An attack of adults of the Hornius grandis species was detected in a roble (Nothofagus obliqua) nursery with young 1 and 2 year-old seedlings. Unable to determine the degree of harm the attack would produce since the life cycle was unknown, its morphological and biological description was carried out. Results indicate that adults and larvae have different seasonal and breeding activities. The last instar presents a period of diapause in the summer thus making it a univoltine species.