Bacillus thuringiensis H-14 (Alex), originally isolated from moribund and dead larvae of the casuarina stem borer, Stromatium fulvum, was tested in the laboratory against the second and fourth-instar larvae of both mosquito species, Aedes aegypti and Culex pipiens. The pathogenicity of this bacillus was established when both insects were found to be susceptible to infection over the whole range of test concentrations (0.005–0.35 mg/1). However, A. aegypti showed a higher degree of susceptibility to B. thuringiensis H-14 (Alex) than C. pipiens. A mortality of 100% was achieved within 24 hr with concentrations of 0.09 and 0.25 mg/1 for second- and fourth-instar larvae of A. aegypti, respectively. Also, younger larvae of both test mosquito strains were more susceptible than mature ones.
