The gypsy moth (Lymantria dispar) is one of the most important forest pests in Croatia. The global significance of this forest pest increased in 1869 when it was introduced into the United States where it was classified as an invasive species. Redefining the significance of the gypsy moth as a forest pest is necessary because of some recent discoveries, and also due to some unknown biological aspects in Croatian habitats. The shifting of forest pest’s population to the North is a clear indication of the climate change. With the discovery of new scientific methods (molecular genetic analysis), the gypsy moth population in Croatia was divided into two populations (Continental and Mediterranean). These two populations differ in terms of outbreak duration periodicity (Continental: 10 – 11 years; Mediterranean: more frequent and irregular). The migration ability of the gypsy moth enables it to spread into a new area. Regarding the climate change and the shift of forest pests to the North, the question arises regarding the adaptation of the Mediterranean population of the gypsy moth to pedunculate oak (Quercus robur). It is also important to clarify the role of the tannin complex in the adaptation of the gypsy moth to pedunculate oak. Various research on this interaction have shown that positive, negative and neutral impact exist. Also, an important fact within the pedunculate oak forest stands in Croatia is the disappearance of the field elm (Ulmus minor) during the 20th century. The gypsy moth egg mass critical numbers were not in use until the outbreak in 2003 to 2005. Until that outbreak, the method of egg mass count on the transect line was practiced (egg mass presence per tree percentage divided into 5 classes). For more precise gypsy moth egg mass critical numbers calculation, important information is the average mass of the leaves which single gypsy moth larvae consumes during its development. Previously assessed average leaf mass consumed per single gypsy moth larva (12 gram) lacks description, missing the description of the used experimental approach and without the specification of the tree species that these data refers to. The influence of defoliation on tree radial increment loss was studied on the pedunculate oak (28 – 30 %), but no data exists for the common beech (Fagus sylvatica). The two rearing experiments were conducted in 2016 and 2017. The first experiment was based on controlled larval growth based on leaf powder diet and the objective was to investigate the adaptation of the Mediterranean population of the gypsy moth to the pedunculate oak. In addition, the biochemical analysis of tannin complex in the fresh leaves used for powdered diet was done. The molecular genetic analysis has the objective to confirm the origin of the coastal source of gypsy moth larvae to the Mediterranean population. The second experiment was conducted with fresh collected leaves, with the objective to determine the average mass of the leaves which one gypsy moth larva consumes (pedunculate oak and common beech). Dendroentomology analysis on the common beech trees was made using the standard sampling methodology via increment cores. The results of first experiment showed that Mediterranean population larvae of gypsy moth can adapt to the pedunculate oak as its food source. It is important to emphasize that the initial larval stages are important for adaptation and survival. In the later larval stages, survival is high in all rearing groups. The biochemical analysis of tannin complex indicated that condensed tannins act as an inhibitory factor for gypsy moth larvae. The molecular genetic analysis confirmed that origin of used larvae in the first experiment belongs to the Mediterranean haplotypes. The results of second experiment resulted with the average mass of the leaves which single gypsy moth larva consumes until pupation. The average mass of leaves is lower than the previously reported 12 g (pedunculate oak 2,91±0,09 g/dry weight, 9,37±0,30 g/fresh weight; common beech 2,65±0,13 g/dry weight; 7,86±0,38 g/fresh weight). The new calculation of gypsy moth critical numbers based on egg masses resulted with an increase for pedunculate oak forest stands. As of the common beech forest stands, gypsy moth egg mass critical numbers represent a completely new data that may be used in future as a tool for damage prediction in common beech stands. The results of the dendroentomology analysis showed that the defoliation of the gypsy moth has its effects on the loss of tree radial increment (-65 %) on common beech trees in the area affected by outbreak and complete defoliation. The results of this research, aside from clarifying some facts that will help to redefine gypsy moth’s population assessment and damage prediction calculations suggest that further research is necessary in Croatia. The migration ability of the gypsy moth Mediterranean population to the North and confirmed adaptation capability to pedunculate oak as a new food source demonstrate the importance of forest pest monitoring in general, an important segment of forestry acting as a valuable and important source of information. Its importance is emphasized when the decision to use (or not) active forest protection measures is required. Also, the importance if monitoring is of critical value in new areas with no earlier record of known forest pests. The tannin complex and its influence on the gypsy moth larval success acquired through feeding trials represents first records about this important physiological trait in Croatian gypsy moth populations. This raises a question regarding the influence of the tannin complex on the population dynamics of other forest pests in Croatia. The average mass of leaves which one gypsy moth larva consumes is a starting point for the calculation of the gypsy moth egg mass critical numbers. The gypsy moth egg mass critical numbers presented in this study need to be evaluated in field conditions. Further research should try to better define the mass of leaves within the tree canopy. Also, remote sensing methods might be used in order to simplify the procedure. Everything mentioned would all together reduce the costs of the forest protection measures. The dendroentomology analysis conducted so far confirmed that the gypsy moth defoliation has effects on the loss of tree radial increment, both on the pedunculate oak and the common beech. Further dendroentomological research related to the effect of other defoliating forest pests would clearly widen and upgrade the knowledge of this important segment of integrated forest protection.