There's a good discussion of the subject
HERE.
For my two cents, the bee population (all types of bees) in South America has barely been affected by whatever's causing the decline up North. I'd say that maybe pesticide use coupled with untested GMO cropping might be a factor - down here the saying is "if you're using chemicals, you're doing it wrong" and almost all of the countries have rejected GM crops.
This, coupled with an abnormally cold winter season may be taking a toll on the bees.