Characterization of Sudan strains of Bacillus thuringiensis pathogenic to the larvae of the house mosquito Culex quinquefasciatus

Abstract

Mosquitoes, as blood sucking insects and vectors for several serious human diseases, continue to be one of the major threats to public health, comfort and economic growth in the Sudan and many other countries. Dengue, yellow fever, and filariasis in addition to malaria are all transmitted by mosquitoes; this pest is controlled using chemical pesticides. The increasing concern about the environment and the hazards resulting from the sole reliance on pesticides and the acquired resistance to one or multiple insecticides forced scientist to seek for safer efficient alternatives or supplements for the chemical pesticides. Bacillus thuringiensis is considered to be one of the important microbial control agents capable of producing insecticidal proteins with specific pathogenicity. In this study, different samples were obtained from soils collected from different locations in Sudan and from stored products dust and dead insects, in addition to mosquito rearing bonds, with the objective of isolating entomopathogenic Bacillus thuringiensis strains. Thirty nine strains were isolated and identified morphologically and biochemically and their toxicity to the house mosquito Culex quinquefasciatus second and third instar larvae was evaluated. The larvae of the house mosquito were introduced to sterile distilled water treated with 500 ppm concentration from each isolate. Significant differences existed between the isolates in their toxicity 42 hours post treatment, where mortality percentages ranged from 25% to 95% compared to 12.5% in the untreated control larvae. About 69% of the 39 Bt isolates were found pathogenic (mortality ≥50%) to the house mosquito larvae. Regression analysis revealed differences in the lethal times between the different isolates. The LT50 values varied from 29.38 hours for isolate Om-5 to 131.9956 hours for isolate GF-18. The practical significance of these findings for management of mosquitoes is discussed. It is therefore concluded that Sudan environment is rich in Bacillus thuringiensis pathogenic to the house mosquito and 5 isolates resulted in cumulative mortality percentages above 80%.