Tag Archives: winter

Oxalic Acid Vaporization – Varroa Mite Treatment

Oxalic acid vaporization is currently my varroa mite treatment of choice. There are several techniques using oxalic acid, spraying, drizzling and vaporization. Most research indicates that vaporization causes least damage to the bees and is the most effective against mites. However all the techniques work well, there are just different pros & cons that ultimately remain the choice of the beekeeper. This section is looking specifically at Oxalic Acid Vaporization.

Oxalic Acid

Oxalic Acid is a naturally occurring organic compound. You can find it in many fruits & vegetables. Rhubarb contains it and gives it that strange tang on the tongue. It is also naturally present in honey (40-400 mg/kg). Is it a chemical? Well sort of, like sugar is a chemical. Is it natural? Yes, it occurs naturally. Is it “organic”, yes it is not only an organic compound but it is also approved as an “organic” method of Varroa Mite Treatment (Oxalic Acid is NOT approved in the USA for bee treatment). It is also a fairly dangerous compound that needs to be treated very carefully. Ingesting it or sucking up the vapor can cause serious harm and can be fatal in high doses. Unlike many other organic acids, which are generally fairly mild ( acetic acid ), Oxalic acid is unusual as being more like a mineral acid ( think hydrochloric acid ) and is fairly vicious and can burn skin badly.

From the beekeepers perspective, Oxalic Acid for vaporization is cheap and readily available as “oxalic acid dihyrate” which is a fine white crystaline powder.

The mode of action of oxalic acid against varroa mites is not really understood but it is believed that it physically damages the mites. The oxalic acid appears to be spread about by body contact between the bees but how it actually kills the mites is simply not known yet.

Oxalic Acid Vaporization

The “Drizzle” and “Spray” methods of application use oxalic acid disolved in sugar syrup, which the bees then ingest. “vaporisation” involves heating up a small amount of oxalic acid dihyrate within the hive using a special device. The oxalic acid dihyrate first liquifies and then becomes a gaseous vapor which permeates the whole hive. The vapor quickly re-crystalises on all the inner surfaces of the hive and the bees as a fine coating of oxalic acid crystals. These crystals are deady to mites. In a heavily infested hive you can expect a drop of over 1000 mites in the first 24hr, but the treatment will remain effective for up to a week and you will see a continued but diminishing drop over the next few days.

Generally this treatment is best done when there is little or no brood because it is only really effective on varroa in the “phoretic” state (on the bees). Varroa in with the brood are generally protected from the effects of the treatment. If there is brood present then repeat treatments are possible to cover the complete bee cycle. Generally one a week for 3-4 weeks should do it. The best time to do a single deadly treatment is between Christmas & New Year when most hives have the least amount of brood and the mites are exposed.

Manufacturers recommend protective clothing, eyeglassess, vapor mask and gloves when you do the treatments. if you are doing a load of hives then this makes real sense. Oxalic acid is a dangerous product – so is bleach and a number of other nasty household chemicals. if you have enough common sense to handle these substances safely and keep them away from children you should be able to cope with oxalic acid fine, without going OTT. I always use gloves and keep well back when vaporizing.

Pros & Cons of Oxalic Acid Vaporization


  • Oxalic Acid Vaporization is over 96% effective (without brood)
  • Oxalic acid naturally occurs in the hive and is considered an “organic” treatment in the UK
  • Unlike other oxalic acid methods, repeat treatments can be given without damaging bees
  • Do not have to open or disturb hive
  • No known negative effects on bees or brood
  • Because the mode of action is believed to be physical, mites are far less likely to be able to develop resistance


  • Oxalic Acid Vaporization is potentially more hazardous to beekeepers than other oxalic acid methods
  • Vaporizers can be an expensive initial outlay – costing up to ┬ú100
  • Slower than “Drizzle” method
  • Like all “acid” treatments, not effective against mites in brood

Why I chose “Vaporization”

I chose Oxalic Acid Vaporization over the other forms of oxalic acid treatment for the following reasons

  • I have enough common sense to handle Oxalic Acid sensibly!
  • It does not appear to cause any damage to bees or brood. The other application methods are known to have some deliterous effects on bees or brood development
  • Don’t have to open hive or disturb cluster
  • I can do repeat treatments and use it whenever varroa mites need to be “knocked back”
  • Ideal for treating packages and nucs

Video of Oxalic Acid Treatment of Beehives for Varroa

Oxalic Acid Vaporization Treatment Guide

Before getting started you will need to prepare your equipment and if necessary the hives. Make sure there is not too much wind and pick a time when there are not to many bees flying. The temperature should be above 4┬░ C. You will need at least the following items:

  • A vaporizer ( I use a Varrox ) and Oxalic Acid crystals which normally come with a 1g measuring spoon.
  • A 12v battery for powering the vaporizer.
  • Damp cloths or foam for blocking / plugging all entrances and ventilation gaps in the hive
  • Bee suit, equipment & gloves
  • Determine wind direction if any ( and make sure you stay “upwind” when vaporizing

The actual process of vaporization is simple:

  1. Seal screened bottom board if present and block rear inspection area with damp cloths or foam
  2. Wearing gloves, Measure 1 g of OA into vaporizer for each story of hive
  3. Insert vaporizer into entrance and block with damp cloths/foam around it. If entrance is to small you can insert through the screened bottom board inspection tray entrance. At this stage the vaporizer shoud be resting at the bottom of your hive and the hive should be completely sealed.
  4. Connect the vaporizer to the 12v supply. You should be standing upwind and as far away from the hive as possible to avoid any vapors that leak from the hive. Two minutes should be sufficient time to burn off all the oxalic acid. Smoke / vapor can sometimes be seen “leaking” from some areas, re-plug any big leaks but stay upwind!
  5. Disconnect after 2 minutes and leave for another 2 minutes before withdrawing the vaporizer. Re-plug the gap immediately. Cool the vaporiser in water if you have other hives to move on to
  6. Leave the hive sealed for 10 minutes, then remove all the damp cloths/foam from entrances and other gaps. JOB DONE!

Oxalic Acid Vaporization – Single Treatment

Oxalic acid vaporization works on mites that are on the bees. Mites in capped brood are protected. Normally 2/3 of mites are in capped brood at any one time. The logic is therefore clear, the most effective time to treat is when there is the least amount of capped brood in the hive. That moment will vary depending where in the world you live, in the UK the ideal time would be just before new year. If the winter is mild a small amount of brood may be present even then. A single oxalic acid treatment at that time can knock back the varroa and give the hive a chance to go into the new season in good shape with minimum impact to the bees.

Oxalic Acid Vaporization- Multiple Treatments

Generally I support the approach of not treating bees but there are occasions when it makes no sense to simply let a hive collapse, having only one hive is just such a situation! Under these circumstances, oxalic acid vapor treatment can be very effective if the crisis arrives mid winter. But this is not normally the case, autumn is the time when varroa can get out of hand and to treat with OA at that time requires multiple treatments because there is still brood present.

Lets look at some bee & mite facts and do some maths! For the purpose of this exercise I will assume full cell size.

  • Brood is capped for between 12-14 days ( workers & drones ), less on small cell
  • Varroa mites stay in the phoretic state (on the bees) for on average 7 days between breeding cycles
  • Oxalic acid vaporization is considered 95% effective on phoretic varroa mites

Therefore 3 oxalic acid vaporization treatments, 5 days apart will ensure that every mite in the hive is exposed to the treatment. Treatments after this will be subject to the law of diminishing returns.

My Experience using Oxalic Acid Vaporization

In late October I realized that with a Varroa count of nearly 100 per 24hrs, my first and only hive would definitely collapse. So I made the decision to treat using Oxalic Acid Vaporization. The weather had been mild and there was brood still present.

I embarked on a series of 6 oxalic acid vaporization treatments 5 days apart. The first treatment produced a drop of over 1000 mites and following days showed a slow reduction in drop rate 700 to around 200. The first 3 treatments showed similar results. It was only after the 3rd treatment that I started to see a real reduction in initial drop. Still several hundred but much reduced.

After the 6th treatment I was seeing initial drops of around 50. Within two weeks of the final treatment I was seeing a varroa drop of around 1-2 per day.

I was fortunate with the weather as it stayed mild, giving the hive an opportunity to produce new brood free of varroa stress. The reason I choose to do 6 treatments was that I had let the situation get out of hand, normally only 3 treatments would have been necessary.

I did see a slight increase in bee mortality – or at least bees more actively removing dead bees and I also noticed an increase in stores uptake.

In late December, when there is normally the least amount of brood and the Varroa are most vulnerable to this treatment, I followed up with a “preventative” treatment, this produced an initial drop of only 21 mites. See: Oxalic Acid Vaporization Annual Treatment.

Bees Starting Winter Cluster

Bees Winter Clustering in Super

Bees Clustering in Super

Its October and the weather has just started changing. Cold winds from the north have brought on a real chill and the bees appear to be preparing for winter by developing a loose winter cluster pattern in the super. This picture is of the bees forming a loose winter cluster.

Some bees are still flying but activity seems to have slowed down markedly.