I don't have the figures in front of me, but the average mite count during the year looked something like those below. This is an average, using a sugar shake, with thousands of hives tested by the Pa. state ag. department.
And this is the same pattern also every year, as conducted and seen by the state inspectors. Mites will have a 4 to 10 times multiplication factor between the middle of August and the middle of October.
You can have low mite counts almost all year long. But when August rolls around, thats when the problems start. And this is why early treatment at that time, prior to the fall brood cycle is best, if your going to treat.
Can I ask, what type bees are you running?