Tom,
Greetings from England.  Bees are always reluctant to leave brood as I found
to my cost last year.  I cleared bees from supers in 2 or 3 hives with
benzaldehyde.  There are always a few left which I shake off on the way home.
This time I found a sizeable knot of bees in one of the supers when I shook
them.  When I came to extract I found the reason why - brood.  I assumed the
queen was also anong the bees I shook off at the side of the road - but that's
another story.
Chris Slade