Ivermectin tolerance and associated costs in the dung beetle Euoniticellus intermedius

Ivermectin is the most common antiparasitic medication used in cattle in some regions of the world, despite residues excreted in dung being highly toxic for non-target coprophagous insects that provide highly valuable ecosystem services, such as dung beetles. Whereas closely related species and distant populations can considerably differ in ivermectin tolerance, this is unknown for dung beetles, which are highly sensitive. Here, we evaluated ivermectin tolerance in the dung beetle Euoniticellus intermedius from six sites and measured fitness-related traits in beetles exposed to different ivermectin concentrations under a common garden experiment in the laboratory. These sites were sampled along an elevational gradient to capture natural variation in life history traits associated with contrasting environmental conditions and land use history. As expected, large variation among sites was observed in LC₅₀ (i. e. tolerance), being more than 200 times higher in the most tolerant compared to the least tolerant site, where nearly zero laid eggs emerged at high concentrations. Despite these differences, no elevational pattern was detected in ivermectin tolerance, although elevation effects on some life history traits were detected. Finally, we confirmed that ivermectin tolerance had associated costs in the most tolerant population, as revealed by a trade-off between offspring number and size that was clearly exaggerated with increasing ivermectin concentration. These findings indicate that susceptibility to ivermectin is highly variable among geographically close sites and that factors other than elevation are driving ivermectin susceptibility. The high fitness costs associated with ivermectin tolerance indicate that ivermectin exerts strong selective pressure on dung beetles, driving the evolution of alternative life histories in contaminated pastures.