A common pesticide used increasingly in recent years for crops such as corn and soybeans is the probable culprit in the destruction of honeybee colonies around the world, a study released Thursday by researchers at the Harvard School of Public Health has found.
The researchers said they found convincing evidence of the link between the pesticide known as imidacloprid and honeybees abandoning their hives, or colony collapse disorder, which they say began occurring in 2006 on a scale and scope never seen before in the history of the beekeeping industry.
Bees pollinate about one-third of crops in the United States, including fruits, vegetables, nuts, and livestock feed. A widespread loss of bees could be devastating to the nation’s agriculture.
“The significance of bees to agriculture cannot be underestimated,’’ said Alex Lu, associate professor of environmental exposure biology at the Harvard School of Public Health, who estimated bees account for about $15 billion in revenue for the agricultural industry.
“It apparently doesn’t take much of the pesticide to affect the bees,’’ Lu said. “Our experiment included pesticide amounts below what is normally present in the environment.’’
Before 2006, the typical bee colony collapse was between 25 and 30 percent; that figure has doubled since then, said Charles Benbrook, chief scientist of the Organic Center in Boulder, Colo., and former executive director of the National Academy of Sciences Board on Agriculture.
Bees are exposed to the pesticide through nectar from plants or through high-fructose corn syrup, which beekeepers use to feed their bees, the researchers said. Corn grown in the United States has been treated with imidacloprid since 2005.
But officials at Bayer, the German chemical and pharmaceutical company that produces more of the pesticide than any other company in the world, said that the study was flawed and that its findings should be disregarded.
They said imidacloprid is used only on a small amount of the nation’s crops, although they could not provide specific figures, and argued the doses used in Lu’s study were excessive.
“It’s a very effective and safe insecticide, much safer than the products it replaced,’’ said David Fischer, director of environmental toxicology and risk assessment at Bayer CropScience, who said the product has been sold since 1994. “All they have shown is if you feed massive amounts of a toxic insecticide to bees that you can cause mortality.’’
Officials at the US Environmental Protection Agency said they are concerned about the results of the study, which will appear in the June issue of the Bulletin of Insectology.
“The prevailing theory among the global scientific and regulatory communities is that the declines in the health of honey bees in general are related to complex interactions among multiple stressors that bees encounter, including inadequate food sources, diseases [such as parasites and viruses], habitat loss, and bee management practices, as well as pesticides,’’ EPA officials said. “While our understanding of the potential role of pesticides in pollinator health declines is still progressing, we continue to seek to learn what regulatory changes, if any, may be effective.’’
The officials said they are collaborating with federal and state agencies, industry, academia, and beekeepers to ensure that pesticides “continue to meet the statutory standard of no unreasonable adverse effects on people and the environment.’’
They have moved neonicotinoids, a class of insecticides that act on the central nervous system of insects, up in their review schedule. Imidacloprid was the first to be reviewed in 2008.
“All the neonicotinoids will begin registration review by the end of this year,’’ the officials said.
In the Harvard study, Lu and his coauthors set up groups of four beehives in five locations in Worcester County in 2010 in an effort to replicate how imidacloprid may cause colony collapse disorder.
Over 23 weeks, the hives were treated with different levels of imidacloprid, while a control hive went untreated. After 12 weeks of exposure to the pesticide, all the bees were alive. But at the end, 15 of 16 of the treated hives were dead. Those exposed to the highest levels of the imidacloprid died first.
Lu said the characteristics of the dead hives were consistent with colony collapse disorder, noting the hives were empty except for food stores, some pollen, and young bees. There were few dead bees nearby. When other conditions cause a hive collapse, such as disease or pests, a large number of dead bees are typically found near the hives.
While officials at Bayer said the study administered more imidacloprid than is typically applied, Lu said it took only low levels to cause hive collapse, less than is typically used in crops or in areas where bees forage.
“The outcome of the study is overwhelming, and I think that outweighs the small sample size,’’ Lu said.
Benbrook, the Organic Center chief scientist, said the study builds on similar findings about the potential dangers of imidacloprid. “This adds an important piece of the puzzle, confirming another important pathway through which bees are getting exposed to these insecticides,’’ he said. “The experimental design is very sound, and the findings are dramatic.’’
He pointed out that bans of the pesticide in parts of France and Italy since 2009 have substantially reduced colony collapse disorder there.
“This isn’t just an issue for bees,’’ Benbrook said. “People, especially children, consume a lot of high-fructose corn syrup. The presence of any pesticides in high fructose corn syrup should be a concern for the general public.’’