Extraction and assessment of Rudbeckia hirta extracts’ larvicidal activity on lackey moth (Malacosoma neustria Testacea): toxicity, nutritional effects and enzyme activities

Abstract

The insecticidal activity of Rudbeckia hirta has been confirmed, however, it has not been adequately studied. In the present study, we optimized extraction conditions and analyzed chemical constituents of ethanol extracts of R. hirta (RHE), the insecticidal activities of RHE on Malacosoma neustria Testacea larvae were investigated, and the safety of RHE for non-target aquatic organisms were evaluated. The results indicated that the optimal extraction conditions of RHE were liquid-solid ratio: 41.4:1 (mL:g), extraction temperature: 41.4°C, and extraction time: 51.99 min, under this condition, the actual extraction rate was 30.27%. RHE contains 22 compounds, flavone and flavonoid derivatives accounting for 75.47% of the total compound contents. More importantly, RHE had a significant biotoxic effect on instar larvae of M. neustria Testacea, the value of LC50 was 6.437 mg/mL at 72 h post-treatment. RHE also significantly affected larval feeding, digestion, and nutrient uptake, six nutritional indices (LWG, RGR, ECI, RCR, AD, and AR) of RHE-treated larvae were suppressed. Under the stress of RHE, the detoxification enzyme activities (ACP, AKP, and GST) were remarkably inhibited, as well as the detoxification enzyme activities (CAT, POD, and SOD) were significantly suppressed in vivo of larvae within 72 h after treatment. Safety experiments showed that RHE was practically nontoxic to Danio rerio. Taken together, RHE has significant insecticidal and antifeeding activities on M. neustria Testacea larvae and can be developed as an environmentally friendly botanical insecticide for M. neustria Testacea control.