Western spruce budworm

Choristoneura occidentalis

The western spruce budworm is an important native defoliator of interior Douglas-fir. Population levels periodically reach outbreak proportions.

On this page

• Description
• Host tree species
• Life cycle
• Damage symptoms
• Identification images

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Description

The western spruce budworm requires one year to complete its life cycle. Certain factors contribute to a stand's susceptibility to outbreaks.

• Western Spruce Budworm Biology (PDF, 402KB)

Host tree species

The primary host of this budworm is interior Douglas-fir. Other tree species such as the true firs (for example subalpine fir), western larch and, to a lesser degree, spruce can also be affected.

Life cycle

Budworm moths mate and lay eggs in late July through August. The eggs are laid in masses (up to 150) on the underside of needles, overlapping like fish scales. They are bright green when laid, measuring slightly less than 1cm in total length, and become translucent when empty. They hatch within 12 days.

The larva pupates from late June to mid July. The reddish brown pupa is 12 to 16mm long, being broad at the head end, tapering to a point at the rear. The pupal stage lasts on average two weeks.

Damage symptoms

The larval stage defoliates host trees and causes visible damage and tree mortality. When trees are stressed they are more likely to die.

Repeated budworm defoliation causes tree mortality over large areas, reduction of growth rates and reduced lumber quality. Sustained attack results in complete defoliation in four to five years.

Once an infestation has subsided, surviving trees take several years to regain full foliage and resume growth. Successive years of defoliation in stands may predispose trees to other insects and pathogens such as bark beetles and root disease.

Identification images