Abstract
Bacillus sphaericus strain 1593 and B. thuringiensis serotype H-14 were evaluated for persistence of toxicity against two species of mosquito larvae, Culex quinquefasciatus and Aedes aegypti, in a selected simulating plot in Bangkok. Both strains of bacteria demonstrated larvicidal activity towards both species of mosquito larvae. In tap water, the toxicity of B. sphaericus strain 1593 was found to be greater towards C. quinquefasciatus larvae than A. aegypti larvae, whereas the toxicity of B. thuringiensis serotype H-14 was found to be greater towards A. aegypti larvae than C. quinquefasciatus larvae. The persistence of toxicity of these two bacteria was found to be different. The lethal concentration of B. thuriengiensis H-14 against A. aegypti decreased from LC 90 to below LC 50 in about 15 weeks when tested in tap water. The decrease was faster in polluted water. The toxicity of B. sphaericus 1593 towards C. quinquefasciatus larvae persisted for at least 9 months in tap water and 6 months in polluted water. The multiplication of bacteria was indicated only in populations of B. sphaericus 1593 tested with C. quinquefasciatus larvae.
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