Treating bees with oxalic acid for varroa mites

My neighbor, Nate, is starting out with his first beehive and was concerned about varroa mites. I made a video of us inspecting the hive and treating it with oxalic acid in sugar syrup.

It turned out that the things he thought were mites were actually hive beetles, but he decided to do a treatment anyway since we know there are at least some mites in the hive.

The syrup is made by putting 35 grams of oxalic acid in a 1 liter glass or plastic jar, filling half of the jar with white sugar, and filling the rest of the jar with warm water. (1.2 oz in a quart is the same ratio.) The oxalic acid needs to be weighed out accurately, but the sugar is not so critical. The recommended treatment is 5 mL between each frame in the broodnest with a 50 mL maximum for the whole hive.

Oxalic acid is one of the stronger organic acids. Chemists use the terms strong and weak to describe what fraction of the molecules with dissociate in water to make hydronium ions, which are the characteristic ions of an acid. Stronger acids have a lower pKa value and are more likely to produce solutions with lots of hydronium ions.

The table below lists two mineral acids and several common organic acids along with their pKa values.

Acid	pKa	Found in
Hydrochloric	-6.0	human stomach
Sulfuric	-3.0	car battery
Oxalic	1.27	rhubarb
Malic	1.92	apple
Phosphoric	1.96	cola
Tartaric	3.02	grape
Citric	3.13	lemon
Lactic	3.86	yogurt
Benzoic	4.2	cranberry
Acetic	4.76	vinegar
Carbonic	6.46	seltzer water
Tannic	~10	cocoa

The pKa value of an acid can tell you whether it is a weak or strong acid, but the final acidity of the solution is described by the pH. A solution's pH is determined by the acid's strength and its concentration. This is a little bit confusing because sometimes people use the term "strong solution" when they are talking about a "concentrated solution." When chemists talk about acids as strong or weak they are talking only about the intrinsic property of the the acid molecules, not about the solution's concentration.

Just for fun, I calculated the pH of a solution made with 35 grams of oxalic acid and a total volume of 1 liter, ignoring the the sugar, and it came out to pH = 1.0. You can try the same calculation using an online calculator. A pH of 1.0 is at the low end of the range of normal pH for human stomach acid, so the oxalic acid solution we used on the bees would be unpleasantly sour to taste and would probably burn like hell if you got it in your eye, but it will not dissolve your hands if you get some on you. All the same, it is wise to handle chemicals with care, protect your eyes from splashes, and wash off any that might get on your skin or clothes.

For comparison I calculated the pH of vinegar, which is 5% acetic acid, and found that it has a pH of 2.4. A 30% muriatic acid solution that you might use for cleaning concrete has pH of -1.0.

Given the way that the bees acted when Nate put the solution on them and the modest pH of the solution, I'm not concerned about hurting either bees or humans with oxalic acid treatments for mites. In the long run it is better if we can breed bees and mites that can get along together without requiring any treatment, so we are doing our best to propagate the local feral bees in our area that have very good mite resistance.