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Montana Field Guide

Montana Field Guides

Northern Blue - Plebejus idas

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 1.2-1.5 cm. Sexually dimorphic. Uppersurface of male blue with white margin and black submarginal chevrons, female brown with whitish margin and orange or brown submarginal chevrons; undersurface light gray ground color, hindwing with black marginal line broken into small spots at vein endings, submarginal black-capped orange chevrons reduced.

One flight; mostly late June to mid-August except late July to mid-August in Newfoundland and Labrador (Scott 1986). Mainly late June to August (Glassberg 2001). Early June to mid-October in Oregon and Washington (Pyle 2002), late June to late August in Oregon (Warren 2005), July to early August in British Columbia (Guppy and Shepard 2001), late April to mid-August in southeastern British Columbia (Threatful 1988).

Diagnostic Characteristics
Best told by a combination of the traits mentioned in the general description above; generally paler and less well-marked than similar species (especially P. melissa).

Species Range
Montana Range

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Range Comments
Holarctic. In North America, Alaska south to central California, southern Idaho, northern Utah, southwestern Colorado, east across boreal and southern Canada to the northern Great Lakes region and northeastern Ontario, with isolated populations in Labrador, Newfoundland, southern Quebec, Nova Scotia (Scott 1986; Opler and Wright 1999: Glassberg 2001). Some authorities recognize P. anna as a distinct species from P. idas (Guppy and Shepard 2001; Pyle 2002; Warren 2005) in which case the range of idas would exclude the Cascades and Sierra Nevada; mostly 2745 m to 3355 m elevation but to 3658 m in Colorado (Brown 1957; Ferris and Brown 1981), 2134 m to 3353 m elevation in Wyoming (Nabokov 1953), usually above 914 m elevation in Oregon and Washington (Pyle 2002), 1158 m to at least 2438 m elevation in Oregon (Warren 2005), above 1000 m elevation in interior British Columbia (Guppy and Shepard 2001), 731 m to 1311 m elevation in southeastern British Columbia (Threatful 1988). In Montana, reported from the montane western 1/3 of the state (Kohler 1980; Stanford and Opler 1993). Locally uncommon to common (Glassberg 2001).

Summary of Observations Submitted for Montana
Number of Observations: 9

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Relative Density



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


Dry hills and flats, montane moist meadows, bog edges, trail sides and road cuts, above treeline in alpine tundra (Scott 1986; Opler and Wright 1999; Glassberg 2001; Guppy and Shepard 2001; Pyle 2002). In Glacier National Park, Montana reported from xeric and mesic montane meadows (Debinski 1993).

Food Habits
Larval food plants include Astragalus, Empetrum, Kalmia, Lathurus, Ledum, Lotus, Lupinus, Vaccinium, and Vicea (Ferris and Brown 1981; Scott 1986, 1992; Guppy and Shepard 2001; Pyle 2002). Adults feed on flower nectar (including Achillea, Anaphalis, Symphyotrichum, Trifolium) and mud (Scott 1986; Pyle 2002).

Reproductive Characteristics
Females lay eggs on host plant stems and in debris on the ground beneath host plant (Scott 1986; Guppy and Shepard 2001). Eggs diapause (overwinter). Eggs hatch in late spring a few days after introduction to warm conditions, develop from L1 instar to L4 instar and pupae in about 30 days (depending on temperature). Pupation occurs on upper surface of host plant leaves (in captivity) or in ant nests (in Europe), adults eclose (emerge from pupae) in about 12 days. Larvae solitary, build no nest, are associated with ants (Scott 1979, 1986; James and Nunnallee 2011). Males patrol throughout the day on flats and hillsides near host plants in bunch grassland, seeking females (Scott 1975b, 1986).

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Citation for data on this website:
Northern Blue — Plebejus idas.  Montana Field Guide.  .  Retrieved on , from