Assessment of short and long-term effects of imidacloprid on the burrowing behaviour of two earthworm species ( Aporrectodea caliginosa and Lumbricus terrestris) by using 2D and 3D post-exposure techniques

► Imidacloprid caused long- and short-term effects on earthworm burrowing behaviour. ► Many effects were detected at concentrations occurring in agricultural soils. ► Burrow depth and burrow length proved to be relevant and sensitive endpoints. ► By doing post-exposure observations, effective burrowing capacities were measured. Adverse effects of agrochemicals on earthworms’ burrowing behaviour can have crucial impacts on the entire ecosystem. In the present study, we have therefore assessed short- and long-term effects on burrowing behaviour in the earthworm species Aporrectodea caliginosa and Lumbricus terrestris after exposure to a range of imidacloprid concentrations (0.2–4 mg kg −1 dry weight (DW)) for different exposure times (1, 7, 14 d). 2D-terraria were used for the examination of post-exposure short-term effects (24–96 h), while post-exposure long-term effects were assessed by means of X-ray burrow reconstruction in three dimensional soil cores (6 weeks). For the latter each core was incubated with two specimens of L. terrestris and four of A. calignosa. Short-term effects on the burrowing behaviour (2D) of A. caliginosa were already detected at the lowest test concentration (0.2 mg kg −1 DW), whereas such effects in L. terrestris were not observed until exposure to concentrations 10 times higher (2 mg kg −1 DW). For both species tested in the 2D-terraria, “total burrow length after 24 h” and “maximal burrow depth after 24 h” were the most sensitive endpoints. 3D reconstructions of the burrow systems made by both earthworm species in the repacked soil cores revealed a significant linear decrease in burrow volume with increasing imidacloprid concentration. Since many of the observed effects occurred at imidacloprid concentrations relevant to natural conditions and since reduced activities of earthworms in soils can have crucial impacts on the ecosystem level, our results are of environmental concern.