Toxicity and Action of Chlorpyrifos and Other Organophosphates in the Eastern Subterranean Termite12

Abstract

Chlorpyrifos and certain other organophosphates were toxic when applied topically to Reticulitermes flavipes (Kollar). Symptoms manifested by termites poisoned with these organophosphates were cholinomimetic. There existed a relationship between in vitro termite cholinesterase (ChE) inhibition by direct inhibitors as measured by pI50 (the negative logarithm of the molar inhibitor concentration yielding 50% inhibition of enzyme) and toxicity as measured by LD50. Generally the shorter alkyl chain organophosphates were the more potent ChE inhibitors and were highly toxic. Conversely, the longer alkyl chain compounds were less potent in vitro ChE inhibitors and were much less toxic. With chlorpyrifos and several related chloropyridyl phosphorothioates and phosphates there was a direct relationship between in vivo ChE inhibition and the severity of the symptoms of poisoning. Thus, the action of chlorpyrifos and certain other organophosphates in the eastern subterranean termite was related to inhibition of ChE.