The effect of ivermectin on soil organisms was assessed in Terrestrial Model Ecosystems (TMEs). Intact soil cores were extracted from a pasture in England and kept for up to 14 weeks in the laboratory. Ivermectin was applied to the soil surface via spiked cow dung slurry at seven concentration rates ranging from 0.25 to 180 mg/TME, referring to concentrations of 0.19–227 mg ivermectin/kg soil dry weight in the uppermost (0–1 cm) soil layer. After 7, 28 and 96 days following the application soil cores were destructively sampled to determine ivermectin residues in soil and to assess possible effects on microbial biomass, nematodes, enchytraeids, earthworms, micro-arthropods, and bait-lamina feeding activity. No significant effect of ivermectin was found for microbial respiration and numbers of nematodes and mites. Due to a lack of dose–response patterns no effect concentrations could be determined for the endpoints enchytraeid and collembolan numbers as well as total earthworm biomass. In contrast, EC50 values for the endpoint feeding rate could be calculated as 0.46, 4.31 and 15.1 mg ivermectin/kg soil dry weight in three soil layers (0–1, 0–5 and 0–8 cm, respectively). The multivariate Principal Response Curve (PRC) was used to calculate the NOECcommunity, based on earthworm, enchytraeid and collembolan abundance data, as 0.33 and 0.78 mg ivermectin/kg soil dw for day 7 and day 96, respectively. The results shown here are in line with laboratory data, indicating in general low to moderate effects of ivermectin on soil organisms. As shown by the results of the bait-lamina tests, semi-field methods such as TMEs are useful extensions of the battery of potential test methods since complex and ecologically relevant endpoints can be included.
