Unveiling insecticidal compounds from the invasive weed Tithonia diversifolia against Spodoptera litura and elucidating mechanisms via transcriptomic analysis.

Tithonia diversifolia, an invasive weed, has potential as a natural alternative to chemical insecticides. However, the specific insecticidal compounds and their mechanisms of action against Spodoptera litura have not been thoroughly investigated. In this study, an extract was prepared using 85 % methanol containing 0.2 % hydrochloric acid, and it was analyzed using UPLC-ESI-MS/MS. A feeding experiment was conducted to evaluate the effects of the extract and its compounds on S. litura larvae, alongside a transcriptomic analysis to explore the underlying mechanisms. Results showed that 18 compounds were isolated and identified from the methanol extract, including eight quinic acid and derivatives, two flavonoids, three sesquiterpenoids, three fatty acids and two lipids. The feeding experiments indicated that the extract and three specific compounds, hispidulin, 1,5-quinic acid and 6-methoxyluteolin, significantly inhibited larval growth, while linolenic acid demonstrated notable insecticide activity compared to the positive control. Larvae treated with the methanol extract and the aforementioned compounds exhibited faded skin. Transcriptomic analysis identified 2708 differentially expressed genes (DEGs), with 1512 genes up-regulated and 1196 down-regulated. GO enrichment and KEGG pathway analyses showed that the DEGs were significantly associated with integral membrane components, cuticle structure and serine type endopeptidase activity. qRT-PCR results indicated significant differences in the expression of six genes related to larval growth between the control group and the methanol extract-treated group. Overall, the results suggested that the methanol extract of T. diversifolia possesss insecticidal properties, with 6-methoxyluteolin, hispidulin, 1,5-quinic acid and linolenic acid identified as active components, and the insecticidal mechanism appears to involve disruption of insect cuticle formation.