Two groups of 50 heifers were given a subcutaneous injection of either abamectin (Avomec) or doramectin (Dectomax) at a dose rate of 200 μg/kg live weight. A third group of 50 heifers remained untreated. Dung samples were collected on 1, 3, 6, 9, 18, 24, 34, and 42 d after injection, and excreted residues were bioassayed using the dung beetle Onthophagus binodis Thunberg. Fewer newly emerged adults of O. binodis survived exposure to dung from cattle treated 3 and 6 d previously with abamectin or 9 d previously with doramectin than from dung of untreated cattle. Both compounds induced a range of sublethal effects on O.binodis. Abamectin residues excreted in dung up to 42 d after injection had a deleterious impact on ovarian condition, brood mass (egg) production, and larval survival. Doramectin residues only had a deleterious effect on these parameters at 3 and 6 d after injection relative to dung from control cattle. Analysis of the dung collected at each date after injection indicated that moisture content, pH, and percent nitrogen were not different from other physicochemical profiles conducted on cattle dung. Doramectin residues attained maximal concentrations of 101.1 μg/kg 3 d after injection followed by a linear decline with an elimination half-life estimate of 15 d. The bioassay data indicated that doramectin concentrations of <60 μg/kg have minimal impact on the mortality and reproductive potential of O. binodis and that deleterious effects to this species will be evident for only 1–2 wk after administration of the drug to cattle. The potential ecotoxic effects of these compounds are discussed in terms of dung beetle activity and strategies for parasite control of cattle in the Australian environment.
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