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Yes ... it was bugging me a bit -it didn't make a lot of sense putting it at the very bottom of the hive where very few bees really go and then I thought the bees spend a lot of time smoothing things out inside the hive anyway - they tend to chew off any rough splinters from my home made frames and I find the debris on the hive floor. If they were that worried about scratching then they would be scratching on splinters and bits of stuff already in the hive.
What I can't reconcile is the video of them apparently rubbing up against the bits of the 'Bee gym' .. so what ARE they doing ? My guess is they are using their bodies to try and determine what the projections are and then, presumably, they'll be breaking their teeth trying to smooth it out -- or perhaps they are just scent marking the projections so that the propolisers can come along later and smooth things out ?
Who knows ... I'm not convinced (even though I lean towards the dark side of beekeeping) that it's an answer. Perhaps my feeder will stay as a feeder rather than being turned into a bee scratching post and varroa trap !