Impact of Andricus pseudomultiplicatus Complex Galls (Hymenoptera: Cynipidae) on the Mortality of Quercus libani Trees in High-Altitude Mediterranean Forests

Oak forests are increasingly threatened by a complex interplay of biological and environmental pressures. Although gall-inducing wasps often produce taxonomically ambiguous galls, they can substantially reduce tree vitality, particularly under drought conditions. However, their direct impact on host tree mortality remains poorly understood. In this study, we investigated the ecological effects of galls of the Andricus pseudomultiplicatus complex on Quercus libani, a drought-sensitive oak species that exhibits elevated levels of dieback and mortality. We combined multiple survey approaches, incorporating extensive field data on climate patterns, gall infestation intensity, host vitality, soil properties, and pathogen presence. Our analysis revealed significant correlation between gall density and key indicators of tree mortality, including diameter at breast height and crown area. Environmental stressors most notably drought, in interaction with the activity of the A. pseudomultiplicatus complex, emerged as major drivers of tree susceptibility and mortality. These findings highlight that the contribution of cynipid wasps to Q. libani mortality has so far been underestimated, particularly in the broader context of climate change.