Endospore morphology and efficacy of indigenous Bacillus thuringiensis strains against okra fruit and shoot borer, Earias vittella

Aim: The aim of this study was to determine the size of endospores of native Bacillus thuringiensis strains and identify an effective strain for managing the okra fruit and shoot borer, Earias vittella. Methodology: Six native Bacillus thuringiensis strains were cultured on T3 agar plates and incubated at 300C for 96 hrs. Subsequently, detailed examination of the endospores was conducted using a NanoSem 450 scanning electron microscope (SEM) to discern intrinsic differences between endospores from six isolates. Bioassays were performed using six native Bacillus thuringiensis cultures at various concentrations. A fruit dip bioassay experiment was conducted with the reference strain, Bacillus thuringiensis subsp Kurstaki (HD 1), to assess the insecticidal toxicity of six native Bacillus thuringiensis isolates against the okra fruit and shoot borer, Earias vittella. Results: Scanning electron microscope studies on Bt endospores revealed a range of sizes, with a notable proportion being spherical. Endospore sizes were quantified using Image J software, showing a normal distribution. Qualitative insecticidal bioassays were used to predict their insecticidal activity. In laboratory evaluations against Earias vittella larvae with the fruit dip method, all native Bt isolate treatments resulted in over 50% mortality. The highest mortality was observed with native Bt isolate 493 (90.00%), while the lowest mortality was recorded with isolate 49 treated larvae (60.00%). Interpretation: In summary, isolate 493 emerged as the most effective native Bt isolate among those tested, exhibiting the smallest endospore size compared to other native Bt isolates. Key words: Bacillus thuringiensis, Endospores, Earias vittella, Okra