Pacific Northwest Moths

Identification

Adults

Pachysphinx modesta is a very large heavy-bodied moth (FW length 50–57 mm) that flies in the northern and eastern parts of the Northwest during the summer. Its forewings are dull olive gray and its hindwings are black with blue marks at the anal angle. The forewings of this giant have strongly and evenly scalloped outer margins and a strongly concave distal trailing margin. The base medial to the median line is paler than the rest of the forewing, and the median area is darkest. The scalloped postmedial line is similar in color to the rest of the forewing but slightly darker. The other lines are faint. The discal spot is small, off-white without a darker outline. The hindwing is gray-brown to red-brown with a dark wine red base. The anal angle and inner margin are lighter tan, with a prominent black and blue area at the anal angle. The anterior of two black marks is triangular, apex anterior, and is larger and darker than the more linear posterior black mark. The head and thorax are olive-gray. The antenna is brown, club-like.

This species can be identified easily by its northerly distribution, very large size, olive-gray forewing with scalloped outer margin, and claret hindwing with black and blue at the anal angle. However, it is similar to Pachysphinx occidentalis, on average a slightly larger and paler species than P. modesta that flies south of northern Washington in our area. These species can be identified by distribution except in the zone of overlap. The most reliable differentiating feature is the shape of the hindwing black marks. They are parallel and nearly equal in strength in P. occidentalis, not asymmetric with a larger anterior triangle as in P. modesta. The larvae of the two species have different skin and caudal horn lengths in the last instars (Hodges 1971; Tuttle 2007).

Larvae

The larva is granulose blue-green with white lateral stripes, and lacks a caudal horn in the last instar.  It is illustrated by Powell & Opler (2009).

Habitat

This species is mostly limited to moist forests of mixed hardwoods and conifers at middle elevations in the Rocky Mountain region where quaking aspen and cottonwoods are common.  It occurs in riparian areas in steppe in the northern Columbia Basin.

Distribution

Pacific Northwest

Pachysphinx modesta is found in forests with aspen in southern British Columbia and northern Washington and Idaho, and near the Continental Divide in the Rocky Mountains.  Its range extends west to the Methow Valley in Okanogan County, Washington and the northern limit of the Columbia Basin near the Columbia River.

Global

This species is widespread in North America. It is found throughout most of the East, except on the Atlantic Coast south of Maryland and the southern parts of Louisiana and Texas. Its range extends west to western Utah, eastern Oregon, and southeastern British Columbia where it overlaps that of P. occidentalis in the Rocky Mountain region. The northern limits of its distribution are reached in southern Labrador, James Bay of Hudson Bay, and northern Alberta.

Life History

Larvae

This species is a foodplant specialist feeding on Salicaceae including both willows (Salix spp.) and cottonwoods (Populus spp.), but appears to particularly favor cottonwoods and quaking aspen (P. tremuloides).

Adults

Adults are single-brooded and can be fairly common from late May until late July in our region.  The are nocturnal and come to lights.  They do not feed as adults.

Economic Importance

None.

Literature

BugGuide

Covell (1984)

Hodges (1971)

Moth Photographers Group

Powell & Opler (2009)

Tuttle (2007)