Methoxyfenozide and Lufenuron Enhanced Insecticidal Activity via 20E Biosynthesis–Immunity Dual Disruption in <i>Spodoptera exigua</i> (Lepidoptera, Noctuidae)

<i>Spodoptera exigua</i> (Lepidoptera, Noctuidae) poses significant threats to global agriculture, necessitating the management of resistance through sustainable alternatives such as Insect growth regulators (IGRs), including Methoxyfenozide (MET) and Lufenuron (LUF). This study elucidates the potential regulatory mechanisms between 20-hydroxyecdysone (20E) and immune signaling under IGR exposure. Sublethal doses of MET, LUF, and their mixture (MML) significantly downregulated the 20E titer by upregulating <i>SeShd</i> and <i>SeCYP18A1</i>. Silencing <i>SeShd</i> decreased 20E, whereas knockdown <i>SeCYP18A1</i> increased 20E; both delayed development and increased susceptibility to IGRs. Furthermore, IGRs activated immune pathways (Toll/IMD), elevating <i>SeDif</i> and <i>SePGRP-LB</i> (<i>SePLB</i>) expression while simultaneously suppressing 20E. Silencing <i>SeDif</i> and <i>SePLB</i> stimulated 20E biosynthesis and heightened susceptibility to IGRs. These findings offer novel insights into the intricate interplay between 20E biosynthesis and the immune response to IGRs. Our work enhances the understanding of the IGRs' molecular mechanism and offers actionable strategies for combating resistance in <i>S. exigua</i>.