I used formic acid last year but going into fall I wasn't happy that the mites were under under control.
[A strong hive in spring - the old mite pad from fall still on the top bars].
I've used Miteaway Quick Strips - called MAQ's. These formic acid gel packs can be placed on a hive while honey supers are on. This is such a relief to beekeeping.
So I planned to treat the hives in mid July to try to get some control over them.
The hard decision is whether to go gangbusters with 2 pads or to do a softer treatment with 1 pad. One pad is called a knockdown and two is a knockout.
The problem is that beekeepers find too much brood are killed and queen deaths occur a lot when using 2 pads.
It's a 7 day treatment and the pad is biodegradable and will be chewed up by the bees so it doesn't have to be removed. I usually remove it well after the 7 days.
This year I did 1 1/2 pads in spring and plan to also treat mid summer and fall. The goal is to get more serious to take down the mites.