This is a post from last year. It was published by Meghan Milbrath from Michigan State. If your bees died, does it fit one of these scenario's?
https://beeinformed.org/2016/03/08/why-did-my-honey-bees-die/
Do a mite check once a month May through October and one week after a mite treatment to see if the mite treatment worked.
How to do a mite check