Damage types caused by two forms of Phytoptus avellanae Nal. ("gall" and "vagrant" forms) and Cecidophyopsis vermiformis Nal., all of which are commonly found in the hazelnut cultivars in the Blacksea Region, were investigated by periodic sampling. Our results show that the gall form of P.avellanae feeds exclusively on generative buds and causes formation of big buds; whereas the vagrant form of P. avellanae does not cause big bud formation, but feeds intensively on both vegetative and generative buds, on catkins and on young fruit clusters which eventually fall. On the other hand, C. vermiformis itself does not cause formation of big buds, but feeds in big buds created by P.avellanae, and not all big buds are infested with C.vermiformis. Big buds with only P. avellanae inside have been termed "spring big buds". Due to intensive feeding and reproduction by C. vermiformis, infested big buds prematurely open and fall down at the end of summer, thus leading to the formation of "summer big buds". The nymphs and adults migrate to spring big buds after the summer big buds dry and form "winter big buds". In addition, C.vermiformis rarely infests vegetative and generative parts which may fall if infestation becomes too heavy.