Antimicrobial Defences Increase with Sociality in Bees 
Biology Letters, Issue Volume 3, Number 4 / August 22, 2007, Pages
422-424 
http://apitherapy.blogspot.com/2007/07/bees-production-of-antimicrobial.html

...Our results suggest that selection by microbial pathogens was
critical to the evolution of sociality and required the production of
strong, front-line antimicrobial defences.

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