WASHINGTON, Sept. 24 (Xinhua) -- A study published on Monday in the Proceedings of the National Academy of Sciences showed the world's most widely used weed killer might kill bees indirectly.
Scientists from the University of Texas at Austin found that honey bees exposed to glyphosate lost some of the beneficial bacteria in their guts and thus were more susceptible to infection and death from harmful bacteria.
They considered it evidence that glyphosate might be contributing to the decline of honey bees and native bees around the world.
"We need better guidelines for glyphosate use, especially regarding bee exposure, because right now the guidelines assume bees are not harmed by the herbicide," said Erick Motta, the graduate student who led the research, along with evolutionary biologist Nancy Moran. "Our study shows that's not true."
Glyphosate interferes with an important enzyme found in plants and microorganisms, but not in animals, it has long been assumed to be nontoxic to animals, including humans and bees.
But the new study suggested that glyphosate could alter a bee's gut microbiome, the ecosystem of bacteria living in the bee's digestive tract, including those that protect it from harmful bacteria, thus compromising the bee's ability to fight infection.
The researchers exposed honey bees to glyphosate at levels known to occur in crop fields, yards and roadsides. Three days later, they observed that the herbicide significantly reduced healthy gut microbiota.
Of eight dominant species of healthy bacteria in the exposed bees, four were found to be less abundant. The hardest hit bacterial species, Snodgrassella alvi, is a critical microbe that helps bees process food and defend against pathogens, according to the study.
The bees with impaired gut microbiomes also are far more likely to die when later exposed to a widespread pathogen called Serratia marcescens, compared with bees with healthy guts.
About half of bees with a healthy microbiome were still alive eight days after exposure to the pathogen, while only about a tenth of bees whose microbiomes had been altered by exposure to the herbicide were still alive.
More than a decade ago, the United States beekeepers began finding their hives decimated by what became known as colony collapse disorder. Millions of bees mysteriously disappeared, leaving farms with fewer pollinators for crops.
"It's not the only thing causing all these bee deaths, but it is definitely something people should worry about because glyphosate is used everywhere," said Motta.
Native bumble bees have microbiomes similar to honey bees, so Moran said it's likely that they would be affected by glyphosate in a similar way.
