I was at LASI (University of Sussex) yesterday for a varroa 'workshop'.
They are great believers in vaporising Oxalic (after ensuring the hive is broodless, and treating in December - not later), and "Hygienic" bees (especially in combination with Oxalic vaporising).
They are prepared to do a second midwinter vaporising, a week after the first, having again ensured there is no brood, and this reduces the surviving varroa in the colony almost to single figures.
They have also tested the efficacy of vaporising when there is sealed brood present, and found that it then 'got' at most about 45% of the varroa. (Tested in April & May.)
Accordingly, they do not advise this as it only 'buys' about 3 weeks (in that time, the varroa population will have rebuilt to pre-treatment levels).
However, they have not examined multiple-repeat vaporisation at 5 day intervals with brood present, but suggest that it might have to be repeated through two entire brood cycles (like Apiguard) to be properly effective.
Asked about vaporising with honey on the hive, Hasan Al Toufailia (the lead researcher on Oxalic) said that although the amount (from a single treatment) was small, it wasn't the sort of thing that he would want to do.
Additional points of possible interest include their now preference for vaporising from below the OMF, leaving the vaporiser in place and (sponge strip) sealed-in for a couple of minutes after switch off before removing, and their insistence on using a mask rated for use against inorganic acid vapours - as no "dust mask" will deal with the tiny particles resulting from vaporising Oxalic. The Health & Safety specialist was insistent that the masks sold by Thornes were utterly useless for this purpose.
The "Hygienic Bees" topics were interesting - especially as one can do a "Freeze-Killed Brood Assay" without using liquid nitrogen. The liquid nitrogen H&S bit was scary interesting ... a bee suit and veil gives very good protection, but tuck your trousers OUTSIDE your boots when handling liquid nitrogen.
The benefits of using 'hygienic' bees (including reduced DWV titres) only really shows up with 95+% removal of Freeze-Killed Brood, and 100% is said to be easily achievable by normal Q selection methods, even with open mating.
The FKB assay needs to repeated, say 3x, to get an accurate score for a queen/colony, and is best done when there isn't a strong flow.
All the various "varroa resistant" bee-breeding programmes so far have produced bees with these high hygiene scores.
Oh, and in South America, it isn't just that the bees are 'africanised' and nasty, they also have a less virulent form of varroa (the Korean/Japanese haplotype).
An interesting and packed afternoon.
/// ADDED - should have mentioned that, despite their opening up for a brood-culling full inspection in December, their winter colony loss rate is running at about 2% ... compared to the 14% reported by the latest BBKA survey.