We have heard a lot about colonies collapsing, presumably most of the mites also die when this happens. This looks like a way of selecting the less virulent mites for survival. (It selects beekeepers too.) Perhaps this is why tracheal mites are, in my experience, quite unusual in England now. We have no registered treatment for tracheal mites and almost no complaints from beekeepers about this. Brian Gant Buckfast, UK