Pacific Northwest Moths

Identification

Adults

Euxoa rockburnei is a medium-sized Euxoa (FW length 16–19 mm) that flies in a variety of habitats east of the Cascade summit during late summer and early fall. It is the largest of its species-group in the Northwest and is placed in the subgenus Euxoa. It has a smoky brown hindwing and a uniform, dark brown forewing with pale-lined spots and black cells. The forewing is very dark brown with a purple tint. The costa and terminal area are slightly darker gray and in some specimens there is a slight red flush in the subterminal area and at the apex. The cell is black. The basal and antemedial lines are black, double with filling of the ground color or a lighter shade. The basal line is irregular, drawn to the base at the cell. The antemedial line is weakly zigzagged, nearly perpendicular to the posterior margin. The median line is dark gray, very faint. The postmedial line is single or double, weakly scalloped, drawn toward the base on the costa, weakly excurved near the reniform spot, and curved to meet the posterior margin at a right angle below the reniform spot. The subterminal line is very weak, comprised of powdery pale yellow spots, slightly uneven, preceded by a blackish shade. The terminal line is black, thin or a series of small spots. The fringe is concolorous with the terminal area, sometimes slightly redder. The orbicular and reniform spots are partially outlined by black, lined by powdery light yellow, and filled with the ground color or dark red-brown. The orbicular spot is round to elliptical, often open toward the costa or both anteriorly and posteriorly. The reniform spot is similar, also often open at one or both ends. The claviform spot is black, moderate-sized. The hindwing is gray, darker toward the margin, with darker discal spot, veins, and terminal line, The hindwing fringe is white with an ochre and gray base. The head and thorax are the same color as the forewing, with a thin black transverse line across the mid-collar. A frontal tubercle is present. The male antenna is strongly biserrate.  Euxoa campestris is a member of the declarata species-group of the subgenus Euxoa.

This species is very close to Euxoa declarata and Euxoa campestris in appearance. The forewing is similar to that of E. campestris which overlaps its range in the far eastern portion of the Northwest and both species have a dark hindwing, but the wing of E. rockburnei tends to be slightly lighter and less even in color. In most areas the hindwing of E. rockburnei is darker than that of E. declarata. These three species can be distinguished by characters of the male and female genitalia. The sacculus of the male valve is more massive than that of E. declarata, and the saccular extension is shorter than that of E. campestris. In females, the sclerotized plate in the ventral ductus bursae of E. rockburnei has rough margins and extends asymmetrically toward the left, whereas those of the other two species are symmetrical with smooth margins. The genitalia are illustrated in Lafontaine (1987).

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 coniferous forests throughout much of western North America.  In the Pacific Northwest, it appears to be most common in dry, open ponderosa pine forests east of the Cascades.

Distribution

Pacific Northwest

Euxoa rockburnei is found throughout most of the Pacific Northwest east of the Coast and Cascade Ranges as far north as the southern part of British Columbia. It is replaced eastward toward the Rocky Mountains by Euxoa campestris.

Global

The range of this species extends south to northern New Mexico, southern Utah, and east-central California (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

Adults of E. rockburnei are on the wing during late summer and early fall. They have been collected during the months of August and September in our region.

Economic Importance

None.

Literature

BugGuide

Lafontaine (1987)

Moth Photographers Group