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The duskywing skippers (genus Erynnis) can be a challenge to identify, particularly in prairie habitat where all three species may occur together. The white spots in the forewing tip are useful characters for separating these species: Persius and E. afranius both have at least two (usually three or four) spots, while the Dreamy Duskywing (Erynnis icelus) has only one spot or none at all. Persius and Afranius are more difficult to separate; Persius is more grey in colour, not brown, and the fore- and hindwing have contrasting grey (forewing) and grey-brown (hindwing) colours; Afranius is more evenly-coloured. Individuals from short-grass prairie habitat in August are almost certainly Afranius, since Persius does not have a second brood. Uncertain specimens must be dissected for diagnostic characteristics of the male genitalia; see Scott (1986) and Guppy & Shepard (2001) for illustrations. Layberry et al. (1998) attribute only subspecies borealis to the Alberta fauna, while Bird et al. (1995) assign northern and southern populations to subspecies borealis and fredericki, respectively.

The egg is pale yellow at first, turning orange as it matures (Bird et al. 1995). Larvae are pale green with a dark dorsal line and a dark brown head, and the pupa is pale-spotted and dark green (Bird et al. 1995). Males perch to await females (Bird et al. 1995). The use of willows and poplars as larval hosts is unusual given the preference for legumes elsewhere, and these host records appear to be restricted to New England populations (Guppy & Shepard 2001). There is no evidence that western populations feed on plants in the willow family (Salicaceae) (Hooper 1973, Guppy & Shepard 2001).

Vetch (Astragalus sp.) is the presumed larval food plant in southwestern Alberta, based on female egg-laying observations (Bird et al. 1995). Lupines (Lupinus spp.) are suspected hostplants in Ontario (Layberry et al. 1998) and BC (Guppy & Shepard 2001). Adults take nectar at locoweed (Oxytropis spp.) and golden bean (Thermopsis rhombifolia) in Saskatchewan (Hooper 1973).

The Persius Skipper occurs in western North America in a broad band from Alaska and northeastern Quebec south to California, Arizona and New Mexico. There are a series of populations disjunct from the western populations, occurring from the western Great Lakes region east to the Atlantic seaboard (Layberry et al. 1998, Opler 1999). The disjunct western and eastern populations, and the apparent differences in foodplants, suggest the taxonomy of these populations warrants further research.