Controlling Varroa Mites with Oxalic Acid Drizzle Method

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By David Bridgers, North Carolina My research project was a collaboration with N.C. State University and Dr. David Tarpy on the control of varroa mites using the oxalic acid drizzle method.

It is essential that beekeepers control mites for hive health and survivability. I used 62 colonies of two- and three-story nucs for this project. Prior to the starting, one colony was removed due to excessive mite load.

The apiary was divided into four groups of 15 colonies. Each group was painted a different color, alternating every hive to minimize drifting. The control group received no treatment; the second group received one treatment of oxalic acid; the third group received HopGuard II treatment; the fourth group received three treatments of oxalic acid once a week for three weeks.

The oxalic acid was applied using the “drizzle” method. The oxalic acid and HopGuard II treatments were administered according to manufacturer instructions and precautions. Three local beekeepers and an NCDA Bee Inspector assisted with the experiment. In the beginning, we installed a sticky board in each colony. Two days later, we collected the sticky boards and samples of bees by bumping a frame of brood with emerging bees in a plastic tub.

We took ½ cup of bees from the tub and placed them in pint jars. Alcohol was poured into each jar and sealed to euthanize bees and mites. We then shook jars vigorously. We counted the mites on the sticky boards and in the jar.

Four weeks later, we inserted the sticky boards. Two days later, we collected the sticky boards and obtained another sample of bees. We did the final alcohol wash and counted the mites in the alcohol and on the sticky boards.

We concluded the three-time oxalic acid drizzle method treatment group had significantly fewer mites compared to the other groups. Treating once with oxalic acid or HopGuard II was not significantly different than doing nothing to control mites. Those groups had an average increase in varroa mite prevalence during the four-week period. I now do an alcohol wash test monthly to monitor the mite load in spring and summer.

I pick a percentage of hives to check and sample these same hives when testing in the spring/summer. When I find a heavy mite load compared to the previous month, I retest. During this retest, I sample additional hives. When I test again, and if the second test is still excessive, I then treat with an approved method. Keeping a watch on the varroa build-up during the early part of the year is one of the most important things a beekeeper can do.