Fairly independent hypotheses relate to the whole matter.  Each one  
has an accumulated mix of experimental results, quantified  
validations, experiences and anecdotes.  Eventually, if we add that  
most inevitable test of time, reality will prevail and it may all be  
clearer.

Here are some obvious hypotheses:

1) Defined defense mechanisms of  resistance in bees (e.g. hygiene,  
grooming) make mite populations decrease or not grow significantly in  
colonies.

2) These defense mechanisms are genetically based, such that  
environmental influences are minor and the traits work uniformly  
regardless of environment.

3) Life history traits of colonies (seasonal growth patterns,  
swarming, timing of drone production) give colonies an advantage in  
dealing with varroa.

4) These life history traits may be more influenced by environment and  
not as reliable in making colonies uniformly resistant.

5)  Isolation of colonies selects for avirulent mites.

6) Crowding of colonies eliminates any advantage of defense mechanisms  
or life history traits in colonies.

7) Outcrossing dilutes the usefulness of different traits at different  
rates.  (This one in itself can be broken down many ways)

What does reality say about each one of these? 

             ***********************************************
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