Resumo. This study reports, for the first time, attack of Idalus agastus (Dyar, 1910) (Lepidoptera: Erebidae: Arctiinae) on the seedlings of Myrtaceae family plants grown in nurseries in southern Brazil. This arctiid is common in the state of Rio Grande do Sul and is a secondary pest of eucalyptus. However, reports of damage to native fruit species in nurseries are unprecedented in this region. Infestation and foliage damage have been observed in Psidium cattleyanum Sabine (strawberry guava), Myrcianthes pungens (O. Berg.) D. Legrand (guabiyu), Myrciaria cauliflora Mart. (jabuticaba), and Eugenia uvalha Cambess. (uvaia) trees during the September/October 2023.