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Montana Field Guide

Montana Field Guides

Western White - Pontia occidentalis

Native Species

Global Rank: G5
State Rank: S5

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General Description
[From Ferris and Brown 1981; Scott 1986; Opler and Wright 1999; Glassberg 2001; Pyle 2002] Forewing 2.0-2.5 cm. Base color milky white. Uppersurface of forewing with black discal cell end bar, 5-6 marginal chevrons ("V" spots) joined on outer third and cell ends, lighter and contrasting with blackish submarginal band. Undersurface for forewing apex and hindwing with veins outlined in gray-green (more strongly in spring flight), never bright green or heavy.

Phenology
One flight, mostly July near treeline, June to early July in far north; two flights May through July at lower elevations and latitudes; three flights April to September in low elevation areas of Colorado (Scott 1986). Mainly two flights April to September, one flight July to August at high elevation (Glassberg 2001). Late May to early September in Colorado (Scott and Scott 1978), mid-June to September near treeline in California (Shapiro 1977), mid-March to mid-September in Oregon and Washington (Pyle 2002), early April to mid-September in Oregon (Warren 2005), June to September in British Columbia (Guppy and Shepard 2001), late April to October in southeastern British Columbia (Threatful 1988).

Diagnostic Characteristics
Best determined by a combination of uppersurface of forewing with 5-6 marginal chevrons ("V" spots) joined on outer third and cell ends, lighter and contrasting with blackish submarginal band; undersurface of hindwing with veins outlined in gray-green (more strongly in spring flight), never bright green or heavy.

Species Range
Montana Range

Year-round
 


Range Comments
Alaska, Yukon, western Northwest Territories south to central California, northern Arizona, northern New Mexico east across boreal Canada to central Ontario, the Great Lakes region, western South Dakota, extreme western Nebraska (Scott 1986; Opler and Wright 1999; Glassberg 2001); 1707 m to 4206 m elevation in Colorado (Brown 1957; Scott and Scott 1978), 12 m to at least 2957 m elevation in Oregon (Warren 2005), 456 m to at least 2500 m elevation in British Columbia (Threatful 1988; Guppy and Shepard 2001). In Montana, reported from all counties (Kohler 1980; Stanford and Opler 1993), to at least 2026 m elevation at Logan Pass in Glacier National Park (FLMNH Lepidopterists' Society database). Range expanding eastward across Northern Great Plains (Opler and Wright 1999) and locally in disturbed habitats such as railroad yards (Pyle 2002). Uncommon to common (Glassberg 2001).

Observations in Montana Natural Heritage Program Database
Number of Observations: 23

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



Migration
Non-migratory.

Habitat
Open habitats, including montane meadows, dry forest openings, dry subarctic and alpine tundra, open plains, railroad yards, fields, roadsides (Shapiro 1977; Scott 1986; Threatful 1988; Opler and Wright 1999; Glassberg 2001; Pyle 2002). In Glacier National Park, Montana reported from mesic montane meadows and above treeline in alpine terrain (Debinski 1993); in the Greater Yellowstone Ecosystem, reported from hilltops, clearings, lowlands, roadsides, alpine terrain (Debinski and Pritchard 2002).

Food Habits
Larval food plants are native and exotic mustards, including Anelsonia, Arabis (several species), Athysanus, Barbarea, Brassica, Cardaria, Chorispora, Caulanthus, Descuriana, Draba (multiple species), Lepidium (multiple species), Phoenicaulis, Rorippa, Sisymbrium, Streptanthus, Thelypodium, Thlaspi; also the non-mustard Cleome (Emmel et al. 1970; Shapiro 1977; Ferris and Brown 1981; Scott 1986, 1992, 2006; Pyle 2002; Graves and Shapiro 2003; Warren 2005). Adults feed on flower nectar (including Achillea, Agoseris, Berteroa, Castilleja, Chrysothamnus, Cirsium, Cleome, Delphinium, Epilobium, Erigeron, Gaillardia, Heterotheca, Heuchera, Hymenoxys, Leucelene, Ligusticum, Machaeranthera, Medicago, Potentilla, Raphanus, Rhinanthus, Salix, Sedum, Senecio, Solidago, Symphyotrichum, Trifolium) and mud (Pyle 2002; Ezzeddine and Matter 2008; James and Nunnallee 2011; Scott 2014).

Reproductive Characteristics
Females lay eggs singly on host plant leaves (often undersurface), stems, flowers (Scott 1986, 1992, 2006; James and Nunnallee 2011). Eggs hatch in about 3 days, develop rapidly (depending on temperature) L5 instar and pupae in 15-20 days, adults eclose (emerge from pupae) in about 10 days; sometime pupate after reach L4 instar (James and Nunnallee 2011). Larvae feed on all parts of host plant, build no nest, overwinter as pupae (Scott 1979, 1986; James and Nunnallee 2011). Males patrol throughout the day on hilltops and around host plants in search of females (Scott 1975b, 1986; Shapiro 1977; Guppy and Shepard 2001; Pyle 2002; James and Nunnallee 2011).

References
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Citation for data on this website:
Western White — Pontia occidentalis.  Montana Field Guide.  .  Retrieved on , from