I have used thymol crystals for varroa since it appeared here in the autumn of 1997. The only other treatment used has been oxalic acid (dripped) for the first two years. This year I have purchased 40 OMFs as I believe that they may have two advantages (already well publicised - mites not being able to return and better overwintering through the increased ventilation). However, I am wondering whether the improved ventilation is likely to reduce the effectiveness of the thymol vapour (method is given here: http://www.stratford-upon-avon.freeserve.co.uk/PENotes/VarroaTreatment.htm). Presumably this would apply to a number of other treatments using substances that evaporate or are dispensed as a fog. Does anyone have any useful experience or sources of information?