Peter, 
   You raise an interesting subject for discussion.  I see two separate but
related issues here.  One; is propolis as effective as it is claimed to be
in contributing to A. melefera’s bio defense system and two; why is it such
a good idea to cycle out old brood comb every three years or so.  
   I always understood the primary function of propolis to be sealing up
cracks that might adversely effect air circulation and where uncleanable
surfaces might occur such as under the ends of frames and places where
pieces of wood are joined.  I was unaware that bees use propolis to line
brood cells.  The possibility that propolis has any anti pathogenic
properties would, I always thought, just add to its perfectness as a
material for this primary function.  
     As far as rotating old brood comb, my understanding was that the main
impetus for this trend arose as a result of the discovery that toxic
chemicals were building up in comb from the use of harsh chemical miticides.
 When it comes to build up of desease causing organisms and their spores, I
would think it’s a matter of either you got it or you don’t.  If you got it,
you probably don’t want to wait too long to get rid of it, right?  If you
don’t got it, why would you go to the trouble and expense.  It seems worth
noting that swarms are attracted to old brood comb.  If it was inherently
bad for them, why would this be the case?
   As usual, I don’t have any answers, only questions.

Steve Noble

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned 
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html