We provide descriptions and illustrations for identification of Phestinia costella Hampson, a stem gall producer on the invasive plant Chromolaena odorata (L.) R. M. King & H. Rob. (Asteraceae) that has been investigated for biological control measures. Specimens collected from C. odorata on Trinidad and Tobago and reported in the literature as Mescinia sp. nr. parvula Zeller were reexamined and identified as P. costella, except for two females identified as an undetermined species of Mescinia Ragonot. We subjected P. costella to comparative examination with species in related genera and concluded that its current generic placement should remain until a broader phylogenetic treatment of these genera can be undertaken. We diagnose the species, describe the previously unknown male, larva, and pupa, and expand the distribution. Biology and rearing procedures are discussed.