Western spruce budworm

Written by V.G. Nealis Revision 2024

French common name: Tordeuse occidentale de l'épinette
Scientific name: Choristoneura occidentalis occidentalis Freeman
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Tortricidae
Partial list of synonyms:
- Choristoneura freemani Razowski
- Choristoneura fumiferana (Clemens)

General information and importance

Western spruce budworm is a native defoliator of coniferous trees in western North America. In Canada, periodic outbreaks occur in coastal and interior forests, including the foothill forests on the eastern slopes of the Rocky Mountains. Historically, outbreaks in Canada were associated with more extensive outbreaks in the Pacific Northwest of the United States, occurring at 30- to 40-year intervals. The duration of outbreaks was shorter in the Canadian portion of the range. However, there have been at least three extensive outbreaks in British Columbia since 1970, the latest beginning in the late 1990s and lasting until 2016. For the first time in survey history, a greater area of forest was defoliated in British Columbia than in the United States. This defoliation included forests farther north and at higher elevations than previously recorded.

Western spruce budworm outbreaks are associated with mature forest stands dominated by primary host trees, such as Douglas-fir (Pseudotsuga menziesii). The intensity of defoliation is greatest in dense stands with closed canopies. Outbreaks occur more frequently in the hottest and driest Douglas-fir forests of the interior.

Distribution and hosts

Western spruce budworm is distributed from New Mexico in the United States to central British Columbia, Canada, westward from the foothill forests on the eastern side of the Rocky Mountains to the Pacific Coast. It typically occurs at higher elevations (>1500 metres) in the southern portion of its range and mid to lower elevations (< 1200 metres) in its Canadian range. Principal hosts are Douglas-fir, true firs (Abies), and to a lesser extent, Engelmann spruce (Picea engelmannii). In Canada, Douglas-fir is essentially the exclusive host, as fir and spruce at these latitudes are at higher elevations than areas historically occupied by western spruce budworm.

Tree parts affected

Current-year buds and needles, immature pollen cones.

Symptoms and signs

Green eggs are laid in overlapping rows on needles. Just before hatch, the dark head of the developing larva can be seen in each fertile egg. Early-stage larvae have reddish to dark brown heads and pale brown bodies, with no distinctive markings. Final-stage larvae are 18 to 25 millimetres in length, with reddish brown heads and bodies distinctly marked with two rows of cream-coloured spots on each dorsal segment. The wingspan of adult moths is 24 to 28 millimetres. The primary coloration of wings is reddish brown, with white and dark brown markings.

Small, early emerging western spruce budworm larvae damage many buds. This is visible only on close inspection. Later, large larvae construct protective feeding shelters by tying needles together with silk. The combination of webbing and damaged needles is noticeable on new shoots in the upper crown. As damaged needles dry in mid-summer, the crown of the tree turns red. This dramatic change in appearance can occur quickly in dry, interior forests. If defoliation persists for several years, the tree crown becomes thinned, and branches may die. Small trees in the understory can also be severely damaged.

Life cycle

Western spruce budworm has one generation per year. Eggs are laid on needles in masses of 20 to 50 overlapping eggs in late July to mid-August, depending on location and year. Eggs hatch in about 1 week. Newly hatched budworms do not feed. They move inwards from the foliage to the interior of the tree, and settle in protected niches, such as bark crevices. They enclose themselves with silk and spend the winter dormant.

Small budworm larvae emerge from hibernation the following May and return to the branch tips in search of fresh buds. Emergence is typically 2 to 3 weeks in advance of bud flush. Small budworms sustain themselves during this period by mining old needles and fresh pollen cones, if available. As current-year buds swell, budworms bore into the bud. Once buds flush, budworm larvae construct feeding shelters by tying needles together with silk. They feed until mid to late July and often pupate in these feeding shelters. Adult moths emerge after about 1 week. The female moth emits a pheromone that attracts males to mate. Moths become active at dusk, often flying above the tree canopy, and may be transported many kilometres on air currents.

The depletion of foliage and changing forest condition during an outbreak reduces the survival of western spruce budworm. Increased impacts by natural enemies, such as predators, diseases, and parasitoids, eventually combine to reduce survival further. Once western spruce budworm populations become low, they may be held in an innocuous state by natural enemies.

Damage

During outbreaks, hundreds of thousands of hectares of forest can be damaged, although complete defoliation of current-year needles in mature trees is an exception. Previous years’ needles of Douglas-fir are seldom damaged and co-dominant trees usually recover. Mortality occurs after several years of defoliation and is only common in the primary Canadian host, Douglas-fir. True firs are more abundant and vulnerable hosts in the United States and account for most of the reported mortality there. Understory trees are also more vulnerable, as they have less foliage and are subject to budworms falling on them from mature trees overhead. Defoliation reduces radial growth of trees irrespective of the species or size. Trees compensate for needle loss by producing more buds on shorter shoots for a few years following defoliation.

Damage to pollen cones may be significant in managed seed orchards. Christmas tree plantations could also be at risk if they are located near outbreaks in larger natural stands.

Secondary effects of defoliation, including attacks by bark beetles, pathogens, or increased fire risk have been suggested, but evidence is anecdotal. Certainly, if mature trees are killed, they will be utilized by many other organisms as they decay. The dry, interior forests where recent western spruce budworm outbreaks prevail are naturally fire prone, so an increase in the risk of fire following defoliation is incremental.

Ecological effects of outbreaks include the selective removal of understory trees, which favours the growth of mature trees. The reduction of smaller, understory trees may reduce fire risk, as these understory trees otherwise serve as ‘laddering’ fuels that enable destructive crown fires. The potential result of significant mortality of mature trees may be the conversion of the stand to a new younger forest consisting of seedlings of the same tree species, or a forest with a different stand composition.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
the value of the host(s) versus the costs of pest management approaches; and
consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

All forests dominated by host trees within the range of western spruce budworm are susceptible to damaging infestations. Vulnerability, however, is a function of severity and duration of the outbreak and the tree species involved. For example, the primary host tree in Canada, Douglas-fir, is much less vulnerable than grand fir (A. grandis), and usually recovers from defoliation.

Forest pest managers monitor population levels by trapping moths with pheromones and sampling branches for egg masses. The main concern of outbreaks is tree mortality, so intervention with insecticides is usually not initiated until after a few years of severe defoliation. Several insecticides have been employed against western spruce budworm. Specific, naturally occurring viruses have been investigated. Much of the annual defoliation is caused by small larvae feeding inside buds where they are relatively protected from pesticides. Both insecticides and pheromones are defined as pest control products and are regulated in Canada. Products registered for use against western spruce budworm under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered products and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Significant levels of mortality only occur after several years of severe defoliation, so accelerating harvest in mature stands is a feasible management option. Salvage harvesting can recover some commercial value where mortality has already occurred.

Silvicultural methods used to reduce damage are limited. Small, understory hosts are more vulnerable than mature trees. This makes multi-story silvicultural practices, where co-dominant trees shelter advanced regeneration, impractical. Reducing the density of host trees and favouring mixed stands may be more effective in reducing impacts. In the Pacific Northwest of the United States, fire suppression favours both regeneration of more vulnerable true firs and advance regeneration. This may contribute to increased stand vulnerability.

Photos

Mature larva of western spruce budworm on a Douglas-fir shoot.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia

Mature larva of western spruce budworm on a Douglas-fir shoot.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia

Close-up of defoliation caused by western spruce budworm larvae.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia

Selected references

Axelson, J.N.; Smith, D.J.; Daniels, L.D.; Alfaro, R.I. 2015. Multicentury reconstruction of western spruce budworm outbreaks in central British Columbia, Canada. Forest Ecology and Management 335: 235–248. https://doi.org/10.1016/j.foreco.2014.10.002

Harris, J.W.E.; Alfaro, R.I.; Dawson, A.F.; Brown, R.G. 1985. The western spruce budworm in British Columbia 1909-1983. Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. Information Report BC-X-257. 32 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/06da2bf6-6f68-4b0d-903a-750832d607f6?fromSearchPage=true

Maclauchlan, L.E.; Daniels, L.D.; Hodge, J.C.; Brooks, J.E. 2018. Characterization of western spruce budworm outbreak regions in the British Columbia Interior. Canadian Journal of Forest Research 48(7): 783–802. https://doi.org/10.1139/cjfr-2017-0278

Nealis, V.G. 2016. Comparative ecology of conifer-feeding spruce budworms. The Canadian Entomologist 148(S1): S33–S57. https://doi.org/10.4039/tce.2015.15

Nealis, V.G.; Régnière, J. 2021. Ecology of outbreak populations of the western spruce budworm. Ecosphere 12(7): e03667. https://doi.org/10.1002/ecs2.3667

Parfett, N.; Clarke, D.; Van Sickle, A. 1994. Using a geographical information system for the input and analysis of historical western spruce budworm in British Columbia. Co-published by Canadian Forest Service, Pacific Forestry Centre, Victoria, British Columbia, and the British Columbia Ministry of Forests. FRDA Report 219. 32 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/f5ad95dd-02b1-48c4-8b36-71cfcac8a9b1?fromSearchPage=true

Shepherd, R.F.; Cunningham, J.C.; Otvos, I.S. 1995. Western spruce budworm, Choristoneura occidentalis. (Chapter 11) Pages 119–121 in J.A. Armstrong and W.G.H. Ives, editors. Forest insect pests in Canada. Natural Resources Canada, Canadian Forest Service, Headquarters, Science and Sustainable Development Directorate. Ottawa, Ontario. 732 p. https://publications.gc.ca/site/eng/9.889231/publication.html

Cite this fact sheet

Nealis, V.G. 2024. Western spruce budworm. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.