Absolutely not suggesting that Icing sugar is suitable as a main treatment nor that is terribly effective in isolation and I think if you start using it you pretty much have to keep using it every inspection throughout the season AND THEN TREAT with something effective.

I'm tyring to find the link for it, but I read a report suggesting that the lower the mite count the more reproductive varroa become.

But I do still believe that Icing sugar has a place if nothing else because I watch our ex inspector use it gauge mite levels on bee samples. Stick bees in jam jar, cover in icing sugar, shake them around a bit and see what drops off.

But let's be absolutely clear here I am not suggesting that icing sugar, in isolation, is a suitable control for varroa any more than having an open mesh floor is varroa control but it has its place as part of an IPM scheme. Whether you judge it to be worthwhile is a different question.