I have used mite-away II since 2005 when it was approved for use in the US. Like any treatment if is vitally important to follow the directions, and one very important direction is temperature during application. Especially during the first week. If the daytime high will exceed 82 degrees you need to remove the pads and re-apply when temperatures fall into porper band, if not high mortality will occur.
I have never experienced any noticeable bee mortality using it. If you are, then reading the directions should solve the problem. Formic acid treatment isn’t new they have been using it in Canada and Europe for over twenty years.
The Busy Bee teaches two lessons: One is not to be idle and the other is not to get stung.