From Scientific American:
“For almost two years we've been documenting and sampling colonies that are dying and examining healthy colonies in the same area, trying to determine what factors are involved,” Pettis says. “I think there are interactions going on, like low-level pesticide exposure and poor nutrition weakening the host honeybees and then pathogens doing the killing. It's similar to a human who might not be eating, or is frail and traveling too much, and as a result is more susceptible to pathogens. If you go into a hospital in excellent health, you don't contract pneumonia, but if you go in weakened, pneumonia kills you.”
Pesticides and fungicides
How much pesticide exposure is too much for a honey bee? Traditionally, Pettis says, manufacturers seek clearance for pesticides by using the LD-50 test, which “essentially applies toxic stuff to bees and sees if half or more of them drop dead.” This brute force test does not, however, gauge long-term systemic effects. “The general feeling is that we need to move beyond mortality testing to sublethal testing that looks at the shortening of life span, disorientation, reduced vigor, and other things,” says Pettis, who has been in discussions with the U.S. Environmental Protection Agency (EPA) about developing newer, more sensitive pesticide tests.
More here.