What is killing off honey bees? Since 2006, there have been significant losses of honey bees from a mysterious condition called Colony Collapse Disorder, or CCD. Now a new study has added to evidence suggesting that a widely used class of insecticides appear to be harming the insects over winter -- especially during colder winters.
"We demonstrated again in this study that neonicotinoids are highly likely to be responsible for triggering CCD in honey bee hives that were healthy prior to the arrival of winter," said lead author Chensheng (Alex) Lu, associate professor of environmental exposure biology at HSPH in a press release.
It appears that something about at least two kinds of neonicotinoids - a class of insecticide -- prompts bees to abandon their hives during winter. Left out in the elements, the bees die.
The finding focused on the two insecticides, imidacloprid and clothianidin, and replicates a 2012 study that had focused only on imidacloprid. The 2012 study determined the chemical had the same effect on the pollinators -- causing them to abandon house in winter.
Lu and his co-authors from the Worcester County Beekeepers Association monitored 18 bee colonies in three locations in central Massachusetts from October 2012 through April 2013. At each location, the researchers separated six colonies into three groups -- one treated with imidacloprid, one with clothianidin, and one untreated.
All three of the bee colonies dwindled as winter set in, as was expected during the cold New England winter months. But by January 2013, bees in the control colony began to increase, while the bees in the insecticide-treated colonies continued to decline. By spring, half of the insecticide treated colonies were lost -- with hives left empty after bees abandoned them.
While the number of bees lost was clearly high, it didn't match the high number of deaths in the 2012 study -- when 94 percent of the insecticide-treated colonies were lost. The researchers believe the higher decline among the 2012 groups could be linked to the fact that the 2011-2012 winter was particularly long and harsh with more frigid temperatures than usual.
The combination of the extremely cold winter and exposure to the chemicals may have delivered a double punch to the insects.
In the most recent study, just one of the control colony of bees was lost and in that hive were thousands of dead bees that appeared to be infested with a common parasite. That finding, the researchers say, counters previous work that had suggested insecticides may be making bees more vulnerable to parasites. Surviving bees in both treated and untreated colonies showed the same levels of parasite infestation.
So what about the insecticides is prompting bees to leave their hives and die? That remains the million dollar question.
"Future research could help elucidate the biological mechanism that is responsible for linking sub-lethal neonicotinoid exposures to CCD," said Lu. "Hopefully we can reverse the continuing trend of honey bee loss."
The study appears online May 9, 2014 in the Bulletin of Insectology.