Dam-spillway breeding ofSimulium damnosumTheobald in northern Ghana

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Abstract
Simulium damnosum. the vector of onchocerciasis in Ghana, was found breeding in large numbers from Sept. to Nov. 1964 on the vegetation-free and obstruction-free concrete spillway of the dam at Soe, east of Bolgatanga in Upper Region, Ghana. S. damnosum laid its eggs where the velocity of the water was between 2.5 and C.2 ft./sec; the heaviest breeding occurred at between 3.5 and 4.3 ft./sec. Eggs were deposited on the concrete below the water line, and larvae and pupae attached directly to the concrete both below and above the water line. The larvae migrated in the direction of higher velocity towards the end of the channel, and from the channel itself when the velocity became unsuitable. Eggs, larvae and pupae were found on canvas strips and grass bundles hung in the channel and trailing just below the surface, but no oviposition or larval attachment occurred on polythene strips tied in the channel. The duration of the egg stage was 30 hrs. Attention was focused on the breeding of S. damnosum in the spillway because many persons in the area were infected with Onchocerca volvulus. The construction of the dam introduced a man-made breeding-place of the vector into an area endemic for onchocerciasis which was previously free from vector breeding.