Medusahead, Taeniatherum caput-medusae ( Poales : Poaceae ), is an annual grass native to central Asia and the Mediterranean region. It is a noxious, invasive weed in much of western North America. During field explorations carried out in Greece in 2017, the new phytophagous eurytomid Tetramesa amica Lotfalizadeh, sp. nov. and its parasitoid Eurytoma amicophaga Lotfalizadeh, sp. nov., also new to science, were recorded for the first time on medusahead. These new species are described and characters that enable to recognize them from their closest relatives are summarized. Tetramesa species are generally species-specific gall-inducers. They induce damages that may have a significant impact on the physiology of infested plants by reducing the productivity of flowering heads and seed weight. Based on these data, T. amica Lotfalizadeh, sp. nov. is currently being investigated as a candidate biological control agent of medusahead.