Impact of Biotransformation and Bioavailability on the Toxicity of the Insecticides α-Cypermethrin and Chlorfenvinphos in Earthworm

Abstract

Knowledge about the bioavailability and metabolism of pesticides in soil organisms facilitates interpretation of its toxicity in soil. The present study relates uptake kinetics and metabolism of two insecticides, the pyrethroid alpha-cypermethrin (alpha-CYP) and the organophosphate chlorfenvinphos (CFVP), in the earthworm Eisenia fetida to their lethal and sublethal toxicity. Experiments were conducted in two soils with different organic matter contents to provide media with contrasting sorption capacity for the insecticides. The results showed that organophosphate CFVP was, when taken up by earthworms, rapidly and irreversibly bound to biomolecules and the fraction of extractable parent insecticide and metabolites was low. In contrast, alpha-CYP was rapidly metabolized by earthworms but did not form conjugates. It seems that the phase II metabolism of alpha-CYP is inhibited in earthworms, resulting in an increasing accumulation of its metabolites. Instantaneous binding of non-altered CFVP to the target site presumably resulted in a higher toxicity compared to alpha-CYP and explains the small difference between lethal and reproduction toxicity. For alpha-CYP, however, accumulation of alpha-CYP metabolites in earthworms during chronic exposure may explain the large observed difference between lethal and sublethal toxicity. Bioaccumulation and toxicity of either insecticide decreased with increasing organic matter content in soil, emphasizing the role of compound sorption on bioavailability and toxicity for soil organisms.