Background and Objectives: A number of plant secondary metabolites can function as botanical insecticides. These compounds exert their effects by disrupting molecular, biochemical, and physiological processes in insects, resulting in reduced feeding, growth, and survival. In this study, the lethal and sublethal effects as well as the impact on the nutritional indices of third-instar larvae of the beet armyworm Spodoptera exigua Hübner, a major pest of sugar beet, were investigated under exposure to three botanical insecticides: matrine, neemarin, and tondexir. Materials and Methods: To determine LC₅₀, sublethal effects, and feeding indices of S. exigua, larvae were reared under laboratory conditions and treated with botanical insecticides including matrine (commercial name: Rui Agro, derived from Sophora flavescens Ait.), neemarin (EC 15%, derived from Azadirachta indica Juss. seeds), and tondexir (EC 80%, an oily extract of Capsicum annuum L.). Following preliminary tests, five logarithmically spaced concentrations of each insecticide were prepared. Treated sugar beet leaves were exposed to third-instar larvae, and larval mortality was recorded after 48 hours. Sublethal effects were evaluated using LC₂₅ concentrations of each insecticide. Treated larvae were monitored until adulthood, and biological parameters such as survival and fecundity were recorded. To compare the effects of different dietary treatments on the nutritional indices of third-instar larvae, food consumption, growth, and frass production were measured. A one-way ANOVA was used for statistical analysis. Lethal concentration values were estimated using probit analysis in SPSS, and life table parameters were analyzed using the age-stage, two-sex life table theory with the TwoSex-MSChart software. Results: The LC₅₀ values of matrine, neemarin, and tondexir against third-instar S. exigua larvae after 48 hours were determined to be 47.61, 153.83, and 221.14 µL/mL, respectively. Sublethal effects were assessed at LC₂₅ concentrations. Results revealed that adult longevity and fecundity were significantly reduced in all botanical treatments compared to the control. The mean oviposition period of adult females decreased from 9.4 days in the control group to 4.90, 7.41, and 7.78 days in matrine, neemarin, and tondexir treatments, respectively. The net reproductive rate (R₀) declined from 283.60 offspring/female/generation in the control to 80.5, 162.57, and 184.22 in matrine, neemarin, and tondexir treatments, respectively. The intrinsic rate of increase (rₘ) was 0.159 day⁻¹ for matrine and 0.167 day⁻¹ in the control. Other life table parameters including the finite rate of increase (λ), gross reproductive rate (GRR), and mean generation time (T) were also significantly reduced in all treatments compared to the control. Furthermore, nutritional indices such as the Relative Consumption Rate (RCR), Relative Growth Rate (RGR), Efficiency of Conversion of Ingested food (ECI), and Efficiency of Conversion of Digested food (ECD) were all significantly lower in larvae fed with treated diets. Overall, matrine showed superior performance in all three categories—lethal, sublethal, and nutritional—compared to neemarin and tondexir. Conclusion: The findings of this study indicate that the tested botanical compounds significantly affect lethality, sub lethality, and nutritional indices of S. exigua. Matrine, neemarin, and tondexir may serve as environmentally friendly and sustainable alternatives for the integrated management of beet armyworm. Their application could contribute to reducing reliance on synthetic insecticides and lowering environmental risks, thereby improving pest management strategies. |