Toxicity of Chlorfenapyr to Folsomia candida and Enchytraeus crypticus in natural and artificial soil.

Pesticides with vastly different modes of action are needed to combat pests that have become multi-pesticide resistant. One proposed alternative is the pyrrole insecticide chlorfenapyr. This pro-pesticide needs bioactivation by CYP450 enzymes to form the more potent metabolite tralopyril. However, its effects on non-target soil organisms are largely unknown. This study aimed to assess the toxicity of chlorfenapyr to Folsomia candida and Enchytraeus crypticus, two model species for soil ecotoxicology, following Organization for Economic Co-operation and Development (OECD) guidelines. Tests were performed in Lufa 2.2 natural soil and in OECD artificial soil containing 5% sphagnum peat (OECD5). F. candida were exposed for 28 days, E. crypticus for 21 days. High toxicity of chlorfenapyr was observed to F. candida, with LC10s of 1.88 and 0.64 mg/kg dry soil, and LC50s of 2.44 and 1.01 mg/kg dry soil in Lufa 2.2 and OECD5 soils, respectively. For reproduction, EC10s were 0.36 and 0.25 mg/kg dry soil, and EC50s 0.79 and 0.35 mg/kg dry soil in Lufa 2.2 and OECD5 soils, respectively. Chlorfenapyr was not toxic to E. crypticus within the tested concentration range (ie No-Observed Effect Concentration (NOEC) >900 mg/kg dry soil), indicating no immediate hazard to enchytraeids in the field. The observed effect concentrations for F. candida are within the predicted environmental concentrations for agricultural use of chlorfenapyr and therefore indicate an immediate hazard towards Collembola. The differences in toxicity observed between the two test soils and the two test species show the importance of using different soils and different test organisms in the environmental risk assessment of pesticides.