The most widely sprayed herbicide in the world kills honeybees, according to a new report.

Glyphosate, an herbicide and active ingredient in Monsanto’s (now Bayer’s) Roundup weed killer, targets enzymes long assumed to be found only in plants. The product is advertised as being innocuous to wildlife. But some bacteria also use this enzyme, including a microbiome found in the intestines of most bees. When pollinators come in contact with glyphosate, the chemical reduces this gut bacteria, leaving bees vulnerable to pathogens and premature death.

“The bee itself has no molecular targets from glyphosate,” Nancy Moran, a biologist at the University of Texas at Austin and a coauthor of the study, told Environmental Health News. “But its gut bacteria do have targets.”

Moran and other scientists liken glyphosate exposure to taking too many antibiotics—and upsetting the balance of good bacteria that supports immunity and digestion.

“We all know that glyphosate is an antibiotic. It’s very toxic to bacteria. It’s even patented as an antibiotic,” says Nathan Donley, a senior scientist at the Center for Biological Diversity. “But very few researchers have actually dived into this issue. The good thing is, that’s starting to change.”

To show glyphosate’s effects on gut microbiome, Moran and her team exposed honeybees to various levels of the herbicide, which measurably decreased total gut bacteria. Treated bees were then exposed to a common pathogen, and those with reduced bacteria were more likely to die prematurely. They repeated the experiment on other hives with similar results and published their findings early last week in Proceedings of the National Academy of Sciences.

Photo courtesy of Vivian Abagiu/University of Texas at Austin

Glyphosate is the most commonly used weed killer worldwide. A 2016 study found that 3.5 billion pounds of the herbicide have been used in the U.S. since 1974, when Monsanto first put it on the shelves in Roundup. Two-thirds of that volume has been sprayed in the last decade.

“Glyphosate is everywhere,” Donley says. “We’ve contaminated just about every open space that exists in the States.”

Only recently have scientists started to unlock how this widespread use of glyphosate affects not only pollinators and wildlife, but also human health. Researchers have shown that glyphosate also impacts gut microbiome in rats, while a study published last year in the Journal of American Medical Association revealed a 1,000 percent spike of glyphosate levels in the human bloodstream over the last two decades, with untold consequences.

Bayer, which recently acquired Monsanto, is already feeling the heat. In August, a California court ordered the chemical company to pay $289 million in damages after jurors ruled that Roundup caused a terminally ill man’s cancer. On September 18, Bayer asked the court to dismiss the verdict, though the World Health Organization has labeled glyphosate as carcinogenic. The recent ruling also inspired an influx of similar allegations against Bayer’s Monsanto. Over 8,000 people have sued the company for not disclosing its cancer risks.

Glyphosate isn’t the only Monsanto ingredient charged with bee mortality either. The company also sells seed treatment with neonicotinoids, a class of insecticides scientifically shown to kill pollinators like bees and butterflies. In 2013, the European Union banned neonics from outdoor use. The U.S. Fish and Wildlife Service did the same in the wildlife refuge system in 2014, though that ban has recently been rolled back.

Such studies linking glyphosate to bacterial health in bees could lead to a similar pause in the continued use of glyphosate. “We need better guidelines for glyphosate use, especially regarding bee exposure,” Erick Motta, the UT-Austin grad student who led the research, said in a university statement.

But aside from bees, Donley sees potential in this emerging research into microbiomes. No studies have yet shown glyphosate’s effects on human gut bacteria, he says, but the weed killer is now a proven harmful antibiotic, and the potential is there. “In 10 years, the case for glyphosate affecting our microbiome could be just as strong as the case for glyphosate causing cancer.”