Combining tea saponin and matrine botanical insecticides is highly effective against fall armyworm Spodoptera frugiperda

As one of the most important invasive and destructive pests in the world, the fall armyworm Spodoptera frugi-perda (Lepidoptera: Noctuidae) reduces crop yield and causes economic loss. It is critical to find an appropriate controlling approach. This study aimed to determine the insecticidal activities of botanical insecticides (30% tea saponin, 0.6% matrine and a mixture thereof) on fall armyworm larvae, as well as the effects of sublethal concentrations of an optimal ratio of the mixture, on the survival rate, developmental duration, pupation rate, emergence rate, and body weight of fall armyworm. In addition, we evaluated the control effect of the insecticides (30% tea saponin, 0.6% matrine, optimal ratio of mixture along with 0.3% azadirachtin and 36% chlorfenapyr) and toxicity to natural enemies in field conditions. The results showed that all botanical insecticides and mixtures were toxic, and the insecticidal bioactivity of 10:1 volume ratio mixture was the highest at 96 hours, with a median lethal concentration (LC50) of 56.561 mg/mL. Meanwhile, the sublethal concentration of a 10:1 volume ratio of the mixture significantly affected the survival rate, prolonged developmental duration and reduced emergence rates and pupal weights, but it did not affect the pupation rates of fall armyworm. Furthermore, it has harmless or slightly harmful effects on natural enemies. Our study provides a novel environmentally friendly and natural enemy-friendly approach to controlling fall armyworms.