Models to assist the evaluation of the impact of avermectins on dung insect populations

Avermectins are widely employed by farmers and veterinarians to control both internal and external livestock parasites. Following treatment of livestock, avermectins are gradually eliminated in faeces where they retain many of their insecticidal properties. However the timing and extent of typical parasite control measures, coupled with the eventual excretion and in situ degradation of avermectin residues, strongly suggests that only a proportion of the local cophrophilous insect population will be exposed to the active compound. This paper outlines one of the first formal attempts to quantitatively estimate the degree of exposure of dung insects to avermectins and the subsequent impact of this compound on their populations. An analytical model was employed to clarify the relationship between key variables, while a detail-rich, simulation model provided estimates of insect mortality in cattle farms under more realistic sets of assumptions. The impact of avermectins on dung insect populations was shown to be highly dependent on factors such as the proportion of cattle treated, the length of time faeces remain attractive and the time taken by cattle to excrete all active residues. In typical cattle farming systems, estimates of the maximum cumulative insect mortality in a given season were rarely