Elevated CO2 and Water Limitation Alter the Primary Metabolite Composition of Mealybug Honeydew, Reducing Parasitoid Fitness.

Honeydew excreted by phloem-feeding insects is a major plant-derived carbohydrate resource in agroecosystems, yet its chemical sensitivity to climate change remains poorly understood. We examined how elevated atmospheric CO2 and water limitation affect honeydew primary metabolites and a honeydew-feeding parasitoid. Using a tritrophic system comprising grapevine (Vitis vinifera), the vine mealybug (Planococcus ficus), and its parasitoid (Anagyrus vladimiri), plants were grown under ambient or elevated CO2 and well-watered or water limitation conditions. Honeydew was quantified, primary metabolites were profiled by GC-MS, and parasitoid longevity and egg load were measured. Honeydew quantity was unaffected by treatments, but elevated CO2 reduced sugar concentrations, including those of sucrose and erlose, while water limitation increased raffinose and myo-inositol concentrations. These chemical changes were associated with reduced parasitoid longevity and fecundity, demonstrating that climate-driven alterations in honeydew composition may compromise its nutritional quality, with implications for biological control in vineyard systems.