What is causing the decline of honey bee populations?

The population decline of honey bees started in this country in the mid 1980’s when two new parasitic mites were introduced. Most of our bees have pretty good resistance now to one of these, the tracheal mite, but there are still some bees killed by them. The Varroa mite continues to kill our bees. We use plastic strips with chemicals in our hives to kill the mites. They have virtually wiped out the feral honey bees and the number of managed hives in Indiana has declined by almost two thirds.
The only good study on the size of the decrease was in California where about 90% of the feral colonies were killed by mites in the first two years after they arrived. This is happening all over the US and also in Europe and other places, like Mexico, Canada etc. It is a worldwide problem.
The Varroa mites came from the Asian honey bee, Apis cerana, which can tolerate the mites. There are about 6 species called honey bees, in the genus Apis. You cannot cross them with each other so we cannot breed the resistance into our bees. There are about 20 subspecies of our Apis mellifera, and they all look fairly similar.
Probably the major cause of the decrease is Varroa mites, and the viruses associated with what is called “parasitic mite syndrome”. The mites feed on pupae and ride on adults. They are big enough to see with the naked eye.
However, a recent phenomenon referred to as “colony collapse disorder” has been reported to kill many hives in 2006 and 2007. The set of symptoms includes rapid dwindling of the population, resulting in just a handful of bees, a queen and no dead bees around the hive. Much brood and honey may be present. It remains to be seen if this problem will persist. Similar phenomenon have been observed in the past and referred to as “disappearing disease” but no one has determined the cause.
– Greg Hunt, Purdue University