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A medium-size (approx. 3.5-4.0 cm wingspan) dark red-brown moth. There are two forms. The "normal" form has dark red-brown forewings, with a broad contrasting lighter yellow-brown subterminal band. The normal lines and spots are poorly defined, except for the orbicular which is outlined in lighter brown and the prominent narrow boomerang-shaped reniform, which in outlined and partly filled with bright white. The white scales frequently run along the veins, connecting the lower end with the pale subterminal band. Form atra has almost completely dark brown, almost black, forewings, unmarked except for the white or pale yellow reniform. The normal pattern is visible only with close scrutiny. The hindwings in both forms are lighter sooty brown. Antennae are simple and the sexes are similar. Specimens of form atra are very easy to mistake for darker specimens of Apamea cogitata. Apamea cogitata shows no trace of the short black basal streak, faint claviform spot and antemedian and postmedian lines that can barely be made out in atra.

The adults are nocturnal and come to light. There is a single brood annually, with adults flying in late summer and early fall. The larvae are borers in the stems of monocots, in particular sedges (Carex sp.). Reported to sometimes damage young corn in Ohio. They overwinter in the egg stage.

Helotropha reniformis shows few biogeographic affinities, and has been collected from the Lake Athabasca area in the northern boreal forest south to Writing-on-Stone Provincial Park in the arid short-grass prairie region. It's apparent absence in the mountains is difficult to understand. It is noteworthy for the two very different looking forms.