>>Oxalic in late fall is not a safe for the coming winter but for the winter after that. This is an excellent point. OA is very effective - from 85% to 100% depending on the study - in eliminating varroa. It's a pleasure to see colonies of vibrant honey bees the following spring. Barring mite re-infestation and perhaps susceptible genetics, these colonies will continue strong until the next fall. That next fall I like to identify the best hives, in terms of honey production and mite tolerance, for queen rearing the following spring. Hopefully, one day I'll be able to either skip the OA treatment or do it every other year. As far as the effectiveness, I believe, if one uses precisely the prescribed method (ie. the 3.5% concentration and 4-5 ml of solution per a full Langstroth bee space) in the broodless period, one consistently approaches a 100% mite kill w/o serious harm to the bees. Waldemar -- Visit www.honeybeeworld.com/bee-l for rules, FAQ and other info ---