Mythimna amblycasis is a moth of the family Noctuidae. It is endemic to Kauai, Oahu, Molokai, Maui, Lanai and Hawaii. Larvae feed on various grasses, bunch grass and sugarcane. The caterpillars become full-grown in about a month from the hatching of the eggs. The full-grown caterpillar is about 38 mm. It is usually cream colored with pale brownish mottlings. Pupation takes place in an earthen cell a little below the surface of the ground, or beneathstones or rubbish. The pupa is about 19 mm long and 5 mm thick. The moth emerges from the pupa in about two weeks.