Although that would of course be much more convenient when trying to solve the problem, research shows Colony Collapse Disorder, or CCD, that sudden die-off of bee populations in Europe, North America and Asia, cannot be attributed to one single cause.

Instead there are many factors at play, possibly ranging from climate change (and the spread of exotic plant species and general biodiversity decline it causes) to our use of cell phones (as paradoxically bees seem not to like the buzz).

But rather than one big heavy pile of environmental stressors adding their weight to eventually crush the beehives, a new picture is emerging, of several alliances of silent assassins.

On Thursday an interdisciplinary team of French parasitology and toxicology experts has uncovered the names of another such couple: Nosema ceranae & Firponil.

Lowering the bees’ defenses

In their publication in open-source science journal PLoS ONE they conclude the one, a pesticide, in very low doses [over ‘100 times less than LD50’ – the toxic  dose that would singlehandedly wipe out half the bee population] sufficiently lowers the bees’ immune defenses to let the other, a microscopic pathogen, successfully invade their colonies – where it wreaks havoc and may lead to sudden collective bee mortality.

The researchers, of the Université Blaise Pascal in Clermont-Ferrand and the Laboratoire de Toxicologie Environnementale in Avignon, find this could well be a pattern instead of a unique mortal combination. A chemically very different insecticide, thiacloprid, would – again, in sub-lethal doses – be equally successful in inviting the bee pathogen.

A short history of Bonnies & Clydes

It is not the first time researchers discover a deadly cocktail in the open environment of honeybee populations. In fact we have come to a list of such symbiotic killers.

Only a week ago a publication in PNAS, the journal of the US Academy of Sciences, pointed to the combination of a pest and a pesticide, in this case the mite varroa destructor – and two mite-killing acaricides, namely tau-fluvalinate and coumaphos.

(They also conclude beekeepers faced with varroa destructor do good to keep spraying the chemicals, as the pesticides should only hurt the bees, while hopefully wiping out the mites. Here too however they do warn adding several non-lethal doses of poison can have a ‘surprising’ cumulative effect on bees’ health – so don’t try that in the backyard.)

Earlier this year a Dutch bee researcher of Wageningen University stated the widely used pesticide imidacloprid is not in itself [as some had previously suggested] the main cause of CCD – but that it may promote distribution of that same varroa mite, currently perhaps the most devastating of bee-killers.

Non-poisonous cocktails

Defining a killer combination does not always require a pesticide or another chemical pollutant. In January there was a PNAS study that named one more deadly couple, though this time hardly one that operates silently.

It involves a different family member of the Nosema pathogens (N. bombi) which in recent years could suddenly conquer the world thanks to perhaps the most unstoppable force of modern times: globalisation.

Nosema bombi is native to European bee populations, where it does not seem to do much damage. Beekeepers have however ‘imported’ the single-celled creature to North America – and this time it is not the honeybees, but the various wild American bumblebee populations that seem to suffer the deadly consequences.

And in 2010 another French study, published in Biology Letters, showed how forced dietary changes caused by a decrease in flower biodiversity makes honeybees vulnerable to viruses they might otherwise be able to withstand.

One too many cocktails

Also in 2010 another PLoS ONE publication quite confidently suggested CCD was caused by a combination of one virus (Iridoviridae) and two fungi. The virus is common in bee populations, but would only turn lethal with help of either one of the fungi.

This study however was later discredited when an investigative CNN journalist discovered the lead author had some comfortable ties with Bayer, the German pharmaceuticals giant, that is of course very willing to solve the world’s pollination problems by mass-producing something to kill a virus or a fungus, or preferably both – no matter if these chemicals would in turn -collaterally- provide us with yet one more CCD cocktail.

© Rolf Schuttenhelm | www.bitsofscience.org