Weedkiller glyphosate blocks the symbiotic relationship between bacteria and insects, according to a new study published in Nature (1). Insects exposed to glyphosate no longer receive essential compounds, like amino acids, that they need from bacteria. It’s generally accepted that animals are not affected by glyphosate, but it turns out glyphosate can harm insects indirectly by targeting their bacterial partners, potentially contributing to further losses in diversity.
Glyphosate is one of the most used pesticides in agriculture, despite many controversies about its potentially dangerous side effects. In plants and some bacteria, this pesticide blocks growth by inhibiting the synthesis of some amino acids through the shikimate pathway.
In contrast, animals don’t have this pathway and therefore, in theory, should not be directly affected by glyphosate (there are studies confirming that glyphosate can be harmful to a variety of animals, but not through the shikimate pathway).
Indirectly, however, is a different story. Many animals rely on a relationship with bacteria, which in turn rely upon the shikimate pathway to produce amino acids to give back to the animals. In other words, animals are indeed affected by glyphosate through the shikimate pathway.
Take saw-toothed grain beetles (Oryzaephilus surinamensis), for example. These insects live in symbiosis with bacteria, which provide them essential compounds, including an amino acid known as tyrosine. These compounds are vital for the beetles to form their exoskeleton, protecting them from enemies and avoid desiccation. With the help of their bacterial friends, these beetles can survive in a wide range of environmental conditions – particularly in terms of humidity – thanks of this exoskeleton.
A team of researchers from the Johannes Gutenberg University and the Max Planck Institute showed that using glyphosate completely destroyed the relationship between the saw-toothed beetle and bacteria, leaving these insects without any protection against stress drought or predator attack. Not surprisingly, the team confirmed that the beetle’s bacterial partners rely on the shikimate pathway to synthesise some amino acids.
From these results, the team wondered whether this was a one-off example of a more widespread issue. “When we observed the detrimental impact of glyphosate exposure on this symbiotic association, we wondered whether glyphosate poses a general threat to insects that depend on their microbial partners”, explained Tobias Engl, one of the lead authors of the study.
Using genetic analysis, the team realised that many bacteria associated with different insects indeed have the shikimate pathway, suggesting that this susceptibility to glyphosate represents a serious problem for insect survival.
This could have serious long term effects. Every day in the news, we read about the impact of the disappearance of bees, beetles and other insects. Insect diversity and abundance is decreasing to dangerously low levels. Now there’s one more problem to add to the list: glyphosate used in agriculture endangers the symbiotic relationships between bacteria and insects, posing a severe threat to their survival and the environment.
(1) Kiefer, J.S.T., Batsukh, S., Bauer, E. et al. Inhibition of a nutritional endosymbiont by glyphosate abolishes mutualistic benefit on cuticle synthesis in Oryzaephilus surinamensis. Commun Biol 4, 554 (2021). https://doi.org/10.1038/s42003-021-02057-6