Oxalic acid is an organic acid and traces are found in honey. It also forms a vital part of the Integrated Pest Management plan and is effective, relatively inexpensive, and complicated as it all sounds below, is quick and easy to use. It should be used once a year, it does not treat varroa under capped brood and it dissolves in honey so it should be applied after honey stores have been removed and when there is no sealed brood, with temperatures above freezing. The textbooks say that brood rearing is dependent on day length but many beekeepers noted brood throughout last winter, which was notably mild. Given our proximity to the Gulf Stream, it would seem logical to treat close to the shortest day, some time in December.
The concentration of the solution is critical and the BBKA advice is to purchase a prepared solution that is weak and therefore relatively safe. Otherwise purchase oxalic acid bihydrate and mix the solution with great care: oxalic acid is poisonous and corrosive, it should not be inhaled and, if spilt onto the skin, it should immediately be washed off thoroughly. When mixing the solution, acid resistant gloves, goggles and a P2 dust mask should be used. It is mixed in the ratio of 1 litre of water:1kg of sugar:75g oxalic acid; add the acid to the liquid and then keep it in a secure plastic screw top container that is clearly labelled and stored well out of reach of children. It will keep in a cool dark place for 6 months, in reality; once all the beekeeper’s hives have been treated the remaining solution should be poured down the drain. Ordinary kitchen scales are slightly inaccurate so weighing out small amounts can be problematic, oxalic acid is cheap enough to prepare a larger quantity and discard some or share it between beekeepers providing it is transported with care.
By December the colonies will have been closed up for several weeks and the boxes may be well sealed up with propolis. At a home apiary it is worth loosening but not opening up the boxes at the level of the cluster the day before. At an out apiary where a one off visit is more likely, loosen (but do not open) all the brood boxes in turn and then return to the first hive to apply the acid when the bees have had a few minutes to settle and calm down. Opening the hive up in winter is not a great idea, so, ideally choose a sunny warm day and if the boxes are pre-loosened it is possible to operate without causing a major disturbance. Wear all your bee gear and rubber gloves to protect from both bees and oxalic acid and use a little smoke.
The solution should be trickled between the top bars at the top of the cluster at a dose of 5ml per space. It is important to dribble the solution over the bees and space, not over the top bars. The solution should be lukewarm and it is best to apply it with a syringe (bought from a pharmacy). It makes life much easier and safer to take a wide necked deep cylindrical plastic container to the hives and transfer some oxalic acid solution into it and then stand it in a cardboard box. The wide base means it is much less likely to be knocked over and the wide neck allows syringe and hand to be inserted to draw the solution up, the cardboard box provides a safe and stable zone for working so the whole set-up reduces the risk of a spill over the beekeeper. If travelling to an out apiary the ‘stock bottle’ of acid can then be insulated and kept warm with a gel block.
If you are in the habit of giving your bees candy for Christmas, feed as you reassemble the hive to save disturbing them again when it is likely to be colder.