Effects of the parasiticide ivermectin on the structure and function of dung and soil invertebrate communities in the field (Madrid, Spain)

Abstract

The effects of the parasiticide ivermectin on dung and soil fauna as well as dung decomposition were assessed in a field study conducted near Madrid (Spain). Groups of cattle were treated with an injection of Ivomec ® at a dosage of 200 μg ivermectin kg −1 body weight at four different time points before collecting the fresh dung of all cattle groups on day 0. Dung pats prepared from the dung of the ivermectin-treated cattle were exposed in the field from day 0 onwards in parallel with dung pats that were prepared from the dung of untreated cattle and that was either spiked with a high concentration of ivermectin (positive control) or not spiked (negative control). Ivermectin concentrations in freshly excreted dung ranged from 0.31 to 0.81 mg ivermectin kg −1 dung dry weight. Ivermectin soil concentrations were highest below ivermectin-spiked dung in the uppermost soil layer with up to 0.085 mg ivermectin kg −1 soil dw. No ivermectin-related effects on collembolans and mites were found. A significant decrease in the abundance of adult dung beetles was observed at 0.81 mg ivermectin kg −1 dung dw, but the two most abundant species showed contrasting responses. For the dung beetle species, Volinus distinctus, a no observed effect concentration (NOEC) and a median effect concentration (EC 50) of 0.50 and 0.62 mg ivermectin kg −1 dung dw, respectively, were determined. Dung fly larvae were the most sensitive dung fauna group as their abundance was significantly reduced in all ivermectin treatments, resulting in a NOEC of <0.31 mg ivermectin kg −1 dw. Staphylinid beetles were abundant in the dung, but apparently not affected by ivermectin. Dung from ivermectin-treated cattle degraded more slowly than dung from untreated cattle, resulting in a NOEC of <0.78 mg ivermectin kg −1 dung dw for this functional endpoint. Adult dung beetles of both dominant species were attracted to dung pats spiked with ivermectin. The mobility of ivermectin appeared to differ between dung from ivermectin-treated cattle and dung spiked with ivermectin. This finding challenges approaches to assess the fate and effect of veterinary pharmaceuticals in higher tier tests by using spiked dung. This field study detected consistent results regarding effects of ivermectin on structural and functional endpoints in the dung compartment but effects on soil microarthropods are of little relevance in comparison. This study further advances guidance on higher tier tests that is required to make the results more applicable to the environmental risk assessment of veterinary pharmaceuticals.