New screening test to predict the potential impact of ivermectin-contaminated cattle dung on dung beetles

According to European Union recommendations, a test method has been developed to evaluate the effects of veterinary pharmaceuticals on dung feeding insects. This test method was evaluated with the dung beetle Aphodius constans by using fecal residues of ivermectin after a pour-on administration. Dung of different age (and thus containing different concentrations of ivermectin) as well as mixtures of highly-contaminated spiked dung with untreated control dung were studied in five test runs in two laboratories. The concentration of ivermectin (active substance; a.s.) in the dung samples was verified analytically. The main test endpoint was the survival of first instar larvae. The LC$_{50}$ using dung directly obtained from treated cattle ranged from 470 to 692 $\mu $g a.s. kg$^{-1}$ dung (dry weight; d.w.) and 67 to 97 $\mu $g a.s. kg$^{-1}$ dung (fresh weight; f.w.). Using mixtures, the outcome of two tests was almost identical: 770 to 781 $\mu $g a.s. kg$^{-1}$ dung (d.w.); 109 to 132 $\mu $g a.s. kg$^{-1}$ dung (f.w.). In comparison to the LC$_{50}$ values obtained when ivermectin was spiked in control dung at several concentrations (LC$_{50}$ 880–985 $\mu $g a.s. kg$^{-1}$ dung (d.w.)), the LC$_{50}$ values were again very similar. Three conclusions can be drawn from these results. The proposed test method seems to be robust and allows for the initiation of an international validation process (including ringtesting). Because of only small differences found in tests in which the test substance was spiked into control dung and those in which dung from treated cattle was applied, the use of a standard test method is proposed. The effects of ivermectin on ecologically relevant dung beetles obtained in a standardised test method reflect the results from field studies and are in the range of environmentally relevant concentrations