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By Jean-Daniel Charriere, Anton Imdorf and V'Erena Kilchenmann, Beekeeping dept Fam. Schwarzenburgsrasse 155. CH 3097 Liebefeld

Translated from the Schweizerische Bienen Zeitung 115(8) pp 463-469 1992


In 1979 Kunzler et al published the first results on the effectiveness of formic acid against Varroa jacobsoni. The present anti - Varroa treatments using formic acid make it possible to waive the application of synthetic acaricides with their negative side effects (residues in the hive products, development of resistant Varroa strains and environmental contami nation). On the other hand the formic acid treatment has disadvantages such as different effectiveness, loss of young larvae and bees, in some cases even queen loss and a somewhat greater labour input. Beekeepers for this reason are reluctant to use formic acid. The experience gained from large scale trials with formic acid indicate however, that the Varroa mite can be treated effectively using this organic acid.

In order to offer the beekeeper an attractive titleernative to the synthetic acaricides the methods of application of formic acid will have to be improved. To discover the weaknesses in the current methods of application it is necessary to establish the course of the formic acid concentration in the hive atmosphere during treatment. The trial sought to find a method by which the actual course of the formic acid concentration could be followed.

Known Measurements

Ritter and Ruttner confirmed in laboratory trials in 1980 that formic acid had a low toxic effect on bees but a high toxic effect on Varroa mites. The mite mortality increased with the amount evaporated per day and the exposure time. Wissen and Maul(1980) believe that the Varroa killing effect of the formic acid depends on the concentration in the spaces between the combs to a greater degree than on the applied amount of formic acid. Kunzler et al (1979) measured the formic acid concentration in colonies using a titration method and confirmed that with 50ug formic acid per litre of air (circa 26 ppm) within one hour 20% of the mites could be killed. Adelt et al (1986) have ascertained that the formic acid concentration in colonies treated with Illertisser plates does not in general exceed 180ppm. Muller (personal communication Liebig) measured 220ppm in the Hohenheimer Hive one hour after treatment with the Illertisser plate.

Material and Methods

Hive Design and Colonies

The research was carried out using the two most popular hive designs in use in Switzerland. The Dadant-Leaf Hive which is single walled, is arranged ‘cold way’ with 12 frames and having a volume of 65 litres and an entrance 45cm wide (fully open during the trials). The Swiss hive however is a rear-opening design arranged in ‘warm way’ with 10 - 12 frames and an entrance 15cm wide. An additional chamber was added over the brood box for the formic acid trial to give a 65 litre hive volume.

Both hive designs were fitted with hive floor inserts under the mesh floor board frame. The race of honey bee treated was the Carniolan, all colonies were equal in strength and were in the same apiary site. The same colonies were used for the first and second treatment applications, except for one colony in the Dadant hive.

Application of the Formic Acid

The colonies were treated using a modification by the Beekeeping Dept, Liebefeld (1991) of the Illertisser Method. The treatment for the Dadant hive was carried out from below. The viscose sponge was soaked with 30 ml 85% formic acid and placed on the Varroa insert. The Swiss hive was treated from above. The sponge in this case was soaked in 20 ml 60% formic acid in August and 30 ml 60% in September, due to the high temperatures and placed on the brood frames. All the treatments were carried out in the evening. The absorbent cloths were left in place until the end of the trials. The first treatment took place in August and the second in September.

Abstraction of the Air Sample and Measurement of the Formic Acid Concentration

The day prior to the start of measurements, two teflon tubes of 2mm inside diameter were inserted into the colonies. The two sample positions were located in the middle of the brood nest half way up the comb and spaced two combs apart.

The air sample was abstracted using an electronically controlled air pump (Supelo PAS - 30003) with a capacity of 60 - 85 ml/min. The contaminated colony air was drawn directly through the suction tubes(Dragerwerk AG Lubeck Type No 6722101. The reagent was coloured by the indrawn air. From the length of the coloration and the measured volume of the sampled hive atmosphere the formic acid concentration in the hive atmosphere was able to be calculated. In order to avoid saturating the reagent in the front part of the tube component, on the advice of the Drager company, after each sample was abstracted clean air was drawn through the sampling kit.

Temperature Measurement

The air temperature was measured electronically (Hamster Mesomatic AG Zurich). The heat probes in the Swiss hive were positioned in the upper part of the comb 3 cm under the absorbent sponge. I n the Dadant hive it was placed on the Varroa insert close to the absorbent sponge 4 cm under the mesh floor board.

Results and Discussion

Course of the Formic Acid concentration in the hive atmosphere during the first six hours of the trial.

The formic acid concentration rose rapidly in both hives during the early hours of the trials. The peak values measured exceeded 400 ppm many times. The highest recorded value was 565 ppm. During treatment at the beginning of July using Illertisser plates in Zander hives Muller(personal communication) likewise confirmed high concentrations on the early hours of treatment. This rapid rise in formic acid concentration in the early stages of the treatment is due to the high evaporation of the acid and the failure of the bees to ventilate the hive atmosphere efficiently. During the following five hours the acid content in the hive atmosphere fell gradually.

In the three simultaneously and similarly treated colonies different acid concentrations were recorded. Moreover the values between the two probe positions also differed in the same colony. These variations are probably due to different ventilation characteristics of the bees.

The measured acid concentrations in the Swiss hive were on average higher than those in the Dadant hives, despite the applied dosage being lower. The higher levels could be explained by the treatment being administered from above, which resulted in the steadier and more intensive evaporation of the acid on the one hand and the peculiarities of the hive design on the other hand. In “Warm Way” (Swiss hive) each comb represents a baffle for the out flow of acid vapour through the entrance. In the “Cold Way”(Dadant hive) each space is open to the entrance. This guarantees an effective removal of the vapour to the outside atmosphere. Since the absorbent cloth is placed on the insert under the mesh floor, most probably a portion of the vapour is directly lost to atmosphere without reaching the brood nest. Besides the entrance of the Dadant hive is circa three times larger than the Swiss hive entrance.

Different Results in August and September

At the 4th September 1991 the ambient temperature in the Swiss hive was around 3 C lower than the 12th August. Due to this the amount of formic acid for the September treatment was increased from 20ml to 30ml. Despite this the acid concentrations measured in the colony in the late summer was on average 47% lower than in high summer.

In the Dadant hive the same formic acid amounts were used for both applications. The treatments were carried out 9 and 12 days later than those for the Swiss hive. In this case the values recorded during the initial 5 hours with the late summer treatment was almost 70% lower than the high summer values, titlehough at 16/9/91, during the first six treatments an almost 2 C higher temperature was measured at the insert than on 21/8/91. The results lead to the conclusion that as well as temperature there are still other seasonally conditioned factors present such as colony strength, (amount and age of the brood) and the behaviour of the bees(ventilation activity after the harvest or feeding!).

Course of the Formic Acid Concentration in the Hive Atmosphere after the first six hours of the trial

The formic acid content of the Swiss hive during August was measured at different times over a period of 16 hours and in September over a period of 44 hours(top application). After an initial high concentration a gradual steady decline of the acid level was confirmed. No further rise at any time later was observed..

In the Dadant hive(treatment from below) the acid concentration was measured over a 60 hour period for both treatment periods. In this case during the two days after the start of the treatment on 21/8 in the warm day time hours a dramatic further increase in the acid values was measured. This phenomenon could also be observed to a lesser degree after the treatment on 16/9.

These differing results in the two hives depend less on the hive design than on the different application methods. In the Swiss hive the treatment plates were placed on the brood frames. The variations in temperature in this position are low and the temperature is rarely lower than 28 C. In the high floor of the Dadant hive the temperature variation was much greater, since it was directly influenced by the weather. At low temperatures especially at night the evaporation rate of the acid decreased. The observations of Wachendorfer et al(1985) where the evaporation of the acid in the Illetisser plates in a free standing hive treated from above took 4 - 8 hours, confirms our suppositions.


The trial allowed the course of the formic acid concentration in the hive atmosphere during the treatment to be followed. From the work done by Bolli( in preparation for publication) the formic acid concentrations which we measured in the experimental colonies are not high enough to adversely affect the breathing of the larvae or the adult bees. Therefore the occasional loss of young larvae and queen bees despite the correct application of the formic acid dose is not caused by the formic acid itself but by the behaviour of the colonies themselves( neglecting the brood, balling the queen etc). There were considerable variations in the formic acid content of the hive atmospheres, the treatment and even between the different sampling points within the a colony. This indicates that the colonies themselves react very differently to the formic acid treatment. The rapid rise and the level of the formic acid during the initial stages of the treatment in instances of overdosing and high temperature in the proximity of the formic acid plate can result in the loss of bee life.


A method was developed to ascertain the formic acid concentrations in the hive atmosphere. Colonies in Dadant and Swiss hives were treated with a modified version of the Illertisser Plate during August and September. The Dadant hives were treated with 30ml 85% formic acid from below. The Swiss hives were treated with 30ml 60% formic acid the plates being placed on the frame tops. The Swiss hives indicated higher peaks(565ppm) than the Dadant hives(440ppm). After the first hour the acid concentration reduced. The Dadant hive indicated a further increase in acid concentration in the days following, when the daytime temperatures were high. While in the Swiss hive no such rise was observed.

In August in the Swiss hive the average maximum value was 370ppm and in the Dadant hives it was 279ppm. In September in contrast to this in the Swiss hive the average was 250ppm and in the Dadant the average was considerably lower at 113ppm. The concentration curves showed distinct differences during the simultaneous treatments on colonies in the same apiary. Large concentration differences between the different probe positions in the hive were also confirmed.

Formic Acid in Practical Beekeeping

In order to avoid the negative effects of formic acid treatment it is necessary for the beekeeper to be experienced and have an ‘intuitive feel’ in the use of the substance under different weather conditions and hive designs. Formic acid as an anti-Varroa treatment has been, due to the somewhat greater labour input required, criticised by many beekeepers. On the positive side are the advantages that neither residues nor short term resistance result from the use of formic acid. Therefore better application methods have to be developed with which the rapid and high rise in acid concentration at the start of the treatment may be avoided. Many years of experiment by Kramer (1990) and an ‘in house’ trial at Liebefeld have indicated that treatment with low concentrations of formic acid over a number of days offers a promising solution. Such a method of application reduces the labour requirement dramatically since possibly two treatments(after the honey harvest and at the end of September) will suffice. Such ‘gentle’ acting formic acid carriers, which may be made by the beekeeper himself at a reasonable cost are however not yet available on the market.


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