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The rice stem borer (RSB), Chilo suppressalis (Walker), poses a significant threat to rice crops in China, with a rapid increase in chlorantraniliprole (CHL) resistance observed over the past decade. However, the co-evolutionary relationship between metabolic and target-site resistance mechanisms underlying high-level CHL resistance remains poorly explored. This study monitored CHL resistance in RSB populations from the Fuyang (FY) district of Hangzhou city, Zhejiang Province, from 2020 to 2023, revealing a resistance ratio of 749- to 1820-fold. Two known mutations (Y4667D/C and I4758M) were detected, and the I4758M mutant genotype, accounting for over 73%, displayed a gradual decrease in homozygosity over the past four years. Additionally, there was a significant 2.30-fold increase in P450 activity in FY23 compared to a susceptible strain. Furthermore, four significantly overexpressed P450 genes (CsCYP321F3, CsCYP324A12, CsCYP6CV5 and CsCYP9A68) were identified in FY populations, displaying relatively stable levels between years, except for CsCYP6CV5. In this context, we screened a plant-derived synergist, magnolol (MA), which inhibited P450 activities by 29.09%-33.53% and reduced the mRNA expression of CsCYP6CV5 by 56.96%-60.28% from second day post-treatment. Field trial results indicated that a formulation of 25% MA suspension concentrate (SC) mixed with 200 g/L CHL SC substantially reduced the dead heart rates by 53.02% and increased the panicle-bearing tiller rate by 39.09% compared to the insecticide alone in early-season rice. These findings reveal the evolutionary mechanisms driving high-level resistance to diamides in lepidopteran pests and provide practical insights for managing CHL resistance in RSB field populations. [ABSTRACT FROM AUTHOR]