Pacific Northwest Moths

Identification

Adults

Euxoa ridingsiana is a small to medium-sized Euxoa (FW length 13–16 mm) that flies in dry forests during the summer. It has a dark gray-brown and ochre-tan forewing with light gray veins lining the cell, pale-filled spots, and a two-toned, off-white hindwing. The forewing is relatively short and rectangular. It is gray-brown. The costa is dull ochre-tan, striped with light gray veins and darker brown. A dull yellow streak is present in the fold and the subterminal area is streaky yellow tan and darker gray-brown. The radial and cubital veins and their proximal branches are pale gray. The terminal area is brown-gray. The antemedial line is brown and black, irregular and angled strongly toward the outer margin below the cubital vein, often bulging far lateral between 1A+2A and the posterior margin. The median line is absent. The postmedial line is gray followed by ochre-tan, weakly scalloped, relatively faint, pulled far basad on the costa, bluntly excurved around the end of the cell, and angled 45° toward the base below the level of the cell. The pale ochre subterminal line is powdery, often broken into spots, slightly irregular. It is preceded by a blackish gray shade on the costa and multiple black wedges opposite the cell and in the fold. The terminal line is dark gray, linear or broken into spots. The fringe is gray with a thin tan base. The orbicular and reniform spots are black with a thin lining of light whitish-gray, filled centrally with gray. The orbicular spot is round or a tilted oval and the reniform spot is kidney-shaped. The claviform spot is black, moderate-sized and narrow. The hindwing is pale yellowish-gray with a fairly well-defined gray marginal band, darker in females, with two-toned gray and white fringe. The head and most of the thorax are dark gray-brown. The base of the collar is brown, separated from the darker edge by a thin black line. A frontal tubercle is present. The male antenna is weakly biserrate. Euxoa ridingsiana is placed in the subgenus Orosagrotis.

This species can be recognized by its late summer flight, relatively small size, gray-brown and dull ochre forewing with light gray veins near the dark cell, streaked costa, and pale-rimmed small forewing spots. It is almost identical to Euxoa maimes, a smaller rarely-collected species found in eastern British Columbia in the Pacific Northwest. According to Lafontaine (1987), E. ridingsiana is larger than E. maimes (FW length 11–14 mm). The antenna of E. ridingsiana is moderately biserrate and that of E. ridingsiana is narrower weakly biserrate. Although less helpful for identification of individual specimens than entire populations, the relative percentages of individuals with gray and red-brown color of the thoraxes differs between species. That of E. maimes is dark gray in 90%, while that of E. ridingsiana is dark gray in 50%. Biological differences between these species are also known. Euxoa ridingsiana favors forests, and E. maimes prefers dry grasslands. Euxoa ridingsiana emerges about a week earlier in the year than E. ridingsiana at locations where they occur together. Euxoa ridingsiana is also similar to Euxoa flavicollis, another member of the same subgenus. The forewing of E. ridingsiana is lighter and streakier than that of E. flavicollis and its costa is lighter gray and ochre compared to bright ochre or orange in E. flavicollis. The light form of another rare Pacific Northwest species in the subgenus Orosagrotis, Euxoa manitobana, is also resembles E. ridingsiana but has a longer wing with a dentate postmedial line that E. ridingsiana lacks. Finally, Euxoa olivalis, a member of the subgenus Euxoa, resembles E. ridingsiana and size, color, and streaky markings. It differs in having an elongate bar-like orbicular spot, not oval as in E. ridingsiana.

Euxoa is a very large genus with many similar species, many of which are also quite variable. The genus is defined by a saccular extension of the valves in males and sclerotized plates on the dorsal and ventral ductus bursae in females. The genus was revised by Lafontaine (1987) in the Moths of North America series and is divided into eight subgenera based on structural characters. These moths are among the most difficult to identify. Even though the forewing color and strength of various markings can vary significantly in some of the species, the shape of the lines and spots and the color of the hindwings (often different in the sexes) are more constant. The habitat and flight period are also important in helping to narrow the possibilities.

Habitat

This species is widely distributed in dry forest habitats of the Rocky Mountain region, particularly in quaking aspen forests.  In the Pacific Northwest, it is relatively rare and sporadic in open pine forests east of the Cascades.  However, it also occurs in desert regions at low elevations of the Great Basin region, but only in wet meadows or near wetlands such as marshes, creeks, and rivers.

Distribution

Pacific Northwest

Euxoa ridingsiana has been found in widely separate localities in south-central and southeastern British Columbia, northern and southern Washington, eastern Oregon, central and southern Idaho, and western Montana in our region. 

Global

This species has a fairly wide range in western North America. It occurs as far north as Alaska and Northwest Territories. The range extends to east-central California near the Pacific Ocean, to Utah and north-central New Mexico in the Rocky Mountain region, and to North Dakota and Montana in the Great Plains. This area is larger than that occupied by E. maimes (Lafontaine 1987).

Life History

Larvae

No information is presently available regarding larval foodplants of this species, but it is probably a soil-surface feeding cutworm that feeds on herbaceous vegetation based on related species.

Adults

This species flies during late summer, usually during July and August. It is nocturnal and comes to lights.

Economic Importance

None.

Literature

Lafontaine (1987)

Moth Photographers Group