I made up some oxalic acid solution this afternoon with the intention of administering it to my hives over the next week or two in order to knock the varroa mites on the head. Here’s the recipe: take a plastic cider flagon and clean it out. Measure a litre of water, pour it into the container and mark where the water level is. Empty the container and pour in ordinary table sugar until it reaches the line. It should be near enough a kilo bag of sugar.
Empty your kettle. Go into the garden and fill it from the rain water barrel. The reason for using rain water is because it is soft, ie contains no dissolved chalk which would precipitate as calcium oxalate and thus waste some of your oxalic acid. Boil the kettle and add the water little by little to the sugar, giving it a shake every so often to dissolve it. You will notice that the level goes down as the sugar dissolves. Eventually the sugar solution will reach the level you first marked.
Now leave it to get cold. A little of the sugar may precipitate out, but don’t worry. When it is cold add about 40 grammes of oxalic acid crystals and shake it up. If you prefer to measure by volume rather than weight, fill an empty 35ml film container of the type that is becoming a rarity now everybody uses digital cameras.
Lastly, use sticky labels to mark the container with a skull and crossbones and the message that it contains poison, namely a 3.2% solution of oxalic acid acid in sucrose solution (3.2 rather than 4% because there is some water in the crystals). Then put it in the fridge or other cool place until you are ready to use it The reason for this is that over time and with warmth a substance called hydroxymethylfurfuradehyde (HMF for short) will form. This is said to be harmful to bees although I have yet to see any science confirming this. In my view, bees must have been exposed to small doses of HMF practically for ever and ought to be able to cope with it. You can see when your solution is getting too much HMF as it turns brown. A litre solution is probably too much for you to use up in one year unless you have a lot of hives or share your solution with your friends in the BKA. I managed to keep mine for two winters before it started to become discoloured.
I’ll tell you next time how to administer it.
Since writing what’s above, Ruary Rudd (who has a scientific mind) has sent me an email saying that my methods are a bit wonky and querying whether a film container really does hold that amount. I agree that my methods are rough and ready, but they work. There are various schools of thought as to the best strength of the solution. At a lecture at Apimondia in Dublin we were told that the syrup strength is more important than that of the oxalic – strong is better – and that 3.2 % oxalic is better than 4.5. I did check the film container. It does hold 40 grammes of oxalic crystals provided it is lightly patted down with the finger. At present nobody is very sure exactly how/whether the oxalic works against the varroa mites (it might be the placebo effect!) and recommendations and recipes vary so for the average non-scientific beekeeper, near enough is near enough.