Larvicidal Activity and Midgut Histopathology of Bacillus thuringiensis Whole Cells and Spores Against Culex pipiens (Diptera: Culicidae) Larvae

Abstract

Culex pipiens is a vector of several human pathogens, and the chemical control of its larvae is losing ground to insecticide resistance. This study measured the larvicidal activity of whole cells and spores of a local Bacillus thuringiensis isolate against third-instar Cx. pipiens larvae and described the damage they cause to the larval midgut. Larvae were collected in Al-Hillah, central Iraq, between November 2024 and February 2025 and were reared in the laboratory for three generations. Bacillus thuringiensis was isolated from non-clinical food samples and identified by colony morphology on HiCrome^TM Bacillus agar, Gram staining, biochemical tests and 16S rRNA gene sequencing. Three whole-cell concentrations (1.5 × 10⁸, 7.5 × 10⁷ and 3.7 × 10⁷ CFU/mL) and three spore concentrations (1.5 × 10⁷, 1.5 × 10⁶ and 1.5 × 10⁵ CFU/mL) were tested, each with three replicates of ten larvae. Beta-cypermethrin served as the positive control and dechlorinated water as the negative control. Dead larvae were counted at 24, 48 and 72 h, and mortality was corrected with Abbott's formula. Whole cells killed every larva at 72 h at all three concentrations (100 % corrected mortality) against 10 % in the negative control. Mortality was very low at 24 h and rose sharply between 48 and 72 h. Spores gave 100 %, 95.72 % and 89.30 % corrected mortality at 1.5 × 10⁷, 1.5 × 10⁶ and 1.5 × 10⁵ CFU/mL, so their activity was close to that of whole cells. Histological sections stained with haematoxylin and eosin showed damage of the midgut that increased with the severity of the treatment: the epithelial lining was disrupted, muscle bundles were fragmented, and vacuoles, cellular debris and spore clusters filled the tissue. Control larvae kept a normal tissue architecture. The local B. thuringiensis isolate is therefore an effective, time-dependent larvicide against Cx. pipiens.