The CCD connection: what I believe about colony collapse disorder
Hardly a day goes by when someone doesn’t mention colony collapse disorder to me, either in person, in an e-mail or comment, or on the phone. “I hear they found the cause of CCD!” is a statement I’ve heard dozens of times over the last few years.
I’ve deliberately avoided writing about it—partially because I didn’t know much about it myself and partially because there was (and continues to be) a great deal written by people who know very little. It’s hard to compete with the press.
But, at this point, I believe competent research by a legion of intelligent scientists has given us some answers. Consequently I am now going out on a limb to give you my personal interpretation of that science. Although it is a complex subject, I will try to make it as simple as possible. So here goes.
Virtually all living things carry viruses, including plants, animals, and bacteria. The organism in which the virus lives is called the host.
The CCD Connection: Honey bees are hosts to least 18 known viruses. Most of these viruses have been recognized for a long time.
Viruses survive by taking over the genetic machinery of the host. Simply put, they instruct the host to stop doing whatever it is doing and replicate the virus instead. While viruses cannot walk, swim, or fly they are easily transmitted by contact between individuals or by a vector. A vector is simply a carrier that transmits an infectious agent.
The CCD Connection: The introduction of Varroa mites gave bee viruses the boost they needed to spread rapidly and widely throughout the honey bee population. Varroa mites carry many bee viruses in their bodies and transmit them with their bite.
However, viruses are usually inactive. They live in the host organism but don’t do harm because they are suppressed by the immune system of the host. The amount of a virus that lives in an organism but does no harm is known as the background level. [Think of a human “cold” virus. Although cold viruses are everywhere, we usually show no symptoms of a cold because our immune systems suppress the virus.]
The CCD Connection: Many bee viruses can be found in healthy colonies of bees. As long as the colony remains healthy, the viruses remain in the background and cause no problem.
Sometimes the immune system becomes ineffective at suppressing the viruses. Immune systems become less effective when an organism is stressed. Apparently, the energy of the host goes into compensating for the stress rather than maintaining the immune system. [So although cold temperatures by themselves don’t cause colds in humans, becoming chilled may suppress the immune system long enough for the virus to take over.]
The CCD Connection: Honey bee colonies are becoming more and more stressed. Many of the stressors are similar to human stressors, for example poor nutrition, exhaustion, chilling, and disease. In addition, honey bees are exposed to environmental toxins such as pesticides, and the excessive fear, noise, and incarceration imposed by migratory beekeeping. Worst of all,Varroa mites not only distribute viruses but weaken the bees by sucking their hemolymph (insect blood.)
When groups of organisms live together, just a few individuals with compromised immune systems may manufacture enough excess virus to infect other individuals. Instead of the usual background levels of virus, there are now overwhelmingly large numbers of virus.
The CCD Connection: Disease spreads quickly whenever individuals live close together–and honey bees are no exception. Honey bees live close together within the colony, but they also live close to other colonies within an apiary, close to hundreds of other colonies on the back of a truck, or thousands of other colonies in a vast orchard.
The presence of many sick individuals (another stressor) combined with an overwhelming number of virus organisms suppresses the immune function of the remaining hosts and, before you know it, the entire group is susceptible to any number of other infections. Other viral diseases that had been living in the background start to replicate and other organisms, such as bacteria, fungi, or microsporidia set up housekeeping in the compromised body of the host.
The CCD Connection: Collapsing colonies are usually found with multiple viral infections or viral infections in tamdem with some other pathogen, such as the microsporidian Nosema ceranae. And nearly all collapsed colonies have been found to be infested with Varroa mites. This evidence works well with the model described above.
In summary, I believe that the ultimate answer to the CCD question will be some combination of multiple disease organisms and multiple stressors. Unquestionably there is more to know. There may be some viral pathogen that is worse than others, or some combination of pathogen/parasite/stressor that sets off a domino effect. But are we going to find one stressor, one organism, or one management error we can fix overnight? I think not.
As I’ve said before, CCD has served honey bees well in that it has focused attention on apicultural and agricultural practices that are simply unsustainable. Even if we ultimately “fix” CCD, unless we clean up our act another catastrophe is sure to follow.