Environmental assessment of veterinary avermectins in temperate pastoral ecosystems

Abstract

SummaryAvermectins and their metabolites are excreted mainly in the faeces; they do not readily move from the site of dung deposition because of their low solubility in water and their tight binding to organic matter. Avermectins degrade in the environment through photodegradation and aerobic breakdown by soil organisms. The dung mesofauna potentially exposed to avermectin residues includes insects, earthworms, springtails, mites and nematodes. These organisms occupy a variety of different niches within the ecosystem and the faunal composition changes as the pats age. Some members of this fauna act in concert with soil microbial flora and assist in the breakdown of dung and consequent nutrient re‐cycling on pasture. There are marked seasonal patterns of faunal abundance and behaviour which affect their relative importance in the decomposer community. Some species of the coprophagous insect fauna, particularly in the larval stages, can be adversely affected by the presence of avermectin residues in the faeces. Veterinary use patterns of avermectins in temperate regions indicate that peak periods of insect activity and peak times of avermectin use are often asynchronous. When avermectin usage and insect activity do coincide, the heterogeneous patterns of administration to livestock and the focus of treatment on young animals result in the deposition on pasture of faeces which are predominantly free of avermectin residues. Results of large scale, long term studies indicate that, even under conditions of relatively high levels of avermectin use in cattle, the impact on non‐target insect populations and their function is limited.