>His impression is that the removal of "diseased" trees has eliminated the number of available cavities for free-living bees, but that they are still out there.
He tells me that all the cutouts that he does are of dark bees, which suggests that a free-living population continues to exist.

In the UK, I'd keep an eye on this young man - he is doing a lot of research on the scope and nature of feral colonies in the UK (and worldwide):

https://www.researchgate.net/profile/Oliver-Visick

He's got this one teed-up in preprint right now:

'Ancient, veteran and other listed trees as nest sites for wild honey bee, Apis mellifera, colonies'

https://assets.researchsquare.com/files/rs-2718809/v1/54a515d1-8203-49a7-b9b3-6fd4412d11ce.pdf?c=1680216366

As it relates to our recent discussions, it is noted:

'There are approximately 270,000 managed honey bee colonies in Britain (c. 1 per km2) (NBU, 2021) and our results suggest that wild colony numbers may approach or even exceed those of
managed colonies in areas with a high density of TSIs (> 50 per km2). This will have implications for the role of natural selection on colony survival and the contribution of wild colonies to pollination services.'

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