Drone Brood Foundation is great for trapping varroa mites!
This is one piece plastic frame/foundation for drone cells. Mites have to reproduce in a capped brood cell, and they prefer drone cells because drone stay in their cells for 24 days. Place a couple of these frames in your brood box and once the cells are capped, then you have trapped the mites. Now, remove the frame and place it in a bad in the freezer overnight. The mites and drones are killed. Scrap open the cells then place back in the hive. The workers will clean out the dead drones along with all the dead mites. This frame/foundation is a bright green so you can easily identify it in the hive.