Glyphosate dangerous to beetles as well as plants

The commonly used herbicide glyphosate might not be just bad news for bees. New research has found it could have broader ramifications for the survival of other insects by affecting their bacterial allies.

The study, published in the journal Communications Biology, showed that glyphosate inhibits a biochemical pathway in bacteria that was previously only thought to affect plants, making them unable to supply the growing sawtoothed grain beetle with essential nutrients.

This beetle relies on symbiotic bacteria to provide amino acid building blocks for its protective exoskeleton. Without the bacteria, the beetle’s coating is much thinner, making it vulnerable to drought and predators.

Some might say this is a good thing, as the grain beetle (Oryzaephilus surinamensis) is a global pest that – as its name suggests – gets into stored grain and grain products.

But there’s more to it, as this beetle’s reliance on nutrients from its bacterial friends is more widespread in the insect world.

“The sawtoothed grain beetle is a model system that is easy to maintain and manipulate and allows us to study other host symbiont associations,” explains senior author Tobias Engl from Germany’s Institute of Organismic and Molecular Evolution in Mainz.

“Thus the implications of our findings have to be seen not only in the limited, anthropogenic focus on a single grain pest, but in the larger context of the entire diversity of insects in our environment.”

It comes down to the shikimate pathway which glyphosate selectively inhibits in plants, preventing the biosynthesis of aromatic amino acids and thus plant growth.

Although this pathway doesn’t occur in animals, it does in microorganisms. This includes the gut bacteria of bees, a discovery that linked glyphosate to their demise. Engl’s team, including first author Julian Kiefer, sought to understand how widespread this sensitivity is.

“An impact of glyphosate on animals via their essential bacterial partners that use or even specialise in the shikimate metabolic pathway seems obvious,” says Engl, “once the interaction of both partners is understood.”

The team sequenced the genome of the bacterial symbiont to identify its ability to metabolise nutrients, explains Kiefer. Then they compared the amounts of amino acids in normal and symbiont free beetles.

They raised the beetles with or without their symbionts on diets supplemented with glyphosate or synthesised amino acids.

Results showed that the bacterial symbiont’s genome is specialised for making precursors for the essential aromatic amino acids tryptophan, phenylalanine and tyrosine via the shikimate pathway.

These amino acids are highly prevalent in the insect cuticle, which relies especially on tyrosine in late pupae for its synthesis.

Beetles reared on a diet with glyphosate were unable to establish normal symbiont numbers, resulting in a deficient, thinner cuticle equivalent to that produced by antibiotics. This was somewhat mitigated by supplementation with aromatic amino acids.

“These findings highlight the importance of symbiont-mediated tyrosine supplementation for cuticle biosynthesis in insects,” write Kiefer and colleagues, “but also paint an alarming scenario regarding the use of glyphosate in light of recent declines in insect populations.”