Six species of Cecidomyiidae are described from five galls on mountainmint, Pycnanthemum spp.: Ametrodiplosis geminata n. sp. and Dasineura laquerrarum n. sp. from two leaf bud galls, Lasioptera lorrainae n. sp. and Neolasioptera truncata n. sp. from two stem galls, Ametrodiplosis recondita n. sp. from rhizome galls and Hypogeadiplosis (n. g.) intrusa n. sp., an inquiline in the rhizome galls. The genus Ametrodiplosis is redescribed and the five previously described other Nearctic species, including Ametrodiplosis piperitae (Felt), new combination, are reviewed. The several Neolasioptera species that occur on Lamiaceae are reviewed. Neolasioptera menthae Felt is unidentifiable and is considered a nomen dubium. Changes are made to the generic key of Gagné 2018 to accommodate the new genus Hypogeadiplosis.