The parasitoid wasp genus Alphomelon Mason, 1981 is revised, based on a combination of basic morphology (dichotomous key and brief diagnostic descriptions), DNA barcoding, biology (host data and wasp cocoons), and distribution data. A total of 49 species is considered; the genus is almost entirely Neotropical (48 species recorded from that region), but three species reach the Nearctic, with one of them extending as far north as 45° N in Canada. Alphomelon parasitizes exclusively Hesperiinae caterpillars (Lepidoptera: Hesperiidae), mostly feeding on monocots in the families Arecaceae, Bromeliaceae, Cannaceae, Commelinaceae, Heliconiaceae, and Poaceae. Most wasp species parasitize either on one or very few (2-4) host species, usually within one or two hesperiine genera; but some species can parasitize several hosts from up to nine different hesperiine genera. Among species with available data for their cocoons, roughly half weave solitary cocoons (16) and half are gregarious (17); cocoons tend to be surrounded by a rather distinctive, coarse silk (especially in solitary species, but also distinguishable in some gregarious species). Neither morphology nor DNA barcoding alone was sufficient on its own to delimit all species properly; by integrating all available evidence (even if incomplete, as available data for every species is different) a foundation is provided for future studies incorporating more specimens, especially from South America. The following 30 new species are described: cruzi, itatiaiensis, and palomae, authored by Shimbori & Fernandez-Triana; and adrianguadamuzi, amazonas, andydeansi, calixtomoragai, carolinacanoae, christerhanssoni, diniamartinezae, duvalierbricenoi, eldaarayae, eliethcantillanoae, gloriasihezarae, guillermopereirai, hazelcambroneroae, josecortesi, keineraragoni, luciarosae, manuelriosi, mikesharkeyi, osvaldoespinozai, paramelanoscelis, paranigriceps, petronariosae, ricardocaleroi, rigoi, rostermoragai, sergioriosi, and yanayacu, authored by Fernandez-Triana & Shimbori.
Keywords: Biology; DNA barcoding; Microgastrinae; Nearctic; Neotropical; morphology.