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Western spruce budworm

The western spruce budworm (Choristoneura occidentalis) is a moth native to North America and is found in western Canada.

It mainly feeds on Douglas-fir trees in British Columbia, but also attacks white spruce, Engelmann spruce, grand fir and subalpine fir trees.

The western spruce budworm has many natural predators, parasites and diseases that help keep the population low. Climate might also have a significant role in controlling population numbers.

Quick facts

  • The western spruce budworm occurs in coastal and interior British Columbia south of Quesnel, in the foothills of Alberta and in the Cypress Hills on the Alberta-Saskatchewan border
  • The western spruce budworm feeds on the growing shoots of trees
  • Early in the growing season, shoots and needles are encased in silk and turn a distinct reddish colour by late summer. The tops of some trees may appear needleless
  • Most trees survive years of defoliation though grow slower than non-infested trees and often develop defects reducing their value as timber
  • After 4 to 5 years of persistent and severe defoliation trees usually die, especially understory trees
Larvae of the western spruce budworm.

A western spruce budworm caterpillar

Larger image


Aerial view of a forest affected by western spruce budworm.

Defoliation caused by the western spruce budworm near Marshall Creek, British Columbia

The western spruce budworm significantly affects Canada’s forests:

  • Since 2000, the western spruce budworm defoliates, on average, more than 500,000 hectares of mapped forest annually
  • An outbreak may last several years, which can cause high levels of tree mortality, growth loss and timber defects in mature softwood forests
  • Significant losses of important timber and non-timber resources can occur, negatively affecting the forest industry and forestry-dependent communities

CFS scientific research

Scientists working at the Canadian Forest Service (CFS) have studied the lifecycle of the western spruce budworm and have identified the following:

  • In mid- to late-summer, adult moths lay eggs in overlapping rows on the needles of trees
  • Eggs hatch in a week and the newly-hatched caterpillars (budworms) find niches on the tree where they build a silk shelter and hibernate during the fall and winter. At this time the budworms do not feed on the tree
  • Budworms emerge the following April to May and forage for new buds by walking or ballooning on silk throughout the forest canopy
  • As the buds expand, the budworms use silk to tie the needles together and construct a feeding shelter. They continue to feed on new shoots in June and July
  • Once feeding is complete, budworm larvae pupate within the feeding shelter. Adult moths emerge one- to- two weeks later
  • Both male and female moths are capable of flying several kilometres to new locations

Working collaboratively to develop solutions

Through the Canadian Forest Service, Natural Resources Canada collaborates with the University of Victoria and the Chinese Academy of Sciences, about ongoing research that includes tree-ring studies aimed at describing the long-term history of western spruce budworm outbreaks and their natural range of variation.

This research will help determine scientists determine if the recent trend toward longer and more extensive outbreaks is related to climate change.

Want more information on the western spruce budworm?

Contact Brian Van Hezewijk of the Pacific Forestry Centre.

Find out more
Canadian Forest Service publications
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