There are four species of Erynnis in Washington. A fifth species, Afranius Duskywing (Erynnis afranius) was previously thought to have been found in a few locations in eastern Washington and Idaho, but those specimens have since been ruled out as mis-identified Persius Duskywings (E. persius).
Dreamy Duskywing (E. icelus) is the smallest of these species, and the only one without a series of white spots ("hyaline spots") on the dorsal forewing. It sometimes has one or two light gray, almost white spots on the forewing, but never a full series of four spots with one near the center of the forewing as in the other three species. Dreamy Duskywing larvae feed on willow, aspen, and poplar.
Propertius Duskywing (E. propertius) is the largest of these species, and is usually much lighter than the other three species. Females have large white spots and heavy dusting of gray scales on the forewings. Larvae feed on oak leaves.
Persius Duskywing (E. persius) is separated from Dreamy by the presence of white spots on the dorsal forewings. It is usually slightly larger than Dreamy. The dorsal forewings of Persius are more evenly covered with gray scales, separating it from the easily confused Pacuvius. Persius Duskywing larvae feed on legumes, primarily lupine species.
Pacuvius Duskywing (E. pacuvius) is easily confused with Persius, but the forewing is mostly brown from the body to the row of white spots, most of the gray coloring is restricted to the marginal area. Larvae feed on Ceanothus species (mountain balm, buckbrush) and adults are primarily found near these shrubs.