Pathogenecity testing of indigenous Bacillus thuringiensis isolates against chickpea pod borer, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) in Ethiopia

Abstract

Helicoverpa armigera (Hübner) is the most significant insect pest of chickpea in Ethiopia. Sole reliance on insecticide causes both human and environmental health problems. A common soil bacterium, Bacillus thuringiensis used as a bioinsecticide to control numerous Lepidopteran larvae. Therefore, indigenous isolates were collected, screened, tested for pathogenicity against H. armigera with a reference Bacillus. var. thuringiensis. From 121 soil samples collected, 23 Bacillus species were isolated and purified. Bt index ranged from 0.33 to 1.00 and the mean was 0.73 average Bt index. Absorbance values ranged from 0.46 to 0.98. Screening biotoxicity assay was conducted at the rate of 1 mL against 3rd instar larvae of H. armigera. Experiments were replicated three times; ten larvae were used per replicate and reputed thrice. Ten indigenous Bt isolates were screened based on larval mortality which ranged between 27.78% and 75.83% and LT50 values of 45.71–143.9 h. The highest mortality of 75.83% and LT50 of 45.71 h was recorded from reference Bt followed by indigenous Bt isolate KDL with mortality percentage of 69.16% and LT50 of 48.49 h. Statistically no significant difference was observed among the tested bacterial isolates. Biotoxicity assay was conducted at five concentrations ranging from 0.25 to 2 mL. The LD50 and LD 90 values were 0.84–1.24 mL and 1.54–1.98 mL, respectively. The lowest LD50, 90 of 0.84 mL and 1.54 mL were recorded from KDL Bt isolate compared to reference Bt whose LD50 and LD90 were 0.84 mL and 1.66 mL. Results of Bt growth under varied values of temperature ranged from 15oC to 40 °C. All the Bt isolates showed growth at all the values of the tested temperatures with steady growth from 15 °C to 20 °C and uniform growth observed from 20 °C to 30 °C. After 35 °C, the growth of all the Bt isolates declined indicating that 30 °C–35 °C is the optimum growth temperature range. Protein content ranged from 0.43 to 0.97 (mg/mL). However, protein content and larval mortality was not linearly correlated. It was concluded that all the tested indigenous Bacillus isolates were virulent enough and identified as a promising candidate bioinsecticide against H. armigera. Hence, recommended for further intensive evaluations under greenhouse and field trails.