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, averm ectins are eliminated in the livestock faeces where they also have a w ide range of harmful affects upon certain characteristic insects that breed in dung, few of which are pests, and many of which are beneficia l. The effects range from acute toxicity in larvae and adults, through disruption of metamorphosis, to interference with reproduction. Diffe rent methods of drug administration lead to different concentrations o f drug residues in the faeces, which in turn influence the responses o f non-target organisms. Higher Diptera are particularly sensitive to d rug residues and show a wide range of responses from death of larvae t o developmental abnormalities in the adults. Larvae and immature adult s of Coleoptera show some mortality in the dung of recently treated an imals, while delayed effects upon.reproduction and physiology have bee n observed in adults feeding on dung at longer post-treatment times. A lthough the impact of lethal doses has been described in some species, the effects of sub-lethal doses have hardly been recognised at the pr esent time. Correlated with the deleterious effect upon dung-breeding insects, a retardation in the rate of loss of biomass of dung pats fro m avermectin-treated cattle has been observed following the various fo rms of drug administration. Differences in methodology, inappropriate statistics, and/or extremes of climatic conditions prevailing at the t ime of testing, explain the results of those studies where such delays have not been observed. It is short-sighted to consider only dung dis persal in relation to avermectin usage, a practice that overlooks the impact on the insects themselves and their diverse roles in pasturelan d ecology.