For many decades, the macrocyclic lactone drug ivermectin is extensively used in veterinary medicine and agriculture, as well as human medicine. Residues of ivermectin excreted in cattle dung remain persistent in soils (Mougin et al., 2003), biologically active and threaten non-target soil and coprophagous organisms such as dung flies and beetles (Lumaret et al., 2012). Ivermectin affects highly beneficial and taxonomically diverse groups inhabiting dung pats, including flies, parasitic wasps, as well as coprophilus and predatory dung beetles (Villar et al., 2022). Ivermectin resistance is well document in insects, but it seems to take longer and to be less effective than resistance to insecticides or other antiparasitic drugs, because of different physiological mechanisms involved in resistance (Seaman et al., 2015).

In that context, Gonzalez-Tokman et al. (2024) compared the reproductive success of a line of dung beetles (Euoniticellus intermedius, Scarabaeinae) exposed to a moderate concentration of invermectin during 18 generations, and a control line of beatles that was maintained free of antiparasitic drug. They carried-out toxicity experiments with increasing ivermectin concentrations to determine if sensitivity to ivermectin was reduced after some generations of exposure, possibly by acquiring resistance by means of transgenerational effects. Thus, dung beetles did not generate resistance to ivermectin after 18 generations of continuous exposure, and quantitative genetic analyses showed only low genetic variation in response to ivermectin.

The results published by Gonzalez-Tokman et al. (2024) indicated a low potential of beetles for adaptation to the drug, and suggest for non-target invertebrate groups a possible risk of extinction in ivermectin-contaminated pastures. These effects can greatly impact grassland ecology, lower their quality and reduce the area available and palatable to livestock.

References

Mougin, C., Kollmann, A., Dubroca, J., Ducrot, P.-H., Alvinerie, M., Galtier, P., 2003. Fate of the veterinary medicine ivermectin in soil. Environ. Chem. Letters 1, 131-134. https://doi.org/10.1007/s10311-003-0032-9

Lumaret, JP., Errouissi, F., Floate, K., Römbke, J., Wardhaugh, K., 2012. A review on the toxicity and non-target effects of macrocyclic lactones in terrestrial and aquatic environments. Current Pharmaceutical Biotechnology 13(6), 1004-60. https://doi.org/10.2174/138920112800399257

Villar, D., & Schaeffer, D.J., 2022. Ivermectin use on pastured livestock in Colombia: parasite resistance and impacts on the dung community. Revista Colombiana De Ciencias Pecuarias, 36(1), 3-12. https://doi.org/10.17533/udea.rccp.v36n1a2

Seaman, J.A., Alout, H., Meyers, J.I., Stenglein, M.D., Dabiré, R.K., Lozano-Fuentes, S., Burton, T.A., 471 Kuklinski, W.S., Black, W.C., Foy, B.D., 2015. Age and prior blood feeding of Anopheles gambiae influences their susceptibility and gene expression patterns to ivermectin-containing blood meals. BMC Genomics 16, 797. https://doi.org/10.1186/s12864-015-2029-8 

González-Tokman, D., Arellano-Torres, A., Baena-Díaz, F., Bustos, C., Martínez M., I., 2024. Ivermectin resistance in dung beetles exposed for multiple generations, bioRxiv ver. 3 peer-reviewed and recommended by Peer Community in Ecotoxicology and Environmental Chemistry. https://doi.org/10.1101/2023.05.08.539900