Simplification of insect communities developing in dung of cattle treated up to 25 weeks previously with an extended-release formulation of the parasiticide eprinomectin

Cattle treated with a novel formulation of the parasiticide eprinomectin (sold as LongRange®) faecally excrete insecticidal residues for an extended period post-application. We examined the non-target effect of these residues by comparing insect communities developing in dung pats of untreated cattle (Wk 0) to those developing in dung pats of cattle treated 1, 2, 4, 8, 12, 16, 20 and 24/25 weeks previously. Results from two experiments showed that dung of untreated cattle supported more total insects (beetles, flies, parasitoid wasps) and more species than did dung of cattle treated ≤ 12 weeks (Experiment 1) and ≤ 24/25 weeks (Experiment 2) previously. Across the two experiments, an effect of residue on individual taxa was either not detected (9 cases) or was determined to suppress insect development in dung of cattle treated 8-12 weeks (2 cases), 12-16 weeks (3 cases), 16-20 weeks (2 cases) or ≤ 24/25 weeks (6 cases) previously. Flies were particularly sensitive to residues with suppression often at or near 100%. We conclude that cattle treated with LongRange® in spring will excrete residues in their dung for the entire grazing season with an associated simplification of the dung insect community. The effect of this simplification on the long-term health on dung-insect populations and dung degradation was not examined in the current study, but merits future research.