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Montana Field Guide

Montana Field Guides

Rocky Mountain Dotted Blue - Euphilotes ancilla

Native Species

Global Rank: G5
State Rank: S5

Agency Status

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General Description
[From Ferris and Brown 1981; Scott 1986; Opler and Wright 1999; Glassberg 2001; Pyle 2002] Forewing 1.1-1.3 cm. Fringes white and weakly checkered. Uppersurface of male deep blue with medium black border, hindwing aurora usually absent but sometimes present. Female brown, with orange aurora varying wide to absent, terminal line thin. Undersurface light blue-gray, forewing often suffused with smoky dark-gray, hindwing with prominent black spots smaller than on forewing, submarginal orange aurora usually composed of unconnected spots, capped with inward-pointing black.

One flight; mostly June to July (Scott 1986). Late May to July (Glassberg 2001). Late May to early July in Oregon (Warren 2005). Early season cohort early May to early July, late season cohort mid-July to mid-August in southern Nevada (Austin et al. 2008).

Diagnostic Characteristics
Difficult to distinguish in field from closely-related species based on gross external characters, necessitating data on date, locality, larval hostplant, ultimately morphology of adult genitalia (Glassberg 2001; Guppy and Shepard 2001; Austin et al. 2008). Helpful characteristics include presence of the submarginal orange aurora on both surfaces of the hindwing with orange in disconnected spots on the undersurface, the wing fringes white and weakly checkered, undersurface of forewing often suffused with smoky dark-gray, hindwing with prominent black spots smaller than on forewing.

Species Range
Montana Range Range Descriptions


Range Comments
Southern Alberta and southern Saskatchewan south through southern Idaho, Wyoming, Colorado and northeastern Utah in the Rocky Mountain region to extreme northern New Mexico, and along the eastern base of the Cascades and Sierra Nevada from northern Washington to southern California and southern Nevada; not yet reported in British Columbia (Opler and Wright 1999; Glassberg 2001; Guppy and Shepard 2001; Pyle 2002; Warren 2005); 1676 m to 2590 m elevation in Colorado (Brown 1957; Ferris and Brown 1981), 1341 m elevation in southeastern Oregon (Warren 2005). In Montana, reported from at least 15 counties in the western half of the state (Kohler 1980; Stanford and Opler 1993) to at least 1554 m elevation, since then reported from extreme southeastern Montana (FLMNH Lepidiopterists' Society database). Locally rare in the north, common to abundant in the south (Glassberg 2001).

Observations in Montana Natural Heritage Program Database
Number of Observations: 2

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



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


Rocky slopes and hilltops, exposed flats, desert washes, canyons, sagebrush steppe, open woodland (Scott 1986; Opler and Wright 1999; Glassberg 2001; Pyle 2002). In Glacier National Park, Montana reported from transition habitat between low elevation and alpine (Debinski 1993), in the Greater Yellowstone Ecosystem reported from xeric montane meadows dominated by sagebrush (Debinski et al. 2013).

Food Habits
Larval food plants include several species of Eriogonum, especially E. umbellatum in the Rocky Mountain region (Ferris and Brown 1981; Scott 1986, 1992, 2006). Adults feed on flower nectar (including Chrysothamnus, Eriogonum, Heterotheca, Medicago) and mud (Scott 2014).

Reproductive Characteristics
Females lay eggs singly on host plant flowers or flower buds (Scott 1986, 1992, 2006). Larval development poorly described (but see Austin et al. 2008), is likely similar to E. enoptes of which it was once considered a subspecies (see Ferris and Brown 1981; Scott 1986). Eggs of E. enoptes hatch about 5 days post-oviposition, develop from L1 instar to L4 instar and pupation in about 23 days, pupae overwinter (diapause), usually in soil or debris at base of host plant. Larvae build no nest, eat host plant flowers and fruits, are tended by at least five species of ants (Scott 1979, 1986; Austin et al. 2008; James and Nunnallee 2011). Males patrol throughout the day over all kinds of topography near host plants while seeking females (Scott 1975b, 1986).

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Rocky Mountain Dotted Blue — Euphilotes ancilla.  Montana Field Guide.  .  Retrieved on , from