Two widely used neonicotinoids appear to significantly harm honeybee colonies over the winter, particularly during colder winters, according to new research.
The study, from Harvard School of Public Health (HSPH), replicated a 2012 finding from the same research group that found a link between low doses of imidacloprid, Bayer Crop Science’s embattled systemic insecticide, and colony collapse disorder (CCD), in which bees abandon their hives over the winter and eventually die.
But Bayer has rejected the findings of the research as “totally unrepresentative of what is happening in agriculture”.
The new study also found that low doses of a second Bayer neonicotinoid, clothianidin, had the same effect.
The insecticides are two of the three active neonicotinoid ingredients covered by the European Commission’s partial, two-year ban, which came into force on 1 December 2013, following concerns by EU scientists about their link to bee decline.
“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 Harvard.
Lu and his team 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.
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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.
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.
By April 2013, six out of 12 of the neonicotinoid-treated colonies were lost, with abandoned hives that are typical of CCD.
Only one of the control colonies was lost – 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 – 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.
“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,” said Lu.
“Hopefully we can reverse the continuing trend of honey bee loss.”
Commenting on the new study, Friends of the Earth’s senior nature campaigner, Paul de Zylva said: “Sudden deaths of entire honey bee colonies are a persistent concern in North America.
“Comprehensive research into the role pesticides play in bee decline is urgently required – including how they may compound other pressures, such as a lack of food and loss of habitat.
“The UK government has accepted the need for a national action plan to reverse bee and pollinator decline. But its draft plan is dangerously complacent on pesticides, placing far too much trust in chemical firms and flawed procedures.
“If the UK National Pollinator Strategy is to be a success it must turn the current rising curve of pesticide use in the UK into a downward arrow.”