Western Spruce Budworm in Utah: Identification, Management, and Control

Dr. Michael Kuhns, USU Extension Forester

Introduction
The western spruce budworm (Choristoneura occidentalis) is a serious defoliator of Douglas-fir, spruces, and true firs in western North America. Its larvae feed on young developing buds, needles, shoots, and other plant parts. Damage varies but can be severe. Control can be accomplished with insecticides on selected trees but is difficult in forest stands.

Hosts
Preferred species are Douglas-fir and the true firs (white and subalpine or alpine firs in Utah). Spruces also may be attacked (Engelmann and blue spruces in Utah).

Damage
Heavy feeding causes partial or complete defoliation of new growth and deforms shoots. Cones and seeds also may be damaged or destroyed. Though a single defoliation does not cause death, multiple defoliations weaken trees and may kill tops or even entire trees. Trees weakened by spruce budworm may be predisposed to damage from other agents like bark beetles. Though spruce budworm damage usually is restricted to native trees, ornamental trees also can be affected.

Budworm outbreaks occur in cycles lasting 3 to 10 years in moist stands and up to 30 years in dry stands. Outbreaks may be more common and more severe in overmature stands and stands dominated by shade tolerant climax species, such as true firs.

Identification
Adult moths of the western spruce budworm are about one inch across, with brown to orange-brown forewings and light tan hind-wings (Figure 1). Mature larvae (caterpillars) are one inch long with dark heads, and an olive-brown to red-brown body with two white spots on each segment (Figure 2).

Life Cycle
The western spruce budworm's life cycle generally takes 12 months, producing one generation per year. At high elevation sites the life cycle can take two years to complete. Exact timing of the life cycle is greatly influenced by temperature.

The budworm overwinters as a small larva in a silky hibernaculum (cocoon-like shelter) on the bark. Larvae emerge in the spring and feed on buds, developing cones, and needles from previous years. They progress to feeding on new needles and twigs as growth begins. Larvae go through six stages of development, called instars, as they mature, each time shedding their outer skin or cuticle.

Larvae mature and pupate (see pupa in Figure 3) about 30 to 40 days after beginning feeding in the spring. Larvae pupate in feeding webs or on twigs or branches. Adult moths usually emerge in July or August, migrate (females can fly as far as 50 miles), mate, and lay eggs in clusters on needles. Eggs are laid on the undersides of needles in shingle-like groups. Eggs hatch in about 10 days and the new larvae immediately spin hibernaculae in bark crevices.

Management and Control
Complete control of western spruce budworm is not possible, desirable, or necessary. This insect has many natural enemies that help control populations, including parasitic wasps and flies and predators like birds and spiders. Adverse climatic conditions also reduce numbers. Insects that defoliate trees generally are much less serious than those that bore into stems or damage root systems. Foliage can be replaced fairly easily, while woody stems and roots cannot.

Outbreak severity can be reduced over the long term by forest management strategies that favor young, vigorous, even-aged stands (see Figure 4). Favoring non-host tree species and thinning to reduce stand density also can help reduce outbreak severity. The following table lists some of the factors that affect susceptibility of forests to spruce budworm outbreaks

Table 1. Stand and site factors that affect susceptibility of forest stands to spruce budworm outbreaks. Factors are listed from most to least susceptible.

The risk of a western spruce budworm outbreak should not be the only factor considered when managing your forest. For example, though young even-aged stands are less susceptible to a spruce budworm outbreak, they lack diversity important for some wildlife species. In such cases having a wide variety of stand ages, structures, and species may be more important than reducing the risk of spruce budworm attack.

Neighboring property conditions are very important when you are managing for a healthy forest. Managing for healthy conditions on your 160 acres will be less effective if you are surrounded by thousands of acres in poor condition.

In severe outbreaks or on high-value trees insecticides can be used to reduce spruce budworm populations and defoliation. Individual trees can be sprayed from the ground, or aerial application can be done over wide areas. Spraying is impractical in many cases and can kill non-target insect species.

Registered insecticides for control of western spruce budworm are Bacillus thuringiensis (B.t.; a biological insecticide for caterpillars; may cause less problems for non-target species), Sevin, Dursban, and Tempo 2. Insecticides are applied in spring (just as buds begin to open; timing depends on temperature and elevation) when larvae are feeding on old needles and before they enter the buds. Applications in June and July control larvae on new growth. Always read the insecticide label for directions on pests controlled, application, and disposal.

More Information
The Utah Division of Forestry, Fire & State Lands (801-538-5555) provides assistance in developing a management plan for your forested land. General forest management information is available from your local USU extension agent or e-mail Darren McAvoy at Darren.McAvoy@usu.edu.

Credits: This article was written by Dr. Michael Kuhns, USU Extension Forester, under a cooperative agreement with the Utah Division of Forestry, Fire & State Lands. It is current as of June 1998. Thanks to Ms. Colleen Keyes and Dr. Jesse Logan for their reviews and to Dr. Diane Alston for her assistance. Some illustrations were obtained from the USDA-Forest Service publication Western Spruce Budworm, Technical Bulletin No. 1694, 1987.

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Larva feeding on young Douglas-fir cone.