6-Methoxyluteolin from the Invasive Plant Tithonia diversifolia Suppresses Melanization and Impairs Epidermal Integrity in Spodoptera litura Larvae

Simple SummaryInvasive plants often produce chemical compounds that help them compete in new environments, and some of these compounds can harm insect pests. Understanding how these compounds affect insects could help us develop safer, more environmentally friendly alternatives to conventional chemical pesticides. In this study, we investigated how a compound called 6-methoxyluteolin, extracted from the invasive plantTithonia diversifolia(Mexican sunflower), affects the integument of the agricultural pestSpodoptera litura(common cutworm) at different stages of larval development. We found that this compound reduced the activity of a key pigment-forming enzyme and decreased melanin levels in the larval integument across multiple developmental stages. Histological analysis further revealed progressive thinning and structural disorganization of the outer cuticle layer with increasing compound concentration. These results suggest that 6-methoxyluteolin disrupts the melanization process essential for cuticle integrity in insect larvae, and support its potential as a targeted botanical insecticide.The invasive plantTithonia diversifoliais a promising source of botanical insecticides, yet the mechanisms by which its active compounds impair insect physiology remain incompletely characterized. This study investigated the effects of 6-methoxyluteolin (6-ML), a flavonoid isolated fromT. diversifolia, on melanization and cuticle integrity inSpodoptera lituralarvae. Cuticle melanization, mediated by tyrosinase-catalyzed oxidation of tyrosine-derived substrates, is essential for cuticle sclerotization and immune defense in insects. Whether 6-ML directly targets this pathway has not been previously examined at the biochemical and histological level. Tyrosinase activity and melanin content were measured across 3rd–6th-instar larvae exposed to a concentration gradient of 6-ML (1.625–100 μg/mL). Histological sections of 4th- and 5th-instar larvae were prepared by paraffin embedding and hematoxylin and eosin (H&E) staining. 6-ML significantly reduced tyrosinase activity and melanin content in a concentration-dependent manner across all instars examined. Histological analysis revealed progressive cuticle thinning and loss of pigmentation granules with increasing 6-ML concentration. These findings provide the first biochemical and histological evidence that 6-ML targets the melanization pathway inS. litura, supporting its potential as a melanization-inhibiting botanical insecticide.