AbstractBreeding of Simulium damnosum Theobald s.1., the vector of Onchocerca volvulus, had been eliminated by 1977 from about 654 000 km2 of the Onchocerciasis Control Programme in the Volta River Basin Area (OCP) of West Africa. Nevertheless, migrating adult females continually invaded the controlled area, being blown on the prevailing south-westerly winds from uncontrolled breeding sites beyond the south-western border of the OCP area. Graphs of numbers of females caught per man per day (the daily biting rate) throughout the wet season, March to October, at 16 sites in 1977 within the OCP controlled area were remarkably similar in pattern from site to site over a range of about 500 km downwind. In 1978, only seven sites within the OCP area were similarly monitored, and the graphs were different in pattern from those in 1977, when they each consisted of three or four well-defined waves or cycles of daily biting rate which could be easily identified and traced across country, their times of occurrence lagging increasingly as the distance of the site from the south-western border of the OCP area increased. Four methods were used to demonstrate and estimate the lag: visual comparison of seasonal graphs; comparison of the mean dates of cycles at different sites and the regression of these dates on distance from the south-western border; the dates when particular cumulative percentages of the total season's catch occurred at each site and their regression on distance; and principal coordinate analysis of the data and its relation to distance from the south-western border. Statistically significant lags were demonstrated and averaged one day for every 10–30 km from the border in 1977, which indicated an average speed of migration across country. In 1978, a rate of one day per 7–35 km was indicated. Where some graphs at outlying sites were anomalous, possible alternative sources of immigrant flies are considered. The possible behaviour of flies in causing the lag is discussed.
