The latest on honeybee “colony collapse disorder” (CCD) may be disappointing to some (it’s still not cell phones), but it is not really surprising (except perhaps to conspiracy theorists). So far, at least, there is still no evidence for a single culprit. The leading evidence up to this point suggested a virus, but it seems to be more complicated than that – in fact, the latest study implicates the combined effect of multiple viruses. This is not a satisfying conclusion which will lead to an easy remedy, but then again it is also not unrealistic.
A group led by May Berenbaum at the University of Illinois (of X-Files referential fame) compared immune response gene expression in the guts of healthy bees with those in victims of CCD (because most pathogens enter insects through the gut). They found a whopping 65 markers (bits of gene code) that were associated with CCD bees.
Furthermore, they found no direct evidence that pesticides were potential culprits, based on expression of genes associated with pesticide resistance. The problem with the found immune response, though, was that there was no consistent pattern among the afflicted bees; if there had been, a specific pathogen could be identified. Because there was not, the suggestion is that afflicted bees have been overrun by multiple viruses at once, and what the particular viruses are depends on where the bees were from.
They also found curious RNA fragments that were more abundant in CCD victims. At this point they are only able to speculate on the origin of the fragments. They conclude only that these fragments combined with the presence of multiple viruses may allow people to identify colonies afflicted with CCD.
It’s an incremental step, but it’s a step. Some may find the idea fantastic that an insect species could be infected with so many viruses at once. But in truth, all living organisms, humans included, are constantly being bombarded with pathogens that can infect us. Our immune systems are crucial gatekeepers preventing constant disease – anyone with a compromised immune system (such as an AIDS victim or the “bubble boy”) could tell you this is so.
But why should bee immune systems in particular be compromised? One theory has been proposed often in the scramble to determine the cause of CCD: we are just stressing our domesticated bees too much by overworking them. But beekeepers have been working honeybees hard as long as they’ve been keeping them. True, the fewer colonies (due to other pests such as varroa mites) that are left have more work to do, but this alone probably is not the answer.
But there are lots of ways to compromise an immune system. Although no pesticide response was found, a chemical could have an indirect effect that would not have shown up on their assays. A compound that changes the acidity of the insects’ guts slightly could make them much more susceptible to multiple viruses – physical barriers to infection are as important as a physiological immune response (think of your skin’s susceptibility to infection, in an unblemished area versus at the site of a cut). A beekeeper I know suspects that the pesticide routinely used in bee colonies to combat the varroa mites may be involved (not the mites themselves, which has been theorized). Growing use of these chemicals has occurred more recently.
Or, a virus analogous to our own HIV could be throwing the immune system out of whack. If a retrovirus (like AIDS) is co-opting the bees’ own genetic machinary, this might explain the RNA fragments (which were not viral, but from ribosomes – structures that viruses themselves lack). At the very least, the immune responses in the different geographical groups need to be matched up with the viruses that occur there, to see if the responses are normal, if not consistent. Presumably that is a direction currently being pursued by the Illinois group.
Putting together a combination of chemical and virus, it seems possible that, say, one of the mite pesticides could have created conditions in which a currently unknown virus is able to enter a bee’s gut and compromise its immune system.
In any case, the pieces of the puzzle are clearly not all there yet. A simple explanation would have been found by now, if it were available. But it now seems likely, based on this study, that we may be encountering a type of AIDS in bees – which incidentally, is not transmitted via sex, because honeybee workers are all sterile females.
R. M. Johnson, J. D. Evans, G. E. Robinson, and M. R. Berenbaum, 2009.
Changes in transcript abundance relating to colony collapse disorder in honey bees (Apis mellifera). Proceedings of the National Academy of Sciences (published online before print August 24, 2009, doi:10.1073/pnas.0906970106).