[log in to unmask] wrote about a hive that refuses to recover (my
words, not his).

He described the hive as "went queenless, then laying worker, ... combined
them with a
queenright nuc on top, settled down, but then faltered ... I suspected that
the hive may have been too close to an apple orchard, and that pesticide
spraying might be affecting them...."

I highly suspect that there is a pathogen present in your combs.  It could
be that the original hive was the demise of the queenright nuc rather than
your new colony was done in by apple orchard spraying.  What you describe
sounds like a classic case of AFB infected equipment being ure-used.  A
possibility is that the queenless hive harbored AFB, uniting with the nuc
gave the new queen an opportunity to lay in the infected equipment which
developed and then succumbed.  If you can't test yourself (no microscope,
don't know how, whatever), you might want to send some comb out for testing
before giving the twice failed equipment another try.

Submitted as a possible alternative.

Sincerely,
Aaron Morris