The hostplant, Schinus molle L. (Anacardiaceae) and the last instar of the Arctiinae Amastus walkeri (Rothschild, 1922) are reported for the first time. Several caterpillars with abundant setae and gregarious behavior, were observed feeding on the leaves of Schinus molle L. in the department of Arequipa, southwestern Peru. Ten caterpillars were collected and raised in laboratory condition. Two adults were recovered. According to the external morphology of the adults, they correspond to the species identified as Amastus walkeri (Rothschild).