I operate by the Clean Slate Approach (tm) (not really), and the Assume the Worst and CYA approach for fall/winter treatment.
For both the spring Clean Slate Approach and fall/winter CYA approach:
In fall/winter, I treat until each colony has a mite drop 2 days post treatment of 50 mites or less on the bottom board, 2 times in a row. Some colonies (all 2 medium sized boxes, but 10 12.75" frames) will need only 3 treatments, about 3 weeks apart. Some colonies will need at least 4, and will show mite drops of 1000 + twice in a row. They all overwinter fine.
I interpret the high sustained mite drops as symptoms of an incoming mite bomb because my greedy girls went out and robbed. Starting this year, those get tossed from the gene pool. Hence the "assume the worst and CYA" aspect of my treat-everyone philosophy in fall/winter - there are no alcohol wash mite checks post mid August in my neck of the woods, cuz I don't want to lose winter bees to an alcohol wash.
This sets me up really well to have a clean slate in Spring/summer. I mean, if there were 50 mites on the bottom board, and that 50 represents 90% of the total mites dead, then there aren't that many left... I might even be getting down to 10 mites left.
For the spring Clean Slate Approach:
In May, my threshold for treating is 1 mite in an alcohol wash of 300 bees. I'm quite shocked at the high thresholds listed here; I found that using a higher threshold in May resulted in runaway mite levels by August.
In August, I'm a bit more relaxed, because Winter (treatment) is coming. I have had colonies make it through winter just fine with 13-20/300 in August (0/300 in May, total fall mite drop post OAV was less than 1500 mites, in one hive's case, which tells you how many mites accumulated over the spring/summer), so my threshold in August is higher. It's really challenging to treat the colonies in August because it's too hot to use Formic Pro without a chance of roaching the queen, so my economic threshold there is over 13 mites/300 bees.
For me to be so relaxed in August, I have to be sure in May they are pretty clean; hence the 1/300 threshold in May.
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