The focus is mostly on dangers to human health – but a study has now made it clear again that the notorious weed killer glyphosate also threatens insects: it can inhibit symbiotic bacteria that are necessary for the formation of the exoskeleton of some species, according to studies Beetles. Due to this weakening, glyphosate could also play a role in insect death. Against the background of planned application bans, the following applies: When looking for alternatives to glyphosate, possible effects on symbiotic partners of living beings must be taken into account, according to the study.
There is a simple connection behind the immense consumption of herbicides in agriculture: where weeds sprout, the yield decreases. That is why the chemical club is swung – glyphosate is a particularly powerful one. It is a so-called total herbicide – it destroys all greenery. After the substance has broken down, the desired crop can then be planted on the cleared field. Since glyphosate attacks an enzyme that is not found in animals or humans, it has long been considered harmless. But then this picture changed: some studies attested the substance’s carcinogenic effect – others contradicted this finding, however, which sparked fierce controversy. They were also fueled by evidence that glyphosate harms bees by affecting their intestinal bacteria.
One aspect came into play that was initially hardly considered: although glyphosate only inhibits the so-called shikimate metabolic pathway, which does not occur in animals, some microorganisms also use it in addition to plants. Thus, animals are only supposedly safe from the effect: “An influence of glyphosate on animals via their essential, bacterial partners that use the shikimate metabolic pathway is in principle obvious as soon as one understands the interaction between the two partners,” explains Tobias Engl from Max -Planck Institute for Chemical Ecology in Jena. It has already been shown that glyphosate has negative effects on microorganisms in the gut of honeybees, making them more susceptible to various stress factors.
Targeting symbiosis partners
Engl and his colleagues now focused on another symbiosis of insects and bacteria: many species have specialized microbes in certain cells that produce building blocks for them. They form aromatic amino acids for the structure of the exoskeleton (cuticle) of insects. If the supply of these substances is inadequate or absent, the outer shell cannot guarantee its protective function against drought or enemies. What harms the microbes therefore also weakens the insects.
In order to investigate the possible effect of glyphosate in this context, the scientists looked at the flat grain beetle (Oryzaephilus surinamensis) as a model. Through experiments they were initially able to confirm: If the test animals were exposed to practice-relevant amounts of glyphosate, the establishment of the symbiotic bacteria was inhibited during the entire beetle development, whereby the benefit of the microbes for the formation of the cuticle was completely negated. Specifically, this means that they developed a weak protective cover. The tests clearly showed the role of aromatic amino acids: By supplementing food with these substances, the loss of the symbiotic partner could be compensated for. “Since we were able to observe how glyphosate damages the symbiotic community, we asked ourselves whether glyphosate also poses a general danger to other insects that depend on their microbial partners,” says Tobias Engl.
Potentially far-reaching significance
To answer this question, the researchers sequenced the genome of the bacterial partner of the tip beetle. It was shown that it contains the genetic instructions for the formation of aromatic amino acids via the shikimate pathway. Comparisons with other known genome data then initially provided indications that symbiotic relationships with a comparable function have developed independently of one another in different bacterial strains and distantly related beetle families. Apparently, the importance goes beyond this group of insects: with the help of phylogenetic analyzes, the researchers showed that the shikimate metabolic pathways of many indispensable symbiotic bacteria that are associated with very different host insects contain a glyphosate-sensitive enzyme.
The new findings make it clear that the use of the weed killer glyphosate in agriculture endangers the vital symbiotic relationships between insects and microorganisms and can thus pose a serious problem for ecosystems. Possibly this is a factor in the sometimes dramatic decline in insect populations in recent decades, say the researchers. Against this background, it is encouraging that at least an end to the approval of glyphosate in many countries is in sight. But that is why alternative weed killers are now also in demand. The results of the study send out a clear message: “The use of herbicides with potential side effects on animals or their associated microorganisms should be carefully considered,” the scientists conclude.
Source: Max Planck Institute for Chemical Ecology, Article: Communications Biology, doi: 10.1038 / s42003-021-02057-6