Oxalic acid treatment is an effective method of reducing the number of Varroa mites in honey bees. In this in-depth article, with the help of the experts of the OXALIKA team, we will review the current knowledge and research on oxalic acid treatment. Its effectiveness against Varroa mites, and we will also check some valuable tips to use it correctly and avoid problems for your hives.
What is oxalic acid?
Oxalic acid is a natural compound found in many plants and produced by the metabolism of animals. It can also be synthesized in the laboratory. It is also a component of the bile and sweat of mammals, including humans. However, this must not mislead us. In fact, despite being a natural compound also present in various foods, including honey, the use and vaporization of oxalic acid present some risks. Oxalic acid is toxic to humans. The damage it causes can be severe, even fatal, and it is, therefore, necessary to protect your skin, eyes, and respiratory tract during its use.
Oxalic acid as a Varroa control method
Oxalic acid works by killing parasitic mites, with limited adverse effects on bees, when used correctly. This treatment is effective because oxalic acid penetrates the mite tissues and disrupts cell metabolism. Some synthetic drugs (e.g., Apistan® and Bayvarol®) are often used against Varroa of honey bees. Still, unfortunately, these drugs, in addition to developing resistance in mites, leave residues in honey and wax, and in some cases, they can even be dangerous to human health. Unlike these drugs, properly used oxalic acid leaves no residue and so far does not appear to have promoted resistance in Varroa.
Oxalic acid in organic production
Oxalic acid is considered a biological control method for Varroa. Several scientific studies have proven its effectiveness as a Varroa treatment, and it is generally considered safe for bees when used as directed. Be warned that any treatment applied incorrectly can cause harm to your bees, and oxalic acid is not exempt from this warning. Using oxalic acid in the hive is generally safe for bees as long as you follow the manufacturer’s instructions and use specific procedures and precautions.
Use on bees
The treatment of honey bee colonies with oxalic acid has been used in various parts of the world since the 1980s (the USA approved the use of oxalic acid in 2015). In some countries, the law may not consent to the use of oxalic acid yet, or it may be allowed only in specific formulations. Every beekeeper needs to check the legislation in their country before administering oxalic acid to their bees. Oxalic acid can be applied mainly in two ways:
- As a solution of sugar and oxalic acid to drip onto the hives;
- Employing vaporization (sublimation).
Oxalic acid can also treat “packaged” bees before shipping them to customers. Varroa-infested “packaged” bees are a big problem for beekeepers, and using a spray with a solution of sugar and oxalic acid is a popular way to tackle this problem.
Effectiveness of oxalic acid treatment against Varroa mites
The efficacy of oxalic acid treatments is still the subject of much research. A study carried out in England in 2016 showed that sublimation is superior to other application methods, achieving higher Varroa mortality at very low dosages of oxalic acid (0.56 or 1.125 g, per colony):
- Drip method: 20.57% mortality;
- Spraying 25.86%;
- Sublimation 81.97%.
Furthermore, with a higher dosage of oxalic acid, 2.25g per colony, the mortality rates of Varroa are between 93-95% for all methods, up to 97.6% with sublimation.
Oxalic acid sublimation (vaporization) results in lower worker bee mortality (between 3 and 12 times) in the ten days following application, compared to dripping or spraying, and colony mortality below mid-spring. Colonies treated by sublimation also showed a larger brood area four months later than those treated by drip or spraying compared to untreated control colonies.
Safety of Varroa treatment with oxalic acid
Oxalic acid treatment is generally considered safe for honey bees, but some precautions exist. In particular, treatment should be performed when temperatures are lower, and bees are less likely to be re-infested with Varroa. In addition, it is essential to follow the manufacturer’s instructions carefully and ensure that oxalic acid does not come into contact with your skin or eyes.
In addition, oxalic acid should preferably only be used in late autumn or early spring when brood presence is reduced or at other times of the year after brood lockout. Oxalic acid can damage the brood of bees and not reach the mites inside the closed cells. Oxalic acid can also be used as a “cleansing” treatment for Varroa after applying a different acaricide where varroa infestations remain problematic.
Beekeepers using oxalic acid must wear:
- Long-sleeved shirt and long pants;
- Socks and shoes;
- Chemical resistant gloves;
- Safety goggles;
- Respirator with N95 or N99 particulate filter and a cartridge suitable for organic vapors.
Limitations of oxalic acid treatment for Varroa in honey bees
As we have seen, using oxalic acid treatment to control Varroa mites may have some limitations. In particular, the cure may be less effective in some conditions, such as in the presence of a brood. Furthermore, oxalic acid treatment is not recommended for bees during the larvae growth and development phase, as it may interfere with the normal development of the larvae.
Another necessary precaution: remove the supers before applying oxalic acid and wait 1-2 days before reassembling them. This precaution allows any residual oxalic acid to be eliminated from the hive, thus avoiding contamination of the honey.
Mitigation of the adverse effects of long-term treatment with oxalic acid
As with any mite treatment, there may be an inherent risk associated with excessive use of oxalic acid. This risk is due to Varroa’s ability to develop resistance to any prolonged treatment. Oxalic acid remains a popular and widely effective treatment when used intelligently. For this reason, use oxalic acid wisely in an integrated pest management plan.
An integrated control plan consists in varying the use of treatments against mites over time. Varroa will be less likely to develop resistance by changing and rotating the various treatments used.
When to do oxalic acid treatment
One correct and safe approach is to use oxalic acid when there are few Varroa mites within the brood cells. For this reason, oxalic acid is often applied in late fall or winter and after brood lock. Varroa present in the hive during these times is an easy target for the treatment. Estimates vary, but you can expect approximately 90% efficacy in eliminating exposed Varroa phoretic.
A common practice, in the presence of broods, is to administer repeated doses of oxalic acid by sublimation within a few days of each other.
In this regard, one study conducted in 2020-2021 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8825467) showed how the administration of 1 gram of oxalic acid every five days for a total of 7 treatments, although well tolerated by bees, does not lead to a reduction in the number of varroa presents but merely prevents their increase.
Another study, also from 2021 (https://www.researchgate.net/publication/349318139), showed that it is possible to obtain a significant reduction in the number of Varroa by administering 4 grams 3 times seven days apart, even in the presence of brood.
It would be helpful to clearly understand the long-term effects on bees due to administering such high doses of oxalic acid.
In conclusion, the advice we give to everyone is:
- Administer the doses recommended by the drug manufacturer or imposed by the state’s laws in which you operate.
- To obtain greater efficacy, administer oxalic acid only when the brood is absent or decreasing in an integrated pest management strategy consisting of alternating treatment types during the year.
- Consult the guidelines of specialized associations in your area for treatment methods and thresholds to limit the risk of new infestations.
Methods of administration of oxalic acid against Varroa
As mentioned earlier, there are several ways to apply oxalic acid to your hives. The two most popular options are drip and sublimation (vaporization) methods. Let’s see in detail how to do them correctly.
This approach involves using a veterinary syringe to drip the oxalic acid mixture onto the bees between the hive frames. There are several recipes to prepare the mix, in which the amount of water, sugar, and oxalic acid is variable. In some countries, the quantities of acid, water, and sugar are set by law.
We advise beekeepers to follow the formulation recommended by the manufacturer of the drug permitted in the country in which it operates.
By way of example, a typical formulation that allows you to treat about ten hives is the following:
- 300 ml of water
- 300 gr sugar
- 30 gr oxalic acid
Another example with a lower concentration of oxalic acid is the following:
- 300 ml of water
- 300 gr sugar
- 17 grams of oxalic acid
Materials for applying oxalic acid by dripping
To use oxalic acid by the drip method, you will need:
- A mixing container (do not use food or drink containers);
- A scale for measuring oxalic acid;
- A syringe;
- A plastic or wooden stirrer;
- Protecting clothes, N95/N99 particulate mask, antacid gloves, protective goggles;
- Hot water;
Here is an example of how to prepare and administer a mixture suitable for ten hives
Preparation and application:
- Weigh 30 grams of oxalic acid and place it in the mixing container;
- Add 300 ml of warm water and mix until the oxalic acid crystals are completely dissolved (do not shake);
- Add 300 g of sugar and mix again until the sugar has dissolved;
- Close the container hermetically and label it clearly (e.g., “Warning Oxalic Acid”);
- We strongly recommend that you also bring water with you so that you can quickly dilute any spilled mixture you may accidentally come into contact with;
- Put on your protective clothing;
- Draw 50 ccs of the mixture into your syringe (this is the maximum dose for each hive);
- Drop five ccs of the mixture into the space between the frames occupied by bees.
Generally, the drip method is considered more suitable for a limited number of hives, as it requires more time and weather conditions ideal for opening the hives. It is particularly recommended after the block of summer brood as a buffer treatment as, compared to sublimated use, it does not require the same levels of protection for the operator, which, given the high ambient temperatures, would be rather uncomfortable. Furthermore, by opening the hives, it is possible to take advantage of a general check of the colonies.
Sublimation method (vaporization)
The sublimation method of application is prevalent and has several advantages. When heated to the proper temperature, solid oxalic acid melts and evaporates. In contact with the air, it forms a mist of tiny crystals deposited on the bees and the internal parts of the hive.
A suitable oxalic acid sublimation device must allow you to obtain proper heating, effectively fumigating the hive. A key advantage over dripping is that you can apply the treatment without opening the hive. This detail can be crucial if you are treating in the fall or winter when low temperatures may not be favorable for the hive to open. In addition to avoiding disturbing your bees, using particular sublimation device models, the sublimation is considerably faster than the drop approach. Some research also suggests vaporization is less harmful to bees and broods than dripping.
On the other hand, the presence of oxalic acid vapors means that you must be careful to ensure you don’t inhale the fumes. For this reason, it is essential to use a suitable mask and other personal protective equipment.
For information and instructional videos on the correct use of OXALIKA sublimation devices, you can visit this page: https://www.oxalika.com/how-it-work/