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A fairly large (4.0-4.5 cm wingspan) grey-brown moth, one of the largest Euxoa. There are several forms of adults; the most common with a black basal dash, black between the orbicular and reniform spot, a contrasting paler grey or yellow costa and a pale streak beyond the claviform spot. A less common form has an evenly-colored pale grey or pale brown forewing. Yet another form has dark brown forewings with white outlined reniform and orbicular spots. Auxiliaris belongs to the subgenus Chorizagrotis, characterized by long, apically spatulate saccular extension and short harpes. The only other member of the subgenus in Alberta is E. adumbrata, which can be separated by it's longer harpes (more than 5 times as long as wide vs 3 times in auxiliaris).

After emerging in late spring or early summer, the adults migrate to higher, cooler elevation for the summer, returning in late summer and early fall to lay eggs, which hatch after a suitable rain. Eggs are laid directly onto the soil, with bare soils such as cultivated or overgrazed areas being preferred. The eggs hatch after a suitable rain, and they overwinter as partly grown larvae. The larvae resume feeding in the spring, pupate, and the adults emerge in late spring early summer. The larvae feed above ground at night and on cool days, hiding in the soil or under clods during the day. Many of the adults which emerge in spring migrate to high elevations to aestivate, returning to the lowland to lay eggs in late summer and fall. There is but a single brood of long-lived adults each year. Auxiliaris make up a large portion of the masses of moths that aestivate in rockslides in the mountains in Wyoming and Montana, and which are an important seasonal food for grizzly bears.

A wide variety of plants and nearly all field crops including wheat, alfalfa, barley, potato, sugarbeets, other vegetables and grasses (Hein et al, undated). They have been found to prefer broad-leaved weeds over wheat in wheat fields (Peairs, 1999)