Assessment of the effects of imidacloprid on the behavior of two earthworm species (Aporrectodea nocturna and Allolobophora icterica) using 2D terraria

Earthworms are “ecosystem engineers” and changes in their main activities (creation of burrows and burial of organic matter) due to pollutants can have important effects on soil functions and indirectly on other components of the soil ecosystem. Using 2D terraria, we studied the behavior of two earthworm species (the endogeic Allolobophora icterica and the anecic Aporrectodea nocturna) exposed to sublethal concentrations (0.5 and 1 mg kg−1 of dry soil) of imidacloprid, a widely used neonicotinoid insecticide. A second experiment was carried out to determine whether such imidacloprid concentrations were avoided by the two earthworm species we studied. In polluted soils, significant weight loss was observed in both species. No significant avoidance of imidacloprid was detected, whereas several modifications of the behavior were observed directly (burrowing dynamics, number of oscillations in the burrow system) or indirectly in the resulting burrow system architecture (area, topology, sinuosity maximal depth) according to the species and concentration tested. When exposed to imidacloprid (0.5 or 1 mg kg−1), A. icterica almost stopped burrowing after 24 h, whereas A. nocturna burrowed continuously but with a significantly lower rate compared to the control. Moreover, in both species, the burrow systems were always shallower when worms were exposed to imidacloprid. Since this last endpoint is easy to measure and is linked to water transfer in soils, it could be used in ecotoxicological studies provided it could be validated for a wide range of pollutants and concentrations.