Peter The screened Bottom boards are not for the purpose of eliminating V Mites. Their function is the ventilation feature as well as keeping debris normally accumulating off the bottom board. The board ( Can be Cardboard) beneath the screen serves the purpos of monitoring the mite " drop off". The side effect of the SBB is a reduction of mite propagation because the fallen mites do / cannot return to the bees. The mite population increases with the seasons time element , causing eventual demise of the colony. Quality of stock or breed does not seem to enter the equation. I keep adequate control with the application method of FGMO and have not lost a colony to either V or T mites.Of course It is up to the individual to be the judge of her or his method of treatment. Best regards JDF