MODELS TO ASSIST THE EVALUATION OF THE IMPACT OF AVERMECTINS ON DUNG INSECT POPULATIONS

Avermectins are widely employed by farmers and veterinarians to contro l both internal and external livestock parasites. Following treatment of livestock, avermectins are gradually eliminated in faeces where the y retain many of their insecticidal properties. However the timing and extent of typical parasite control measures, coupled with the eventua l excretion and in situ degradation of avermectin residues, strongly s uggests that only a proportion of the local cophrophilous insect popul ation 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 t his 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 farm s 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 r emain attractive and the time taken by cattle to excrete all active re sidues. In typical cattle farming systems, estimates of the maximum cu mulative insect mortality in a given season were rarely > 25%. (C) 199 8 Elsevier Science B.V. All rights reserved.