Changes in the structure of dung insect communities after ivermectin usage in a grassland ecosystem. II. Impact of ivermectin under high-rainfall conditions

Abstract

Abstract A large-scale field study was carried out to assess the ecotoxicological effect of ivermectin, a broad-spectrum veterinary agent, on dung insect communities under normal extensive farming conditions in South Africa. Dung insect communities were monitored: i) one year after a first treatment of entire herds with a single standard injection of ivermectin (200 μ·kg−1) in the 1992/93 season; and ii) for three months after a second single standard injection in the 1993/94 season. Two herds were treated with a single standard injection of ivermectin while two herds remained untreated as controls. Each herd was held in a paddock of about 80 ha. Field work was carried out in the rainy season of 1993/94, when the study area received above-average rainfall. The impact of ivermectin was examined using a variety of community measures, including univariate, graphical and multivariate methods. No effect of ivermectin on dung insect communities was observable one year after the 1992/93 treatment. Seven days after treatment in the 1993/94 season, fewer hydrophilid larvae, and scarabaeid and dipteran pupae were present in pats from treated animals in comparison with untreated controls. One and three months after treatment, there were no effects that could be attributed directly to the treatment with ivermectin. The results of the study indicate that the seriousness of the impact of ivermectin depends on several factors, including climatic conditions, spatial scale of treatment and number of animals treated in a herd.