The world’s most commonly used weed killer makes honeybees more susceptible to disease, according to a new study from researchers at the University of Texas at Austin. Glyphosate, the active ingredient in Monsanto’s Roundup herbicide, weakens bees’ gut bacteria, making them more likely to contract deadly infections.

“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 Erick Motta, a graduate student and a leader on the study , which was published in the Proceedings of the National Academy of Sciences.

Bees, which are vital for pollination and food production, are in decline. The environmental advocacy group Greenpeace reports that since 2006 the U.S. honeybee population has declined by 40 percent.

“We need better guidelines for glyphosate use, especially regarding bee exposure, because right now the guidelines assume bees are not harmed by the herbicide,” Motta said. “Our study shows that’s not true.”

Glyphosate is also the active ingredient in Monsanto’s Ranger Pro herbicide.

“Claims that glyphosate has a negative impact on honeybees are simply not true,” Bayer, Monsanto’s parent company, told The Guardian . “More than 40 years of robust, independent scientific evidence shows that it poses no unreasonable risk for humans, animal, and the environment generally.”

California lists glyphosate as a known chemical to cause cancer. Scientists have also identified links between glyphosate and non-Hodgkin lymphoma.