A very interesting study confirming the obvious:

Bee image via AFP.Chronic exposure to pesticides has a bigger knock-on effect on bees than conventional probes suggest, according to a new study on Sunday touching on the mysterious collapse of bee colonies.
Biologists at the University of London carried out an exceptional field study into bumblebees exposed to two commonly used agricultural insecticides. ….
… In the colonies exposed to imidacloprid, fewer adult workers emerged from larvae and a higher proportion of foragers failed to return to the nest, the investigators found.
In those exposed to gamma-cyhalothrin, there was a higher death rate among worker bees.
And colonies that were exposed to both kinds of pesticides were likelier to fail.
The experiment was exceptionally long and detailed, the scientists say.

Pesticides are bad for bees.  What else would one expect?