An attempt to eradicate Glossina pallidipes Austen from the Lambwe Valley of western Kenya by sequential aerial application of endosulfan aerosol spray is described. The operation was ultimately unsuccessful, even after nine sprays of relatively heavy dosages of insecticide. Survivors were present throughout after each spray, and females mated and bred between sprays. The population was reduced by over 99·9% in main habitats of thicket and woodland and by about 90% in conifer plantation. The outcome of a model of spray effectiveness suggested, however, that the final reduction in population size was the cumulative effect of only about a 90% kill per spray application. Post-spray monitoring showed that the tsetse population in thicket returned to its pre-spray level in little more than 12 months and became stabilized thereafter. Elsewhere, recovery was much slower and more variable. Evidence was found for population regulation by density-dependent emigration of flies. Calculations of population growth rate gave an instantaneous rate of increase of 0·0148/day, equivalent to a finite rate of increase of 1·568/month or a doubling time of 47 days. This permitted a 90-fold increase in population size over a ten-month period of unrestricted growth. The failure to achieve eradication is discussed in terms of certain operational shortcomings and environmental constraints which exist in the Lambwe Valley.
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