In the fall of 2006, some beekeepers in the U.S. reported losing 30-90% of their colonies and the symptoms associated with the dead colonies did not match those produced by know bee pests / pathogens. Since then several groups of scientists have tried to find a cause for it.
CCD symptoms often associated with collapsing (weakening) colonies may include:
an insufficient number of bees to maintain the amount of brood in the colony;
the workforce is composed largely of younger adult bees;
the queen is present; and
the cluster of bees is reluctant to consume food provided to them by the beekeeper.
In collapsed (dead) colonies, CCD may produce the following symptoms:
the complete absence of adult bees in colonies with few or no dead bees in / around colonies;
the presence of capped brood; and
the presence of food stores that are not robbed by other bees or typical colony pests.
After 5 years of studies it seems like CCD is caused by a combination of a variety of factors, possible including mites, viruses and pesticides. Some studies have also checked the influence of cell phone signals and climate change. While global warming may not directly challenge a species of insect that can prosper from very cold climates to the equator, climate change may result in more stress on the bees. Increased periods of dry, hot weather or cold, rainy weather, could limit availability and access to those important pollens. The bees will have to rear their brood at the expense of their body nutrient reserves. It does not seem to affect feral bee populations, which would point towards something that is used in commercial beekeeping.
Further studies are ongoing. For the latest finding check out the US Dept. of Agriculture or the Bee Health websites.