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?