Current research by Jeff Pettis  shows that the most widely used pesticide, imidacloprid, compromises bees’ immune system even when present in sub-lethal doses. Bees thus weakened are very susceptible to infection with other pathogens.
Both of the groups that had been exposed to imidacloprid harboured an average of 700,000 parasite spores in each bee. Bees from the control colonies, by contrast, harboured fewer than 200,000 spores in their bodies. The insecticide, in other words, was exposing bees to infestation, and thus to a much greater chance of dying prematurely.
Other research [2,3,4] had similar findings. In fact, the pesticide may be enough in producing colony collapse disorder even without the presence of a pathogen, merely by affecting their nervous system, as suggested by Lu & al. :
[T]o test the effect of even small amounts of these pesticides on western honeybees …, Harvard researchers treated 16 hives with different levels of imidacloprid, leaving four hives untreated. After 12 weeks, the bees in all twenty hives—treated and untreated—were alive, though those treated with the highest does of imidacloprid appeared weaker. But by 23 weeks everything had changed: 15 out of the 16 hives (94 percent) treated with imidacloprid underwent classic Colony Collapse Disorder: hives were largely empty with only a few young bees surviving. The adults had simply vanished. The hives that received the highest doses of imidacloprid collapsed first. Meanwhile the four untreated hives were healthy
They manage to replicate the exact conditions of colony collapse disorder. This suggests that their explanation is likely the correct one.
 Jeffery S. Pettis, Dennis vanEngelsdorp & al., Pesticide exposure in honey bees results in increased levels of the gut pathogen Nosema, Naturwissenschaften, Jan 2012. DOI 10.1007/s00114-011-0881-1
 Chensheng Lu, Kenneth M. Warchol, Richard A. Callahan. In situ replication of honey bee colony collapse disorder. Bulletin of Insectology. 2012.
 Penelope R. Whitehorn, Stephanie O’Connor, & al., Neonicotinoid Pesticide Reduces Bumble Bee Colony Growth and Queen Production, Science, Mar 2012. DOI: 10.1126/science.1215025
 Mickaël Henry, Maxime Béguin, & al., A Common Pesticide Decreases Foraging Success and Survival in Honey Bees, Science, Mar 2012. DOI: 10.1126/science.1215039