Western spruce budworm, Choristoneura occidentalis

Fig. 1   Western spruce budworm	Fig. 2  Western spruce budworm damage to a Douglas-fir stand.

Distribution:  The western spruce budworm is found in the coast, montane and Columbia forest types of southern B.C. at 350 m to 1460 m elevation.

Tree Species Attacked: The host is primarily Douglas-fir, though spruce, western larch, and lodgepole pine are attacked occasionally. All ages of trees are susceptible.

Insect Description & Damage Symptoms: Young larvae are very small and light yellow-green. As they grow, larvae become brown with a dark brown head capsule, and four conspicuous, paired, ivory-coloured spots appear on each body segment. In the last instar, some larvae turn olive-brown. Full-grown western spruce budworms are 1.6 to 3.2 cm long. Larvae pass through six instars before they pupate. The larvae pupate within webbed foliage from late June to mid-July. Pupae are dark reddish-brown and 1.2 to 1.6 cm long. The adult moth is mottled grey to rusty brown, with a wingspan of 2.4 to 3 cm long. The female moths lay eggs in masses on the underside of needles in a shingle-like pattern in August. The egg masses are bright green when laid, and translucent white when empty. The life cycle is completed in one year. 

Initially, larvae mine the needles, buds, and new cones, and feed on current foliage after bud flush. Older larvae prefer current foliage but will feed on older foliage if current foliage is depleted. Feeding takes place from late April through June. The crowns of damaged trees appear reddish-brown from June to September. The initial symptoms of defoliation may be seen in tree tops and branch tips, where chewed needles accumulate in webbing.

Damage: Tree mortality can occur after several successive years of severe defoliation, particularly on immature or suppressed trees. Other damage includes top-kill (resulting in stem defects), reduced seed production due to damaged cones, and height and volume loss.

Similar Damage: Damage can be confused with other defoliators, such as early feeding by the tussock moth, or several species of cone worms. The prominent ivory-coloured spots on the western spruce budworm larvae are very distinctive. The different host species and provincial distribution distinguish the various species of budworm from each other.

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Contact Tim Ebata if you have comments on the presentation of this information.

BC Ministry of Forests
Forest Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136

Last updated March 04, 2002