Larvicidal Activities of Indigenous Bacillus thuringiensis Isolates and Nematode Symbiotic Bacterial Toxins against the Mosquito Vector, Culex pipiens (Diptera: Culicidae)

The incidence of mosquito-borne diseases and the resistance of mosquitoes to conventional pesticides have recently caused a panic to the authorities in the endemic countries. This study was conducted to identify native larvicidal biopesticides against Culex pipiens for utilization in the battle against mosquito-borne diseases.
Larvicidal activities of new indigenous Bacillus thuringiensis isolates and crude toxin complexes (TCs) of two nematode bacterial-symbionts, Photorhabdus luminescens akhurstii (HRM1) and Ph. luminescens akhurstii (HS1) that tested against Cx. pipiens. B. thuringiensis isolates were recovered from different environmental samples in Saudi Arabia, and the entomopathogenic nematodes, Heterorhabditis indica (HRM1) and He. sp (HS1) were iso­lated from Egypt. Larvicidal activities (LC50 and LC95) of the potentially active B. thuringiensis strains or TCs were then evaluated at 24 and 48h post-treatment.
Three B. thuringiensis isolates were almost as active as the reference B. thuringiensis israelensis (Bti-H14), and seven isolates were 1.6–5.4 times more toxic than Bti-H14. On the other hand, the TCs of the bacterial sym­bionts, HRM1 and HS1, showed promising larvicidal activities. HS1 showed LC50 of 2.54 folds that of HRM1 at 24h post-treatment. Moreover, histopathological examinations of the HS1-treated larvae showed deformations in midgut epithelial cells at 24h post-treatment.
Synergistic activity and molecular characterization of these potentially active biocontrol agents are currently being investigated. These results may lead to the identification of eco-friend mosquito larvicidal product(s) that could contribute to the battle against mosquito-borne diseases.