New evidence suggests that using neonicotinoids to control soybean pests may be hurting farmers.

Thiomethoxam, a neonicotinoid nerve poison, is widely applied to soybeans as a seed coating. After a bean seed is planted and it germinates and grows into a plant, every cell of the plant becomes infused with the neonicotinoid pesticide. In theory, when a pest feeds on the soybean plant, it dies from neonicotinoid pesticide exposure.

However, a 2014 study by Margaret Douglas of Pennsylvania State University and coworkers in the Journal of Applied Ecology demonstrates that slugs — a major soybean pest — are unaffected when they feed on neonicotinoid-treated soybean plants. However, beneficial ground beetles — naturally abundant predators that eat slugs and normally keep them in check — experience seizures, paralysis and death when they eat slugs from plants containing these nerve poisons.

In field trials, the researchers measured a 5 per cent yield reduction in neonicotinoid-treated soybeans compared to soybeans that did not contain the pesticide.

This is not the first study showing that using neonicotinoids on soybeans may actually reduce yields. However, it goes beyond previous studies by demonstrating in detail one mechanism by which this yield reduction occurs — namely, killing off beneficial predatory insects.

Neonicotinoids are highly toxic to bees, earthworms, and many other organisms. Toxicity varies widely according to which group of organisms is exposed, and can also vary among species within a group. Certain pest species may be unaffected, while many non-target organisms are harmed.