A common weed killer, considered harmless to animals, can harm bees worldwide | Science

Glyphosate, the most widely used herbicide in the world and a product long considered harmless to animals, could have adverse effects on honey bees. The chemical appears to disrupt the microbial community of the bees' digestive system, making them more vulnerable to infection. The discovery adds another potential reason for the alarming decline of honey bees in some parts of the world, as well as for that of other pollinators living in colonies, such as bumblebees.

"It's really critical," says Fred Gould, an entomologist at North Carolina State University in Raleigh, who did not participate. The study challenges the conventional wisdom that animals are immune to glyphosate because they target a particular cellular mechanism to plants and certain bacteria. "I was surprised."

Glyphosate kills plants by blocking an enzyme that they use to make several key amino acids, the building blocks of proteins. Animals do not produce this enzyme, but some bacteria use it.

This sparked the interest of Nancy Moran, an evolutionary biologist at the University of Texas at Austin, who spent a decade examining the gut microbiome – the population of bacteria that inhabit the intestines of animals – honey bees (Apis mellifera). She and her colleagues took about 2,000 bees from a hive and fed sugar syrup and others syrup containing glyphosate at levels similar to those they might encounter in the world. environment during foraging. Three days after returning to the hive, the bowels of glyphosate-fed bees were weaker Snodgrassella alvi than bees that have not been exposed. Some results were confusing; the bees that obtained the most glyphosate had a microbiome of more normal appearance after 3 days than those with lower doses. Moran says it's unclear if this is because more bees with the highest dose have died, leaving behind those that are more resistant to the herbicide.

In later tests, bees that consumed glyphosate had five times fewer bacteria. In a petri dish, most strains of S. alvi slowed or stopped growing after a high dose of glyphosate.

This change in the microbial habitat of a bee seems to make it more vulnerable to deadly infections. When tested on several hundred bees, only 12% of insects fed glyphosate survived Serratia marcescens– a bacterium found largely in trace amounts in hives and bee casings, which can cause infections by invading other parts of the body of a bee, against 47% who have not consumed.

It is unclear why a microbiome disturbed by glyphosate would make bees more vulnerable to infection, says Moran. S. alvi lines of the wall of the intestine, and could create a protective barrier. It also secretes a chemical that could attack invading bacteria.

The results – reported today in the Proceedings of the National Academy of Sciences"Gene Robinson, a honey bee geneticist at the University of Illinois at Urbana, did not participate in the study. In recent years, US commercial beekeepers have seen almost a third of their hives fail in winter, more than double the historical rate. Researchers believe that pesticides, pathogens, parasites and nutritional problems all play a role. One of the major strengths of the new paper is that it indicates a mechanism – the disruption of intestinal microbes – of how a pesticide could affect bees, he says.

The discovery also raises questions as to whether glyphosate affects the microbiome of other animals, including humans. The role of microbes in the human gut has many similarities with the bowels of bees, says Moran. More research is needed; humans have different microbes in their intestines, they have considerably larger bacterial populations and are probably exposed to much lower doses of glyphosate than bees.

The new research will definitely make a controversial herbicide a flash point. Some also warned that it could be bad for people. Public health agencies have offered conflicting assessments to find out if the chemical is a probable carcinogen.