Insecticidal activities of Streptomyces sp. KSF103 ethyl acetate extract against medically important mosquitoes and non-target organisms

A potentially novel actinobacterium isolated from forest soil, Streptomyces sp. KSF103 was evaluated for its insecticidal effect against several mosquito species namely Aedes aegypti, Aedes albopictus, Anopheles cracens and Culex quinquefasciatus. Mosquito larvae and adults were exposed to various concentrations of the ethyl acetate (EA) extract for 24 h. Considerable mortality was evident after the EA extract treatment for all four important vector mosquitoes. Larvicidal activity of the EA extract resulted in LC(50) at 0.045 mg/mL and LC(90) at 0.080 mg/mL for Ae. aegypti; LC(50) at 0.060 mg/mL and LC(90) at 0.247 mg/mL for Ae. albopictus; LC(50) at 2.141 mg/mL and LC(90) at 6.345 mg/mL for An. cracens; and LC(50) at 0.272 mg/mL and LC(90) at 0.980 mg/mL for Cx. quinquefasciatus. In adulticidal tests, the EA extract was the most toxic to Ae. albopictus adults (LD(50) = 2.445 mg/mL; LD(90) = 20.004 mg/mL), followed by An. cracens (LD(50) = 5.121 mg/mL; LD(90) = 147.854 mg/mL) and then Ae. aegypti (LD(50) = 28.873 mg/mL; LD(90) = 274.823 mg/mL). Additionally, the EA extract exhibited ovicidal activity against Ae. aegypti (LC(50) = 0.715 mg/mL; LC(90) = 6.956 mg/mL), Ae. albopictus (LC(50) = 0.715 mg/mL; LC(90) = 6.956 mg/mL), and An. cracens (LC(50) = 0.715 mg/mL; LC(90) = 6.956 mg/mL), evaluated up to 168 h post-treatment. It displayed no toxicity on the freshwater microalga Chlorella sp. Beijerinck UMACC 313, marine microalga Chlorella sp. Beijerinck UMACC 258 and the ant Odontoponera denticulata. In conclusion, the EA extract showed promising larvicidal, adulticidal and ovicidal activity against Ae. aegypti, Ae. albopictus, An. cracens, and Cx. quinquefasciatus (larvae only). The results suggest that the EA extract of Streptomyces sp. KSF103 has the potential to be used as an environmental-friendly approach in mosquito control. The current study would serve as an initial step toward complementing microbe-based bioinsecticides for synthetic insecticides against medically important mosquitoes.