Synonyms and other taxonomic changes
Pontia sisymbrii (Boisduval)
Orig. Comb: Pieris sisymbrii Boisduval, 1852
Most like other Pontia species. P. beckeri is usually larger, with wider and distinctly green patterning on the lower hind wing, while P. sisymbrii has pattern distinctly dark grayish, even if there is a hint of green. Also, the green pattern in P. beckeri is broken in the middle where there is a pale patch, while in P. sisybrii the dark vein edging continues through this patch.
P. protodice & P. occidentalis are also usually larger, are varied in pattern, but unless they are plain white below (which P. sisymbrii never is), have at least part of a dark line running along the middle of the cell between veins Cu2 and A2 (=2V) where vein A1 would be if it existed. P. sisymbrii never shows this dark line (P. beckeri sometimes does). The color of the dark pattern on the under hind wing of P. protodice varies greatly with gender, individually, and seasonally, but often it is not much grayish, but rather greenish, brownish, or yellowish.
P. sisymbrii flies early in the season (late winter / spring) and is rarely seen after June even at high elevation, while other species are multi-brooded and can be seen year-round as the climate and weather allow.
Not always diagnostic, but useful are the facts that the dark bar at the end of the discal cell is very narrow (usually over twice as long as wide), and the underside usually has the veins contrastingly pale (often white or yellow) in P. sisymbrii. In other Pontia species the dark forewing bar is usually much wider, and the under hind wing veins are rarely white, sometimes yellow, but generally are not either.
Most often in rugged terrain of mountains and canyons at Upper Sonoran and Montane elevations, often in semi-arid areas dominated by open brush or woodland.
February-July, early in season, flying earlier at low elevations and in south than higher up or more northern parts of range. One flight per year.
Recorded host plant genera include: Arabis, Caulanthus, Sisymbrium, Streptanthus...all in Brassicaceae (the Mustard Family).
Larvae feed on members of mustard family containing sulfur compounds called "mustard oils" which are presumably sequestered and deter predation. Males patrol hilltops, as well as along canyon bottoms.
Very similar to members of the genus Baltia in Asia, and perhaps closely akin.
Western White, Pontia occidentalis
Brock and Kaufman, pp. 48-49 (1)
Garth and Tilden, pp. 104-105, plate 21c (2)
Shapiro, A. M. (1981). Egg-mimics of Streptanthus
(Cruciferae) deter oviposition by Pieris sisymbrii
(Lepidoptera: Pieridae). Oecologia 48: 142–143. (PDF here