Why do herbicides affect animals?

Maanantai 17.10.2022 klo 16.50 - Mikko Nikinmaa

The development of herbicides has long concentrated on finding molecules that affect biochemical pathways not present in animals. As a consequence, it has been considered that they do not cause harm to insects and vertebrates. Studies on animal cells have usually confirmed this supposition: since the biochemical pathway is not present, the compound is not toxic to animals.

Because of this, the increasing number of findings suggesting that herbicides have toxic effects on animal populations have largely been labelled as trash. This conclusion does not, however, take into account that the animal body has more microbes than body’s own cells. The animal’s microbiome, as it is called, influences nutrition, humoral and immune functions, metabolism and even neural behaviour. Many microbes in our body have the biochemical pathways targeted by the herbicide molecules. Consequently, herbicides affect the species distribution of microbiome, whereby the functions affected by the microbiome can also be influenced in animal body, and the herbicide be toxic to animal.

In recent review by Ruuskanen et al. in Trends in Ecology and Evolution (doi.org/10.1016/j.tree.2022.09.009) give a detailed description of the role of microbiomes in shaping the responses of non-target organisms to herbicides. An important point to note is that the sensitivities of organisms that are susceptible vary markedly. Also, the speed by which resistance is developed is highly variable. Further, the fact that different conclusions are reached if animal cells or whole animals are used in the studies casts significant doubt on the animal protection thesis that instead of intact animals, cell cultures should be used in toxicological studies.