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A medium size moth (forewing length about 36 mm) but one of the larger species of Agrotis. The forewings are pale yellowish grey or tan with dark blackish or reddish brown along the leading edge. The antemedian and postmedian cross lines are essentially obsolete, indicated by a few dark scales only. The claviform, reniform and orbicular spots are also less prominent and partially lined with lighter scales. The hindwings are dark brown. The predominantly pale forewings will separate stigmosa from other species of Agrotis. Most likely to be confused with Agrotis volubilis, which flies at the same time. Agrotis volubilis has dark forewings and pale hindwings; this pattern is reversed in stigmosa.

Agrotis stigmosa is single brooded with adults in Alberta from mid May through mid-June. The adults are nocturnal and come to light. The larvae are described by Lafontaine (2004). Larvae have been reared on Yarrow (Achillea sp.) but like other Agrotis, they are likely generalists on low-growing herbs.