Menthol is used primarily and exclusively for the control of tracheal mites. It does not work well or at all on Varroa mites because of their development cycle and where it takes place. Varroa adults are just too big to be affected significantly by menthol vapors.
Although this seems like a simple question, a couple of thoughts come to mind:
- If you treat once a year, the chance of your own population of mites building up increases because of the time between treatments. Plus you have to be aware that drifting bees will be bringing mites into a particular hive all the time.
- If you treat too often, then you are accelerating the selection, (survival of the fittest) of the mites most resistant to the mite treatment. These mites breed and, of course, you have a population of mites that is resistant to a particular miticide. There has to be a happy medium somewhere. We suggest starting off with treating in the spring before buildup and honeyflows and in the fall in preparation for winter. Twice-a-year treatments are important in areas with a high density of colonies as well as high varroa mite populations. You will have to adjust your treatments as you get a feel for the seriousness of the varroa problem in your area.