For millennia the Varroa mite was kept away from Apis Mellifera by geographical separation, but subsequent to the activities of human beings Varroa has now spread throughout most countries of the world and currently poses one of the biggest challenges facing beekeepers.
With regard to how best to deal with Varroa, beekeepers have become entrenched into two opposing camps: one which advocates treatment, and the other advocating being 'treatment-free' ... which is usually synonymous with being chemical-free, although there are several other types of treatment available, such as physical, and procedural (being essentially methods of brood interruption), but although some of these may be very effective methods at a local level, the overall problem continues to persist.
It is common knowledge that drones from one hive freely enter other hives, regardless of whether that other hive be in the same apiary or not. And although it is generally considered that guard bees prevent foreign foraging bees from entering, this guarding activity is not 100% effective, for sometimes foreign foraging bees DO manage to enter. I first suspected this (the entry of foreign foragers past guard bees) as being the source of communication which leads to robbing under conditions of nectar dearth, and it was effectively 'confirmed' when - having sealed all my hives at midnight on several occasions - one or two bees were observed waiting patiently to be let into their hive entrances the following morning. But where had they spent the night ? These bees were observed to always be in good condition, and so had very clearly spent the night somewhere warm, and most likely had been 'topped-up' with some kind of carbohydrate in order to fuel their return flight home. Spending the night in another hive, at another apiary, is the only obvious conclusion which can be drawn.
This is why 100% mite-kills in (say) a Thermo-Solar hive, or even by the use of numerous Oxalic Acid Vapourisation applications, are doomed to failure if a neighbouring apiary is being run treatment-free, and where a small residual population of mites are being tolerated. Thus, an apiary might well be mite-free immediately after applying the treatment of choice - but will not remain mite-free for very long after that. For beekeeping is a communal activity, in the sense that your bees will always affect neighbouring apiaries, and vice-versa. The philosophies of treatment and treatment-free can then be seen as opposing philosophies and as such cannot co-exist effectively. That is why the problem of Varroa continues to persist, even in those areas where feral colonies are absent or few in number.
To be treatment-free for a number of years may sound like a success story - and I suppose it IS when viewed at the individual apiary level, but when viewed from a wider communal point-of-view, it is just so many years of fostering a seed source of this parasite which may well have been spread during that time by natural inter-apiary foraging activity.
I cannot offer any solutions to this riddle, but it is very obvious to me that the current scenario of some treating for Varroa and some not treating, has generated an untenable situation which is analogous to the constant bailing-out of water from a leaky boat - insomuch as it ain't solving the underlying problem.