Pseudochalcura alba Heraty & Heraty, sp. n. is described from Santiago del Estero and Catamarca provinces in northwestern Argentina. The male and female have long dorsal rami on all of the flagellomeres basal to the terminal segment, which is a unique feature within the genus and shared only with some species of Rhipipalloidea. Antennal modifications are compared across the Stilbula clade, of which all are parasitoids of Camponotini (Formicinae). A phylogenetic hypothesis for the group is proposed based on an analysis of 28S and 18S sequence data for 28 species. Ramose antennae are derived independently in both males and females across the clade, but with fully ramose female antennae restricted to the New World prolata group of Pseudochalcura and to some species of the Old World genus Rhipipalloidea. A sister group relationship between these genera is proposed based on both morphological and molecular data. Female antennae in other species of these genera, and other genera in the clade are at most dorsally lobate or serrate, but more commonly cylindrical. Monophyly of species of Obeza and Lophyrocera is supported and linked to a behavioral trait of oviposition into fruits as opposed to flower heads or leaf buds. Within the Stilbula clade, a dichotomy between New and Old World taxa suggest relatively recent post-Miocene exchanges across the Northern Hemisphere.