Bees could be dying as a result of exposure to the world’s most popular weed killer, new research suggests.
Scientists found evidence that glyphosate, the active ingredient in Roundup, may be contributing to the decline of honey bees around the world.
The study shows that worker bees exposed to the weed killer lose beneficial gut bacteria.
This in turn is likely to leave the insects vulnerable to deadly infection by harmful bugs, the scientists believe.
Lead researcher Erick Motta, from the University of Texas, US, said: “We need better guidelines for glyphosate use, especially regarding bee exposure, because right now the guidelines assume bees are not harmed by the herbicide.
“Our study shows that’s not true.”
But Bayer, the parent company of Roundup’s makers Monsanto, denied the claims and said evidence showed its product was safe.
Glyphosate is generally thought to be non-toxic to animals, including humans and pollinating insects.
But in a test case last month a US jury found in favour of a groundsman who claimed glyphosate herbicides were partly to blame for his cancer.
Dewayne Johnson is to receive $289 million (£226m) in damages from agrichemical giants Monsanto. The company, now owned by German pharmaceutical firm Bayer, has vowed to appeal and maintains that its glyphosate products are safe.
The weed killer narrowly escaped a ban in the European Union after more than 1.3 million people signed a petition calling for its removal.
In the new study scientists exposed honey bees to glyphosate at levels known to occur in crop fields, gardens and road sides.
Bees had their backs marked with coloured dots so they could be tracked and later recaptured.
After three days, the exposed bees were found to have significantly reduced levels of healthy gut bacteria.
Of eight dominant bacterial species, four were less abundant. The most affected benign bug, Snodgrassella alvi, is critical to bee well-being, helping the insects process food and keep out dangerous infectious agents.
Bees with impaired gut “microbiomes” – microbe populations – were also far more likely to die when later exposed to a harmful invasive bacterium, Serratia marcescens.
After eight days, Serratia had killed half the bees with healthy microbiomes, but 90% of those had been exposed to the weed killer.
Professor Nancy Moran, also from the University of Texas, who co-led the research, said: “Studies in humans, bees and other animals have shown that the gut microbiome is a stable community that resists infection by opportunistic invaders. So if you disrupt the normal, stable community, you are more susceptible to this invasion of pathogens.”
The scientists, whose results appear in the journal Proceedings of the National Academy of Sciences, urged farmers and home owners to avoid spraying glyphosate herbicides on flowering plants likely to attract bees.
In 2006, vanishing worker honey bees in North America led to the term colony collapse disorder being introduced to describe a phenomenon that was decimating hives.
European beekeepers encountered the same problem, to a lesser degree. In Northern Ireland there were reports of declines greater than 50%.
Glyphosate weed killer may be implicated in colony collapse disorder, according to the researchers.
“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 Ms Motta.
Since bumble bees have microbiomes similar to those of honey bees, they could be affected by glyphosates in the same way, the scientists warned.
British expert Professor Dave Goulson, from the University of Sussex, said: “Those of us that study bees have long ago come to the conclusion that colony health is adversely affected by a number of interacting stressors, including exposure to cocktails of insecticides and fungicides, impacts of pathogens, and effects of poor nutrition.
“It now seems that we have to add glyphosate to the list of problems that they face. This study is also further evidence that the landscape-scale application of large quantities of pesticides has negative consequences that are often hard to predict.”
But a spokesman for Monsanto’s parent company Bayer said: “Claims that glyphosate has a negative impact on honey bees are simply not true. More than 40 years of robust, independent scientific evidence shows that it poses no unreasonable risk for humans, animal, and the environment generally.
“Regulatory authorities carefully consider these issues when they approve new pesticides for use.”