Many woody species, such as Quercus robur L., have interannual variability in seed production. This phenomenon, known as “masting” can make a few disadvantages for the natural regeneration by reducing recruitment opportunities in the years of low fruit production. Studies on seed production not only have shown significant variability between years but also among individuals. Our aim was quantifying the percentage of acorn losses for pre-dispersive predation. For this, we have been estimated for three years the number of acorns that reach the ground. Of all the acorns that produces the tree, only part reach the soil in perfect viability to germinate and establish itself as seedling. A significant number fall to the soil before completing its development, probably because failures during this process or by self-regulatory mechanisms of the tree itself, which only keeps the seeds that can withstand according to the resources at its disposal. Another important part is consumed by predators on the tree, and finally an even more important part of acorns is attacked and predated by insect larvae. In the oak species, most are coleopteran of the genus Curculio and lepidopteran of the genus Cydia. In years of abundant production, the acorns reach the ground, viable to germinate and establish themselves as seedlings, ranging between 5% and 33%. The larvae consume resources stored in the seed, reaching its maximum development when the acorn has completed its maturation and falls. The damages are not only caused by the direct consumption of cotyledons and embryo but, even in cases in which they remain intact, the larvae generate cavities and galleries in the seed, which facilitates entry of fungi, bacteria, and other insects. In conclusion, pre-dispersive acorns predation by insects, it could place itself as one of the main constraints for natural regeneration of Quercus species.