A preliminary assessment of some immediate and long-term effects of aerial spraying of endosulfan on Glossina pallidipes (Austen) in the Lambwe Valley, Kenya

Abstract

In 1981 an attempt was made to eradicate a 300 km² fly-belt of Glossina pallidipes (Austen) in the Lambwe Valley of western Kenya by means, principally, of sequential aerial application of endosulfan aerosol spray. The progress of spraying operations was monitored entomologically by surveying for fly with biconical traps sited in main habitats of thicket, Acacia woodland and coniferous plantation, and by age-grading females caught between spray cycles.

Although nine spray applications of relatively heavy dosages of insecticide were carried out, the campaign ultimately failed to accomplish its objective. Females which had clearly survived and bred in the intervals between spray cycles were found throughout the area. By the end of operations, the tsetse population appeared to have been reduced by 99.9% in thicket and woodland, and by 99% in coniferous plantation. Nevertheless, on the basis of a crude estimate of total fly numbers in the Lambwe Valley, several thousand flies possibly remained after spraying.

Monthly post-spray monitoring showed that the population in thicket fully recovered in about 12 months, but to only 20–30% of its original level in woodland and plantation two years after spraying. The population in thicket maintained an approximate equilibrium density after recovery, indicating that density-dependent processes had re-established a regulatory effect. There was some evidence that this was brought about in part by fly movement. Calculations of the population growth rate gave a value for r, the instantaneous rate of increase, of 0.015 per day; equivalent to a finite rate of increase of 1.610 per month, and a population doubling time of 45 days. Some possible ecological reasons for failure to achieve eradication are discussed.