Agree with the others. I have screen bottom boards where I can check the mite drop every couple weeks. When I see 10ish dropped mites, I treat the Oxalic Vaproizer pronto. I keep my hives pretty darn clean of mites.
However, it has amazed me how quickly a mite load of 2,000 dropped mites can happen, like 2 or 3 weeks. Like Jacobs, every year in the beginning of the fall season, I notice some deformed wings virus. Let Me Tell Ya! That can happen in a blink of an eye, and I am on it like lighting.
Sorry to hear of your lost bees :(