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Western, Eastern and Black-headed budworms

The pest

Budworms are the larvae, or caterpillars, of certain moth species. They are so-called because of their habit of eating the buds of flowers.

These four budworms (there are two species of black-headed budworm) are serious and destructive pests of conifer trees, especially spruces (Picea spp.), firs (Abies spp.), larches (Larix spp.) and Douglas fir (Pseudotsuga menziesii).

Their scientific names are:

  • Choristoneura freemani (formerly C. occidentalis - Western spruce budworm);
  • C. fumiferana (Eastern spruce budworm); and
  • Acleris gloverana and A. variana (Black-headed budworms).

The threat

Budworms are present in eastern, western and northern states of the USA, and parts of Canada. They cause high mortality to fir, spruce, Douglas fir, larch and a number of other conifer species.

Suitable host tree species are present in the UK, and some are important forestry species here. The UK climate is also suitable for them. It is therefore considered that they would pose a threat to conifer trees and forests if they entered the UK.


Defoliation (needle loss) is the main symptom of budworm attack.

Western spruce budworm is one of the most serious forest defoliators in western North America. Heavy and repeated attacks can lead to top death of the trees and whole-tree deaths.

Feeding starts on buds and new foliage and, if populations are relatively small, is confined to those parts of the tree. If populations are high, all new foliage can be destroyed. Feeding then takes place on older foliage, which can lead to complete defoliation and death of the tree.

Landscape-scale damage of grand fir in Oregon caused by Western spruce budworm (C. freemani). Source: USDA.

Landscape-scale damage of grand fir in Oregon caused by Western spruce budworm. Source: USDA.

Eastern spruce budworm is one of the most widely distributed forest insects in North America, and is a highly destructive defoliator of fir and spruce forests in eastern USA and Canada. Severe outbreaks have been recorded since the mid-1930s in Ontario, Quebec and the Atlantic region, and many trees have been killed.

Defoliation and damage on spruce caused by Western spruce budworm

Defoliation and damage on spruce caused by Western spruce budworm (C. fumiferana). Source: J. O’Brien USDA;

Black-headed budworm: depending on population density, defoliation by black-headed budworms can be extensive, with a tendency for the effects to be greatest near the tops of trees and on the canopy edges. This reflects the adults’ preference for attacking dominant and co-dominant trees within a stand. Partial feeding, resulting in needle breakage, but not complete consumption, results in reddening of the foliage.This starts in June and continues through July, depending on latitude. Top death and, at very high attack densities, whole-tree death, can occur.

Foliage discolouration after attack by black-headed budworm (A. gloverana). USDA Forest Service;

Foliage discolouration after attack by black-headed budworm (A. gloverana). USDA Forest Service;

Susceptible species

Western spruce budworm occurs principally on Douglas fir and other forest trees, such as fir species. Several significant host plants of western spruce budworm are widely grown in European forests and plantations, including British forests.

Eastern spruce budworm occurs mainly on spruces and firs, but can also be found on Douglas fir, pines and, occasionally, on hemlocks and larches. It particularly attacks balsam fir, white spruce and red spruce in eastern North America, and sub-alpine fir, Engelmann spruce, white spruce and Douglas fir in western North America.
Black-headed budworms infest hemlocks, firs (mainly balsam fir), spruces (especially Sitka spruce), and Douglas fir.


  • USA:
    • Western spruce budworm: Western North America
    • Eastern spruce budworm: Northern America
    • Black-headed budworm: Western USA northwards towards coastal British Columbia.
  • Asia: No record exists for this area.
  • Europe: No confirmed record exists for this area. None of the pests is known to be present in the UK.

Control measures

Identified pathways for international movement of all four species include plants for planting and cut branches (including foliage) of host plant species. These could hold all life stages other than adults (i.e. over-wintering eggs, young feeding larvae within buds or expanding needles, older feeding larvae, and pupae within silk-covered groups of needles).

Current EU regulations prohibit imports of planting material and cut branches (including foliage) for all identified tree hosts of the four budworm species and, if applied rigorously, these controls should prevent the introduction of the pest from North America to the UK and the rest of Europe.


Our contingency plan outlines the actions which would be taken in the event of an outbreak of any of these budworms being found in the UK.

Report a sighting

Although none of these budworms is believed to be present in the UK, there is a risk of their being accidentally introduced. We therefore urge the public, and especially tree and plant professionals and importers of wood and plant materials, to remain vigilant for signs of them, and to report suspicious symptoms to us.

Tree Alert iconPlease report suspected cases to us with our Tree Alert on-line pest and disease reporting form.

Last updated: 13th January 2018