Western spruce budworm
Choristoneura occidentalis Freeman (Lepidoptera: Tortricidae)
Orientation to pest
Western spruce budworm, Choristoneura occidentalis Freeman, is a tortricid moth native to western North American that feeds on various conifers, including species of fir (Abies), larch (Larix), spruce (Picea), and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco). The life history of this moth is similar to that of the spruce budworm (Choristoneura fumiferana [Clemens]). Adults fly and lay eggs in summer (July and August) on the host plant. Young larvae do not feed but rather immediately spin a hibernacula under bark scales, where they overwinter. The next spring, larvae become active and feed by mining old needles until the buds swell. Larvae then bore into the buds and feed on the expanding needles. Later in the spring, they loosely web together growing tips and feed upon the new needles, where they later pupate. There is one generation per year. This is considered one of the most damaging pests in western North American forests, and large, sustained outbreaks sometimes occur. Damaged trees may be killed entirely or just the tops may die, destroying the commercial value of the trees.
Hosts commonly attacked
This species feeds on various confers including Abies concolor (Gordon) Lindley ex Hildebrand, Abies grandis (Douglas ex D. Don) Lindley, Abies lasiocarpa (Hooker) Nuttall, Larix occidentalis Nutt., Picea engelmannii Parry ex Engelm., Picea glauca (Moench) Voss, Picea pungens Engelm., Douglas-fir, and juniper (Juniperus).
Distribution
This moth is found in several western U.S. states, including Colorado, Idaho, Montana, Wyoming, Colorado, Arizona, Utah, New Mexico, Oregon and Washington, as well of British Columbia and southwest Alberta, Canada.
 USDA Forest Service • Forest Insect & Disease Leaflet 53 |
| Figure 1. Distribution in western North America of the western spruce budworm |
Images of western spruce budworm
| Figure 2. Adult of the western spruce budworm, Choristoneura occidentalis | Figure 3. Silvery egg mass of the western spruce budworm on needle | Figure 4. Very young larva of western spruce budworm feeding in bud (left) and on new foliage (right) | |
| Figure 5. Webbed foliage, the feeding site of young larvae of the western spruce budworm | Figure 6. Mature larva of western spruce budworm | Figure 7. Pupae of the western spruce budworm | |
| Figure 8. Damage caused by western spruce budworm to a grand fir | Figure 9. Stand of grand fir in Oregon defoliated by western spruce budworm | Figure 10. Stand with dead trees 10 years after a western spruce budworm outbreak in Oregon | Figure 11. Eggs of western spruce budworm parasitized by the egg parasitoid Trichogramma minutum Riley |
Important biological control agents related to this pest species
As with other Choristoneura species, the western spruce budworm is affected by a large complex of natural enemies, especially parasitoids, which, along with adverse weather, partially limit the pest's density. However, population outbreaks occur when climatic and forest stand conditions are favorable. There are more than 40 species of insect parasitoids of the western spruce budworm, of which four or five species are most common. See Bellows et al. (1998) for species names.
Web links for information on western spruce budworm
- Forest Insect & Disease Leaflet 53 | USDA Forest Service
- Fact Sheet | Colorado State University Extension
- Forest Health Note | Washington State University Cooperative Extension
Provides information for the non-industrial private forest landowner - Forest Health Note | Oregon Department of Forestry FHP
Articles
- Schmidt, W. C. and D. C. Fellin. 1973. Western spruce budworm damage affects form and height growth of western larch. Canadian Journal of Forest Research 3(1): 17-26.
- Jennings, D. T., F. B. Knight, S. C. Hacker, and M. E. McKnight. 1979. Spruce budworms bibliography. Misc. Rep. 213. University of Maine, School of Forest Resources, Life Science and Agricultural Experiment Station. Orono, Maine, 687 pp.
- Bellows, T. S., C. Meisenbacher, and R. C. Reardon. 1998. Biological Control of Arthropod Forest Pests of the Western United States: A review and recommendations. FHTET-96-21. USDA Forest Service, Morgantown, West Virginia, USA
