The study aim was to identify the impact of the major defoliator pests of oaks, in correlation with the main oaks phenological phenomena and the development of various pests stages, to better understand the vitality of forest ecosystems, the causes and effects of stress factors. It was determined that the most aggressive pests of oaks in the studied stands were on the leaves: insects of fam.Cynipidae, Totrix viridana, Erannis defoliaria, Operophtera rumata, Tischeria complanella, insects of fam.Argidae; and on the fruit - Balaninus glandium. The actions of the insects were directly dependent on abiotic and biotic factors. The use of pheromone traps has proved to be an effective method in monitoring the pests of the investigated stands and has allowed the determination of outbreaks for the development of sp.Totrix viridana and Lymantria dispar. During the growing season, through crown defoliation, trees have gone from healthy tree to moderate and severe ones. The study suggests the need to develop effective strategies and measures to protect forests in the face of environmental changes.