Essential Oils from Neolamarckia cadamba: Methyl Salicylate-Rich Stem Bark Oil as a Multi-Functional Biopesticide with Insecticidal and Antifungal Efficacy

The escalating challenges of insecticide resistance and environmental pollution underscore the urgent need for sustainable and multi-functional biopesticides. This study reveals the chemical diversity and potent bioactivity of essential oils (EOs) from Neolamarckia cadamba, highlighting their potential as a valuable source of bioactive agents. Gas chromatography–mass spectrometry analysis revealed a striking contrast between the essential oils: the stem bark EO was dominated by methyl salicylate (MeSA, 97.61%), representing the first report of MeSA as a major constituent in this species, while the leaf oil exhibited a complex profile enriched with diterpenoids (25.09%) and fatty acids (23.21%). Both EOs exhibited significant insecticidal efficacy against Aedes aegypti, demonstrating rapid knockdown with median knockdown times (KT₅₀) of 1.36–1.97 min—surpassing the synthetic dimefluthrin. Additionally, they demonstrated pronounced toxicity, with median lethal concentrations (LC₅₀) of 73.41–75.27 μg/mL and fumigant toxicity values of 0.20–0.22 μL/L. Notably, the major component MeSA in the stem bark EO demonstrated obvious insecticidal potential, exhibiting rapid knockdown activity (KT₅₀ of 2.29 min), fumigant toxicity (LC₅₀ of 1.55 μL/L, 5 h), and poisonous activity (LC₅₀ of 92.67 μg/mL, 24 h). Meanwhile, both the stem bark EO and MeSA exhibited strong antifungal activity against the phytopathogen Rhizoctonia solani, with median effective concentration (EC₅₀) values of 48.70 and 53.91 μg/mL, respectively. This efficacy surpassed that of the commercial fungicide physcion (EC₅₀ of 93.34 μg/mL). Additionally, the EOs demonstrated moderate antioxidant activity in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. Mechanistic investigations revealed that the antifungal action of MeSA involved severe cellular disruption, including ultrastructural damage, membrane peroxidation, and critical metabolic suppression via the inhibition of succinate dehydrogenase activity. Our results clearly established N. cadamba EOs, particularly the MeSA-rich stem bark oil, as potent, plant-based, and multi-target agent with significant potential for integration into sustainable pest and disease management strategies.