Nontoxic effects of thymol, carvacrol, cinnamaldehyde, and garlic oil on dung beetles: A potential alternative to ecotoxic anthelmintics

by José R. Verdú, Vieyle Cortez, Rocío Rosa-García, Antonio J. Ortiz, Urcesino García-Prieto, Jean-Pierre Lumaret, Carmelo García Romero, Francisco Sánchez-Piñero

The sustainability of the traditional extensive livestock sector will only be possible if healthy dung-decomposing insect communities are preserved. However, many current pharmaceutical anthelmintics are harmful to dung beetles, their presence can have a negative impact on biological systems. Phytochemical anthelmintics are an alternative to ecotoxic synthetic pharmaceutical anthelmintics, although ecotoxicological tests of their possible indirect effects on dung beetles are required to demonstrate their viability. In this study, the potential ecotoxicity of thymol, carvacrol, cinnamaldehyde and garlic oil (diallyl disulfide and diallyl trisulfide) were tested for the first time. Inhibition of antennal response was measured as a relevant parameter by obtaining relevant toxicity thresholds derived from concentration‒response curves, such as the IC₅₀. All phytochemical compounds tested were demonstrated to be suitable alternative candidates to the highly ecotoxic compound ivermectin, considering their non-toxicity to nontarget organisms. Residues of the phytochemical antiparasitics found in cattle droppings were extremely low, even undetectable in the case of diallyl disulfide and diallyl trisulfide. Furthermore, our results showed that none of the phytochemical compounds have ecotoxic effects, even at extremely high concentrations, including those almost 1000 times higher than what is most likely to be found in dung susceptible to ingestion by dung beetles in the field. We can conclude that the four selected phytochemical compounds meet the requirements to be considered reliable alternatives to ecotoxic veterinary medicinal products, such as ivermectin.