CHANGES IN THE STRUCTURE OF DUNG INSECT COMMUNITIES AFTER IVERMECTIN USAGE IN A GRASSLAND ECOSYSTEM - II - IMPACT OF IVERMECTIN UNDER HIGH-RAINFALL CONDITIONS

A large-scale field study was carried out to assess the ecotoxicologic al effect of ivermectin, a broad-spectrum veterinary agent, on dung in sect communities under normal extensive farming conditions in South Af rica. Dung insect communities were monitored: i) one year after a firs t treatment of entire herds with a single standard injection of iverme ctin (200 mu.kg(-1)) in the 1992/93 season; and ii) for three months a fter a second single standard injection in the 1993/94 season. Two her ds were treated with a single standard injection of ivermectin while t wo herds remained untreated as controls. Each herd was held in a paddo ck of about 80 ha. Field work was carried out in the rainy season of 1 993/94, when the study area received above-average rainfall. The impac t of ivermectin was examined using a variety of community measures, in cluding univariate, graphical and multivariate methods. No effect of i vermectin 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 prese nt in pats from treated animals in comparison with untreated controls. One and three months after treatment, there were no effects that coul d be attributed directly to the treatment with ivermectin. The results of the study indicate that the seriousness of the impact of ivermecti n depends on several factors, including climatic conditions, spatial s cale of treatment and number of animals treated in a herd. (C) Elsevie r, Paris.