Prior posts have discussed the various research done and theories put forth related to colony collapse disorder (see http://en.wikipedia.org/wiki/Colony_collapse_disorder for an overview). The range of potential causes includes pesticides, global warming, disease, and habitat destruction.
The problem has been getting worse since 2006. Once again, researchers believe they may have an answer. It is intriguing, and may even be correct. As often happens with answers to scientific questions, it is multifactoral.
Researchers knew that the pesticide imidacloprid (see http://toxnet.nlm.nih.gov/cgi-bin/sis/search/f?./temp/~bZTZKk:1 and http://en.wikipedia.org/wiki/Imidacloprid) adversely affected a honeybee's ability to learn; imidaclopid is a systemic neonicotinoid insecticide. They wondered if the insecticide had other adverse effects. In particular, because colonies that go on to collapse appear to have a greater and more varied load of parasites and pathogens than other honeybee colonies, they wondered if there was an impact on the bee's immune system (see, for example, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1847501/ and http://www.apimondia.com/apiacta/articles/2003/glinski_1.pdf).
To gain insight they set up an experiment using 30 hives. The bees were feed a protein food (rather than pollen, which is fed to developing larvae) that was spiked with imidacloprid. For ten of the hives the concentration of insecticide was 10 ppb; for another ten, 20 ppb. Previous experiments have demonstrated that neither dose perceptibly harms the honeybee. An additional 10 hives were given unspiked protein food, providing the necessary experimental control. When young bees emerged, they were collected and feed spores of the fungal parasite Nosema (see http://en.wikipedia.org/wiki/Nosema_apis). 12 days later the bees were killed and assessed for their infestation.
Both groups of bees that were feed the spiked protein food harbored an average of 700,000 parasite spores per bee; bees from the "control" colony, less than 200,000. Though the insecticide was at levels thought to be harmless, it was impacting the bee's immune system and causing a more significant infestation to manifest.
Whether this is the actual cause of colony collapse disorder remains to be seen. As they say, only time will tell. However, the thesis is plausible.