A Nemertean-Derived Peptide Toxin Exhibits Oral Insecticidal Activity Against Spodoptera litura

Spodoptera litura is a polyphagous lepidopteran pest with rapidly evolving insecticide resistance. Peptide toxins from venomous organisms, many stabilized by the inhibitor cystine knot motif, offer bioinsecticidal potential but show limited oral efficacy. Seven previously reported candidates were recombinantly produced in Pichia pastoris and screened against S. litura by injection and leaf-disc feeding bioassays, followed by whole-plant spray assays. The marine nemertean toxin nemertide α-1, designated A1, showed the strongest oral activity, with a median lethal concentration of 3.97 μg μL^-1. Conversely, the engineered spider-venom peptide U1-AGTX-Ta1b R9Q (T1) was most toxic by injection but showed no detectable oral activity, demonstrating that injection toxicity did not predict oral efficacy. High-cell-density fermentation yielded an estimated A1 titer of 1.20 g L^-1. Whole-plant foliar application of A1 showed concentration-dependent efficacy. Mortality reached 92.07% at 8 μg μL^-1, whereas lower concentrations suppressed larval growth, reducing the mean weight of surviving larvae by 41.55% and 67.58% at 1 and 2 μg μL^-1, respectively, and mitigated feeding damage. These findings extend the documented oral insecticidal spectrum of nemertide α-1 to S. litura and demonstrate whole-plant efficacy following foliar application, supporting its development as an active ingredient for foliar-applied peptide bioinsecticides.