A Synergistic Terpenoid-Phenolic Defense Network Confers Resistance to Dendrolimus superans in Hybrid Larch

To elucidate the resistance of hybrid larch (Larix spp.) clones to Dendrolimus superans and their chemical defense mechanisms, 46 clones were subjected to field feeding-infestation experiments. Insect resistance was comprehensively evaluated by the membership function method from three perspectives: insect weight change, frass weight and defoliation rate (damage consequences); pre- and post-infestation physiological indices (constitutive and induced resistance). High- and low-resistance materials screened from each perspective were further analyzed by widely targeted LC-MS metabolomics. The three damage indices differed highly significantly among clones (P<0.01) and were positively correlated, indicating abundant genetic variation in resistance. Clones such as -30-C, -35-A and -21-A showed strong overall resistance, whereas -28-E and -20-A showed high constitutive and induced resistance. Defence enzyme activities (POD, SOD, CAT, PPO and PAL) correlated closely with secondary metabolites (total phenolics, tannins and total flavonoids) and nutrients, and rose synergistically after feeding, reflecting synergy between constitutive and induced resistance. In total, 3,577 metabolites were identified, predominantly terpenoids (>90%). The three groups yielded 1,010, 1,086 and 922 differentially accumulated metabolites, and PCA separated high- from low-resistance materials in all groups. These metabolites were significantly enriched in metabolic pathways, biosynthesis of secondary metabolites, monoterpenoid and diterpenoid biosynthesis, and phenylpropanoid biosynthesis. In conclusion, resistance of hybrid larch to D. superans is jointly conferred by the defence enzyme system, secondary metabolite accumulation and a 'terpenoid-phenolic' metabolic network: terpenoids and phenolics act synergistically via the terpenoid and phenylpropanoid pathways, together forming the core of chemical defense and supporting the breeding of insect-resistant clones.