Laboratory experiments with Bacillus sphaericus strain ABG 6232 against Culex pipiens molestus and culex antennatus larvae proved that this larvicide was highly toxic to Culex antennatus larvae compared to Culex pipiens
molestus larvae and the 2nd instar larvae of the latter species were more susceptible than the 4th instar larvae. Treating Culex pipiens molestus larvae with Bacillus sphaericus strain ABG 6232 for different periods ranging from 1 hr to 48 hr indicated that, the bacterial suspension has a slight effect on the larval mortality percentage during the first hours but after 12 hours it has no significant effect on the larval mortality percentage especially when the larvae were exposed to high bacterial concentrations. The histopathological examination of treated fourth instar larvae of Culex pipiens molestus with Bacillus sphaericus strain ABG 6232 revealed that the mid-gut epithelial cells and the peritrophic membrane were the sites of action of this pathogen.