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Oxalic acid treatment and vaporizer

Recently I bought a new oxalic acid vaporizer. I have used vaporizers before, and often to very good effect, but the treatment method does have its drawbacks sadly, which I hope to have been largely addressed with this new model.

To my knowledge, Oxalic acid vaporization has originally been used in the Soviet Union (that’s how old that method really is) after the Varroa Mite appeared there somewhen during the 80s. Western beekeepers in contrast moved towards synthetic miticides. Their efficiency is waning however, and costs for treatments are going up. Beekeepers are desperate for alternative treatments.

What is oxalic acid and how does it work against Varroa Mites?


Oxalic acid is a so-called organic acid, and naturally occurs in many plants as well as vegetables (Rhubarb, Spinach, Carrots…). In more concentrated form it is a relatively potent acid and can be used for applications like bleaching wood or removing algae from boat hulls. Since it is relatively abundant in nature (and foods) it is not particularly toxic (but it is not completely harmless either).

I have to add the obligatory safety advice: You are dealing with an acid here, wear safety glasses (you really do not want it in your eyes), and gloves! For vaporization and spraying tiny droplets or fine dust is created which can easily get into your lungs. Theoretically this can cause damage, OA is also very irritating, and you will immediately know if you have inhaled some. Definitely wear respiratory protection. Safety data sheets will typically ask for a respirator to be worn so that is what you should do. I personally (being careless) get away with wearing a normal face mask, that protects against dust.

How does oxalic acid treatment work?

How oxalic acid kills Varroa mites is a subject of ongoing research, but the most likely mechanism is that it acts as a contact poison. I will try to keep this as simple as possible:

Honeybees are covered by an exoskeleton and any dry oxalic acid crystals cannot permeate through it to enter their hemolymph (their “blood”). This makes Honeybees essentially immune to dry oxalic acid. Note that any oxalic acid solution in water will be able to dissolve (how fast depends on the concentration) the exoskeleton and harm the bees that get in contact with it, but this is only relevant for dribbling.

Varroa mites are covered by such an exoskeleton too, but they have an “Achilles heel”. Their feet have microscopic moist “sticky pads”, and if they walk over any oxalic acid crystals these pads will absorb some of that acid, which will allow it to get into their hemolymph essentially poisoning them.

Essentially if you can make the Varroa mites walk over tiny crystals of oxalic acid they will die, while said crystals are totally harmless for bees.

Thus, the aim for treatment of a beehive with Oxalic acid is to finely distribute as many tiny oxalic acid crystals within the hive (and on the bees themselves) as possible, to ensure that the mites walk over them and die. Every single treatment method is trying to achieve that, with different means.

What is oxalic acid dribbling?

Dribbling, does use a syringe to dribble a few millilitres of oxalic acid-sugar solution (around 3.5% concentration of OA) onto the bees directly. Note how we mentioned earlier that dissolved OA will actually be harmful to bees? That is the case here, but generally you will only “hit” a few hundred bees with the dribbles and not cause too much of an issue if you do not overdo it. Due to the sugar the solution is sticky, and the bees will start to clean themselves and get it off of them. This behavior will help spread the oxalic acid within the cluster of bees. Since it always is warm in a beehive the water in the solution will eventually evaporate leaving behind many crystals of dry oxalic acid, distributed within the hives by the bees themselves. Dribbling can be done reasonably quickly, but does have the highest amount of adverse effects on the bees, of all the oxalic treatment methods, which is why I personally do not use it.

What is oxalic acid spraying?

With Spraying, a solution of Oxalic Acid (around 2.5 to 4.5% of OA in water) is sprayed directly onto each side of each frame (with the bees on it) using a simple spray bottle. It is important that the spray nozzle is adjusted such that it creates a very fine mist. The water in those tiny droplets evaporates quickly, again leaving tiny oxalic acid crystals everywhere. In contrast to dribbling the bees do not (significantly) contribute to the dispersion of said crystals, thus it is important to spray both sides of every frame with bees on them. Spraying is very time consuming, as every hive has to be opened and every frame pulled out and sprayed. But it is very efficient and does not produce any adverse effects on the bees. Spray bottles are incredibly cheap, and a suitable oxalic acid solution can be created with the means typically found in every kitchen.

Oxalic acid vaporization

Vaporization does work slightly different. Here oxalic acid is heated up until it sublimates. Sublimation means the solid crystals directly turn into vapour. This vapour by the way is fully transparent and invisible. But after the vapour exits the vaporizer, it quickly cools down below its sublimation point, thus the OA vapour goes solid again. Just now, due to this process it consists of tiny crystals finely distributed in warm air, which creates the “white smoke” typically associated with oxalic vaporization. If this stream of warm air and oxalic is directed into a hive it will distribute a fine layer of OA dust everywhere.

One of the drawbacks of any OA treatment is that the bees will very quickly (within days) remove most of the crystals, as they are keen to keep their hive tidy. Thus the effect of one treatment will not last very long.

Oxalic and glycerine strips are a very elegant way of achieving the same effect. Glycerine is un-toxic, and slightly sticky. Cardboard or fabric strips soaked with a mixture of both substances are inserted into a hive and left for weeks to months. The bees will be walking over these strips and thus spread this mixture everywhere. This is akin to laying an oily rag on the ground in the middle of your house and walking over it all the time, eventually the oil is going to end up everywhere, but this process takes time. The advantage is that these strips create a continuous stream of OA crystals deposited everywhere, essentially creating an ongoing treatment. To be efficient, they need to be placed in areas with a lot of activity, within the brood nest (which is also where most mites will be).

Drawbacks of Oxalic Acid

Oxalic Acids biggest drawback is that it does not work on Varroa Mites that are inside capped brood cells (which is where they recreate). By the way the only treatment that does kill mites inside capped brood is formic acid. When a hive is engaged in heavy brood rearing during spring and summer, most mites that are infesting a hive will be in the capped brood cells, thus each oxalic treatment can only kill a small fraction of the total number of mites. But each capped brood cell will eventually hatch (usually after 21 days for workers and 24 for drones), exposing any mites inside to the outside.

That is why pretty much all treatments (including synthetics) are to be done over the span of at least 3-4 weeks, ideally more than that to go over multiple brood cycles. They essentially create a “death zone” outside of the capped brood, which as the Varroa mites are forced to leave the cells eliminates them.

The efficiency of this “death zone” determines how fast the reduction of mites happens. In theory if it was 100% efficient (i.e., any exposed mite guaranteed to die instantly) one could completely eliminate all mites inside a hive within 21 days (24 if they are raising drones).

Going back to Oxalic acid, depending on the dosage and application method a single treatment can reach up to 95% (i.e., killing 95% of all mites outside of brood cells pretty much instantly). However, this immediate killing effect will only last for 1 – 2 days. That is why one treatment of OA will not do anything for a hive that is rearing brood, most mites simply are not affected by it.

To have an effect you either need to make sure the hive does not have any capped brood (for example by using the split and treat method) or do multiple treatments over the span of at least 21 days. OA-Glycerine strips will do this by design as the OA is continuously released until you or the bees have removed the strips from the hive, but the total dosage of OA distributed within the hive at any given point (i.e., “deadliness” of the “death zone”) is less, thus requiring a longer treatment time to make up for this. This is the reason why strips, while being a good method for hives that do not have a high level of infestation yet have a major drawback: If you are late in your treatment, and infestation rates are high, signs of PMS are manifesting you will want to reduce mite numbers quickly.

In that case vaporization or spraying (which achieves the same effect, just requires more effort to do) are a feasible treatment method. It simply needs to be done multiple time over the course of a few weeks.

For a quick knockback one treatment every third day does seem to work well for me. And yes, this can become quite labour intense if one has many hives. But it is a 100% organic, efficient, and cheap (per dose, not including vaporizers) way of treating your beehives. Also, there are no known cases of mites developing any resistance to organic acids.

This finally brings me to the topic I actually wanted to write about: my new oxalic acid vaporizer.

It is the Hungarian-designed InstantVap, which works with 18V Batteries commonly used for cordless drills. So far, I am really impressed by this device. Once heated up (which only takes a few minutes) a treatment of one hive takes around 1 minute. The device is immediately ready to go after each treatment, and the very innovative plunger makes dosing of the OA a matter of seconds. I'll post a video of it soon as well.

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