Combined effect of local isolate Spodoptera littoralis nucleopolyhedrosis virus and Bacillus thuringiensis on Culex pipiens L. larvae (Culicidae: Diptera)

A local isolate of Spodoptera littoralis nucleopolyhedrosis virus (SlNPV) showed pathogenicity against Culex pipiens larvae, its long incubation period was broken using a combination between SlNPV and Bacillus thuringiensis subsp. kurstaki (Btk). Laboratory bioassay tests revealed that Culex pipiens 3rd larval instars were susceptible to the applied combination. Data revealed that the addition of Btk generally increased the toxic effect of SlNPV since LC50 decreased from 1.3 × 103 PIB/ml (for SlNPV alone) to 3.6 × 102 mixed polyhedra and spores/ml (SlNPV-Btk combination), indicating a synergistic ratio of 3.6. LC95 was surprisingly 137-folds dropped as well. The addition of SlNPV generally increased the toxic effect of Btk at a low concentration, where the LC50 outstandingly decreased from 2.5 × 105 spores/ml (for Btk) to 3.6 × 102 mixed polyhedra and spores/ml (for SlNPV-Btk combination), indicating a synergistic ratio of 6.9 × 102.