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Oxalic Acid Vaporisation - as a Treatment against Varroa

By Klaus Klebs, Bummerten 279, 4315 Zugen

Translated from the Schweizerische Bienen Zeitung, 3/2001, pages 23 - 24. by A.E.McArthur MIL


Using a small vaporiser, connected to a 12 volt battery, 64 bee colonies were treated with oxalic acid last autumn. The treatment introduced via the entrance or roof ventilator took around 5 minutes per colony.

By virtue of the air circulation produced by the fanning of the bees the oxalic acid vapour penetrates into every nook and cranny inside the hive, resulting in 200 - 1400 mites per colony falling down onto the floorboard insert over a period of two weeks. All of the colonies tolerated the treatment very well. The bees were slightly restless for around 20 minutes after the treatment, in that fanning occurred and food up take increased. The bee mortality rate at treatment and during the subsequent eight weeks was comparable with the untreated control colonies.

The treatment can be carried out at temperatures above O° Celsius and when even when flying is not possible and is also no affected by adverse weather conditions.

Use of Oxalic Acid

The traditional treatment for Varroa using oxalic acid is either the labour intensive method of spraying each comb occupied by bees with a 2.1% water solution of the acid or trickling 20 ml of a 2.1% acid/sugar solution into the frame spaces where the bees are clustering during the broodless period in November - January when the ambient is above 8 Celsius and flight is possible.

Vaporisation of Oxalic Acid

It is possible by applying high temperature to produce a gas directly from the solid form of oxalic acid (powdered). [Sublimation - at 157° Celsius]. By vaporising oxalic acid at the side of the winter cluster the vapour penetrates passively due to the fanning of the bees into every corner of the hive and every external body part of the bees themselves. The gas forms an aerosol with the air within the hive, allowing the lowest oxalic acid concentration of aerosol to achieve the most optimum Varroa mite reduction effect.

The vaporisation unit is protected from mal-function by a safety device. The high temperature required for the vaporisation of the oxalic acid only occurs locally within the vaporising unit. The temperature of the immediate environment at a distance of 0.5 cm from the unit rises to no more than circa 40 Celsius, which is not even high enough to melt wax!

Method of Treatment

The treatment can be carried out via the entrance or the roof ventilator. For safety reasons a face mask and gloves should be worn (oxalic acid in particular concentrations can damage internal organs and be absorbed by skin).

The oxalic acid dihydrate dosage is placed on the surface of the vaporiser and amounts to 1 gm for strong colonies and 0.5 gm for nuclei or artificial swarms.  The vaporiser is then placed at the side of the winter cluster at a distance of 6 - 10 cm from it. The entrance and roof ventilators are sealed using foam rubber and the interfaces between chambers can be taped up using masking tape so that no gas escapes from the hive.

When all preparations have been carried out the connections on the vaporiser are coupled to the terminals of a 12 Volt battery. The acid powder begins to vaporise after around 45 seconds. The whole process of vaporisation takes about 4 minutes. The hive remains closed up for a further 5 minutes for optimal distribution of the vapour. To speed the treatment procedure up the hives to be treated could all be pre-prepared in readiness for the treatment. Also the use of 2 or more vaporisers would of course add to the time savings per colony.


Over a period of 2 weekends at midday with a temperature of 3 - 6° Celsius 64 of the author's colonies  were treated. The subsequent 14 days were so unfavourable due to cold wet weather that flying for the bees was not possible. The bee mortality which was checked daily was no different from that of the untreated colonies. The bees were a little restless during and for around 20 minutes after the treatment, in that fanning and an increased food uptake was noted.

In the first two groups the Varroa 'drop' in twelve colonies was checked daily, always at the same time. This daily count was carried out before and after the two treatments using formic acid as well as in the corresponding time in September and October for the other remaining colonies being tested.

The eight colonies which were treated using Bayvarol the mite fall was between 400 - 1400. In four 'honey gathering strength' colonies, which had been treated earlier using 85% formic acid the mite fall was between 200 - 700. Nine 'honey gathering strength' colonies which were treated a week later, indicated similar results. The largest number of mites dropped within the first week (especially after the 2nd and 6th day of treatment). Feedback from beekeepers who used the vaporiser to treat a total of around 200 colonies confirmed similar results relative to mite fall and bee tolerance to the treatment. The vaporisation treatment showed a higher efficiency relative to the spray method. This finding was confirmed in four colonies which were treated at the end of September with the oxalic acid spray method and then treated with the vaporised oxalic acid in November. The daily mite fall in these tested colonies rose from 1 - 2 mites per day to 10 - 20 mites per day.


Honey naturally contains between 40 - 400 mg/kg of oxalic acid. A 1 gm amount of acid vaporised in an overwintering colony which has 8 food combs (circa 16kg stores) corresponds to around 60 mg/kg of introduced oxalic acid. This amount is negligible and besides the bees will consume all of these stores over the winter/early spring period.

Compared to the acaricides and pyrethroids, the organic acids, due to their different method of attacking the mite will not promote resistant mites.

Biotechnical Methods

By virtue of the ease of operation of this new application the beekeeper is now able to eliminate the residual population of mites from the overwintering bee colony.

Another use can be made of the device in the treatment of nuclei and artificial swarms in the spring/early summer. There is also the possibility to rid the bees of mites after removal of the final honey harvest. The use of this method in spring and summer has still to be researched.

The Vaporiser may be purchased from the author, cost (Fr 180.-)

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