An article on Phys.org's website reviewed a recent study at the University of Illinois in which the researchers found a plausible correlation between feeding of corn syrup and Colony Collapse Disorder.
"The researchers aren't suggesting that high-fructose corn syrup is itself toxic to bees, instead, they say their findings indicate that by eating the replacement food instead of honey, the bees are not being exposed to other chemicals that help the bees fight off toxins, such as those found in pesticides."
"Specifically, they found that when bees are exposed to the enzyme p-coumaric, their immune system appears stronger—it turns on detoxification genes. P-coumaric is found in pollen walls, not nectar, and makes its way into honey inadvertently via sticking to the legs of bees as they visit flowers. Similarly, the team discovered other compounds found in poplar sap that appear to do much the same thing. It all together adds up to a diet that helps bees fight off toxins, the researchers report."