| Summary: | Abstract Background The invincible effects of the entomopathogenic fungi (EPF) under appropriate circumstances compensate for the flaws of chemical insecticides in the control programs. Beauveria bassiana is one of the most elaborated EPF of pest control all over the world. The potential of using the B. bassiana isolate (Y-F_ITS1) was examined against different larval instars (L2, L3, L4, and L5), pupae, and eggs of the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae), using different concentrations under laboratory conditions. Results In bioassay, 2nd instar larvae of S. littoralis were significantly the most susceptible stage to B. bassiana isolate at P ≤ 0.05. It is apparent from the results that the cumulative mortality percentage increased with elapsing time with typical symptoms of infection and sporulation. High efficacies ranging from 85.0 to 99.0% corrected mortality rates were recorded for 2nd instar larvae at the spore concentration (1 × 109 spores ml−1). The mortality percentage reached 90.0% for the 3rd instar larvae treated with the same concentration after 5 days. The least mortality rates results were recorded at the 5th instar larvae. B. bassiana isolate showed an ovicidal effect to the eggs of S. littoralis that reached 100% mortality when treated with 1.0 × 108 and 1.0 × 109 spores ml−1, while the concentration 1 × 107 and 1.0 × 108 caused 65.0 and 87.0%, respectively. Reduction of adult emergence reached (0%) in case of treatment with the highest concentrations (1 × 108 and 1 × 109 spores ml−1). Morphogenetic abnormalities were also recorded. Conclusion Results of mortality rates and lethal concentration values resulted from the experiments indicated that the tested Y-F_ITS1 isolate was efficient and can be recommended as a potential biocontrol agent against S. littoralis. Further field evaluations are still needed.
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