Insect activity was significantly reduced in dung from cattle treated with a recommended topical dose (500 ug/kg) of the endectocide, ivermectin. Reductions affected an ecologically and taxonomically diverse group of insects including coprophagous flies, parasitic wasps, and both predacious and coprophagous beetles. Whereas some species appeared unaffected by faecal residues, the emergence of other species was reduced in dung voided < 12 weeks post-application. The species most affected were the flies, Sepsis sp. and Coproica mitchelli (Malloch), eucoilid wasps, and the beetles, Cercyon quisquilhis (Linnaeus) and C. pygmaeus (Illiger). Results were consistent within and between years. Similar reductions have been reported previously, but typically for a much shorter post-application period. The greater duration of effect observed in the current study is attributed to a more accurate bioassay, species-level identifications, and the use of sample sizes generally larger than those used in previous studies. Reduced insect activity was associated with slower dung pat degradation. When ivermectin was added directly to dung, at levels previously reported to occur in dung from treated cattle, the treated dung had not appreciably degraded after 340 days in the field. In contrast, untreated dung pats deposited at the same time and place had largely degraded after 80 days in the field. The effect of ivermectin use on pasture quality is probably 'moot' in southern Alberta because of the current pattern of use. Should this pattern change, however, results of the present study validate the need for future studies to assess the off-target effects of ivermectin use on pasture quality.
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