Glyphosate doesn’t just kill plants. It also harms beetles.
All over the world, insects are in decline. Ubiquitous species, like the rust-spotted bumblebee and the monarch butterfly, are now rarities, and studies in Europe have found catastrophic accidents in the total number of flying insects.
The causes of the decline overlap, from habitat loss to electric light. Agricultural chemicals are almost certainly essential, such as neonicotinoid pesticides implicated in the catastrophic collapse of monarch populations. But it’s not just pesticides: new research implicates glyphosate, one of the world’s most common herbicides, as part of the problem, albeit unexpectedly.
In the well-studied sawtooth beetle, high doses of glyphosate, better known by its American brand name, RoundUp, disrupts a crucial relationship with the symbiotic bacteria, weakening the beetle’s exoskeletons.
It is generally believed that glyphosate only kills plants because it targets a specific molecular pathway, called the shikimate pathway, which does not exist in animals. (Some studies have linked the chemical to human cancer, although other peer-reviewed research – funded in part by Monsanto – has not found a link.)
But sawtooth beetles rely on a symbiotic relationship with a special type of bacteria (unnamed) to build their shells. This bacterium in turn uses the shikimate path to make the rough building blocks that beetles need.
In fact, it’s one of the only things he does. According to Engl, the bacterium appears to have taken refuge inside the beetles for so long that it has lost much of its genome, leaving it “entirely dependent on the host”.
“They were under selective pressure to just perform this unique function,” says Tobias Engl, who studies insect symbiosis at the Max Planck Institute for Chemical Ecology and is one of the study’s lead authors.. “Now they only live to synthesize aromatic amino acids.”
Beetles depend on bacteria for these amino acids – especially the one called tyrosine – during their metamorphosis from larvae to adulthood. During this time, they need tyrosine to harden their extra-hard exoskeleton, or cuticle. This puts them in a bind: While they are shapeshifting, they cannot eat. But they also can’t store enough tyrosine to avoid them, because the chemical is toxic in high concentrations.
“They need another way to cope,” Engl says. “Either by creating non-toxic storage conditions, or by having a partner who delivers the components when they are needed” – bacteria.
Glyphosate appears to kill these partners. After exposure to the chemical, the beetles make softer and weaker shells. It doesn’t kill them outright, but it makes them more vulnerable. “They do a lot worse,” Engl says. “Their cuticle is thinner, which creates a higher risk of desiccation and higher mortality.”
Researchers exposed beetles to high levels of glyphosate, comparable to when a field is sprayed with the chemical. Engl says more research will be needed to show how beetles are affected by lower and persistent levels. But before the study, it was not clear that beetles would even transmit environmental glyphosate to their internal bacterial partners.
The sawtooth beetle is a pest of crops, but Engl says it’s a model for all kinds of other beetles, which make up about a quarter of all known animal species. “[The beetle] is generally used by us to understand associations between insects, their microbial partners, and their ecological importance, so it is an approximation for many insects, ”Engl says.
Previous research has found similar symbiotic relationships in other beetle species as well as in ants, which have the same hard cuticles. This research did not specifically study glyphosate, but the chemical was found to disrupt symbiosis in tsetse flies and disrupt the intestinal biome of bees.
“They’re part of the natural variety of insects out there. Yes [other insects] are sensitive to the same things, so that’s a worrying thought given the widespread use of glyphosate, ”Engl says.