Effects of dietary juglone on the oriental fruit moth Grapholita molesta (Lepidoptera: Tortricidae): growth performance and protective enzyme activity.

The oriental fruit moth, Grapholita molesta, is expanding its host range to walnut (Juglans regia), a non-Rosaceous species producing the allelochemical juglone. This study assessed juglone toxicity to G. molesta and its physiological adaptation. Seasonal juglone in walnut husks was quantified biweekly using high-performance liquid chromatography (HPLC). Neonates were reared on diets containing 0, 1.25, 2.5, 5, 7.5, or 10 mg/g juglone to determine development, survival, fecundity, and population parameters. Additionally, protective enzyme activities were compared between larvae fed control and juglone diets. Juglone in husks peaked at 42.28 mg/g in late May, decreased to 6.02 mg/g by early August, and remained stable. Juglone exerted concentration-dependent effects. Larval duration increased from 20.4 d (control) to 36.0 d at 10 mg/g, and survival declined from 50.5% to 4.4%. Fecundity decreased from 217 eggs/female (control) to 168 at 2.5 mg/g and 19 at 10 mg/g; hatchability dropped from 92% to 43% at 7.5 mg/g. Pupation, emergence, and mating rates were affected only at ≥7.5 mg/g. At 10 mg/g, most individuals failed to complete their life cycle. This threshold aligns with the seasonal minimum juglone (6.02 mg/g), indicating host colonization is constrained to a biochemical window when juglone falls below 7.5 mg/g. Underlying these fitness costs, juglone suppressed peroxidase (POD) and superoxide dismutase (SOD) at ≥7.5 mg/g, while catalase (CAT) was inhibited only at 10 mg/g. These findings elucidate the toxicity of walnut metabolites to G. molesta and clarify physiological mechanisms potentially facilitating its host range expansion.