Bacillus thuringiensis Sublethal Concentration Effectively Alter Cytogenetic Response of Spodoptera frugiperda Larvae

Bacillus thuringiensis (Bt)-based microbial insecticides are crucial for controlling the maize pest, Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae). While the insecticidal efficacy of Bt has been well-documented, its sublethal cytogenetic toxicity on midgut cells remains less explored. This study aimed to evaluate the sublethal concentration toxicity of Bacillus thuringiensis, var. Kurstaki (Btk), on survival percentage, genotoxicity, histopathology, and ultrastructure of the midgut cells of S. frugiperda larvae. By calculating lethal concentration (LC) values (LC₂₅, LC₅₀, LC₇₅, and LC₉₀) in the laboratory, the insecticidal efficacy was ascertained. The Btk lethal concentration was toxic to third-instar S. frugiperda larvae after 48 h, with an LC₅₀ of 109 µg/100 mL (106-112 µg/100 mL). Subsequently, the results demonstrated that LC₂₅ resulted in a 22% reduction in larval survival percentages, so larval midgut tissues treated with the sublethal concentration (LC₂₅) were isolated for further cytogenetic analysis. Consequently, Btk treatment markedly disrupted midgut cells, inducing micronuclei (MN) formation and reducing SfDnmt2 mRNA gene expression considerably. Furthermore, cytotoxicity has been shown in S. frugiperda midgut epithelial cells by cellular disorganization, fragmentation and protrusion, peritrophic matrix rupture, and microvilli degeneration. Btk sublethal concentration also induced cytotoxicity by causing apoptosis and necrosis in S. frugiperda midgut cells. These findings suggest that sublethal concentrations of Bt induce significant cytogenetic stress, which may complement the role of Bt-based biopesticides in integrated pest management strategies as a promising, environmentally safe method for decreasing the survival rate and controlling the pest S. frugiperda.