Gall-inducing insects may cause multiple physiological changes in host plants, such as the loss of photosynthetic pigments and reduced photosynthetic capacity. However, the direction of these changes is usually insect species-dependent. Therefore, the objective of this research was to characterize the indirect effects of galls induced by asexual generation of Neuroterus numismalis (Fourc.) and N. quercusbaccarum L. on photosynthesis by comparing changes in photosynthetic and photoprotective pigments and chlorophyll a fluorescence in foliar tissue with and without galls in naturally growing pedunculate oak trees (Quercus robur L.). The presence of galls of both Cynipidae species caused a significant decrease of chlorophyll a, b and carotenoids contents. Moreover, photosynthetic parameters (F0, Fm, Fv/Fm, Y, qP, qN) were significantly decreased. These results provide valuable information for diagnosing the oak infections using a noninvasive method, such as chlorophyll a fluorescence and predicting the effect of infections on photosynthetic productivity.