This study tested a hypothesis that the bacterial immunosuppresants enhance BtI susceptibility of two mosquitoes, the forest mosquito (Aedes albopictus) and the house mosquito (Culex pipiens pallens). Three symbiotic bacteria Xenorhabdus nematophila (Xn), X. hominickii (Xh), and Photorhabdus temperata subsp. temperata (Ptt) were isolated from their symbiotic nematodes and cultured in nutrient broth to allow them to produce the secondary metabolites. BtI gave significant toxicities to A. albopictus and C. pipiens pallens larvae: 50% of lethal concentration to be 2.9 × 105 spores/mL and 2.2 × 105 spores/mL at 16 h after treatment, respectively. Addition of each bacteria-cultured broth significantly enhanced BtI toxicity to the mosquito larvae by lowering LC50 values of BtI to A. albopictus larvae (1.5 × 105 to Xn, 1.7 × 105 to Xh, and 1.9 × 105 to Ptt, respectively) and to C. pipiens pallens larvae (1.2 × 105 to Xn, 1.3 × 105 to Xh, and 1.5 × 105 to Ptt, respectively). Based on these results, we developed a new mosquitocidal Bt formulation called ‘Dip-Kill’, which consisted of 80% Xn-cultured broth, 10% BtI (1010 spores/mL), and 10% preservative. Only 400 ppm of Dip-Kill showed 100% mortality to fourth instar larvae of A. albopictus and C. pipiens pallens 16 h after treatment.

An assessment was made of the toxicity of 12 insecticides, three essential oils and Bacillus thuringiensis var. israelensis (Bti) alone or in combination with the oil major constituents, (E)-anethole (AN), (E)-cinnamaldehyde (CA) and eugenol (EU), to third instars from bamboo forest collected Aedes albopictus and rice paddy field collected Anopheles sinensis resistant to various groups of insecticides. The toxicity of the test insecticides, essential oils and binary mixtures of Bti and the oil constituents (1:1 ratio) was evaluated using a direct-contact mortality bioassay. Binary mixtures of B.t.i. and CA, AN or EU were significantly more toxic against Ae. albopictus larvae (LC50, 0.0084, 0.0134 and 0.0237 mg/l) and An. sinensis larvae (0.0159, 0.0388 and 0.0541 mg/l) than either B.t.i. (1.7884 and 2.1681 mg/l) or CA (11.46 and 19.43 mg/l), AN (16.66 and 25.11 mg/l) or EU (24.60 and 32.14 mg/l) alone. Based on the co-toxicity coefficient (CC) and synergistic factor (SF), the three binary mixtures operated in a synergy pattern (CC, 140.7–368.3 and SF, 76–213 for Ae. albopictus CC, 75.1–245.3 and SF, 40–136 for An. sinensis).The binary mixtures of Bti and essential oil constituents described, particularly (E)-cinnamaldehyde, merit further study as potential mosquito larvicides for the control of malaria vector mosquito populations in light of global efforts to reduce the level of highly toxic synthetic insecticides in the aquatic environment.