Calamoncosis carncrossi Nartshuk, new species, is described and illustrated from the Montezuma National Wildlife Refuge Complex in the Finger Lakes region of central New York. Larvae and puparia of C. carncrossi were discovered in Phragmites australis americanus, a North American native subspecies of the European wetland grass Phragmites australis. Characters of the adult, larva, and pupariun of C. carncrossi are differentiated from those of C. aprica, a closely related Palearctic species. This new species also represents the first Nearctic record of a described species of the largely Palearctic genus Calamoncosis.