Until this year I've been dosing with VOA once a year during one of the warm days we often get here in mid-winter - which has proved enough to keep the Varroa problem in check - but had been planning to engage in a program of multiple-dosing during the season from now on ...
But as someone pointed out when I mentioned my use of anti-robbing screens and wider spacing between hives to help stop intra-apiary infestation - none of these measures will stop the bees from bringing back into the apiary mites they pick up from outside of it.
This has caused me to challenge (in the academic sense) the rationale behind attempting to eliminate every last mite from a colony with 4 doses every 5 days, because of this subsequent mite importation from outside.
I was particularly impressed by a talk given by Marion Ellis about the Varroa mite [https://www.youtube.com/watch?v=q4WvPNmS7uc] in which he graphically demonstrated the consequences of treating mites at various times of the year - i.e. the mite populations which later resulted - the object of doing so being NOT to eliminate every last mite, but to decimate the mite population sufficiently such that it never has enough time in which to recover the numbers required to cause a colony collapse.
I'm now wondering whether such a strategy of 'acceptance' might be preferable to the pursuit of a complete (albeit temporary) elimination - with a single dose (say) being given monthly, or bi-monthly throughout the season - that is, for those with really large numbers of colonies.