In hives affected by Colony Colapse Disorder (CCD), first observed on sunflowers in France, there may only be a few bees around the queen, or none at all. Bees are not seen leaving the hive, and where they go is a mystery. Clinical evidence for hives collapsing due to disease pathogens such as the varroa mite, Nosema ceranae or viruses is lacking. Varroa and its proliferation in hives is clearly visible with the naked eye, but the hive does not disappear so quickly as those affected by CCD. A correlation between the varroa infestation level of an apiary and the disappearance recorded has not been demonstrated. Bees in apiaries infested by Nosema ceranae do not have the key symptom: diarrhea. The Acute Paralysis Virus is too virulent and would kill the infected bee before contaminating other hosts or vectors. An infection with the Deformed Wing Virus is clearly visible (atrophied wings and a small stomach) and does not explain the rapid collapse of hives during sunflower harvesting.
Source: Janine Kievits (2007) Pesticide News 76: 3-5