Oxalic acid (OA) is a natural compound found in various plant sources and is also produced synthetically.
It has been used very extensively to combat Varroa destructor mites in Europe and many other countries, with records of it being used in the early 1980s in Japan (Nanetti, Büchler, et al. 2003).
Ellis and Aliano found that OA is about 70 times as toxic to mites as it is to adult bees.
Mode of Action.
Oxalic acid kills mites through a combination of physical and chemical mechanisms: -
- Contact Poisoning: When oxalic acid comes into contact with the mites, it can penetrate their exoskeletons and interfere with their cellular processes. This disrupts the mites' ability to regulate water, causing them to dehydrate and die.
- Acidification: Oxalic acid is an organic acid that can lower the pH of the mites' body fluids, disrupting their internal environment and leading to metabolic dysfunction. This disruption can be lethal to the mites.
- Oxidative Stress: Oxalic acid can induce oxidative stress in the mites' cells. Oxidative stress occurs when there is an imbalance between the production of reactive oxygen species (ROS) and the mites' ability to detoxify them. This can damage essential cellular components, ultimately leading to cell death.
- OA contact with the empodia of the mites will cause destruction of the cellular membranes, and absorption into the haemolymph.
- Disruption of Calcium Regulation: Oxalic acid can form insoluble crystals with calcium ions. Mites rely on proper calcium regulation for various physiological processes. The formation of these crystals interferes with the mites' ability to maintain calcium homeostasis, leading to dysfunction and death.
- Respiratory Inhibition: Oxalic acid can interfere with the mites' respiratory system by affecting the tracheal system (the equivalent of lungs in insects). This disruption can hinder the mites' ability to breathe properly, leading to suffocation and death.
About Oxalic Acid.
There are many common vegetables that contain oxalic acid or oxalates.
The natural concentration of oxalic acid (oxalates) based on fresh weight in spinach is 0.3-1.2%, in rhubarb 0.2-1.3%, in tea 0.3-2.0%, and in cocoa 0.5-0.9% (Fassett 1973).
A standard serving of Spinach is 75 – 100 grams per person. That equates to 0.75 – 1 gram of Oxalic acid consumption.
For the following reasons, oxalic looked like a sure winner for mite control: -
- It’s a natural part of our diet, so we have a metabolism to deal with it.
- It’s a natural part of the environment, so we’re not going to be poisoning anything but the mites.
- Since it is not lipid (fat) soluble, it will not build up in the wax of the combs.
- It is a natural component of honey, and treatment with oxalic does not appreciably increase the concentration of it in honey (Brødsgaard, 1998).
Do Oxalic Acid treatments cause excess residues in honey?
No. See info below.
Oxalic acid is a natural constituent of honey; values between 8 and 17,000 mg/kg have been found in different honeys (Mutinelli et al. 1997; Bernardini and Gardi 2001; Bogdanov et al. 2002).
The application of oxalic acid-based solutions (Bienenwohl or a self-prepared oxalic acid solution with sugar) to control Varroa destructor resulted in no relevant changes in the oxalic acid content of honey produced the following year, compared with honey samples from untreated colonies from the same location. The range of oxalic acid content in honey was 5–68mg/kg in oxalic acid treated and 5–65mg/kg in untreated colonies. The oxalic acid content of the honey was positively correlated with its electrical conductivity and thus with its original nectar or honeydew source.
Indeed, there is no risk of honey residues after all types of oxalic acid treatments.
(Radetzki 1994; Mutinelli et al. 1997; Del Nozal et al. 2000; Bernardini and Gardi 2001; Radetzki and Barmann 2001; Bogdanov et al. 2002).
A very comprehensive petition sent to United States Department of Agriculture: -https://www.ams.usda.gov/sites/default/files/media/OxalicAcidTR.pdf
Maximum Residue Levels. (MRL)
There are no MRLs of Oxalic Acid imposed on honey in New Zealand or USA.
Oxalic Acid Extended-Release strips. (OAE)
This involves the use of a special cellulose fibre matrix to absorb the Oxalic/Glycerine solution.
The cardboard Beequip Strips, as sold by Beequip NZ, are very commonly used for this application in NZ and other countries.
The strips are suspended over the brood frames for the bees to walk on and carry the acid around the hive.
As the solution gets used up on the surface, more glycerine/OA solution flows to the surface from within the strip.
The glycerine is an excellent carrier and allows the solution to stick to bees, so it is spread throughout the beehive.
A food-grade plant-based glycerine is used during the manufacture of oxalic acid extended-release strips.
Glycerine is used extensively in the food manufacturing industry.
Glycerol is a natural constituent in honey.
Randy Oliver Field Data for OAE.
In more recent years, a lot of trials have been conducted worldwide with OAE.
Randy Oliver is a biologist and ex commercial beekeeper with the website https://scientificbeekeeping.com/
He has a lot of work documented on the website about varroa control and OAE. Most likely more than anyone else.
He is not selling any products, and he publishes the data from field trials exactly as it is.
Click here to see the results of a very extensive field trial in 2023 by Randy Oliver.
The Beequip Strips were the only matrix that he used in a staple shape over the frames. He emphasises that this must be done in single brood boxes. The other matrix’s will not work if placed horizontal over a single brood box.
See another test that Randy Oliver did in 2022:-
2022 Extended-Release Oxalic Update Part 2 https://scientificbeekeeping.com/2022-extended-release-oxalic-oae-update-part-2/
It has a section on Beequip Strips. Randy calls them chip board strips.
Some beekeepers think that OAE strips are not successful in geographical areas of high humidity.
Randy Oliver’s answer to this: -
My answer to these opinions is that the bees work hard to control the humidity in the brood nest.
Outside weather conditions have some effect but not that much.
The feedback that I get from U.S. commercial beekeepers in high-humidity areas -- both warm humidity of the East Coast, and cool humidity in the Pacific Northwest -- is that OAE works great for them.
It also works great in countries in Latin America and Europe.
What we do see, is some degree of year-to-year and yard-to-yard variation -- something that is commonly observed with any varroa treatment.
Can Varroa Mites get resistant to Oxalic Acid?
Currently, there is no known varroa resistance to OA globally. Some scientists say that they don’t believe it is possible for varroa mites to get resistance to OA.
A field trial showed that varroa mites exposed continuously to Oxalic Acid over 8 years were more susceptible to OA treatment than mites that had never had contact to it.