European oak metabolites shape digestion and fitness of the herbivore Tortrix viridana

1. Plants harbour a wide range of leaf-feeding insects whose survival and fitness are influenced by both energy-rich molecules and phytochemicals in the host foliage. Yet, how leaf host chemical diversity and insect microbiota-key factors in ecological and physiological processes-impact insect nutrition and fitness are still poorly understood. 2. To study the effects of leaf metabolic composition on insect herbivory resistance and performance, we fed the larvae of the specialist herbivory Tortrix viridana with leaves of susceptible and resistant Quercus robur trees that are characterized by contrasting metabolomes. We analysed the larval performance and mortality, the metabolomes in plant leaves, and in the insects' saliva and faeces by non-targeted metabolomics. Using chemometrics, mass difference network analysis and metabarcoding, we show the metabolome changes and chemical reactions associated with the different diets as well as their impact on insect fitness and gut microbiota. 3. In the saliva and faeces of larvae, plant secondary metabolites (e.g. flavonoids) persisted more the insect digestion while compounds from primary metabolism were more depleted. In addition, metabolic reactions within the larvae indicated different degradation pathways used on the two plant metabolic types (syn. metabotypes), including sulfation and sulfonation. We show that feeding insects with resistant oak leaves, enriched in secondary metabolites and depleted in primary metabolites, impaired insect performance and mortality. Although the insects' gut microbiota was slightly different upon the contrasting diets, overall, it was fairly stable. Despite the impact of host chemicals on herbivores, larvae were generally highly efficient in nutrient assimilation (feed conversion ratios of 3.3-3.6) and able to minimize plant defences (78% of secondary metabolites were converted, broken down or sequestrated). 4. The comparison of the oak metabotypes showed how the foliar composition of resistant oaks affected insect fitness by influencing their digestion. Herbivores feeding on resistant oaks were less efficient due to their lower ability to metabolize and detoxify higher levels of host phytochemicals, whereas those on susceptible oaks were more efficient as they could degrade the host metabolome. This study highlights the importance of the oak leaf chemical composition to insect digestion and fitness of a specialized herbivore.