Friday, January 24, 2014

Colony collapse disorder and TRSV - an answer or just another data point?

A new study of what's killing honey bees, reported a few days ago in the New York Times and elsewhere, suggests that a virus that normally infects plants might be involved.  The virus has apparently mutated and jumped to bees, much like influenza can jump from birds and pigs to humans.

The virus was found by serendipity, in a screen of the viral load of a sample of bees and the pollen they had collected.  Researchers were looking for both rare and frequent viruses and found the tobacco ringspot virus (TRSV), which is presumed to be transmitted via pollen.  The virus presumably works by attacking the bee's nervous system.

It seems that bees pick up TRSV when they forage for pollen. They then may share it with larvae when they feed them "bee bread", a mixture of saliva, nectar and pollen. In addition, mites that feed on the bees may also be part of the chain of transmission, presumably of other infectious agents as well as TRSV.

Tobacco ringspot virus; Forestry Images

Other RNA viruses are involved in colony collapse, but this is the first one that has been found to be transmitted by pollen.  TRSV infects many different species of plant, and can be devastating.
Of a number of plant diseases caused by TRSV, bud blight disease of soybean (Glycine max L.) is the most severe. It is characterized by necrotic ring spots on the foliage, curving of the terminal bud, and rapid wilting and eventual death of the entire plant, resulting in a yield loss of 25 to 100%.
Bees and other pollinators can transmit the virus between plants, but infected seeds are another mode of transmission.  The virus has been found throughout the honeybee body, and in an ectoparasite of the bee, Varroa destructor.  And, it is correlated with winter colony loss.  This is the first RNA virus that has been found to infect plants and animals.
Of ten bee colonies included in this study, six were classified as ... strong colonies and four were classified as weak colonies. Both TRSV and IAPV [Israeli acute paralysis virus] were absent in bees from strong colonies in any month, but both were found in bees from weak colonies. As with other detected viruses, TRSV showed a significant seasonality. The infection rate of TRSV increased from spring (7%) to summer (16.3%) and autumn (18.3%) and peaked in winter (22.5%) before colony collapse. .... The bee populations in weak colonies that had a high level of multiple virus infections began falling rapidly in late fall. All colonies that were classified as strong in this study survived through the cold winter months, while weak colonies perished before February
The authors of this study carefully do not claim to have found the cause of colony collapse disorder.  Instead, they suggest they've found a new mode of transmission of viruses to insects, and further suggest that TRSV is but one possible cause of bee decline.

Fundamental questions remain -- are weak colonies weak for unknown reasons, but thus susceptible to viral and parasitic infections, or do viral and parasitic infections weaken colonies?  Can bees and colonies withstand a certain amount of infection, but over a certain threshold more infection or parasite infestation is devastating?  Is there still a single cause of colony collapse to be found? 


Manoj Samanta said...

I thought they already blamed sophisticated pesticides for it.


"Early in 2013, the European Food Safety Authority issued a declaration that three specific neonicotinoid pesticides pose an acute risk to honeybees, and the European Commission (EC) proposed a two-year ban on them.[85] David Goulson, who led one of the key 2012 studies at the University of Stirling said that the decision "begs the question of what was going on when these chemicals were first approved." The chemical manufacturer Bayer said it was "ready to work with" the EC and member states.[86] In April 2013, the European Union voted for a two-year restriction on neonicotinoid insecticides. The ban will restrict the use of imidacloprid, clothianidin, and thiamethoxam for use on crops that are attractive to bees. Eight nations voted against the motion, including the British government which argued that the science was incomplete.[87]"

Anne Buchanan said...

Well, the problem with the neonicotinoid explanation is that it doesn't explain CCD in many instances. And, the ban is not nearly long enough for resulting data to reveal much of anything. (We blogged about this at the time.) I think CCD is another example of the scientific method flailing about in a sea of complexity. It's just like trying to understand, say, the rise in asthma in rich countries starting in the 80's. It seems like there must have been some simple environmental change with an obvious effect, but 'the' cause is still unfound.

rk said...

Wonder about your thoughts on this from today's NYT

Gather from what I've read in your very interesting posts that scientists don't really know enough to actually pinpoint a genetic bullet against a particular "pest"

Anne Buchanan said...

I think it's a never ending arms race. It's certainly possible, at least in theory, to develop a genetic bullet against a particular pest and RNAi is as good a way to do this as any. But, the pest will evolve resistance, and the race will start all over again. That's probably the way it has to be, but it's a short term fix.

And there will be economic and social sequelae -- this is another way in which farmers will become dependent on Monsanto, and high-tech agriculture. But it's not even clear that the varroa mite is the major problem with colony collapse disorder. It may be one of the straws that breaks the camel's back, but it may not be. As with all the other 'causes' of CCD, I'm betting there are colonies that haven't been infected with the mite that have still died out. So, Monsanto will create a dependency on this technology that may not even be effective.

I think better to revisit this when CCD is understood. It may turn out there are low tech solutions, as it's turning out there are for weed and other pest problems. But Monsanto isn't going to invest in those.

rk said...

"... this is another way in which farmers will become dependent on Monsanto, and high-tech agriculture."

Agreed; anything to ensure a captive market.