more than 2 mites natuaral drops in 24 hours. TREAT! Sorry to be that short. if you do not treat you will loose your hive. Hivelife from first infection to dead 3 years. And my advice don't relay on fancy treatments. If you use apistan and byvarol strictly follow the written labels. otherwise the risk will be resistant varroa.
Danish treatment:
Spring formic acid
Summer drone removement
autoumn when the hive is broodless oxalic acid