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Residues in Honey after Treating against Varroa using the Thymol Frame

By Stephan Bogdanov, Verena Kilchenmann, Anton Imdorf, and Peter Fluri, Apicultural Dept, FAM, Liebefeld, Switzerland.

Translated from the Schweizerische Bienen Zeitung, 4/98, pp 224–226

By A.E.McArthur MIL

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Thymol has shown itself to be an effective treatment against Varroa and it is tolerated well by the bees. This substance has been used in Switzerland since 1994 mainly in the form of “Api-life Var” which is registered by the International Office for the Regulation of Medicines. The treatment is carried out after the removal of the honey harvest during late summer into autumn for a period of 8 weeks. The repeated use of this method even over a number of years causes no excessive residues to build up in either the wax or honey.

Over the last 3 years a modification to this treatment has appeared in the shape of the “Frakno-Thymol Frame” designed by Franz Knobelspies. He recommends contrary to the “Api-life Var” method that the colonies should be treated on a continuous basis with thymol. This of course begs the question of whether the quality of the honey might be affected by thymol residues. This would not be a health problem as such, however the taste of the honey could be influenced and this would contravene the current food regulations. The food regulations in Switzerland and also those of the EU prohibit any additions to honey which would alter its natural taste.

Tests on the Taste of Honey

In Switzerland the tolerance level for thymol is 0,8mg/kg of honey and at this level the consumer palate is unable to taste the thymol. The threshold for taste being altered is about 1,1mg/kg.

The reason for the current work was concern for the quality of honey where thymol was resident for all of the year in the colony. The Apicultural Institute of the University of Hohenheim released results in 1997 which showed that a fifth of German honey from colonies treated using the thymol frame indicated a thymol content above the Swiss tolerance threshold of 0,8mg/kg. The aim of our current study is to establish the level of thymol residues present in honey after the long term use of the Thymol Frame by Swiss beekeepers. Although the study was only carried out over a single season and the results are therefore only provisional, it was decided because of their current importance they ought to be made available to beekeepers now.

Samples and Analysis

The various honey sorts came from the apiaries of 17 beekeepers (14 using Swiss Standard hives and 3 using magazine hives) and was harvested in 1997. The Varroa treatment was carried out during 1996 and 1997 (in one case since 1995) using the protracted thymol treatment recommended by Knobelspies. The special frames remaining in the brood chamber for the whole year, being recharged with 12g of thymol crystals twice annually ( as a rule in March/April and again in July/August). The active substance evaporates steadily from the frames over the months. One beekeeper removed the frames at the start of April for the whole honey gathering period. A total of 22 samples were tested. The sources of the honey were established by measuring the conductivity of the samples. Accordingly there were 6 flower honey, 6 honeydew and 10 flower and honeydew mixtures samples. The thymol content was established using gaschromatography. The detectability limit for thymol is 0,02mg/kg.

Results and Discussion

1.Residues after long term application of thymol

The results of the Liebefeld(1997) and German(1996) research are shown in fig 1. The average value of the 22 Swiss samples is 0,33mg thymol/kg honey ( the minimum value <0,02mg/kg, the maximum value is 0.83mg/kg ). One sample is slightly over the upper tolerance limit of 0,8mg/kg. The residues measured in the German honey at Hohenheim (Klaus Wallner) in 1996 were in general higher than those in the Swiss samples: Average value 0.63; Minimum value 0.07; Maximum value 2,00mg/kg. The reasons for the higher German values were not investigated deeper. A factor could be presumed to be the greater number of years of treatment in the German apiaries.

2. Affect of the Number of Years of Treatment

In one apiary in Switzerland samples from colonies which had been subject to long term thymol treatment were compared in the second and third year: The 3rd year sample showed a markedly higher level of residues. This result will have to be confirmed by further analysis.

3 Variations within the Types of Honey

The different Swiss honey types from 1997 were compared and the residue results tabled below:

  Avg mg/kg Min mg/kg  Max mg/kg
Flower Honey (6 samples) 0,52 0.40 0,83
Mixture Honey(10 samples) 0,25 0,08 0,54
Honeydew(6 samples) 0,29 <0,02 0,63
Thymol Frame.
Long term application of
2 – 3 years (22 samples)
0,33 <0,02 0,83
Apilife VAR
8 weeks in autumn
1 – 5 years (28 samples)
0,16 <0,02 0.48

The residues after treatment with Apilife VAR were significantly lower
(p = 0,005) than with the Thymol Frame.

From these values it can be seen that the spring honey has a higher lever of residues than the honeydew from later in the summer. Whether this will hold true in future sampling must await a repeat of these analyses in subsequent years.

4. Withdrawal of the Thymol Frame during the Nectar Flows

There was a question on the effect on the residue levels of removing the thymol frame during the nectar flow. The frames were removed at one apiary during the nectar flow period. In this case the residue levels for the second honey harvest were considerably lower than those at other apiaries. This finding cannot of course be considered as definitive, because no other measurements are available.

5. Comparison with Apilife VAR

It is interesting to compare the residue levels after long term treatment using the thymol frame, with the residues resulting from the application of Apilife VAR during the 8 week period in the autumn (see table above).

6. Effectiveness of the Frakno Thymol Frame

The effectiveness of the Frakno Frame was not considered during this research. According to the views of the beekeepers, there was no cause to believe that the frame was ineffective in its use in the 17 test apiaries. Of course when the recommended long term treatment is interrupted (removal of the frame during the weeks or months of nectar flows), there could be a fall off in the effectiveness of the frame whose consequences are not known.

Conclusions

After long term treatment of colonies using the thymol frame a number of the honey samples showed a thymol residue level in the region of the tolerance limit (0,8mg/kg) or above this limit. It is possible that the honey taste could also be affected.

The removal of the thymol frame during the honey flow may probably reduce the residue levels. Doing this would reduce the effectiveness of the treatment and possibly cause it to be ineffective against the mite.

Due to the misgivings mentioned regarding honey quality the use of the THYMOL FRAME in general beekeeping practice cannot at this point in time be recommended.

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