Ecotoxicological effects of a synthetic and a natural insecticide on earthworms and soil bacterial community

Earthworms and microbial communities are essential non-target soil organisms that are useful to assess the collateral impact of pesticides. The present paper reports three laboratory experiments performed to investigate the effects of sub-lethal doses of two insecticides, a biologically-derived (spinosad) and a synthetic organophosphate (chlorpyrifos), on earthworm Eisenia foetida and microorganisms in organic soil. The effects were studied in terms of behaviour, reproduction, survival, and DNA damage (comet assay) in earthworms, and Next Generation Sequencing-Illumina was employed to detect the changes in the microbial community. In addition, the influence of earthworms on the degradation kinetics of insecticides and on microbial diversity was evaluated. The weights, reproductive activity and behaviour of earthworms were particularly compromised and followed a dose-dependent trend in chlorpyrifos trials, where the insecticide’s degradation wasn’t affected by the presence of Eisenia foetida. However, earthworms contributed to spinosad’s metabolisation without significantly impacting their health. Early DNA damage was estimated in earthworms exposed to chlorpyrifos, while the impact of spinosad was significant only at the end of the toxicity test. The analysis on the microbial community indicated the buffering effect earthworms had on the bacterial communities starting from earliest sampling until the end of the trial, as well as bacterial community members’ degradation response to pesticides over time.