Residues in honey after application of thymol against Varroa using the Frakno Thymol frame (1998)
Stefan Bogdanov, Verena Kilchenmann, Anton Imdorf and Peter Fluri
(Translation: Ron. Crocker)

Thymol shows a good effect against Varroa mites and high tolerance by bees.[1]. In Switzerland it is used mainly as the active ingredient of the registered product Apilife VAR. (inserts). [2]. The treatment is given after the honey flow in the late summer to autumn, for the duration of eight weeks. No excessive residues accumulate in the honey or in the wax even after several years of repetitive treatment.

For about three years a further method has been used in Switzerland, the "Frakno Thymol Frame" named after the German master beekeeper, Franz Knobelspies. [3]. Knobelspies recommends that, in contrast to the treatment with Apilife VAR., colonies can be treated all-year round with thymol. The question then arises, whether the quality of the honey is affected by thymol residues. These would pose no health risk but might affect the taste of the honey, this would contravene food legislation. Swiss and European Union regulations forbid any additions to honey that change its natural taste.

Sensory test
A tolerance concentration (in Switzerland) of 0.8mg thymol per kg honey has been established because this cannot be tasted by the consumer. The threshold for taste perception lies at about 1.1mg/kg of honey.

The occasion for the existing investigation were concerns regarding honey quality after an all-year thymol application. The Bee Institute of the University of Hohenheim reported their 1997 results [4], where a fifth of the honey from colonies treated with thymol using the long-term thymol frame treatment showed a value above the Swiss tolerance value of 0.8 mg per kilo. The aim of our study is to know the thymol residues in honey after longer-term application with the thymol frame under Swiss conditions. Although the first investigation was accomplished during one bee season and consequently is not complete, beekeepers should be informed of the relevance of the results so far.

Samples and analyses
The honeys originated from apiaries of 17 beekeepers (14 with Swiss hives and 3 with moveable hives) from the 1997 crop. Varroa treatment was carried out in 1996 & 1997 (in one case since 1995) using year-long thymol treatment according to Knobelspies. The special frames remained suspended all year in the brood chambers of the colonies and were filled twice annually (as a rule in March/April as well as in July/August) with 12g crystalline thymol. The active agent evaporated over the course of several months from the top compartment of the frames.

One beekeeper had removed the frame at the beginning of April for the duration the honey season. Altogether 22 honey samples were investigated. The origins of the honeys were determined by means of measurement of conductivity.

Subsequently 6 blossom and honeydew honies were tested, and also 10 blended honies from blossom and honeydew. Thymol content was measured by gaschromatography. The limit of detection for thymol in honey was 0.02mg/kg.

Results and discussion
1. Residues after long term treatments with thymol
The results of the Liebefeld investigation of 1997 and the measurements from Germany in 1996 are presented in thediagram. The average from the 22 Swiss samples was 0.33mg thymol per kg honey (minimum 0.02mg/kg, i.e. below the limit of detection; maximum 0.83mg/kg). One sample lay slightly above the tolerance-value of 0.8mg/kg. The residues that were measured by the Bee Institute in Hohenheim (Klaus Wallner) in German honeys, were generally higher than those in Swiss samples: Average 0.63; Minimum 0.07; Maximum 2.0 mg/kg. The reasons for the higher values in Germany were not examined. It is suspected that the greater number of treatment years in German apiaries could play a role.

thymol_us.gif (5229 octets)

Residues of thymol in honey after year-long treatment with the thymol frame, after Knobelspies
CH = 22 samples from Switzerland
D = 18 samples from Germany
Analyses of Klaus Wallner [4]

2. Influence of the number of treatment years
At one apiary in the Swiss investigation, honey tests were compared from colonies after 2 and 3 years’ long term thymol treatment: residues were clearly higher in the third year. This finding must be confirmed by further tests.

3. Differences in honey types
When different honey types are taken into account, the following thymol residues in Swiss samples of 1997 were found (see table)

From these values the tendency can be seen, that spring honey tends to show higher residues of thymol than the honeydew honey of the summer. Whether this can be confirmed, can only be seen after repeat analyses in future years.

thymol usTable 1

4. Removal of the honey frames during the honey flow
The question arises, whether removing the thymol frame during the honey flow would reduce the residues in the honey. In one apiary the frames were removed during the honey flow. Here the values of thymol residues in the two honey harvests were clearly lower than in the other apiaries. These findings cannot be considered as definitive results because no other measurements exist as yet.

5. Comparison with Apilife VAR
It is interesting to compare the residues after all year use of thymol frames with the ones after application of Apilife VAR. during 8 weeks in autumn (see table below).

thymol us 2
Table 2

The residue after the application of Apilife VAR. was significantly lower, from (p= 0.005), than with the application using the thymol frame.

6. Effectiveness of the Frakno Thymol Frame
In the present investigation the degree of efficiency was not measured.

According to the statements of beekeepers on the 17 apiaries there were no indications of a lack of effectiveness. If however, there is deviation from the recommendation of the all-year treatment (interruption for some weeks or months during the honey flow), a decrease in the effectiveness must be expected and consequences cannot be predicted.

After the whole-year treatment of the colonies with the thymol frame some honey samples show thymol residues in the area of the tolerance value of 0.8mg/kg or above. The possibility that the taste of the honey may be changed cannot be ignored.

Presumably the removal of the frame during the honey flow can decrease the thymol residues but the success of the treatment might be reduced and be no longer sufficient.

Because of the concerns regarding honey quality, the use of the thymol frame cannot yet be recommended for practical application.


  1. Imdorf, A. et al., 1995. Toxicity of thymol, camphor, menthol and eucalyptol in Varroa jacobsoni and Apis mellifera in laboratory tests. Apidologie 26, p.27-31
  2. Imdorf, A. et al., 1994. Apilife VAR. - A new varroacide with thymol as the main ingredient. Bee World 76 77-83 (1995)
  3. Knobelspies, F., 1996, Varroa mites and thymol application in summer. ADIZ, 1996, (6), p. 20-21.
  4. Wallner, K., 1997.In: Report of the Federal Institute of Beekeeping of the University of Hohenheim for the year 1966, ADIZ (3).

Federal Dairy Research Institute - LiebefeldBee Department
CH-3003 Berne Switzerland