Has the cause of Colony Collapse Disorder been found?
Answer: Not quite yet, but we have a big lead.
The ancient Greek philosopher Plato famously described our limited human perception of reality by making an analogy to viewing shadows in a cave. Imagine being chained to a rock, unable to move or even turn around, facing the cave wall. Others behind you would build big fires and cast light onto the wall, and in doing so would project the shadows of themselves and the objects that they carried. The only knowledge of your surroundings, and of what was going on behind you, would only come from an incomplete picture of vague shapes, curious noises, and poorly defined shadows. Piecing them all together into a cohesive, clear understanding of what is happening behind you can therefore be a challenge simply given the means by which you obtain information.
In many ways, science is very similar in its approach to understanding how things work. We do not see distant galaxies move directly; physicists measure the "red shift" of far-away objects to infer their trajectories. We cannot see covalent bonds being formed among atoms; chemists infer the change in structure and properties of elements as they are mixed together. Studying honey bees is no different. For example, we cannot smell queen pheromones, but we can test different chemicals within beehives and see if workers react to them as they would their queen. The quality of what we know is only as good as the resolution of the information given to us.
A new study, making a lot of headlines in the past weeks, has provided yet another piece to the puzzle that is CCD, or colony collapse disorder. Conducted by scientists at the University of Montana and the US Army Research lab, among others, the study suggests that there may be an interaction between Nosema ceranae (the "new" species of microsporidian that infests the gut of adult bees) and one or more viruses (specifically a newly identified invertebrate iridescent virus, or IIV). They used a screening technique, called mass spectrometry-based proteomics (MSP), to quantify a huge number of species-specific proteins among samples of honey bees. Their samples derived from numerous sources, including CCD colonies collected by the original CCD working group in 2006-2007 (31 colonies), another set of CCD colonies from Florida in 2009 (nine colonies), and colonies from either packaged bees from Australia or an isolated, non-migratory apiary in Montana to serve as (non-CCD) controls. Comparing these sets of either sick or healthy colonies, they found a strong relationship between CCD and both Nosema and IIV; many of the non-CCD colonies had either Nosema cerana or the new virus, but only the CCD colonies had both.
To take things a step further, the scientists tested a colony sampled while collapsing in 2008, as well as caged bees inoculated with various combinations of Nosema spores and virus titers. For the observation colony, they found a steadily decreasing number of forager flights at the hive entrance, as well as positive detections of iridovirus and nosema peptides in captured foragers. While correlation doesn't equal causation here, the association is consistent with there being an effect. In the inoculation trials, they found that after 14 days, about 60% of the uninoculated bees survived, about 35% of the virus- or nosema-inoculated bees survived, but only about 25% of the virus+nosema bees survived. This further suggests co-infection is necessary for the highest (but not complete) mortality.
It should be pointed out that cumulative infections of nosema, viruses (although not IIV), and other pathogens have already been shown by other published (and yet unpublished) studies involving hundreds of CCD vs. non-CCD colonies. The current conventional wisdom is that the bees' immune systems are overly burdened, and that they collapse under the cumulative weight of infection. Compared to previous studies, this newer study utilizes a drastically different technique (MSP rather than PCR approaches), thus quantifying infection using a different measuring stick. The authors argue that IIV may not have been detected previously because of its low density, but that seems very unlikely (rather, the ability to detect it may have been reduced or absent). There is also the question as to how the syndrome is manifest, as it does not seem to be a result of mortality per se but rather the disappearance of adult workers. How these various parasites interact, how they are spread, and how we can treat them are all urgent questions that need to be answered.
Much like Plato's parable of the cave, we are limited by our scientific techniques, sample sizes, and measuring sticks in understanding this very complex problem. Yet it is clear, the more shadows we study, the closer we will get to understanding the objects that are casting them.
Bromenshenk, J. J., C. B. Henderson, C. H. Wick, M. F. Stanford, A. W. Zulich, R. E. Jabbour, S. V. Deshpande, P. E. McCubbin, R. A. Seccomb, P. M. Welch, T. Williams, D. R. Firth, E. Skowronski, M. M. Lehmann, S. L. Bilimoria, J. Gress, K. W. Wanner, and R. A. Cramer. (2010). Iridovirus and microsporidian linked to honey bee colony decline. PLoS ONE, 5: e13181. doi:10.1371/journal.pone.0013181.