In an article in the New York Times http://www.nytimes.com/2012/03/30/science/neocotinoid-pesticides-play-a-role-in-bees-decline-2-studies-find.html common pesticides known as neonicotinoids is named as the culprit in Colony Collapse Disorder.
Here is an excerpt from the article:
"In Thursday’s issue of the journal Science, two teams of researchers published studies suggesting that low levels of a common pesticide can have significant effects on bee colonies. One experiment, conducted by French researchers, indicates that the chemicals fog honeybee brains, making it harder for them to find their way home. The other study, by scientists in Britain, suggests that they keep bumblebees from supplying their hives with enough food to produce new queens.
"The authors of both studies contend that their results raise serious questions about the use of the pesticides, known as neonicotinoids."