A paper accepted for publication in Apidologie describes the  
incorporation of a trait for resistance to mites into an "Italian"  
pollination based bee population.  It is possible, perhaps not easy,  
and takes time.  Also, as some have pointed out, the efforts can get  
diluted quickly in the real world.

http://www.ars.usda.gov/research/publications/publications.htm?seq_no_115=317665

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