Nature's way of telling you...something's wrong.
It's been all over the news in Florida: no bees, no citrus. I've heard of the mites too, but there seems to be another problem here too, "hive collapse syndrome". One day they all just leave. Nobody knows where.
This link thinks it might be related to tracheal mites, and lack of genetic diversity...
http://www.thenewstribune.com/news/columnists/merryman/story/6403792p-5710288c.html