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Honeybees exposed to a certain class of insecticide are more likely to die from Colony Collapse Disorder (CCD), the name given to whatever is causing a mass decline in the bee population over the past six years, according to a new study.

The report, which appears today in the Bulletin of Insectology, recreates a 2012 study which first linked the bee-killing disease with neonicotinoids. The same team of researchers from the Harvard School of Public Health who conducted the 2012 study ran this later one, and their findings bolster their earlier findings. According to lead author Chensheng (Alex) Lu, "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."

To perform the latest study, the researchers examined 18 bee colonies in three different locations in central Massachusetts. They split each colony into three groups — one treated with a neonicotinoid called imidacloprid, one with a neonicotinoid called clothianidin, and one left in pristine condition to serve as a control group. The scientists monitored the groups from October 2012 to April 2013 and found that, by the end of that period, half of the neonicotinoid colonies had been decimated, while only one of the control colonies was destroyed by a common intestinal parasite, Nosema cerenae. None of the bees were affected until winter, the authors write:

We found honey bee colonies in both control and neonicotinoid-treated groups progressed almost identically, and observed no acute morbidity or mortality in either group until the arrival of winter... As temperatures began to decrease in late October 2012, we observed a steady decrease of bee cluster size in both control and neonicotinoid-treated hives continued to decline.

The new study also offered some novel information. When the team conducted the research in 2012, a whopping 94 percent of infested colonies died off. The discrepancy suggests that the harsh winter of the period studied (2010-2011) likely exacerbated the effects of the insecticides.