Surfactin induces membrane damage, organelle impairment and DNA fragmentation accompanied by sustained ROS...

The polyphagous pest species Spodoptera exigua (S. exigua) has shown field populations resistant to most synthetic chemical insecticides. Consequently, there is a pressing need for new active substances or pesticides with new modes of action. Surfactin, a cyclic lipopeptide from Bacillus velezensis, exhibits insecticidal activity. Yet, the exact mechanism by which surfactin affects S. exigua remains unclear. Contact toxicity bioassays revealed that surfactin was the most toxic to third-instar S. exigua larvae, with an LC₅₀ of 136.72 mg/L, followed by pyrethrins (326.82 mg/L), avermectin (347.31 mg/L), and clothianidin (416.87 mg/L). Integrated multi-omics and biochemical analyses showed that surfactin caused a progressive and toxic cascade. Within 2 h, a significant increase in reactive oxygen species (ROS) in larval cells suppressed detoxification and antioxidant defense systems. At the same time, the pentose phosphate pathway was upregulated. Following 4-8 h of exposure, major derangement in central carbon metabolism occurred. Metabolomic data showed a dramatic drop in the levels of glucose-6-phosphate and ATP. G6PDH and PFK activities were markedly inhibited, while genes involved in glycolysis, mitochondrial energy metabolism, and intestinal digestion underwent concerted downregulation. A bioenergy deficit caused widespread cell death. Macroscopic lesions comprised tracheal breakdown, epithelial detachment, disruption of the ventral nerve cord, and intestinal necrosis. At the ultrastructural level, cell membrane rupture, cytoplasmic lysis, and degradation of mitochondria and other organelles were observed, with cells shifting from early TUNEL-positive apoptosis to uncontrolled necrosis. These findings demonstrate a surfactin-induced deadly cycle of oxidative burst, metabolic collapse, and cell disintegration that leads to the death of S. exigua larvae. This study suggests that surfactin may be developed as a multi-target bioinsecticide for sustainable pest management.