Lethal and sublethal effects of imidacloprid on two earthworm species (Aporrectodea nocturna and Allolobophora icterica)

The chemical imidacloprid is the major component of many widely used insecticides and is relatively persistent in soils. A set of experiments was carried out to estimate the lethal (mortality) and sublethal (weight loss) effects of one of these insecticides, Confidor, on two earthworm species commonly found in agricultural soils. A preliminary experiment in the absence of earthworms showed that imidacloprid was not rapidly degraded, with a decrease of less than 10% after 2 weeks, and that it was distributed in a reasonably homogeneous manner throughout the soil (less than 10% of variation between samples). The LC50 of imidacloprid for the anecic species Aporrectodea nocturna and the endogeic species Allolobophora icterica was between 2 and 4 mg kg(-1) dry soil. This result is consistent with previous findings obtained with other earthworm species and natural soils. When sublethal effects were examined, significant decreases in weight were observed at concentrations of 0.5 and 1 mg kg(-1) dry soil for the two earthworm species whereas no effect was observed at a concentration of 0.1 mg kg(-1) dry soil (NOEC value). These concentrations are close to 0.33 mg kg(-1) which is the Predictive Environmental Concentration. Weight loss appears to be a valuable endpoint that can be used with worms freshly collected in the field as long as variability in the response of a control is taken into account.