This was Friday’s last lecture and was delivered by Antonia Nanetti, an apicultural research scientist from Italy.
He told us that Varroa numbers roughly double every month when brood is present and illustrated his point with the old tale of wheat grains on a chess board. Oxalic acid is normally found in food, eg rhubarb, beetroot, peanuts. It is also a natural honey compound, varying with the source: with erica (heather) honey it’s very high.
Sprayed oxalic is about 92/93% efficient. The mix he uses is 4.2 % oxalic in a solution containing 60% sugar, dribbled at a rate of 5mm per seam.. The photo he showed demonstrated that the fluid was dribbled across the frames, not between them.
Casting my mind back over a decade to the 2005 Apimondia, we were told then that the Italians had found that the concentration of sugar was more important than that of the oxalic and that 3.2% Oxalic was as good as 4.2%.
Oxalic is a strong acid with a low pH; stronger than propionic, acetic, citric, formic, tartaric or lactic acids. Trickling is more effective than spraying. Api Bioxal is a registered medicine consisting of 35 grammes of oxalic in 500 mil of 1 kg per litre of sugar solution.
For a summer treatment is is necessary to ensure the colony is broodless and Antonio’s method was to cage the queen for 25 days before trickling the oxalic solution. That is 97 – 99% effective. The number of bees, compared to that of a control, is not greatly affected.
He emphasised that the 25 day queen caging is important. The brood interruption is compatible with honey production and there is a very good brood pattern when the queen has been released.
It is not known why varroa is so sensitive to oxalic acid. It is an external effect.