Cytopathological effects of Bacillus thuringiensis subsp. kurstaki CCT 1306 on the midgut of Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae

Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) is a polyphagous insect pest responsible for significant losses of economically important crops. Traditionally, control of this insect pest has relied on synthetic insecticides; however, this approach is not considered sustainable. An efficient, safe, and sustainable alternative is the use of bioinsecticides based on Bacillus thuringiensis (Bt) (Bacillales: Bacillaceae), a bacterial species that possesses biocontrol potential against several orders of insect pests. This study aimed to evaluate the toxic and cytopathological effects of the Bt isolate subsp. kurstaki CCT 1306 on the midgut of S. frugiperda larvae fed an artificial diet contaminated with five different concentrations (1.50, 3, 6, 12, and 24 mL L -1 ) of the entomopathogen. The larval mortality was evaluated after 168 h. Insects were collected 24 h and 48 h after the bioassays for histological and ultrastructural analyses. All tested concentrations were toxic to S. frugiperda larvae; the highest concentrations (6, 12, and 24 mL L -1 ) resulted in significantly higher mortality rates, eliminating 48.77%, 57.89%, and 64.87% of the insects, respectively. The estimated lethal concentrations were LC 50 = 9.53 mL L -1 and LC 90 = 36.95 mL L -1 . Severe damage to the midgut epithelium, including protrusion of columnar cells into the intestinal lumen, vesiculation, autophagic vacuoles, loosening of the basal lamina and muscle fibers, degeneration of microvilli, cell lysis, and epithelial rupture, was observed after exposure. These results demonstrated that Bt subsp. kurstaki CCT 1306 exhibits entomopathogenic activity against S. frugiperda larvae, adversely affecting their digestive system, supporting its use in sustainable pest management.