Ecotoxicity evaluation of thymol in tunneller dung beetles (Coleoptera: Geotrupidae) as a natural alternative to ecotoxic anthelmintics.

Synthetic anthelmintics widely used in livestock production are largely excreted unchanged in faeces, where they can exert sublethal and lethal effects on non-target organisms such as dung beetles. Phytochemical compounds, including thymol (THY), have been proposed as environmentally compatible alternatives; however, information on their ecotoxicological safety across different dung beetle taxa remains limited. We assessed the sublethal effects of dietary thymol on Thorectes lusitanicus (Coleoptera: Geotrupidae), a flightless tunneller dung beetle considered functionally sensitive to veterinary drug residues. Antennal sensory responses were evaluated using electroantennography, and immune function was assessed by measuring haemolymph total protein content, phenoloxidase and prophenoloxidase activities. Ivermectin was included as a reference compound due to its well-documented ecotoxic effects on dung beetles. Thymol ingestion did not affect antennal olfactory responses or immune parameters across a wide range of concentrations, and no neurotoxic symptoms were observed. Significant inhibitory effects were detected only at concentrations substantially higher than thymol residues measured in cattle dung following dietary supplementation. In contrast, ivermectin significantly impaired olfactory function and induced clear neurotoxic symptoms. Overall, these results indicate that thymol is unlikely to cause ecotoxicologically relevant sublethal effects in tunneller dung beetles under the exposure scenarios evaluated. Our findings suggest that thymol-based phytochemical anthelmintics may represent environmentally compatible alternatives to conventional veterinary drugs.