Screening and Synergistic Effects of Phenoloxidase Inhibitors in Spodoptera litura

Phenoloxidase (PO) is a key factor in insect immune defense and a potential target for immunosuppressive insecticide synergists. This study used RNA interference, immune assays, computer-aided drug design, and bioactivity assays to clarify the role of PO in Spodoptera litura immunity and identify PO-targeting synergists. PO knockdown significantly impaired larval hemocyte phagocytosis, nodule formation, and encapsulation, indicating that PO plays an important role in maintaining immune function in S. litura. Bioassays showed that 18 candidate compounds enhanced λ-cyhalothrin toxicity to varying degrees. AG690 (PubChem CID: 3119969) showed the strongest synergistic effect, with synergistic ratios of 1.73, 1.82, and 1.64 at 24, 48, and 72 h, respectively. AG690 reduced PO expression and PO enzymatic activity by 65.79% and 52.41%, respectively, and suppressed cellular immune responses as well as SOD, POD, and lysozyme activities. These findings support PO-targeted immune suppression as a strategy for developing insecticide synergists.