The toxicity of benomyl and some related 2‐substituted benzimidazoles to the earthworm Lumbricus terrestris

Benzimidazole and its 2‐amino analogue were non‐toxic to Lumbricus terrestris when administered orally. The fungicidal 2‐substituted benzimidazoles (benomyl, carben‐dazim, fuberidazole and thiabendazole) and l‐(benzimidazol‐2‐yl)‐3‐butylurea were highly and equally toxic. The equitoxicity of compounds with widely differing substituent groups suggests a common mode of action—the benzimidazole moiety being the active part of the molecule. It is likely that there is a common mode of action for benzimidazole analogues for both fungicidal activity and earthworm toxicity. There was no detectable effect on the cholinesterase levels of earthworms poisoned by benomyl. But it is confirmed that the cholinesterase inhibitory activity of benomyl in vitro is due to formation of butyl isocyanate.