A controversial weedkiller that has been linked to cancer in humans could also be killing bees, scientists have warned.
Research from the US suggests glyphosate, the active ingredient in Roundup, a well-known weedkiller, may be adding to the decline of honey bees around the world.
The study says that bees exposed to the substance lose beneficial gut bacteria, which could leave them vulnerable to fatal infection by harmful bugs.
Lead researcher Erick Motta, from the University of Texas, 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.”
Scientists exposed honey bees to the chemical at the same levels known to occur in crop fields, gardens and road sides.
After three days, those bees had significantly reduced levels of healthy gut bacteria.
Those bees with impaired gut microbiomes were more likely to die when later exposed to a harmful bacterium, Serratia marcescens.
After eight days, the bacterium had killed 90% of the bees that had been exposed to glyphosate.
Scientists believe the chemical could be linked to “colony collapse disorder”, a term that describes the decimation of bee hives, in America and Europe.
However, Bayer, the parent company of Roundup’s makers Monsanto, dismissed the findings, saying that evidence showed its product was safe.
A spokesman 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.”
But in a landmark case last month, a US jury ruled Monsanto was partly liable for a terminally ill man’s cancer and was ordered to pay $289m (£226m) in damages.
Dewayne Johnson, 46, had used Roundup in his work as a groundskeeper and pest manager in Benicia, north of San Francisco.
He claimed that him using the product, often several times a day, caused non-Hodgkin lymphoma, a blood cell cancer.
Monsanto said it would appeal and maintains that its glyphosate products are safe.
In 2015, the International Agency for Research on Cancer, found that the chemical was “probably carcinogenic to humans”.
However, a later study by the World Health Organisation (WHO) and the UN concluded it was unlikely to pose a health “risk to humans from exposure through diet”.
The European Food Safety Authority, the European Chemicals Agency and other organisations also found no link to cancer in humans.
There have been calls to ban the herbicide, which is registered in around 130 countries, including the UK, where it is commonly used to spray pests.
Last year, the European Union narrowly voted to renew use of the chemical, despite a petition signed by more than 1.3 million people calling for a ban.