Aphicidal Activity and Biosafety Assessment of Small Peptides in Soybean Plants

Developing eco-friendly pesticides is important for sustainable agriculture, especially against pests like soybean aphids (Aphis glycines). Peptide-based compound IV-3 (cinnamic acid-Phe-β-Ala-Trp-Gly) can offer a promising potential insecticide, but further research is needed on the aphicidal activity of its components and concentration effects on soybean plants. Here, the aphicidal activities of the small peptides (Trp-Gly, β-Ala-Trp-Gly, and Phe-β-Ala-Trp-Gly) were first investigated against A. glycines, as tripeptides showed similar activity to tetrapeptide, offering a more cost-effective alternative. In addition, the different effects of a chemical link using an amide bond (cinnamic acid–tetrapeptide = IV-3) and a physical mixture (cinnamic acid + tetrapeptide (LC50 = 30.21 mg L–1)) on insecticidal efficacy were studied. Chemically combined molecule IV-3 (cinnamic acid–tetrapeptide) contributed significantly more to the insecticidal efficacy than their physical mixture, highlighting the importance of maintaining structural integrity. The intact IV-3 peptide demonstrated superior aphicidal activity (LC50 = 5.92 mg L–1) against soybean aphids compared to that of its individual components, emphasizing the importance of structural integrity. Biosafety tests confirmed that IV-3 seed treatment and foliar application at 12–100 mg L–1 are safe for plants, with 12 mg L–1 promoting growth without phytotoxic effects. We indicated that the small peptide IV-3 showed promising potential for soybean health in sustainable agriculture by offering strong aphicidal activity and good safety while concurrently enhancing plant growth at low concentration.