They have had regular oxalic vapes as and when required, which always seemed to reduce the drop rates which I used to keep an at least weekly eye on as part of assessing the hives (rather than opening up hives all the time).
My remaining hive had two, late Dec/early Jan, despite me not finding much sign of any drop - I put that down to the difficulty of checking with the skirt/funnel in place and nowhere to slide in a board (or vape tool for that matter).
If I ever try a bottomless hive again I will do a bit more prep on how to provide an adjustable bee entrance and a sealable slot for an observation board/vaping. (I had to retrofit the bee entrance when I realised the returning foragers had nowhere to get in!)