For the past three years, a mysterious die-off of honeybees in the U. S. has gripped public attention and led to fascinating theories about its origin.
Suggested causes of colony collapse disorder (CCD) included pesticides, diseases, changing weather patterns, inadequate nutrition, environmental stress and plain overwork. Some blamed radio waves from cellphones for causing bees to vanish from hives. A few conspiracy theorists even offered alien abduction as the reason.
Whether wacky or plausible, all explanations sought to pin the blame on a single agent – something new that was killing bee colonies.
As it turns out, none of them may be right.
CCD is the result of many forces working together and is not a disorder in itself, bee experts say.
“Most scientists now agree there’s not a single cause or causal organism resulting in this phenomenon. It’s an interaction between a number of different stresses found in the hive,” said Rob Currie, a University of Manitoba entomologist.
“CCD is really just a set of symptoms that describe a condition where bees are basically missing from the hive.”
What happened, though, is that CCD became a generic term for declining bee populations for which people felt there had to be a smoking gun. That most likely isn’t true, said Currie, who recently gave a public seminar on CCD at the U of M.
“When colonies started dying in the U. S. and they started investigating it, they were under the belief at the time that there would be one particular pathogen causing this to happen. As science progressed and they started looking into this in more detail, they’ve pretty much come to the conclusion that it’s multiple factors and interactions between different compounds.”
It’s true that bee numbers in the U. S. have been strangely declining for several years, although the impact wasn’t fully noticed until late 2006.
A U. S. Department of Agriculture 2007 survey estimated average losses of 35 per cent among the 2.44 million honey-producing colonies in the U. S. – a sudden and abnormal drop.
Even more baffling was the way bees disappeared. They just seemed to vanish into thin air. Beekeepers reported opening hives to find adult bees gone.
Often, foraging bees will raid abandoned colonies, robbing them of their honey and nectar. But, according to some reports, bees avoided these colonies as if they were haunted.
Such stories, whether documented or apocryphal, only heightened speculation about reasons for the apparent mystery.
But in an interview, Currie said CCD symptoms can be explained naturally.
Classic CCD – bees vanishing for no apparent reason – did occur. But that can happen for a number of reasons. Currie said bee colonies that get sick because of disease or mites will sometimes either relocate or go someplace to die.
As for bees avoiding deserted colonies, that may depend on the time of year, he said. If colonies collapse in summer when the nectar flow is strong, other bees already have adequate supplies of nectar and honey, so they don’t need to raid other colonies.
CCD has not been reported in Canada, although bee populations in this country have also fallen. An industry survey last winter found 25 per cent of Manitoba’s 85,000 honeybee colonies either died or emerged too weak to be viable. Alberta reported even higher losses.
FEEDING ON BEES
Currie blamed the parasitic varroa mite, endemic in Canadian bee colonies, as the biggest culprit.
“These mites feed on the bees, they feed on both the adults and immature stages of the bees. That in turn weakens the bees, stresses them, compromises their immune system and makes them more susceptible to other diseases and pathogens. They can also inject viruses and pathogens in places where they haven’t been injected before. A lot of them are moving around and infecting bees in ways they’ve never been able to in the past.
“My personal opinion is that if we could get this particular pest under control, we would eliminate a lot of other problems.”
A leading CCD expert in the U. S. also agrees honeybee colonies suffer from compromised immune systems – and pathogens may take advantage of that.
Dennis vanEngelsdorp, an entomologist with the Pennsylvania Department of Agriculture, wrote in a recent paper that one CCD symptom – the lack of dead bees in empty hives – is more common in larger operations.
“This suggests that a highly contagious condition may be responsible for this symptom, a problem that would be compounded by the crowded conditions that often exist in commercial beekeeping operations as they move colonies to and from pollination or honey-producing sites,” wrote vanEngelsdorp, who has studied CCD since it first appeared. [email protected]