Biocontrol Efficacy of Bacillus thuringiensis Isolates Against Early Instar Larvae of Spodoptera exigua on Beet

Abstract

The beet armyworm, Spodoptera exigua, is a highly destructive lepidopteran pest affecting beet cultivation, with early instar larvae causing severe defoliation and yield losses. The present study evaluates the biocontrol efficacy of indigenous Bacillus thuringiensis (Bt) isolates against early instar larvae under laboratory and semi-field conditions. Bt strains were isolated from soil and characterized through standard microbiological and molecular methods, including crystal protein observation and PCR amplification of cry genes. Bioassays were conducted using different concentrations of spore-crystal suspensions against first- and second-instar larvae, and mortality, feeding inhibition, and larval developmental disruption were recorded. Results indicated significant variability in insecticidal activity among Bt isolates. Highly potent isolates caused rapid larval mortality within 48–72 hours and significantly reduced feeding activity compared to untreated controls. Mortality rates were dose-dependent, with higher concentrations producing near-complete larval suppression. Sublethal effects included delayed development, reduced pupation rates, and malformed pupae, indicating strong physiological disruption. Molecular analysis revealed the presence of cry toxin genes associated with lepidopteran specificity, supporting the observed insecticidal activity. Semi-field trials further confirmed the effectiveness of selected isolates in reducing larval populations under natural environmental conditions. The study highlights the potential of native Bt isolates as sustainable and environmentally safe alternatives to chemical insecticides for managing S. exigua in beet ecosystems. These findings emphasize the importance of strain selection and toxin profiling for developing effective biopesticide formulations. Overall, the results demonstrate that B. thuringiensis provides a reliable biological control strategy for early instar suppression, contributing to integrated pest management programs aimed at reducing chemical pesticide dependence in beet production systems.