Follow-up question about oxalic acid post

This is a question, and my reply, that I received as a result of my June 20th post regarding the registration of oxalic acid as a varroa mite control product. I appreciate the question, as I have been hearing a great amount of discussion among beekeepers on how to avoid purchasing a labeled product by mixing up the chemical themselves.

A beekeeper in Kentucky writes,


Myself and others have been referred to to purchase oxalic acid, since the bee supply companies can’t sell it in most states yet. The problem is, CAS# 144-62-7, or Oxalic acid dihydrate (the crystalline form), not wood bleach, is marketed online as being 99% pure. So I was very surprised to read from you that the approved solution for bees is 3%. I don’t think very many people are aware of that. Any suggestions?


Phil’s reply

I am sure that a number of U.S. beekeepers have been using oxalic acid for years, just as many have been using (and I’m certain some still do) homemade formulations of formic acid. Though purchasing and using non-labeled forms of either chemical for varroa mite control is illegal, the employees of state pesticide departments are very busy monitoring the application of legal pesticides, and it is rare for a beekeeper, especially a small scale beekeeper, to be cited for what we call off-label pesticide use. However, the odds of getting caught are not the only consideration.

The issue you inquire about, of getting chemicals in a proper, safe, and effective concentration, is one of the major reasons for pesticide labeling regulation. Before a product can be registered, a great deal of research goes into determining the optimum concentration and method of application for both safety and effectiveness. I have heard horror stories of beekeepers burning off the tips of their fingers or damaging their lungs while handling full strength formic acid. If pesticides in too strong a concentration can be that dangerous for beekeepers, imagine what it could do to your bees. On the other hand, too low a concentration will be ineffective at killing mites. While some beekeepers may dilute formic acid properly, I’m certain some do not.

I wasn’t sure how hazardous oxalic acid is compared to formic acid, so I consulted with a friend who is both a university chemistry professor and a beekeeper. Here is his response:

I have worked with both formic and oxalic acid in the past.  Oxalic acid is somewhat safer than formic acid, but both are dangerous.  In the solid, crystalline form, oxalic acid would not be a problem, but in either concentrated form and in powder form, it can be nasty.  It is pretty reactive with proteins and any nitrogen-containing compounds.  It can be used to cross-link proteins, for example.  On the other hand, there are many enzymes that readily inactivate it.  When working around it, in either concentrated form or in powder applications, I would wear face-mask and gloves, and not just safety glasses.

As you can see, it’s important to know what form of the chemical you’re dealing with. The crystalline form, which you asked about, poses less risk to the user, but it still must be diluted precisely to ensure the demise of the tiny mites without harming their only slightly less tiny hosts.

You asked for my suggestion. My advice is to wait for the registered product to become available through Brushy Mountain. You may decide the cost saving is not worth the risks of do-it yourself pesticide formulation. If you do decide to purchase and dilute raw oxalic acid, you should seek the advice of a qualified chemist. I do not even know how to make a proper dilution – whether by weight or volume – and the results could vary greatly.

I’d like to offer a last comment regarding wood bleach. I know that’s not the specific form of oxalic acid you wrote about, but I have been hearing a great deal of discussion about it among beekeepers. It may be more problematic than I had suspected. Oxalic acid is the primary ingredient, but I have been told that it can also contain inert ingredients such as peroxide, sodium hydroxide and/or hypochlorite, all of which may be harmful to bees. I am still researching this issue, and will follow-up when I have more information.

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