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A medium-size Abagrotis (2.8-3.0 wingspan) with rather stubby red-brown forewings and dull black hindwings. The most prominent marking is the narrow dark bar-shaped reniform, in many specimens accompanied by a dark but less developed orbicular spot. The doubled antemedian and postmedian lines are usually indicated by a series of fine dots, and dark scales are peppered over the remainder of the wing. The fringe on both wings is red-brown. Males can also be separated from other Abagrotis sp. by the cylindrical apex of the valves, the sharp pointed apex of the uncus and the bead-like antennae. Females of trigona do not have the dorso-ventrally flattened abdomen found in females of other Alberta Abagrotis, and are also the only species lacking a signum on the bursa. See also the superficially similar Abagrotis cupida and A. placida.

Western South Dakota and southwestern Manitoba west across southern Saskatchewan and Alberta to Vancouver Island, south to the Mexican border. There is also a disjunct population in Ohio. In Alberta trigona has been collected in the grassland valleys and foothills north to about Red Deer.