When it comes to Varroa control, beekeepers have always been concerned about mites’ resistance to commercial treatments available on the market. It seems the arms race never ends, but changing up treatments throughout the year can help ensure that resistant mites don’t get a foothold. There is a lot of interest in alternative mite control methods, and one that may be a useful addition to the beekeeper’s toolbox is oxalic acid.
Oxalic acid is an organic, naturally occurring compound which can be found in high concentrations in certain plants, notably spinach, rhubarb, and and the aptly named Oxalis. These plants use it as a deterrent against herbivores by making tissues sour and unpalatable (try munching on a raw rhubarb stalk for a demonstration). Oxalic acid is approved for use by beekeepers in Europe, but is still not approved in the U.S. The advantages of using oxalic acid for mite control are as follows: it’s naturally occurring and organic, relatively easy to apply, and is not fat-soluble and therefore does not build up in the wax. The most common method of applying oxalic acid is mixing it with syrup and using a “dribble” technique in the fall or early spring. Mites in sealed brood are not affected, therefore oxalic is not usually used in summer or when there is a significant amount of brood present.
Recently the NorCal Bee Team had the opportunity to assist with a trial of an experimental oxalic acid vaporizer here in NorCal. Oxalic acid clogs normal off-the-shelf foggers, but a local beekeeper/inventor designed a special vaporizer that supposedly does not clog and can save beekeepers time and money when treating for mites. We were called in to assist with the trial by looking at mite levels before and after the fogger was used. The supposed benefits of fogging over dribbling are less ingestion by bees, quicker time to treat (less than 1 minute per hive), and even distribution of acid crystals throughout interior surfaces of the hive. The video below gives an idea of how the fogger is used.
Unfortunately we cannot say if this vaporizer is effective or not because it turned out that there were very few mites to begin with. Another demonstration is being planned so stay tuned!