There are many thoughts around this. Personally I think that all things are accumulating to cause the hive collapse. Whether that be GMO, Neonicticides, pesticides and insecticides.
To Matt's list I'd add loss of habitat diversity, too many bee colonies per square mile, excessive artificial feeding, trucking thousands of miles, the Varroa mite which our bee is not fully adapted to yet, etc.
Here. Have a potato. I grew it in my armpit. And from my other armpit, this tiny ad:
