Popular Weed Killer May Be Responsible for Global Bee Deaths

A popular ingredient in weed killers may be wiping out honey bees by attacking their gut, scientists claim.

The chemical glyphosate is the active ingredient in Roundup, a potent herbicide sold by Monsanto, and other similar products. A new study by researchers at the University of Texas at Austin found that it destroys bees’ specialized gut bacteria, exposing them to infection by deadly bacteria.

Glyphosate is the world’s best-selling herbicide, despite its links to cancer in humans. It works by targeting an enzyme in plants and some microorganisms known as EPSPS, or 5-enolpyruvylshikimate-3-phosphate synthase. According to the study, bee gut bacteria also contains this enzyme, which helps to ward off infections and regulate overall health.

When the scientists exposed bees to the Roundup ingredient, the bees’ healthy bacteria decreased, disrupting their microbiome—a mini-ecosystem of gut microbes responsible for all manner of tasks, such as blocking pathogenic invaders and processing food.

Out of eight healthy bacteria species found in bee guts, four were found to have lessened after three days of exposure to glyphosate, the study notes. When also exposed to a harmful and common bacteria called Serratia marcescens—in order to test how glyphosate would impair the bees’ ability to fend off infection—only a tenth of the bees subjected to the chemical were alive after eight days.

Glyphosate has long been marketed by the herbicide industry as “non-toxic” to animals, and Roundup’s manufacturers contest the study’s results.

“No large-scale study has ever found a link between glyphosate and honey bee health issues,” the company Bayer said in a statement on Tuesday.

Bayer claimed the small number of bees tested in the study was not a representative sample. An exact number was not provided by the study, which said that “Hundreds of adult worker bees were collected from a single hive,” and that a portion was exposed to glyphosate.

The study’s authors theorize that glyphosate may be tied to colony collapse disorder, a deadly and enigmatic phenomenon affecting bees in North America, Europe, and Asia. Beekeepers first reported it in 2006 after noticing that up to 90 percent of hives were dying off—mostly worker bees, thus stranding the queen and eventually destroying the hive.

“We need better guidelines for glyphosate use, especially regarding bee exposure, because right now the guidelines assume bees are not harmed by the herbicide,” Erick Motta, a graduate student and the study’s lead author, said in a statement on Monday. “Our study shows that’s not true.”

The continued debate around the widely used weed killer shows that more research is needed about its effects on bee populations. Lawmakers around the world have pushed to ban the chemical with mixed results.

“It’s not the only thing causing all these bee deaths,” Motta said, “but it is definitely something people should worry about because glyphosate is used everywhere.”