Earthworm ecotoxicological assessments of pesticides used to treat seeds under tropical conditions

Abstract

Ecotoxicological laboratory tests (lower-tier tests) are fundamental tools for assessing the toxicity of pesticides to soil organisms. In this study, using these tests under tropical conditions, we quantified the impact of the insecticides imidacloprid, fipronil, and thiametoxam, and the fungicides captan and carboxin+thiram, all of which are used in the chemical treatment of crop seeds, on the survival, reproduction, and behavior of Eisenia andrei (Oligochaeta). With the exception of imidacloprid, none of the pesticides tested caused mortality in E. andrei in artificial soils. The LC(50) of imidacloprid was estimated as 25.53 mg active ingredient kg(-1) of dry soil. Earthworm reproduction rates were reduced by imidacloprid (EC(50)=4.07 mgkg(-1)), fipronil (EC(20)=23.16 mgkg(-1)), carboxin+thiram (EC(50)=56.38 mgkg(-1)), captan (EC(50)=334.84 mgkg(-1)), and thiametoxam (EC(50)=791.99 mgkg(-1)). Avoidance behavior was observed in the presence of imidacloprid (AC(50)=0.11 mgkg(-1)), captan (AC(50)=33.54 mgkg(-1)), carboxin+thiram (AC(50)=60.32 mgkg(-1)), and thiametoxam (AC(50)=>20 mgkg(-1)). Earthworms showed a preference for soils with the insecticide fipronil. Imidacloprid was the most toxic of the substances tested for E. andrei. The avoidance test was the most sensitive test for most pesticides studied, but results varied between pesticides. These results offer new insights on the toxicity of pesticides used to treat seeds in tropical regions. However, they should be complemented with higher-tier tests in order to reduce the uncertainties in risk assessment.