ScienceDaily (Apr. 29, 2005) — Commercial apiarists and state bee inspectors now have a fast new way to check Varroa mites for this honeybee parasite's resistance to the pesticides coumaphos and fluvalinate.

Agricultural Research Service (ARS) entomologists in Beltsville, Md., have developed a "do-it-yourself" bioassay that determines, within six hours, whether Varroa mites are fully resistant to the pesticides, are approaching resistance, or are still vulnerable.

Varroa mites are blood-sucking parasites of honeybees that can weaken or destroy hives. Continuous use of coumaphos and fluvalinate to prevent such damage has prompted the emergence of resistance among some Varroa populations, according to Jeffery Pettis, in the ARS Bee Research Laboratory at Beltsville.

In studies there, Pettis and other ARS entomologists sought to devise a faster, cheaper and more user-friendly alternative to current methods of checking for pesticide-resistant Varroa mites. These methods are labor-intensive affairs that require specialized equipment and the shipping of mite-infested bees.

The ARS scientists' bioassay is intentionally low-tech. Its main parts include glass canning jars in which to contain honeybees, mesh lids through which mites on the bees can fall out and be counted, and index cards that hold strips of either coumaphous or fluvalinate.

A mathematical formula determines the mites' resistance levels or susceptibility to the pesticides. For example, if the chemicals kill 25 percent of the mites, then the parasites can be considered fully resistant. However, if more than 50 percent are killed, then the mites are still vulnerable to the pesticides. This means the pesticide treatments should still be effective against the mites.

According to Pettis, some state bee inspectors have already used the bioassay to document mite resistance in applying for emergency-use exemptions on alternative control products. They're also using it to monitor the spread of pesticide-resistant mite populations.

Varroa mites are a major threat to honey bee health and are becoming resistant to two compounds (coumaphos and fluvalinate) used to control them. Beekeepers now have a simple assay to determine whether mites are resistant and thus ensure use of appropriate control measures. (Photo by Stephen Ausmus)

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