-infested cells.
 
When the infested cell is detected, the bees gang up, pierce or chew through the waxy cap, and then eat the parasitized bee and attached parasitic mites or discard them from the brood. One bee sniffs out the infected cell, and then remover bees follow, consuming the contents of the cell. Since mites carry viruses and diseases, it is likely that the bees exude a sickly odor.

The mite offspring are usually disposed of or destroyed during this process, while the mother mites often escape. The adult female will enter another brood cell and try again to reproduce. The bees will continue to police the brood, eliminating new mites and significantly reducing the total mite population. So far results are promising that the varroa mite threat may be controlled without resulting to chemicals.
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