First Results in the Control of Simulium damnosum Theobald (Diptera, Simuliidae) in Northern Nigeria

Abstract

An experimental control project against Simulium damnosum Theo. in Northern Nigeria using DDT in diesel oil as a larvicide is described. A description, together with a map, is given of the project area, some 1,200 sq. miles in extent, with a population of 32,000 persons, and it is emphasised that this was selected to provide information on the feasibility of control in an area not isolated from other fly foci.Methods used in applying the larvicide to the rivers, which had rocky beds with rapids and falls, and assessing the effects of treatment are discussed. Applications were made at prearranged treatment points, weekly, for a period of 12 weeks in the dry season of two successive years, 1956 and 1957, and the post-treatment density of adult fly was assessed and compared in a Table and in histograms, with that existing in the year prior to treatment.In the first year, a mean dosage per application of 1·4 p.p.m. of p,p′DDT applied over 30 minutes (S.D. = 0·5 p.p.m.) at each of four treatment points resulted in the clearance of all breeding for a distance of 15–23 miles below these points, and a reduction of adult fly density in the area of 96 per cent. as compared with pre-treatment density. But reinfestation of cleared breeding grounds was found to be rapid, these becoming repopulated with immature stages within six months in spite of a mean wet-season fly density of only 0·35 flies per boy-hour.In the second year, with application of 1 p.p.m. of p,p′DDT for 30 minutes, results were less successful than in the first, although three additional treatment points were used with the aim of controlling a greater distance than in the first year, adult density being only reduced by an estimated 82 per cent. of that before treatment in the first year. The reasons for this are discussed, and the lower degree of control is attributed to the partial failure of the insecticide to reach the breeding sites owing to the exceptionally low state of the rivers.The fly-rounds used in assessing adult density and the pre-treatment breeding sites and the positions of the treatment points are shown on maps. Detailed data from the fly-rounds are given in an Appendix.With the exception of slight mortality among fish, thought to be due to accidental impediments to tbfi free downstream flow of the larvicide, no toxic effects on other organisms were observed.The cost of treatment is given, and the results are discussed briefly in relation to the existing knowledge on control of S. damnosum. It is suggested that control by the application of larvicides in non-isolated foci is a practicable possibility, but that annual treatment might be necessary because of rapid re-establishment in cleared areas. The effect of control, of the degree obtained in this work, on the incidence of human onchocerciasis cannot yet be determined.