Hypochrysops piceatus Kerr, Macqueen & D.P.A. Sands, 1969 is currently known from a restricted area in southeastern Queensland, Australia and is listed as Endangered under the Queensland Nature Conservation Act 1992. Although it is well-known that the larvae are monophagous, feeding exclusively on the foliage of mature (old growth) trees of Allocasuarina luehmannii (RT Baker) LAS Johnson (Casuarinaceae) and obligatorily attended by the ant Anonychomyrma inclinata Lewis & D.P.A. Sands, 2021 (Formicidae: Dolichoderinae), the life history has not previously been described and aspects of the species' ecology remain poorly documented. Here we describe the immature stages and clarify the biology of the species. We also report the utilization of saplings of the larval food plant for the first time, a discovery which has important ramifications for conservation management of the butterfly. The attendant ant of the closely related Hypochrysops cyane (Waterhouse & Lyell, 1914) is clarified and found not to be conspecific with that of H. piceatus.