Control of Culex quinquefasciatus by Bacillus sphaericus: Results of a pilot campaign in a large urban area in equatorial Africa

Culex quinquefasciatus, which is sometimes the vector of Bancroft's filariasis, is a harmful mosquito, the immature stages of which live in collections of waste water resulting from human activity. Larval control, the most appropriate method, is at present carried out with chemical insecticides. But the toxicity of these compounds, together with phenomena of resistance, and the cost of substitute insecticides have turned research towards products of biological origin, and one of the most promising is a liquid concentrate of Bacillus sphaericus strain 2362. This was applied experimentally over an area of 200 hectares in a large city in the south of Cameroon, characterized by a short dry season during which mosquito density is at its highest. Spraying was carried out every three months for a year in a concentration of 10 g/m2 and its efficacy evaluated at the level of adult mosquitos through an indirect system of capture on human baits. The results of this study show, in essence, a reduction by 52.7% in the number of females captured in the overall study area treated, and that the impact of treatment is greater during the period of high mosquito density (55.1%) than in the low density period (40.7%). After analysis of the full set of results, the authors conclude that the pattern of rainfall, the conditions in which the insecticide is applied and reinvasions of mosquitos from untreated areas were the principal factors limiting the efficacy of this control campaign. It is very likely, however, that the phenomenon of reinvasion would be considerably reduced if the entire city were to be sprayed and that the efficacy of the treatment would thus be significantly enhanced. It would be worth extending operations in Yaounde in order to test this hypothesis and confirm the rationale for the control strategy proposed by the authors, which consists in limited spraying during the period of maximum mosquito density only. The authors consider that the liquid concentrate of B. sphaericus strain 2362 may already be regarded as a promising insecticide for the control of C. quinquefasciatus in urban environments. No definitive conclusions can be reached, however, until the efficacy of this insecticide has been evaluated in cities where C. quinquefasciatus is the vector of Bancroft's filariasis. Purely entomological considerations will then need to be supplemented by considerations of an epidemiological and parasitological order, making allowance for the existence of other methods of control, including mosquito nets impregnated with insecticides or chemotherapy.