The ecotoxicological effects of ivermectin on dung insect communities

Abstract

Ivermectin is a broad-spectrum veterinary agent active against a range of nematodes and arthropods in livestock. Concern about the environmental safety of ivermectin has arisen because a considerable amount of the agent is excreted in the animals' dung where it remains active and has been shown to affect non-target organisms such as dung-breeding insects. The ecotoxicological effects of ivermectin on dung insects under South African conditions were assessed at three levels of complexity: (1) laboratory, (2) semi-field and (3) large-scale field studies. For all studies cattle were treated with a single subcutaneous injection of 200μgkg- 1 body mass according to the manufacturer's recommendations. Lethal and sublethal effects of ivermectin and its residues in cattle dung were bioassayed in the laboratory using the two dung beetle species, Euoniticel/us intermedius (Reiche) and Onitis alexis Klug, and the dung breeding fly, Musca nevilli Kleynhans. The parasiticide was found to reduce adult emergence and to prolong development. Studies on the effect of ivermectin on the fertility of E. intermedius and M. nevilli also showed a reduction. The duration and severity of the effects varied among species. All field investigations took place in the summer rainfall region of South Africa. Firstly, field trials carried out in the Transvaal showed, in general, no differences in dung decomposition or colonization of pats from ivermectin-treated animals. However, dung decomposition rates varied among trials. Dung from treated animals was decomposed at the same rate, faster or more slowly than the control two, three and seven days after treatment, when the concentration of ivermectin in dung was high. Dung of both types of pats was broken down rapidly, usually within four days after placement. Treatment of cattle with ivermectin is, therefore, unlikely to affect the overall dung decomposition via repellence or attraction of dung insects to pats from treated animals. Secondly, two large-scale field studies to determine the impact of ivermectin on dung insect communities under normal extensive farming conditions were conducted in the Free State Province. For the field studies entire herds were treated. Under drought conditions ivermectin appears to affect dung insect communities primarily through reduction in Shannon's species diversity and Pielou's evenness for up to three months after treatment. Under more favourable weather conditions with relatively high rainfall, little or no effect was observed. The populations of E. intermedius, the most abundant scarabaeine species at the study site, appeared unaffected under both drought and average to high rainfall conditions. The results of the field studies suggest that the seriousness of the impact depends on several factors, including climatic conditions, spatial scale of treatment (i.e. size of paddocks) and number of animals treated in a herd. A qualitative frame-based model is proposed to assist farm management decisions with regard to the treatment of cattle with ivermectin.