Oxalic acid as a Varroa treatment first caught my attention two years ago when beekeeper Mark Luterra showed me a photo of his bottom tray after treatment. What an incredible display of dead bodies! I was intrigued.
Later, I followed up with some research, mainly from Randy at ScientificBeekeeping.com. From him I devised a treatment plan that is simple, inexpensive, and works well.
But first, what is oxalic acid? Basically it is an organic (carbon-containing) compound that is found in nature. A number of foods we eat are rich with oxalic acid, including spinach, swiss chard, rhubarb, beet greens, kale, sorrel, and chocolate. In fact, there is much speculation that the “spinach effect”—that weird mouth feeling some people get after eating spinach—is actually caused by oxalic acid. And we’ve all heard that rhubarb leaves are poison. The reason? Oxalic acid.
Since oxalic acid is found in nature, and because it is a normal component of honey, oxalic acid is considered a “natural” treatment. In fact, even Certified Naturally Grown beekeeping allows the use of oxalic acid for the treatment of Varroa. Oxalic acid is commonly sold as “wood bleach” and can be found in hardware and paint stores. The type I use can be found here: Savogran 10501 Wood Bleach.
However, oxalic acid in the form that works to kill mites is a potent acid, so care must be taken to avoid causing harm to your bees and yourself if you decide to use it. You should begin by reading the new draft label so you know how to handle the acid and how to protect yourself from splashes and spills.
Oxalic acid can be applied as a dribble, a spray, or a vapor. Since I am a hobby beekeeper with a small number of hives, I prefer the dribble. Personally, I don’t want to buy, clean, or store vaporizers or sprayers, so I’m happy with a box of disposable syringes that I bought online for the purpose. The KISS method works for me, especially in this case.
If you use the oxalic acid dribble method, you will need a canister of wood bleach, a syringe that holds at least 50 ml, a small scale that can measure in grams (tenths or hundredths of grams is best), a standard measuring cup, sugar, and a non-reactive container for mixing. Your wood bleach should be between 95 and 100 percent pure. If you don’t know, you can search the web for the MSDS (Material Safety Data Sheet) for your particular brand and it will tell you.
The other item you will need is soft water. Soft water is an excellent solvent, but when the water is filled with hardness minerals (chiefly calcium and magnesium) it tends to form deposits or precipitates instead of dissolving things. If you see a white substance in the bottom of your container after you mix in the oxalic acid, you should toss that batch and try a different source of water. If you don’t have a water softener, you can use distilled, deionized, reverse osmosis, or even rain water.
According to the new EPA label, you need to mix 35 grams of oxalic acid dihydrate crystals into one liter of 1:1 syrup. (This is the same as Randy Oliver’s weak solution, and the one I’ve been using.) You can make a liter of syrup by using 600 ml of water and 600 grams of table sugar.
Time of application
- Because oxalic acid will not kill Varroa in capped brood, I like to apply oxalic acid at times when little or no brood is present but before it is crazy cold outside. For me, this is late fall.
- Treating once per year at the right time may be enough because this system knocks Varroa down to almost nothing.
- Measure 600 ml of hot water into a non-reactive container.
- Add 35 grams of oxalic dihydrate crystals (wood bleach) into the hot water. Stir but do not shake.
- When the crystals are dissolved, add the 600 grams of sugar. Stir until dissolved.
Apply the solution
- Smoke your bees down between the frames.
- Dip the end of your syringe into the medicated syrup and pull back the plunger, filling the syringe to the 50 ml mark.
- Starting at one end of the frames, dribble 5 ml of the solution along a seam that contains bees. (I like to start at the far end and dribble toward me.)
- Once you have dribbled 5 ml, you must go on to a new seam. (A seam is the space between two frames or the space between a frame and a sidewall.)
- After each seam of bees gets 5 ml of solution, you are done.
- In any case, you cannot go over 50 ml per colony. If the hive has more than 10 seams, dribble where the most bees are. Alternately, you can give less than 5 ml per seam and do more than 10 seams, but you cannot go over 5 ml in any one seam or 50 ml total per colony.
- Remember to apply the mixture directly onto the bees. Mixture that lands on the woodenware will be ignored by the bees and not moved throughout the colony.
I strongly suggest that you practice dribbling with plain syrup in advance. The first time I did a test, I squirt syrup half way across the room. Seriously, it takes a little skill to get the hang of moving the syringe along the seam while gently pressing the plunger. Also, practice reading the graduations. My syringes are marked 10, 20, 30 and so on with five divisions between each one, so five ml is 2.5 divisions. I use this type of syringe: Syringe 60cc Luer Lock Tip Sterile (Pack of 10).
Be sure to use 1:1 syrup for your trial runs because plain water behaves differently. I also recommend putting 5 ml (1 teaspoon) of syrup in a dish so you can see what it looks like.
Once you get a feel for it, you will find that moving quickly along the seam is easier than moving slowly. Also, watching the drip end seems to be easier than watching the graduations once you learn how fast it comes out. Five ml doesn’t seem like much when it’s whiskey, but this stuff is different.
So there you have my method. If you want to use a vaporizer or sprayer, I strongly suggest you read the label and Randy’s site for the best information.
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