The new data, published in the Bulletin of Insectology, further supports earlier research that linked the pesticides to the collapse of bee colonies by finding that two widely used neonicotinoids appear to significantly harm honey bee colonies over the winter, particularly during colder winters.
Led by researchers at the Harvard School of Public Health (HSPH), the new study replicated a 2012 finding from the same research group that found a link between low doses of imidacloprid and Colony Collapse Disorder (CCD) - in which bees abandon their hives over the winter and eventually die.
In addition, the new data also suggested that low doses of a second neonicotinoid, clothianidin, had the same negative effect.
"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 of the HSPH.
"Although we have demonstrated the validity of the association between neonicotinoids and CCD in this study, future research could help elucidate the biological mechanism that is responsible for linking sub-lethal neonicotinoid exposures to CCD," he said. "Hopefully we can reverse the continuing trend of honey bee loss."
Last year EU Member States voted in favour of continent-wide suspension of neonicotinoid pesticides that have been linked to bee deaths.
Reports published by the European Food Safety Authority (EFSA) concluded that the pesticides posed a "high acute risk" to pollinators, including honeybees. However, it added that in some cases it was "unable to finalise the assessments due to shortcomings in the available data".
The European ban applies to 3 pesticides belonging to the neonicotinoids family - clothianidin, imidacloprid and thiametoxam – and is set to last an initial two years from 1st December 2013, through to 2015.
Lu and his co-authors from the Worcester County Beekeepers Association, USA, studied the health of 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.
There was a steady decline in the size of all the bee colonies through the beginning of winter—typical among hives during the colder months in New England, noted the authors.
Beginning in January 2013, bee populations in the control colonies began to increase as expected, but populations in the neonicotinoid-treated hives continued to decline, they said. By April 2013, half of the neonicotinoid-treated colonies were lost, with abandoned hives that are typical of CCD.
They added that only one of the control colonies was lost, in which thousands of dead bees were found inside the hive—with what appeared to be symptoms of a common intestinal parasite called Nosema ceranae.
While the 12 pesticide-treated hives in the current study experienced a 50% CCD mortality rate, the authors noted that, in their 2012 study, bees in pesticide-treated hives had a much higher CCD mortality rate of 94%.
That earlier bee die-off occurred during the particularly cold and prolonged winter of 2010-2011 in central Massachusetts, leading the authors to speculate that colder temperatures, in combination with neonicotinoids, may play a role in the severity of CCD.
Source: Bulletin of Insectology
Volume 67, Number 1, Pages 125-130. Full study linked here.
“Sub-lethal exposure to neonicotinoids impaired honey bees winterization before proceeding to colony collapse disorder”
Authors: Chensheng Lu, Kenneth M. Warchol, Richard A. Callahan