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Oxalic Acid – Where do we Stand?

By Anton Imdorf and Jean Daniel Charriere, Bee Research Centre, Liebefeld.  Switzerland

Translated from Schweizerische Bienen-Zeitung, 9/2002 page 14,
by A.E. McArthur MIL

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Oxalic acid has been used as a treatment against Varroa in Eastern European bee colonies for decades. During this time the concept of alternative Varroa treatment in winter with the use of oxalic acid as the key element has become an established practice in our apiaries.

During recent years with much trial and error, initially using the Spray method, then later the Trickle method we have perfected procedures which are suited to the climatic conditions in Switzerland. Initially the optimum dosage and effectiveness were published. The second phase saw the residue levels investigated. During the last two years the main thrust of our research has been focused on the tolerance of the bees to the spray, trickle and sublimation treatments. This work was carried out in close co-operation with the Bee Research Institutes at Kirchhain and Hohenheim.

The effectiveness of large scale trials with electric sublimation was reported over a number of years by the German Association for Natural Beekeeping.

Effectiveness

All three of the oxalic acid treatment methods mentioned, at the correct dosage, achieve an effectiveness of over 95% in broodless colonies. At present there are sublimator devices in use for the sublimation method, whose effectiveness has not yet been sufficiently investigated. If the opening, for the discharging oxalic acid vapour of these sublimators, does not lie directly under the winter cluster, according to the trials results of recent years there is a pronounced reduction in effectiveness (see www.apis.admin.ch under Varroa and oxalic acid).

Bee Tolerance

From the results of exhaustive trials it has been clearly demonstrated, that there is no difference in the bees’ tolerance of the spray, trickle or sublimation methods. This result is valid for a single treatment with the bees in a broodless condition in accordance with the current treatment recommendations. No significant difference in winter survivability or spring development of colonies could be found between the different treatments.

Residues

In our trials conducted over a period of three years, no increase in the natural oxalic acid content of the spring honey occurred as a result of the annually repeated winter treatment either from the spray or the trickle methods relative to the content in untreated colonies. More trials in different countries where in some cases the sublimation method was included produced the same result. Therefore the quality of honey is not affected by a single oxalic acid treatment year on year.

User Toxicity with the Sublimation Method

The question of risk to the beekeeper with the oxalic acid sublimation method was investigated by the Institute of Work and Social Medicine, Tubingen in co-operation with the German Association for Natural Beekeeping. The results of this work are still not available. Without a qualified toxicity statement it is not possible for the Institute for Bee Research to recommend this method to beekeepers. As soon as a positive result from the toxicologists is available we will include the sublimation of oxalic acid in the list of therapeutic treatment for Swiss Beekeeping. The spray and trickle methods are already on this list.

The Institute will report continuously on the current situation on our Homepage ( www.apis.admin.ch under bee diseases and also periodically in the Swiss Beekeeping Magazine.)

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