Oxalic Acid.

[Jon]

I just checked some trays below colonies which got oxalic 6 days ago and there were far more mites than I expected.
Normally when I apply Oxalic I might find a dozen or so mites but the 4 trays I looked at had up to 200 mites on each.
My best guess is that the cool weather late summer meant that the Apiguard was not as efficient as usual.
This could also explain why I have found that clusters are smaller than usual. More mites and virus produces shorter lived bees.