If you do use BT, make sure it is the type in Xentari and not the type in Dipel. I have not seen or read where the Dipel version of BT is effective.
Also, do not use it on wet honey frames. Honey is anti-bacterial and may compromise the effectiveness of the BT. If you are going to use it on frames that have just been extracted, let the bees clean them up before spraying.