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Treating with Oxalic Acid

By James & Chari Elam

The topic of “how to treat” for Varroa mites has been one that has dominated bee clubs, beekeeping seminars and online blogs for many years. With so many options and opinions, one would likely be more confused with the flood of information than convinced about one treatment application over another. Our focus today isn’t to cover all of the options, but to educate you on a method that’s very popular because it’s more temperature friendly than anything else on the market. That product is Oxalic Acid.

So, what is Oxalic Acid? OA (as it’s termed) is a naturally occurring acid found in plants known as wood sorrels. You’ll be surprised to learn, this is what most of us think of as common clover or shamrock! As kids, you may remember chewing on them and experiencing a very bitter/sour taste. I guess you could say, it was our homegrown “sour candy!” More common “real foods” that contain OA are spinach, broccoli, Brussels sprouts, grapefruit, chives, and rhubarb. Originally Oxalic Acid became popular for treating Varroa mites in Europe & Canada. It wasn’t until 2015 the EPA approved it for treating honeybee colonies in the U.S. However, it was up to each individual state as whether or not to approve it. Texas did. It’s not entirely clear why OA works so well in treating Varroa Mites, but it's commonly thought that it is absorbed through the mite’s feet and then moves into the mite’s bloodstream, ultimately killing it. Important to note: Oxalic acid only kills phoretic mites. In other words, a mite must come into contact with it for it to work. It cannot penetrate wax cappings and kill mites that are growing inside of a capped cell.

Honeybee on Oxalis (wood sorrel) Photo Credit: Locolobo13 - Photo site Imgur

Treating with Oxalic Acid In order to treat Varroa Mites with OA we must first know the life cycle of the Varroa mite. Without delving too deeply into bee biology, once a queen lays an egg, it’s an egg for 3 days, larvae for 6 days and capped 12 days as a developing pupa = emergence at day 21 (3 + 6 + 12 = 21) The reason this is important to know, is Varroa mite females lay their eggs in a cell just prior to capping. Once capped, the Varroa mite is protected from an OA treatment for another 12 days. What does the previous paragraph tell us? This tells us we need to treat more than once in most cases. Using the numbers above, we realize 12 days is our focus timeline. Therefore, we would divide the 12 days evenly in order to “in theory” treat all emerging mites in a “brood cycle.” It is suggested – 1st treatment (considered day 1), 5 days later treat again and 6 days later do your final treatment. In doing so you effectively treated 12 days. 1 + 5 + 6 = 12 Being that OA kills only phoretic mites, it stands to reason treatments are most commonly done in Fall and Winter while broodless. If you’re considering using OA during brood seasons, some suggest you can cage the queen for the treatment cycle, preventing her from laying and capping any more brood

Female Varroa mites lays their eggs in the cell just prior to capping at day 9

Approved treatment methods Probably the most commonly used among beekeepers is the Vaporizing method.

Oxalic Acid Vaporizers

This method utilizes a heating tool attached to a power source. Once heated the OA reaches a temperature of around 157* causing the solid to become a vapor. This vapor then travels through the hive and effectively kills Varroa mites with little to no damage to the bees. One of the main advantages to this method is that you don’t have to open the hive making it a good alternative for treating in the fall and winter! In the video attached to this article, the Vaporizing method is 3rd in the series of demonstrations shown.

Another method used is the Dribble Method. This method is just as the name implies. The Oxalic Acid is mixed in a solution and then “dribbled” between the bars in a hive with a syringe, coating the bees and effectively killing the phoretic Varroa mites. Although contact with the solution doesn’t harm the bees, some will likely consume small amounts which can cause some mild harm. Because of this, the dribble method is used in a “single treatment” application only and like all methods, done during a brood break or broodless cycle. This method is shown as the 1st demonstration in the video.

Last of the delivery methods is the Spraying Method. This method is done for package bees only. Obviously, the bees are broodless in this form and easily treated in this manner. This is probably the simplest of the methods in that all that is required is a spray bottle, Oxalic Acid, and protective gear. Watch instruction for this method, 2nd demonstration in the video tutorial.

Now you know the different methods to deliver Oxalic Acid – which is the most effective and showed the lowest bee mortality? A study done by Sussex University examined 110 hives using all three application methods and the results showed sublimation (Vaporizing) method was “far better at reducing mite population and showed no increase in bee mortality.”

Safety Measures a MUST! We can’t stress enough that even though OA is an organic treatment method for honey bees, it IS harmful to humans if we don’t take the proper precautions while using it. ALWAYS protect your skin, eyes, and lungs from Oxalic Acid. Wear rubber gloves, safety goggles, ventilator mask and a long sleeve shirt or bee suit while using it.

It goes without saying…but we’re going to say it anyway - If you have a severe reaction such as breathing difficulty call 911 immediately! Another safety awareness point to be made, is don’t mix treatments! In other words, we wouldn’t use more than one treatment product at a time on our bees. Doing so could harm our bees and after all the reason we’re treating is to help them, right?

The take away on this segment – OA is considered a safe and organic method of treating Varroa mites when following the directions and with safety in mind!

How effective is Oxalic Acid? OA is very effective! For the Vaporizing method treatment, efficacy can exceed 95% and even higher in the dribble or spray methods. However, as with any Varroa treatment, the overuse of a product will eventually result in the Varroa mites building a resistance to it. This is one of the reasons why we always say, “We don’t take antibiotics unless we are sick, right?” Same goes with mite treatments. ONLY treat if you have tested and treatment is warranted.

Storing Oxalic Acid: Dried, unmixed OA should be kept in a cool dry place and will not expire. A mixed solution however can only last up to a week at room temperature and a few months refrigerated.

If any discoloration or a funny smell start to appear, throw it away. This means an “alternate chemical” has formed (hydroxymethylfurfural) which is toxic to bees. It’s really important to point out that ALL Varroa treatments run some risk to our bees. BUT so do Varroa mites! Regardless of which method you use to treat, we can’t emphasize enough to educate yourself on how and when to use it.

By following directions, you not only insure you’ve given your bees the best possible chance for survival, but you’ve protected yourself and your bees from possible harm. For more information on Varroa Mites, treatment options, and videos on how to test and treat with all options go to:

Oxalic Acid

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