From The New York Times:
More than a quarter of the country’s 2.4 million bee colonies have been lost — tens of billions of bees, according to an estimate from the Apiary Inspectors of America, a national group that tracks beekeeping. So far, no one can say what is causing the bees to become disoriented and fail to return to their hives. As with any great mystery, a number of theories have been posed, and many seem to researchers to be more science fiction than science. People have blamed genetically modified crops, cellular phone towers and high-voltage transmission lines for the disappearances.
Honeybees are arguably the insects that are most important to the human food chain. They are the principal pollinators of hundreds of fruits, vegetables, flowers and nuts. The number of bee colonies has been declining since the 1940s, even as the crops that rely on them, such as California almonds, have grown.
Genetic testing at Columbia University has revealed the presence of multiple micro-organisms in bees from hives or colonies that are in decline, suggesting that something is weakening their immune system. The researchers have found some fungi in the affected bees that are found in humans whose immune systems have been suppressed by the Acquired Immune Deficiency Syndrome or cancer.
More here.