Dirphia dolosa Bouvier, 1929 is an endemic moth from southern Brazil, with available information restricted to occurrence data, natural host plant and adult morphology. This study describes bionomic peculiarities of this species, obtained from observations of larval aggregations in the field and in the laboratory. Data on all developmental stages are presented and compared with those of other Hemileucinae. D. dolosa present a facultative pupal diapause and may have two generations per year under the climatic conditions of the Rio Grande do Sul State, Brazil.