michael, we still have plenty of brood down here. i have several hives that are wall to wall brood in at least one deep.
high yesterday was 94 and we'll probably stay in the 80's for another month or so.
honestly, i've never done a mite count and i've never treated for mites. in my worst years between '99 and '06 i probably lost about 1/3 of my hives through the winter. keep in mind that i didn't feed during those years either, if they made it they made it. i was trying to build up survivor bees. unfortunately, several things took me out of beekeeping for a few years so i didn't get to continue.
this year i broke the brood cycle in certain hives at certain times and left others alone. i will feed this winter, though. i made a lot of splits in a poor year for honey so i'm not holding the bees accountable for low stores.
gary, how many frames of brood do you have? bad shotgun pattern? any bees walking around with deformed wings? if you have wall to wall brood but kind of a shotgun pattern and the population just doesn't seem to build as fast as you'd think it would with such a good laying queen you may be infested.
i'd google "uga honeybees keith delaplaine" and see what i came up with if i were you (may not have spelled his name right). that's probably where the most research that would work for our region has gone on that i can think of. someone else may know of more. they shut the apiary studies program down at auburn a long long time ago. george blake was talking about varroa back in the early 80's and the research at that point was to fill the hive with tobacco smoke. he retired in '83 or '84 and the department was gone a few years later. needed room and resources for frat houses and the athletic department more than studying honeybees. oops!
sorry, i got off track. i'm with michael bush, 40 and 60 are just base line numbers. i'd take the count and i'd look at the over all health of the hive before i threw pesticides in it.
let us know what the count is. you've got me curious now.