Sub-lethal toxicity of the antiparasitic abamectin on earthworms and the application of neutral red retention time as a biomarker

The antiparasitic abamectin has been proven effective against both endo- and ectoparasites of farm animals and hence used widely in animal husbandry. It may enter the soil environment with the excreta of treated animals. Very little information is available with regard to the sub-lethal effects of abamectin on soil invertebrates, such as earthworms. The objective of this study was to evaluate the toxic effect of abamectin on earthworms, using Eisenia fetida, by analyzing changes in the survival, growth, reproduction and cocoon hatchability of exposed earthworms. Furthermore, a biomarker of the lysosomal membrane stability, measured by neutral red retention time (NRR-time), was also applied. Abamectin showed significant toxicity on the growth of earthworms with increasing concentrations up to 5 mg/kg. The most sensitive parameter was reproduction (cocoons production and hatchability) and NRR-time. The number of cocoons was reduced at concentrations above 0.25 mg/kg and no cocoons were present at the highest concentration of 5 mg/kg. Cocoons exposed to abamectin exhibited a reduced hatching success at concentrations above 1.5 mg/kg. The NRR-time was reduced significantly at exposure concentrations of abamectin above 0.25 mg/kg. The change in lysosomal membrane stability showed a good correlation with reproduction and may hence be a potential predicator of the effects on earthworm populations.