The mass honeybee deaths referred to as Colony Collapse Disorder (CCD) have been linked to pesticides known collectively as neonicotinoids, according to a study by researchers from the Harvard School of Public Health. The new study replicates and backs up results from a 2012 study that linked exposure to the pesticides imidacloprid and clothianidin to bees abandoning their hives. Last year the European Union banned three neonicotinoids after the initial study.

The new study (pdf) was published last week in the Bulletin of Insectology. Researchers exposed 12 groups of honeybees to low levels of the two pesticides. The doses were not enough to be directly fatal, but the researchers found that six of the 12 neonicotinoid-treated groups abandoned their hives and did not survive the winter. "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," lead author Chensheng Lu said in a news release.

The researchers also used six control colonies that were not exposed to the pesticides. Only one of the six experienced mass fatalities, which they linked to a common intestinal parasite called Nosema ceranae.

The earlier study, also conducted by the Harvard School of Public Health and the Worcester County Beekeepers Association, had a much higher mortality rate of 94 percent compared to the current study's 50 percent. The researchers say the previous study was conducted during a much colder and longer winter, which may have contributed to the higher level of colony collapse. Both studies were conducted in the same region of Massachusetts.

Lu said the new study demonstrates "the validity of the association between neonicotinoids and CCD," but said future research would be required to determine exactly why the pesticides cause bees to abandon their hives. As the authors wrote in their paper, "It is striking and perplexing to observe the empty neonicotinoid-treated colonies because honey bees normally do not abandon their hives during the winter. This observation may suggest the impairment of honey bee neurological functions, specifically memory, cognition, or behavior, as the results from the chronic sub-lethal neonicotinoid exposure."

With this new information in hand, Lu said: "Hopefully we can reverse the continuing trend of honey bee loss."

Colony collapse disorder was first observed in 2006. Affected colonies have experienced mortality rates as high as 90 percent. Previous research has also identified other potential causes, including pathogens, parasites and bee genetics.

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Colony collapse disorder's link to pesticides strengthened by new study
The pesticides, called neonicotinoids, are "highly likely" to be triggering bee deaths, say researchers.