Toxicity and cytopathology mediated by Bacillus thuringiensis in the midgut of Anticarsia gemmatalis (Lepidoptera: Noctuidae)

Abstract

Bioinsecticides and transgenic plants, based on Bacillus thuringiensis (Bt) toxins are important when managing Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae), a soybean defoliator pest. The interaction of these toxins with the caterpillar’s midgut cells determines their efficacy as an insecticide. The objective was to evaluate the toxicity of B. thuringiensis , subsp . kurstaki strain HD-1 and cytopathological changes mediated by these bacterial toxins in the midgut of A. gemmatalis caterpillars. Insecticidal efficacy was determined by calculating lethal concentration values (LC 25 , LC 50 , LC 75 , LC 90 and LC 99 ) in the laboratory. Midgut fragments from A. gemmatalis were extracted after bacterial ingestion and evaluated by light, transmission electron and confocal microscopy. The Bt median lethal concentrations showed toxicity [LC 50 = 0.46 (0.43–0.49) mg mL −1 ] to fourth instar A. gemmatalis caterpillars after 108 hours. Bt induces severe cytotoxicity to A. gemmatalis midgut epithelial cells with increasing exposure over time, causing cellular disorganization, microvillus degeneration, cell fragmentation and protrusion, peritrophic membrane rupture, and cell vacuolization. The cell nuclei presented condensed chromatin and an increase in lysosome numbers. Apoptosis occurred in the midgut cells of caterpillars exposed to Bt. A regenerative response in A. gemmatalis caterpillars was observed 8 hours after exposure to Bt, however this response was not continuous. Toxins produced by Bt are harmful to A. gemmatalis at median concentration with structural damage and death of the midgut epithelial cells of this insect.

Bioactivity of Bacillus thuringiensis (Bacillales: Bacillaceae) on Diatraea saccharalis (Lepidoptera: Crambidae) eggs

“…We observed vacuolization and degeneration in the intestinal epithelium of the D. saccharalis embryo, which is in agreement with the results of previous studies on neonatal larvae of D. saccharalis 23 . These impairments might be associated with the stimulation of the insect's immune system, such as by microbial infection and toxins 86,87 ; similar results have been observed in other lepidopterans 29,88,89 …”

“…23 These impairments might be associated with the stimulation of the insect's immune system, such as by microbial infection and toxins 86,87 ; similar results have been observed in other lepidopterans. 29,88,89…”

Bacillus thuringiensis Bioinsecticides Induce Developmental Defects in Non-Target Drosophila melanogaster Larvae

“…It has been proposed that lepidopteran larvae succumb to Btk spores and toxins because they are unable to maintain their gut integrity. Conversely, for a lepidopteran larva to acquire resistance to Btk bioinsecticides may rely on its capacity to maintain/regenerate its intestinal integrity [ 97 , 98 , 99 ]. Non-target organisms do not die likely because they are able to regenerate their gut lining faster than Btk bioinsecticides destroy it.…”

Bacillus thuringiensis bioinsecticides induce developmental defects in non-target Drosophila melanogaster larvae

“…It has been proposed that lepidopteran larvae succumb to Btk spores and toxins because they are unable to maintain their gut integrity. Conversely, one mean for a lepidopteran larva to acquire resistance to Btk bioinsecticides may rely on its capacity to maintain/regenerate its intestinal integrity (Castagnola and Jurat-Fuentes, 2016;Castro et al, 2019;Dubovskiy et al, 2016). Non-target organisms are likely as such because they are able to regenerate their gut lining faster than Btk bioinsecticides destroy it.…”