Oxalic acid against Varroa Destructor
Oxalic acid is extremely effective for the control of Varroa in brood free bee colonies. When applied properly, the three forms of application, spraying, trickling and vaporizing, have an efficacy of more than 95 %. A three-year study showed that the natural content of oxalic acid in spring honey is not increased. Thus, no residue problems are to be expected. The three treatments have shown a good bee tolerability and thus do not significantly differ in this respect from the untreated control.

Treatment with Oxalic Acid
Oxalic acid is a short-lived treatment that only kills mites that are living on the bees (ie. those in a phoretic state). It does not kill mites that are in the brood. When there is brood present in a hive, only about 15% of the mite population are normally on the bees (ie. the rest, 85% are in the brood). It follows, therefore, that oxalic acid works best on colonies that are broodless at the time of treatment. This is the ONLY CONDITION (broodless) in which oxalic acid should be used. It can of course be used on swarms (both natural or artificial) if they suspected of carrying a heavy load of mites (normally they carry very few mites).
Treatment should be delayed until colonies are in a broodless state. In our area, this does not usually occur until mid-December or even January. After the 'Apiguard' treatment (in August-September), it is recommended that the catch tray should be removed, the insulation taken out of the cover board and some top ventilation re-instated. This will make me hive much cooler (I know this will horrify some people) and ensure the cessation of brood-rearing as the weather gets colder. Discretion should be exercised with weak colonies.
The treatment material is 3.2% oxalic acid in a 1:1 sugar solution. The recipe for making this is as follows:
- Make up a sugar syrup consisting of 1 kg sugar in 1 L of water. To this should be added 70 mg of oxalic acid dihydrate and well mixed. This will make 1.67 L of treatment material. Accurate weighing of the oxalic acid is essential because under-strength will give a poor mite kill and over-strength may kill bees!
This sweet solution is poisonous and should be stored securely out of the reach of children!
Procedure for treatment with Oxalic Acid by the Dribble Method :
Fill the 50 ml syringe with treatment solution (oxalic acid). Remove the roof. Remove the top box (shallow or deep) with cover board in place and rest it on the upturned roof. Treat the lower box with about 5 ml solution/occupied seam of bees. Replace upper box. Remove cover board and treat any seams of bees as for the lower box (the upper box will often have no bees, especially if the weather is cold). Replace the cover board and roof.
This procedure usually takes less than a minute/hive. It is safe to carry out the treatment in cold conditions with the temperature down to 0°C. If the weather is cold and the bees are well clustered, they will usually not even have got moving until the deed is accomplished. However, it definitely does pay to ensure you have the correct bee space between boxes so that they come apart readily without disturbing the bees - it is surprising how many hives to not meet this specification.
After the oxalic treatment has been completed, the top insulation should be re-instated and any top ventilation closed-off. Mites will continue to fall for about a fortnight after which the catch tray can be removed.