Lethal and sub-lethal effects of ivermectin on north temperate dung beetles, Aphodius ater and Aphodius rufipes (Coleoptera: Scarabaeidae)

Ivermectin is an anthelmintic veterinary medicine, and is excreted in the dung of treated livestock in a mainly unmetabolised form. Ivermectin is known to have toxic effects on dung beetles, but most studies to date have been conducted on tropical and sub-tropical species. Relatively few laboratory studies have focused on the specific effects of ivermectin on survival and development of north temperate dung beetles. In this study, we experimentally investigated the effect of ivermectin concentration on various life stages of two Aphodius dung beetle species. Dung was collected from cattle groups that had been treated with a subcutaneous injection of ivermectin. Laboratory bioassays were conducted by feeding adults of two beetle species (Aphodius ater and Aphodius rufipes) with dung that contained different concentrations of ivermectin. Adult survival and oviposition were measured, and the subsequent development and survival of produced larvae was monitored over time. Larval development rates were significantly slowed by ivermectin. Ivermectin had significant negative effects on the survival of larvae. Overall, ivermectin concentration caused large and significant reductions in the cohort size from an individual dung pat that would potentially contribute to the next generation of beetles. In general, ivermectin concentration did not have significant negative effects on adult survival. The number of eggs per female A. rufipes was significantly reduced by ivermectin concentration in one of two bioassays, but the magnitude of the effect was not large. The actual impacts on dung beetle population dynamics in farmland would depend on several other factors, which are discussed.