Archips (Cacoecia) xylosteana L. is a tortricid defoliator common in orchards and broadleaf forests. It is widely distributed in Scandinavia, Central and Southern Europe, Southern Siberia, Asia Minor and Japan. One of the causes explaining the gradation of this insect is definitely the intensive use of chemical treatments applied during the last decades, which caused modifications in the quantity and quality of the populations, modifications often of great importance for the natural biocenoses. This gives us one more opportunity to prove the validity of the statement often met in the literature that brutal intervention in the forest biocenotic complex triggered the gradations of certain totally unexpected pests, the outcome of which cannot be predicted. Archips (Cacoecia) xylosteana L. registered a sudden increase in number during the last years, not only over an extended area in the south of the country, but also in the hilly areas or even at higher altitude, as a permanent companion of the insect Tortrix viridana L. Thus, in a durmast oak (European oak – Quercus petraea ) stand mixed with beech and hornbeam in the Warthe region, there was a medium defoliation in 1970 caused by the larvae and caterpillars of Tortrix viridana L. and Cacoecia xylosteana L. Our research focused on this outbreak, aiming at gathering data necessary for the identification, detection and prediction the defoliator Cacoecia xylosteana L.