French common name: Tordeuse occidentale de l'épinette
Scientific name: Choristoneura occidentalis Freeman
The western spruce budworm is found in coastal and interior British Columbia south and west of the Fraser River, and from the foothills of Alberta east to the Cypress Hills, on the Alberta–Saskatchewan border.
Current-year buds and shoots of conifer hosts.
Hosts in Canada
Hosts in Canada
Principal hosts in Canada
White spruce, Engelmann spruce, grand fir, subalpine fir
Occasional or potential hosts
Amabilis fir, western larch
Adult moths lay eggs in overlapping rows on the needles of host trees in mid- to late summer. Eggs hatch in a week. Newly hatched budworms do not feed but seek niches on the tree where they settle and construct a silken shelter in which to hibernate. Budworms emerge from hibernation the following April and May and forage for new buds by walking or ballooning on silk throughout the forest canopy. Spring emergence is usually in advance of bud-flush, so small budworm larvae feed on older needles or pollen cones, if available. Eventually the budworms bore into new buds. As the bud expands, the budworms use silk to tie the needles together and construct a feeding shelter. They continue to feed on these new shoots in June and July. Once feeding is complete, budworm larvae pupate within the feeding shelter. The adult moths emerge in one to two weeks after pupation. Female moths emit a sex pheromone that attracts the males to mate. Both male and female moths are capable of flying several kilometres to new locations.
In Canada, the western spruce budworm feeds mostly on Douglas-fir. Outbreaks are associated with mature, homogeneous stands of this species, particularly in the drier portions of the range. Contiguous forests with large trees may support very high densities of western spruce budworm.
Weather has a complex influence. The western spruce budworm feeds on growing, current-year foliage, so the synchrony between its life history events and seasonal development of foliage may be critical. Warm, dry autumns may be more stressful to hibernating budworms than severely cold winters.
Many natural enemies, including predators, parasites and diseases, attack budworms but their impact may be more important in keeping populations low than in ending outbreaks.
Attack and damage
The western spruce budworm feeds on the growing shoots of host trees. Its presence is made evident by distorted and discoloured shoots and needle fragments encased in silk. By late summer, damaged stands display a distinct reddish tinge at the periphery of the tree crowns. The tops of some trees may be entirely stripped of needles. Nonetheless, most trees survive several years of visible defoliation, with only a reduction in annual growth. Persistent, severe defoliation will eventually cause tree mortality, particularly in smaller, understorey trees.
Status in Canada
The most recent outbreaks of western spruce budworm have occurred in the interior Douglas-fir forests of British Columbia. In most years since 2000, more than 500,000 hectares of mapped defoliation has occurred. These recent outbreaks have persisted longer and extended further north and to higher elevations than earlier population outbreaks, according to historical records. Some regions where western spruce budworm outbreaks have been particularly severe have been aerially treated with the biological control agent Bacillus thuringiensis (a bacterium).
Dedes, J. 2009. Biology of the Western Spruce Budworm (Choristoneura occidentalis) (PDF, 4.8 MB). Natural Resources Canada.
Maclauchlan, L., and K. Buxton. 2012. 2012 Overview of forest health conditions in southern British Columbia. (PDF, 4.3 MB)Kamloops, BC: BC Ministry of Forests, Lands, and Natural Resource Operations.
Nealis, V.G., M.K. Noseworthy, R. Turnquist, and V.R. Waring. 2009. Balancing risks of disturbance from mountain pine beetle and western spruce budworm. Canadian Journal of Forest Research 39: 839–848.
Thomson, A.J., and R. Benton. 2007. A 90-year seas warming trend explains outbreak patterns of western spruce budworm on Vancouver Island. Forestry Chronicle 83: 867–869.