I don't have much experience with OAV but what I've read suggests that the optimum temperatures to treat would be between 40 and 50F. That way most of the bees are still in a cluster but not so tightly that the vapors can't penetrate. I think, once the temps are above 60, I'd wait and treat early am or late afternoon. My thinking is that you'll get a more effective treatment if most of the foragers are in the house. I think you can start any time. Just my inexperienced opinion.
Performing a "flash" treatment on the hive when there is little to no brood (ie before buildup) will kill the vast majority of the phoretic mites depending on your treatment type. Since, there is little brood, there are also few mites within the brood. Most mites are phoretic during this time.
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