A Mixture of Bacillus thuringiensis subsp. israelensis With Xenorhabdus nematophila-Cultured Broth Enhances Toxicity Against Mosquitoes Aedes albopictus and Culex pipiens pallens (Diptera: Culicidae)

Xenorhabdus and Photorhabdus spp. (Enterobacteriaceae) can synthesize and release secondary metabolites that play crucial roles in their pathogenicity by suppressing the immunity of target insects. The insect immunity contributes to defense against the pathogenicity of Bacillus thuringiensis (Bt). This study tested a hypothesis that bacterial immunosuppresants could enhance the susceptibility of mosquitoes (Aedes albopictus and Culex pipiens pallens) to Bt. Three symbiotic bacteria [X. nematophila (Xn), X. hominickii (Xh), and P. temperata temperata (Ptt)] were cultured in nutrient broth to allow them to produce secondary metabolites. Bacillus thuringiensis israelensis (BtI) was highly toxic to both culicid mosquitoes with median lethal concentration (LC50, spores/ml) of 2.9 × 105 and 2.2 × 105 at 16 h after treatment, respectively. Addition of each bacteria-cultured broth enhanced BtI toxicity to these mosquito larvae. The LC50 values of BtI to Ae. albopictus larvae were reduced to 1.5 × 105 in Xn mixture, 1.7 × 105 in Xh mixture, and 1.9 × 105 in Ptt mixture. The LC50 values of BtI to Cx. pipiens pallens larvae were also reduced to 1.2 × 105 in Xn mixture, 1.3 × 105 in Xh mixture, and 1.5 × 105 in Ptt mixture. Adding benzylideneacetone or oxindole produced from Xn and Ptt also enhanced BtI toxicities to these mosquito larvae. Based on these results, we developed a new mosquitocidal Bt formulation called “Dip-Kill” consisting of 80% Xn-cultured broth, 10% BtI (1010 spores/ml), and 10% preservative. Dip-Kill at 1,000 ppm was superior to a commercial BtI product at its recommended dose.