Phenological Plasticity and Bio-Physiological Impacts of Corythucha arcuata Under Aridity and Edge Dynamics in Southern Transylvania Oak Forests

The invasive expansion of the oak lace bug (Corythucha arcuata) represents a major threat to European oak forests, yet the synergistic roles of climatic stressors remain poorly understood. This study investigates the phenological plasticity and adaptive thermoregulation of C. arcuata in the specific microclimatic conditions of the Rășinari Forest District, Romania. Monitoring across an altitudinal gradient (525–825 m) identified a complex voltinism, characterized by a highly successful second generation (G2) and a restricted third generation (G3, <12% emergence due to early frosts). By utilizing a physiological time scale (GDD), we demonstrated that G2 exhibits a 15% temporal compression in development duration compared to G1. A critical tipping point for host vulnerability was identified at a De Martonne Aridity Index (IAr) value of 20. Below this threshold, oak trees underwent a linear physiological decline, with a 74.5% decrease in chlorophyll content and a 58.8% accumulation of soluble sugars. These findings support the metabolic bait hypothesis, where drought-stressed foliage becomes a high-quality nutritional resource. Furthermore, we established a critical thermal threshold of 32 °C, which triggers active vertical migration from the sun-exposed canopy to shaded interiors to avoid heat stress. Our results provide a predictive framework for sustainable forest management, identifying aridity as an injury amplifier that facilitates pest impacts under a warming climate.