Two new studies show that widely used pesticides - more specifically neonicotinoids, which are more common in Europe, and coumaphos, which are more common in the U.S. - are damaging the brain of honey bees, reducing their capability to learn and remember, and thus making them less effective foragers. Not only that, but while each chemical is damaging on its own, their effects also combine and add up, so that bees exposed to both suffer even worse neurological damage.
The scientists are recommending that their findings be taken into account when the toxicity of a chemical is being tested; it's not enough to see if the chemical will kill outright a bee, other kinds of impacts should be tested for, because bees live in complex societies and if their abilities to perform certain tasks is affected, it could lead to collapse even if individuals don't immediately die.
Sami has been covering everything surrounding bees for a long time. For more on the topic, see these posts.