Oxalic Acid Trickling to kill Varroa Mites

Margaret Cowley

In the depth of winter the queen often stops laying eggs. This means there is a period of up to about a month around New Year when there is no brood (larvae) in the hive. We can take advantage of this to kill a high percentage of varroa mites, using oxalic acid solution, as they are all on the adults, not hiding away in cells sucking the blood of larvae as they are at other times of the year. Oxalic acid solution should not be used when there is brood present as it can cause larvae to be removed. The treatment can be so effective that it need only be used once a year.

Although oxalic acid is a natural poison (we all know not to eat rhubarb leaves which contain it), research reported in 2014 seems to indicate that it does not adversely affect queens or worker bee adults.

Only approved oxalic acid products should be used. To check if a product is licenced for use in the UK check the Veterinary Medicines Directorate Product Information Database. The actual application is quick and easy. See a video I made showing you how. The important points to note are:

1          Apply the solution during a broodless period, according to the maker’s instructions. This will involve making it up into solution from powder by mixing with syrup.

Practising trickling using water

Practising trickling using water

2         Choose a still day and don your beesuit, veil and gloves. Warm the oxalic acid solution in a jug of hot water.

3          Remove the roof and crownboard. Remove the super if you have one, until you are above the bee cluster. Peering down between the seams, you will be able to see the bees below.

4         Dispense by dribbling about 5 ml of the solution per seam (the space between the frames).

5          Replace the super (if you have one on) and then the crownboard and roof. If you noticed bees in the super, reserve some solution to treat them with.

6          Continue to monitor the natural mite fall on the tray below your Open Mesh Floor.

In previous months I have advocated using medications against varroa only if the number of mites exceeded a certain threshold. For this winter treatment, however, I suggest treating even if you only observe a few mites in your monitoring because it is so effective (up to 95% of mites are killed). Research has shown that this organic acid, which is often found naturally in certain honeys, degrades to harmless carbon dioxide and water very quickly. It is especially useful if your neighbour has not read Bee Craft, and has omitted to monitor and treat for varroa. His/her colonies can be a source of re-infection for yours during the autumn months.

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