Insecticidal activity of the ethanol extracts from Rudbeckia hirta: Efficacy on Lackey moth (Malacosoma neustria testacea) and impact on non-target organisms

Abstract

The insecticidal properties of Rudbeckia hirta have been acknowledged, yet they remain underexplored. This study aimed to optimize the extraction conditions, analyze the chemical constituents of ethanol extracts from R. hirta (RHE), assess the insecticidal activity of RHE against Malacosoma neustria testacea larvae, and evaluate its safety for non-target aquatic organisms. The optimal extraction parameters for RHE were determined to be a liquid-solid ratio of 41.4:1 (mL:g), an extraction temperature of 41 °C, and an extraction time of 52 min, resulting in an actual extraction rate of 30.27 ± 0.16%. Chemical analysis identified 22 compounds within RHE, with flavonoids comprising the majority at 75.47%. Bioassays suggested that RHE exhibited significant toxicity against M. neustria testacea larvae, with an LC₅₀ value of 6.44 ± 0.14 mg/mL at 72 h post-treatment. RHE also markedly inhibited larval feeding, digestion, and nutrient absorption. Furthermore, under RHE exposure, detoxification and antioxidant enzyme activities in the larvae were significantly suppressed within 72 h of treatment. Safety assessments indicated that RHE exhibited no toxicity to Danio rerio. These findings suggest that RHE could serve as a natural alternative to chemical pesticides, offering a sustainable approach to managing moth pests in agroforestry systems.