Neonicotinoids, a class of insecticides, are to blame for the destruction of honey bee colonies in large quantities during winter, according to a study conducted by researchers at the Harvard School of Public Health.
The 2012 study conducted by the team found that imidacloprid, in low amounts, resulted in Colony Collapse Disorder (CCD) - where bees desert their hives and eventually lose their lives due to exposure to the harsh cold. The new study identified a second neonicotinoid - clothianidin - which has similar negative effects. These neonicotinoids impair bees' neurological functions.
Previous studies indicated that CCD in honey bee colonies was mostly due to bees' weak resistance to mites or parasites as a result of exposure to pesticides. However, the new study observed similar levels of pathogen in bee hives associated with CCD as in a group of control hives. As a result, the researchers said that the neonicotinoids are triggering some biological mechanism in bees that is leading to CCD.
"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, in a statement.
For the study, the researchers closely observed the health of 18 bee colonies in three locations in central Massachusetts from October 2012 to April 2013. At every location, the researchers divided six colonies into three groups. They exposed the first group to imidacloprid, second one to clothianidin and left the last one untreated.
The researchers found that the volume of all the bee colonies decreased at the beginning of winter, which is a normal trend among hives during colder months in New England. The bee population in the control colonies began to grow as expected beginning January 2013. But populations in the neonicotinoid-exposed hives continued to drop. By April 2013, 6 out of 12 of the neonicotinoid-exposed colonies were destroyed with higher concentrations of abandoned hives, a characteristic of CCD.
The 12 pesticide-exposed hives in the current study were associated with a 50 percent CCD mortality rate, but in the 2012 study, the similar pesticide-treated hives experienced higher CCD mortality rate, 94%. The alarming death rate occurred during the particularly cold and prolonged winter of 2010-2011 in central Massachusetts. The finding indicates that colder temperatures, combined with neonicotinoids, cause CCD-related deaths of hone bee colonies.
"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."
The finding is published in the Bulletin of Insectology.
