Oxalic acid and glycerin for varroa mites
For beekeepers who treat for varroa mites, oxalic acid has become the default favorite miticide. It is inexpensive, a natural component of honey, safe for bees when used as directed, and is drop-dead effective. But being beekeepers, we can’t agree on anything, so the disagreement about how to apply oxalic acid rages on.
The US Environmental Protection Agency (EPA) currently recognizes three ways: spray, dribble, and vaporization. The spray method is often used on packages, but disagreement about dribble vs vaporization for complete colonies continues to fester. Being fundamentally a minimalist, I prefer the dribble method (less equipment, less expense, less danger) but each time I say so, I get trounced by those who thrive on great clouds of toxic fumes. Whatever.
Randy Oliver to the rescue
But now, biologist Randy Oliver offers us hope in the form of a disposable shop towel soaked in oxalic acid and glycerin. In fact, I have received so many questions about Randy’s new system, I’ve decided to write a short summary of his findings. However, I highly encourage you to read his paper in full, which details his methods, results, and statistical analyses. It also contains many photos.
The original idea for dissolving oxalic acid in glycerin came from elsewhere, but Randy took the idea and refined it. He tried various methods of delivery to find the one method that would be safe for bees and beekeeper, deadly to mites, and both economical and quick. So far, his new method has exceeded his expectations and, much to his credit, Randy is now working to get the method endorsed by the EPA.
The basic idea
The original research showed that dissolving oxalic acid in glycerin provided a way to slowly release the oxalic acid over time. Unlike dribbles or vapor where the dose is applied all at once, the oxalic/glycerin mix provides a slow release that is remarkably effective against mites but easy on the bees. Randy has been able to extend one treatment to last about 30 days, which means multiple treatments are not necessary. Mites are killed as they emerge from the brood cells without repeat applications.
Randy has amazing photos of his bees raising brood all around the soaked towels, seemingly unaffected by their presence, yet the mite kill is spectacular. After about 30 days, the bees have removed the entire towel from the hive so the beekeeper doesn’t not have to re-enter the hive to collect them. It’s the closest thing to magic I’ve seen in a while.
The supply list
Here are the supplies used in the experiments. Some are listed in the article and some I assumed. Plastic, glass, or wood utensils would be necessary since metal can be ruined by oxalic acid.
- Oxalic acid dihydrate (wood bleach)
- Food-grade glycerin
- A flat plastic tray for soaking the towels. Randy recommended two 12 x 14.5 x 2-inch InterDesign Refrigerator and Freezer Storage Organizer Trays.
- Blue disposable shop towels. The article doesn’t specify a brand, but Randy says his towels fit nicely into the above trays. Looking over various offerings, I found that Scott 75143 Original Shop Towels are 11 x 13 inches, so I assume something about that size would work.
- A scale for weighing the oxalic acid
- A glass measuring cup for measuring and heating the glycerin and mixing in the oxalic acid
- A plastic or wooden spoon or spatula for stirring
- A glass jar with a plastic lid for leftover solution
- Chemical-resistant gloves (nitrile)
- Protective goggles
How the shop towels were prepared
Randy is quite clear that he is still refining the method, but here are the steps that worked for him in his summer hives. Testing on winter hives is still underway.
- Chemical-resistant gloves and protective goggles are needed when using oxalic acid.
- He used 25 ml of glycerin, 25 grams of oxalic acid, and one shop towel for every hive.
- The shop towels were stacked in a plastic tray.
- The glycerin was heated in the microwave until it was hot but not boiling (about the temperature of a cup of coffee).
- He stirred the oxalic acid into the warm glycerin, mixing thoroughly.
- The warm mixture was poured over the towels in the tray.
- Once the towels were saturated, he removed them to the second tray to drain. Randy says, “Squeeze or press them until you’ve recovered half the solution.” This is important for achieving the proper dose and encouraging the bees to remove the towels.
- The remaining liquid can be stored in a glass jar for later use. Randy warns that the liquid became quite blue.
Once the towels were prepared, he placed one towel across the top bars of the lower hive body of each hive.
Be sure to read the whole article
Remember that this post is my interpretation of what Randy wrote. Be sure to read his .pdf for all the nuances that I may have left out. Also remember that this method is not yet approved by the EPA and is therefore illegal.
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