A new species, Diatraea mitteri Solis, that had been residing cryptically as Diatraea crambidoides (Grote), feeding on eastern gama grass (Tripsacum dactyloides (L.) L., is described. D. crambidoides occurs in the southern United States and Mexico and is an economic pest of corn (Zea mays L.). It has been reported to also feed on sorghum (Sorghum bicolor (L.) Moench), johnsongrass (Sorghum halepense (L.) Persoon), and sugar cane (Saccharum officinarum L.). We confirm that D. crambidoides also feeds on eastern gama grass. Morphological and molecular characters support the status of D. mitteri as a new species. Parsimony analysis resulted in two clades corresponding to D. crambidoides and D. mitteri. We confirm the distribution of D. mitteri from Kansas, Oklahoma, and Texas, but its distribution could be as broad as the remaining range of eastern gama grass in the eastern United States. All the life stages are described and illustrated. A novel host association, Alabagrus imitatus Cresson (Hymenoptera: Braconidae), as a parasitoid of D. mitteri is reported.