Oxalic Evaporator

Evaporators can be used when dribbling can’t.

The Idea

My DIY vent oxalic evaporator.

Oxalic acid can be sublimated into a beehive as an effective varroa mite treatment using cheap oxalic evaporators built from standard plumbing supplies.

The Details


Oxalic acid is distributed in a beehive by two methods.

One method involves spraying or dribbling an oxalic solution. It has the same drawbacks as using powdered sugar.

A second method involves sublimating oxalic acid by heating it to about 300 degrees F. There are very few drawbacks to using oxalic vapors.

DIY Evaporators

Building an oxalic evaporator is simple. It takes less than five minutes and costs less than $10.

This evaporator consist of two parts, a magazine and a distributor. The magazine is filled with oxalic and is screwed on the distributor.

The distributor directs oxalic acid fumes into the hive interior. Different kinds of distributors are built to accommodate different hive configurations. My vent evaporator fits into a 3/4 inch hole drilled into a super. It’s actually harder to describe an evaporator than it is to build and use one.

Distributor and magazine.

Brass Evaporator

The distributor, shown on my finger tips, consists of a brass 3/4″ adapter with the threads ground down on a taper to fit in a 3/4″ hole, a long 2″ nipple, and an L.

Tightly assemble these parts together using Teflon tape and a pipe wrench. Grind the threads on the 3/4″ adapter down on a taper. Leave enough of the threads so that they will securely bite into the wooden edge of the hole in the super.

  • The magazine consists of a long 2″ nipple, and a cap. Tightly assemble these parts using Teflon tape.
  • Make sure the threads seat into the wood which holds the adapter securely in place. If the threads are just seated in propolis or wax, the adapter will fall out when it’s heated.

Copper Pipe Vent Evaporator

Copper vent evaporator.

Here’s  another vent evaporator which costs less than $3 to build.

It consists of some ½” copper pipe, a 45 degree elbow, two caps and five small screws.

Assemble it using Teflon tape and secure it together with the small screws.

Brazing could be used to replace the screws. Don’t solder it together.

The small piece in my palm is a measuring cup. Cut it off at the proper length for the right dosage.

To use it:

  • fill the measuring cup up with oxalic acid.
  • insert it into the mouth of the evaporator.
  • tip it upright.
  • rap the evaporator on a solid surface to dislodge the oxalic.
  • remove the measuring cup.

The thinner metal heats and cools faster. I don’t use water to cool it. There is less condensation in this evaporator. And the measuring cup is easy and cleanly filled.

If the Teflon tape leaks simply replace it. So, far the tape on this evaporator is still functioning.

Copper Pipe Entrance Evaporator

Copper entrance evaporator.

Here’s  an entrance evaporator. It’s constructed like the copper vent evaporator with an extra piece of copper pipe:

  • flatten this pipe on one end.
  • wrap the circular end with Teflon tape.
  • insert it into the elbow after the evaporator has been charged using the measuring cup.

The Teflon on the inserted pipe needs to be replaced occasionally to prevent leaks.

Evaporator Operation

The basic operation is the same for all evaporators.

Read and understand the information in an oxalic acid MSDS. They are available on the internet. Basically, don’t get it in your eyes, breath the vapor or dust, eat it, or leave on your skin. Don’t underestimate the vapor’s effects. It is composed of steam, formic and oxalic acid. If you have handled caustics, it won’t surprise you. But if you haven’t, take every precaution. Use the right safety gear. This is a no-brainer. How many dead mites would you trade for your kidneys and lungs?

Don’t forget fire safety! Be careful. Using the evaporator is not like using  a smoker. The flame is hot. It’s open. And it’s in an outdoor environment where, the slightest carelessness or a gust of wind can set you or the whole county on fire.

And be sure to use a good torch or you might have a chance to repeat this experience.

  • remove the magazine from the vaporizer.
  • if using an entrance evaporator, make sure it isn’t plugged up. I’ve plugged mine by spearing a propolis lump on the bottom board. When plugged, oxalic vapor will vent around the magazine threads. But most of it will condense next to the plug in the tubing creating a super plug. Heat the tubing away from the hive to remove a plug. When in doubt, run a little water through it to check the flow rate.
  • fill the magazine with the proper amount of oxalic acid. Research indicates that 1.5 to 2 grams per hive is sufficient. The open end of the magazine is inserted into a small plastic bag containing oxalic acid powder. The powder is manipulated through the plastic into the magazine. There’s very little exposure oxalic dust and no contact with the skin.
  • attach the magazine to the vaporizer. Hand tighten it.
  • insert the vaporizer into the hive entrance or twist it into the vent hole if using the vent evaporator.
  • seal up the hive. Paper towels can be used if careful not to set on fire. They can be reused for other hives and then tossed.
  • heat the magazine. Start heating the parts of the evaporator nearest the hive and gradually shift the heat downward toward the magazine.Use a propane torch on it’s lowest setting for 4 minutes. Vapors will be given off at about 1 1/2 minutes. At about 3 minutes, most of the oxalic will suddenly vaporize. Oxalic sublimates at about 150 degrees C. It decomposes at about 180 degrees C.
  • remember you are sublimating oxalic not smelting copper or brass. :-)
  • cool and remove the vaporizer. The brass vaporizer can be cooled by pouring water it.
  • Remove the vaporizer from the hive and recharge it. Be sure the brass magazine’s threads are dry before filling or the oxalic powder may stick to them.
  • 10 to 15 minutes after treating the hive, remove all the stuffing and let the bees out.
  • cleanup. Clean the vaporizer with hot water. Then wash hands, etc.
  • store the vaporizer and the oxalic acid powder in a very safe place away from children and pets.

Vent or Entrance Evaporator?

I’ve tested both types of evaporators and found little difference in their effectiveness. But I like using the vent evaporator better when the bees are flying.

  • Hive activity is not disrupted by its use.
  • The beekeeper can stand and effectively treat from any position.
  • The flame and heat are away from the beekeeper, flying bees and ground vegetation.
  • The distributor is maintenance free.

The entrance evaporator will work with hives lacking the vent holes. In theory, it should distribute the oxalic fumes more effectively than the vent evaporator. But I haven’t noticed any difference in practice. It should be more effective when the bees are clustered. If I were treating when the bees aren’t flying, the entrance evaporator is my choice.

12 Volt Electric Evaporators

An electric evaporator has some advantages over one heated by a torch. They are faster, as no cooling time is required between hives. And all the heat/vaporization occurs inside the hive and away from the beekeeper. There are no matches or open flames, an important factor when the fire danger is high. The only disadvantage I’ve found is that they can’t be used in a hive with a migratory bottom. They are just too fat.

Heilyser, a Canadian company, offers several different models. If I had lots of hives to treat, I would strongly recommend using their electric model. These folks make a great product and provide great service/support. I originally bought an evaporator from them. It was a fine piece of machine work, but wouldn’t fit in my hive entrance, as I use migratory bottoms that create a 3/8″ entrance.

Some beekeepers have built electric evaporators using diesel engine glow plugs/igniters. See: drobbins

Commercial Equipment

Several types of oxalic acid vaporizers are on the market. The European version is basically a 12 volt, electrically heated pan. Eastern European models are very elaborate with pumps, heaters, etc.

Cowan has a commercial prototype in the works.

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9 Responses

  1. -dm says:

    Hi Guys,

    With natural comb, varroa mites are no longer a problem. So, I’m not keeping up with the latest and greatest varroa treatments, including oxalic. Randy Oliver does keep up with them and his updated observations are a must read if you are still treating for varroa.

    After transferring mite tolerant bees from small cell hives into clean, large cell hives,I needed a non-contaminating mite treat to keep those same bees alive and the combs clean. I ordered a propane powered model from the Heilyser.

    It was a great device. Expertly machined and rugged. The people at Heilyser provided great service and support. But there was one problem. It didn’t fit in the 3/8″ entrance left by the migratory bottoms which are screwed onto to my hives.

    Rather than take a hammer and mash their device into compliance, I decided to build my own version which is called an ‘oxalic pipe’ by some. It’s purpose was to provide a temporary solution. And to avoid the mite overload that occurs when migrating to small cell comb. After the bees are on small cell comb, the pipe could be tossed.

    All my bees are on small cell or natural comb. So, I haven’t treated for mites. And I’ve retired my evaporators.


  2. -dm says:

    Oxalic Dribbling
    Evaporators – Going…going….gone

    My evaporators were designed as cheap and expedient devices to be thrown away after getting bees established on natural comb. At that time, there was little information about oxalic dribbling. So I went with the evaporators. Today, I use natural cell and don’t routinely treat for mites.

    Dribbling oxalic is harder on the bees if done carelessly or often. And it’s more seasonally restricted than using an evaporator.

    But dribbling oxalic is a safer, cheaper bet. With the proper dose and application, dribbling is as effective and faster than using my home made evaporator.

    • ke6gwf says:

      Sorta off topic, but… :)
      Dennis, I clicked through to your post on Checkerboarding on the bwrangler blog, and then tried to go to another of your pages, and it pops up that I have to log in. I have a wordpress account, but it just keeps popping the login prompt back up and not letting me in. It doesn’t give me any other options either!
      Do you have your blog restricted access? Are they having problems? Do you have a scoundrel filter that I am setting off? I have never run into this before.

      I can get it through Google Cache, so I know it was somewhat open as of a week ago.

      This is perplexing me!
      I am very interested in natural beekeeping and am trying to get as much info as I can, and your blog looks like it has a lot! (from the peeks I could get past the wordpress bouncer)

      Thank you!

      And now, back to Oxalic Acid… (or not!)

      • -dm says:

        Hi Ben,

        Thanks for the heads up. I’ve combined my BWrangling and BeeNatural blogs. Some of the links were corrupted in the process.

        They should be working now.


  3. Stephen Epstein says:

    A great article. Can oxalic acid be vaporized before a honey flow?

  4. Tony says:

    Hi Dennis,
    A few of the commercial beekeepers in my area (central Alberta) use a fan of some type to circulate the Oxalic vapor within the hive. What’s your opinion??


    • -dm says:

      Hi Tony

      Decades ago, the Europeans added small fans to their DIY oxalic pipes. An eastern European firm had a large, commercial blower.

      Cowan is in the process or has already developed a commercial machine.

      Research showed a fan was well worth the effort as treatments were more consistent. And with the larger blowers, treatments were very fast, on the order of ten second per hive.

      On the down side, Pressurized oxalic vapor is hard to contain or control. It requires extensive safety gear. I’ve wondered what it like to treat a holding yard, with hundreds of hives, with oxalic vapor.

      I had a second kind of DIY pipe in the works. It was T shaped. Had a fan on one end. And used halogen bulb for heat. But my need for oxalic is minimal and I never finished the project.

      Probably the cheapest, safest and easiest way to get oxalic into a hive is to dribble it. If dribbling is done in combination with some other open hive management task, it’s inconsequential.


  5. Allen says:

    I have a lot of questions. One, can you vaporize during a flow? Two, where do you get Oxalic acid? Three, is the acid in liquid or crystal state? Four, what is the concentration? And five, does it have to be pure? Thanks ~ Allen

    • -dm says:

      Hi Mlandmann

      1. Can but shouldn’t.
      2. Depends on how much you need. Small amounts, some people use wood bleach available at the box lumber stores.
      3. It’s crystalline.
      4. It’s not a mixture. Use the 99.99% stuff.
      5. There aren’t any commercial chemicals that are 100% pure. But the cleaner the better.

      Vaporize or dribble? If you can still open your hives, dribbling oxalic would be my preferred method. It’s safer, faster and much less hassle. I’d spend the $20 on a small scale and not invest it in the propane torch.


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