Around August, it is a good idea to do a varroa check in your hive. You can either do a mite drop over an hour, or a drone comb check.
I harvested some honey from my Yellow Top Bar Hive. I got a 4 sq inch section of drone in the honeycomb that I cutout. So while I had it, i opened up 30 cells and found a single mite in every 3rd cell. One cell had 2 mites.