A bee carrying a parasitic mite.
Image: ARS/USDA/ÍScott Bauer

When last we visited the issue of sudden colony collapse, which is worrying farmers by emptying beehives across the US, a parasitic fungus was being tapped as a potential cause. An early access publication in today's Science Express revisits the issue and, although it finds the fungus is more frequent in infected samples, the study suggests a virus is the actual culprit. But a global look at the parasite load in these sick bees suggests there are still some unanswered questions.

The new work performed large scale sequencing on a number of samples from colonies that have collapsed, plus a few that have remained healthy. At the bacterial level, everything looked reasonably normal; there were no major differences between the two types of hives. The same was true for a trypanosomal sequence that appears to be part of the Leishmania family. Funguses, including the one previously suggested as a potential cause, also appeared in unaffected hives. Things finally got interesting when viruses were examined. One virus, Israeli acute paralysis virus of bees, had a 95 percent association with colony collapse.

So, mystery solved? Not really. The authors show that the virus appears to have arrived in the US with bees imported from Australia, a practice that began at about the same time as colony collapses were noted. But Australia does not appear to have any problems with its bee population, suggesting something else must be involved. The researchers suggest US-only parasitic mites or the chemicals used to control them as possible enabling factors.

There are two big limiations with the study as things now stand. The first is that it's not quantitative. If one of the pathogens was present at much higher levels in collapsing hives, the study would not detect it. The second is that it's purely correlative. Correlations can be very informative, but the authors themselves produced data that indicates their value is limited in this case. When the researchers looked at a set of four pathogens (including the virus and fungus suggested as causes), normal colonies were likely to have two, while those suffering a population crash had a mean of 3.7. Separating cause, enablement, and opportunism among those pathogens isn't possible with the available data.

So, the cause of sudden colony collapse remains a bit mysterious, although some positive candidates are emerging. If the virus that these authors suspect really is a key factor, then there's good news: up to a third of the bees in Israel, where it was first identified, are already resistant to it.