Western Spruce Budworm

The western spruce budworm (Choristoneura occidentalis) is an important native defoliator of interior Douglas-fir. Population levels periodically reach outbreak proportions.

Outbreaks have been recorded in British Columbia's interior forests from the early 1900s up to the present. In 1987, the budworm's peak outbreak, more than 800,000 ha were under attack, mostly in the southern interior. Outbreaks can last more than 25 years.

Biology & History

The western spruce budworm requires one year to complete its life cycle. Certain factors contribute to a stand's susceptibility to outbreaks.


The insect's greatest impact is among suppressed and intermediate trees. Repeated budworm defoliation causes tree mortality over large areas, reduction of growth rates and reduced lumber quality. Sustained attack results in complete defoliation in four to five years.

Once an infestation has subsided, defoliated trees take several years to regain full foliage. Radial growth therefore requires several years to return to normal rates. Successive years of defoliation in stands may predispose trees to other insects and pathogens. Tree mortality may continue due to root disease, bark beetles, loss of vigour and other causes.


Long-term strategies consist of stand manipulation, such as thinning, harvesting and alternative species. Short-term strategies include spray programs.