I use formic acid USDA approved organic. The mite away II strips are easy to do. We also incorporate screen bottom boards, powder sugar dusting before we put honey supers on and Drone comb that gets switched out every 20 days.
Here you also need to feed syrup in the spring and in the fall with Fumigillan B for Nosema Creana control.
We also had a problem with some of our bee yards next to big farms with contaminated pollen from GMO corn and Beans.
Use the Beltsville Maryland Bee Lab if in doubt what caused your bees to die. It is a free service.