A colony definitely broodless in October is a good candidate for OA* trickling ... there will still be mites left over from the late summer treatment and, if they're all phoretic, they're easy to get. By January, many colonies have started rearing brood again.
You could do some calculations, but I bet that 95% knockdown of
all mites now would be preferable - in terms of mite numbers by late Spring - to 95% of the phoretic population once the colony has brood in early January.
I treated with OA last year in late November after a protracted cold period. I didn't check for brood but someone - FD perhaps? - commented that their colonies were broodless when checked about the same time. My
Varroa levels this year have been really low.
* I mean Api-Bioxal of course
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