Environment

Urban Forestry

Biological Cycle of the Western Spruce Budworm

The life cycle of the western spruce budworm is completed in one year. The adult is active in mid-July through mid-August. It is rather an innocuous looking moth, mottled grey to rusty brown, with a wingspan of about 3 cm.

After mating, the female moth deposits her eggs in masses on the underside of needles in a shingle-like pattern. The egg masses are bright green when laid and turn translucent white when empty. The eggs hatch after about 10 days, and the very tiny larvae (caterpillars) disperse to find shelter on the underside of bark scales or in lichens on the tree. They do not feed at this time, but instead spin little silken cocoons in the fall to overwinter in.

Early May to June the larvae emerge and disperse by spinning out of the foliage on silken webs. If the budworm is in synchrony with the trees, the larvae emerge as buds are beginning to soften and swell and they can mine into the buds.

Figure 1 - Mature western spruce budworm larva

New buds are very nutritious and offer protection from predators and unseasonable temperatures. If larvae emerge when new buds are too tight to penetrate, they are forced to mine into old needles. Old foliage has lower nutritional value and mined needles are often shed by the tree (with larvae still inside them), causing significant budworm mortality.

As the shoots elongate, budworm larvae feed on the new foliage of trees of all sizes, from May through mid-July. They prefer to feed in buds, cones and on new foliage, but larvae will also "back feed" on old foliage when the latter resources are diminished, particularly when insect populations are very high.

Figure 2 - Western spruce budworm pupal case

The larvae pass through six instars (stages), developing from barely visible to about 3 cm in length. As they grow, primarily beige coloured larvae become brown to olive brown with four conspicuous, paired ivory-coloured spots appearing on each body segment.

The larvae then pupate within webbed foliage from late June to mid-July. Pupae are dark reddish-brown and about 1.5 cm long (Figure 2). Moths emerge from the pupal stage in approximately ten days to begin the cycle again.

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Urban Forest
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Kamloops, BC V2C 6V3
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