Defense dynamics in walnut (Juglans regia L.) fruit during walnut husk fly (Rhagoletis completa Cresson) infestation

The walnut husk fly (WHF) is a critical pest affecting global walnut production, causing economic losses of up to 80%. WHF larvae feed within the walnut husk, creating necrotic spots, which compromise kernel quality and marketability. In this research, the metabolic response of the walnut fruit was addressed for the first time in an integrative, multi-level analysis, which included several primary and secondary metabolites, enzymes, and antioxidant activities by using LC-MS, GC-MS, and different spectrophotometric methods. Results revealed localized and tissue-specific metabolic responses, with significant changes up to 200% in volatiles and phenolics, especially naphthoquinones and sesquiterpenes, as well as enzyme activities. Additionally, prolonged infestations severely affected kernel quality by reducing the content of unsaturated fatty acids and carotenoids by 60–70%, potentially due to oxidative stress and arrested fruit growth. Understanding these defense mechanisms is essential to mitigate economic losses and food waste, ensuring sustainable walnut production through targeted interventions and improved pre- and post-harvest treatments. They are also fundamental for future decisions on pheromone-based management of WHF, as well as for breeding efforts or pest control programs oriented to the enhancement of the natural plant defense mechanisms.