Population studies that help to estimate the advance of Rhyacionia buoliana (Schiff.) in Chile have been carried out due to its continuous settlement and consequent damage to Pinus radiata D. Don plantations. A fundamental requirement for these studies has been to determine the larval age stage of the species, considering the following variables: distance between frontal setae and width of cephalic capsule. The study using samples taken from eight areas of the VIII and IX Region, determined that the most reliable estimation of larval age is given by the distance between frontal setae. Based on this parameter, the use of age tables for the different ecological regions of the species in order to determine the stage at which the diapause period occurs is proposed. This should help to face future research at the population level.