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A New Method of Applying Oxalic Acid

By T. Radetzki, M. Bärmann and G. Sicurella

Translated from "Bienenmütterschen" 12, 2000 pages 239-243

By A.E. McArthur MIL


The Teaching and Research Beekeeping Facility at Fischermühle has been pursuing a Research Project for a number of years. The purpose of which was to simplify the application of Oxalic acid. This project was undertaken due to the spreading of resistance in mite populations to the authorised chemotherapy and because of the increasing levels of residues in hive products. A new procedure has been developed in which the oxalic acid is introduced into the bee colony using a vapourising process. The application is made in winter through the entrance without the need to open the hive. The effectiveness is extremely high and is subject to very little variations in success rate.  The method seems to be well tolerated by the bees and operator safety is guaranteed by the new method of vapourising.

Development of the Method

The Beekeeping Facility at Fischermühle has been engaged in experiments with oxalic acid since 1989. In 1994 the high level of effectiveness of the oxalic acid 'Spray' method was published. These results were later confirmed by Imdorf, Liebig and others, as was the tolerance of the bees to the treatment.

Trials using an Oxalic acid Aerosol

In 1994 we carried out the initial trials using oxalic acid aerosols, which were blown into the hive without opening the hive. An aerosol is a mixture of air and very fine droplets or dust particles. In the cold of the hive the droplets do not remain in aerosol form, they condense. This causes a poor result to be achieved.

As a result of a lecture by Dr Wolfgang Ritter and encouraged by experience from members, we returned again to investigate the aerosol procedure. Dr Ritter reported on laboratory trials, which made clear that the Trickle Method of applying oxalic acid resulted in a serious problem for bee health. Laboratory observations showed that some bees would rather die of hunger than take up the sugar solution containing the acid. The assumption is that the acid attacks the digestive system of the bee. Field trials using the Trickle Method also indicate there could be problems with the procedure. Opposed to this Ritter observed that the moistening of the bees with droplets of the acid with increasing fineness of droplet the bees were less affected by the acid. This observation corresponded to the experience with the Spray Method used at Fischermühle. Therefore using modern techniques finer droplets and particles of oxalic acid were produced and blown into the hive in a warm air stream through the entrance and into the winter cluster as an aerosol. A high effectiveness was achieved. However because the aerosol- air stream was blown from outside the hive - and the hive is not a balloon - much of the oxalic acid air stream leaked through cracks and small gaps back into the atmosphere. To achieve a satisfactory level of success using this application method too much acid ended up in the air surrounding the hive being treated. For reasons of operator safety this method was discontinued. Within the scope of the trial the behaviour of bees exposed to different methods of acid application was observed in a glass hive. A special apparatus was designed and constructed by Bärmann, Radetzki and Scurella, which facilitated the introduction of an aerosol without resorting to a very high pressure.

Materials and Method

A small receptacle for the evaporation of oxalic acid was developed, which can be introduced into the hive entrance. The entrance is closed using foam rubber during the treatment, which takes 3 minutes. 'Open' wire mesh floors are closed. The apparatus is removed after it is switched off and the entrance left closed for another 5 minutes.

In the winter of 1999/2000 a field trial using 77 brood free colonies was carried out. The treatment was carried out using hives of 95 litre volume, designed for 17 'jumbo' frames( as a comparison a Zander hive with 2 brood chambers has a volume of 85 litres). At the end of November the bee population was estimated using the Liebefeld Method. Later 7 groups with equally distributed weak and strong colonies were made up. 5 of these groups were treated with oxalic acid. The groups were treated using the new vaporiser with 0.5; 1.0; 2.0; 3 and 5 grams of oxalic acid crystals. One group was used as 'control' and treated with 2 grams of water and a further group was left untreated. The colonies were arbitrarily placed in 5 different apiaries. The treatments took place on 13th and 14th December 1999, at a temperature of 4 Celcius.

Snow fell on the night of the 14th shortly after the treatment was completed. Prior to the treatment the colonies were 'winter clustered' and had not flown for over a week. Subsequently over a period of 4 weeks where no flight took place the mortality rates of mites and bees was checked. The control treatment was carried out on 9th and 11th January 2000 using 2.1% oxalic acid (3% by weight). Over the following 3 weeks the mortality rates of mite and bees was checked. In the determination of the effectiveness of the treatment the sum of the effectiveness of both treatments was considered to be 100%. The bee population after overwintering was estimated between the 20th and 22nd April. The population estimates were all made by Giuseppe Scurella who had also made regular interim estimates. On 4th May honey was harvested and sent to the University of Hohenheim to be analysed for contaminating residues. As a 'control' Werner Gekeler, apicultural adviser, harvested honey from colonies which had not been treated with oxalic acid and which had been located some kilometres distant from the test colonies during the early summer nectar flow.


The procedure is highly effective and exhibits only negligible deviations. The 3 gram treatment and likewise the 5 gram achieved 95% within one week and over a 4 week period the success rate was 99%. Table 1 shows the average effectiveness and the deviation. Bees that left the colony were not considered; however during the whole test period there was never any chance for bee flight due to the inclement weather. With regard to the group treated using 0.5 grams the vaporising procedure indicated a better bee tolerance than the Spray Method. In all the treatments the average deviation was little different from the untreated colonies. These results show a really good bee tolerance to this novel treatment method. There occurred no statistical evidence of a reduction of the hive populations as a result of the oxalic acid 'steaming' procedure. No queen loss occurred in any colony during treatment.

The test results for oxalic acid residues are not yet available. Meantime interesting results regarding the natural content of oxalic acid in honey have come to light. Work done by F. Mutinelli on 32 samples of different Italian honeys show levels of 0.4 - 0.02 grams of oxalic acid/kilogram.


The results clearly show that the vaporisation of oxalic acid in a hive is a highly effective method for treating against Varroa during the brood free period. In contrast to other procedures for vaporising oxalic acid (the use of which we would strongly discourage) operator safety is guaranteed by employing our special applicator for the following reasons:

The acid begins to vaporise at 102 C. On mixing with the cold air in the hive the oxalic acid gas immediately condenses and forms and aerosol consisting of extremely fine dust like particles of oxalic acid.

The whole process takes place in the hive, without the introduction of external air. The amount of acid escaping to the environment where hives are relatively weather tight is minimal.

Gloves and a face mask as well as the need to cover exposed skin with suitable clothing are a safety requirement. (as with the Spray Method)

The proportioning of the amounts of crystals can be carried out in such a manner that no operator contact with them occurs. The procedure appears to be able to be used even during frost. (not that we would recommend this practice) We have carried out treatments at minus 2C without any negative results being noticed.

The cost of this apparatus is not high, besides the vaporiser can be used by any number of other beekeepers. The cost of the acid is minimal. The apparatus employs a 12 volt battery and a car cigar lighter.

This procedure is of course not authorised in Germany, so we are expressly not recommending it. This report is merely a report on a scientific experiment.

To forestall any enquiries: We do not sell this apparatus.

We wish to pursue our work and are seeking beekeepers, that are willing to participate in a scientifically evaluated field trial in order to answer the following questions:

We would like here to thank our Association members for the financial assistance freely given up to this present time!& Since we do not receive any official financial support, we are totally dependent on donations and member support. Please support us in this important work.

Table 1 : Effect of Oxalic acid Vaporisation using different Dosage Levels


  Vaporised oxalic acid (g/colony)


No Treatment Water 2.0 0.5 1.0 2.0 3.0 5.0
Average effect 1.2 4.8 82.8 96.0 97.2 99.0 99.2
Minimum effect 0 1.3 51.7 93.2 91.7 98.1 99.0
Maximum effect 2.7 11.5 96 99.2 99.6 99.7 99.8
Colonies/groups 6 8 12 10 13 11 5

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