When and how often to treat
How often you monitor and/or treat your colonies for mites, again depends on how much of your time and effort you are willing to put into it and/or how susceptible you think your bees may be to varroa. If you want to keep treatments to a bare minimum, we highly recommend that you treat at least passively in autumn, after you have harvested honey (and thus reduced the size of the hive to a maximum of 3 hive bodies; preferably two) and after the first frost. The reason that fall treating is so important is because it is then that treatments will be most effective. Drone production typically stops in the late summer to early fall, which greatly increases the number of mites in the hive that are what is referred to as "phoretic". This means that they are out and about, feeding on adult bees, rather than breeding (hiding) in the capped brood cells. As the weather turns colder and the nectar stops flowing, mass production of worker brood also comes to a halt. The queen simply stops laying eggs because fewer bees, not more, are needed as winter approaches. Although there will almost always be some small number of cells with brood in them, it is at this time that 95% or more of the mites within the hive are phoretic. Phoretic mites are far more vulnerable to all types of treatments than are non-phoretic ones, but even more so with organic treatments. In fact, powered sugar dusting, which is a highly effective means of removing phoretic mites from the hive, will do absolutely nothing to non-phoretic mites.
Although one passive treatment in the fall may suffice as a minimum, depending on whether or not your bees are regressed, how many drones they produce, etc., actively treating in both the fall and the spring may be necessary depending on these same factors. Sometimes, mite levels can build up rapidly in the spring as the bees produce lots of brood, including lots of drones. There is a much higher chance of this happening if your bees are not regressed. The risk you run by not treating in the spring, is that if you find yourself in an emergency situation during the honey flow (if you see bees with DWV), your treatment options will be limited to only powdered sugar dusting in order to prevent contaminating your honey, and by then the hive is likely to be so large that the powdered sugar dusting may not even be viable or effective. An emergency situation such as this might leave you with no option, other than to use harsh chemical treatments in order to save your bees...a dilemma that you surely do not want to be faced with.