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Western Spruce Budworm and Its Management

Western spruce budworm is an insect pest which defoliates Douglas-fir, Engelmann spruce, subalpine fir and western larch. Tree damage associated with budworm defoliation includes growth loss, top-kill, deformity, reduced seed production and tree mortality. Larger diameter trees that survive major budworm outbreaks in a weakened condition may later be killed by bark beetles. It usually tales three-five years of tree defoliation to cause top kill, reduce growth and tree mortality. Seedling and sapling-sized trees are most likely to suffer severe damage.

What to look for:

When tree shoot growth begins in May-June budworm larvae (small caterpillar) web together adjacent shoots. During July reddish brown branch tips will give trees a scorched appearance. The upper portion of the tree crown may appear bare or thin. Budworm larvae feed in buds and foliage from May-July. Older larvae have dark heads and an olive-brown body with whitish spots. Pupal cases are attached to damaged shoots. Orange to gray-brown moths less than an inch across are abundant in late July and August during an outbreak.


Natural controls include ants and birds which eat budworm larvae. Leaving woody debris on the ground for ants and snags as nesting sites for birds can help sustain populations of these predators. Other natural controls are believed to include cold, wet spring weather; viral pathogens, and lack of food following subsequent years of infestation.

Silviculture practices include maintaining or increasing tree diversity in vulnerable stands (increase the relative abundance of non-host tree species), thinning from below to create single story forest stand structures, and thinning to reduce inter-competition between trees and to increase the vigor of retention trees.

Insecticides are most effective when applied as larvae are actively feeding on new foliage in June. For large outbreaks aerial applications of Bacillus thuringensis a bacterium and the chemical insecticide carbaryl (Trade name –Sevin) are generally recommended. Ground based applications of carbaryl and Bacillus thuringensis (Bt) can be effective on smaller trees.

Bt must be eaten by the larvae to cause mortality. A protein crystal formed by the bacterium carries a toxin which is released in the gut of the larvae. When Bt is ingested the toxin is released and the midgut wall is destroyed and the larvae stops feeding. The bacteria enters the blood of the insect causing full scale infection and death of the insect within 3-5 days.

Aerial application of Bt or Sevin:

Aerial application can be accomplished with fixed wing aircraft or helicopters. Fixed wing aircraft are generally less expensive if a landing strip can be located in close proximity to the project area. The application must be timed with the emergence new foliage and presence of feeding larvae to be effective. Generally results are decreased level of defoliation but not complete elimination. Reapplication of chemical and biological treatments will likely be required during the duration of the spruce budworm outbreak. Historically, outbreaks in Montana have duration of 2-5 years. Cost of aerial treatment can vary significantly but generally range from $55-85 per acre.

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