Just picking another, the researchers did in situ replication of pesticide doses known to be prevalent in working hives, along with control colonies that were untreated. Whats happening is that in the last decade beekeepers largely switched to feeding their colonies High Fructose Corn Syrup from corn treated with neonicotinoid pesticides. Their results:

"15 of 16 imidacloprid- treated hives (94%) were dead across 4 apiaries 23 weeks post imidacloprid dosing. Dead hives were remarkably empty except for stores of food and some pollen left, a resemblance of CCD."

While none of the control hives died.

Their conclusion:

"Data from this in situ study provide convincing evidence that exposure to sub-lethal levels of imidacloprid in HFCS causes honey bees to exhibit symptoms consistent to CCD 23 weeks post imidacloprid dosing. The survival of the control hives managed alongside with the pesticide-treated hives unequivocally augments this conclusion. The observed delayed mortality in honey bees caused by imidacloprid in HFCS is a novel and plausible mechanism for CCD, and should be validated in future studies."