Overview: the impact of avermectins on pastureland ecology

Avermectins, a relatively new class of broad spectrum pesticides, are used widely to control livestock parasites. Following treatment, avermectins are eliminated in the livestock faeces where they also have a wide range of harmful affects upon certain characteristic insects that breed in dung, few of which are pests, and many of which are beneficial. The effects range from acute toxicity in larvae and adults, through disruption of metamorphosis, to interference with reproduction. Different methods of drug administration lead to different concentrations of drug residues in the faeces, which in turn influence the responses of non-target organisms. Higher Diptera are particularly sensitive to drug residues and show a wide range of responses from death of larvae to developmental abnormalities in the adults. Larvae and immature adults of Coleoptera show some mortality in the dung of recently treated animals, while delayed effects upon reproduction and physiology have been observed in adults feeding on dung at longer post-treatment times. Although the impact of lethal doses has been described in some species, the effects of sub-lethal doses have hardly been recognised at the present time. Correlated with the deleterious effect upon dung-breeding insects, a retardation in the rate of loss of biomass of dung pats from avermectin-treated cattle has been observed following the various forms of drug administration. Differences in methodology, inappropriate statistics, and/or extremes of climatic conditions prevailing at the time of testing, explain the results of those studies where such delays have not been observed. It is short-sighted to consider only dung dispersal in relation to avermectin usage, a practice that overlooks the impact on the insects themselves and their diverse roles in pastureland ecology.