Spruce budworm

Written by V.G. Nealis Revision 2024

French common name: Tordeuse des bourgeons de l'épinette
Other common names: Eastern spruce budworm
Scientific name: Choristoneura fumiferana (Clemens)
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Tortricidae

General information and importance

Spruce budworm is the most widespread and damaging native defoliator of coniferous trees in North America. In 1975, defoliation was recorded across more than 50 million hectares of forests in Canada. Fossil evidence shows outbreaks dating back several thousand years as boreal forests moved northward with the retreat of continental glaciers at the end of the last ice age. Outbreaks occur at about 35-year intervals and often last more than 10 years in eastern Canada. In the western boreal forest (Manitoba west to the Rocky Mountains and north to the Northwest Territories and Yukon), the outbreak interval appears shorter and more variable, likely because of changes to host forest composition.

Spruce budworm feeds almost exclusively on current-year growth. It can take several years of defoliation to cause mortality in host trees. A notable exception is balsam fir (Abies balsamea), as spruce budworm can consume both new and old foliage. This may result in high mortality levels following only a few years of severe defoliation. Aerial spray programs have been conducted against spruce budworm to protect foliage during every major outbreak since the 1950s.

Distribution and hosts

Spruce budworm inhabits spruce-fir forests from east of the continental divide to the Atlantic coast in every Canadian province and the Yukon and Northwest Territories. Defoliation has been reported as far north as the Mackenzie River delta in Canada and central Alaska in the United States, and as far south as spruce-fir forests of the northeastern United States from the Great Lakes to Maine.

The primary host tree throughout this vast range is white spruce (Picea glauca). However, the most visibly vulnerable host is balsam fir, especially in eastern Canada. Black spruce (P. mariana) is a widely distributed host but is less vulnerable because of its shorter growing season in northern forests. Red spruce (P. rubens) is also a less vulnerable host because of its relatively restricted range in eastern Canada. During severe outbreaks, spruce budworm can be found feeding on other coniferous species including larch (Larix), hemlock (Tsuga), and pines (Pinus).

Host parts affected

Current-year buds and needles, developing pollen cones.

Symptoms and signs

Green eggs are laid in two overlapping rows on needles. Just before hatch, the dark head of the developing larva can be seen in each fertile egg. Early-stage larvae have dark brown to black heads and pale brown bodies with no distinctive markings. Final-stage larvae are 20 to 30 millimetres in length with dark brown to black heads and reddish-brown bodies distinctly marked with two rows of cream-coloured spots on each dorsal segment. Adults of spruce budworm are medium-sized moths with a wingspan of 21 to 30 millimetres. Wings are smoky grey or sometimes reddish brown, with white, grey, and brown markings.

The first signs of new damage appear in early spring, as budworms mine old needles, causing discoloration. They bore into current-year buds and pollen cones, leaving evident entry holes, silk, and frass on the surfaces. At very high population densities, strands of silk may be seen hanging from the foliage as small budworms forage in the tree canopy for the earliest available buds. Once current-year shoots elongate, feeding shelters are constructed by budworm larvae by tying needles together with silk. These feeding shelters can be seen on the tips of branches.

Damaged needles begin to dry in mid-summer, causing the crown of the tree to turn brick red. These damaged needles eventually fall off, causing branch tips to appear sparse. This also causes the tree crown to appear grey. Trees compensate for needle loss by increasing the production of smaller buds on shorter shoots for a few years following defoliation.

During moth flight, millions of moths may congregate around outdoor light fixtures.

Life cycle

Spruce budworm has one generation per year. Eggs are laid on needles in masses containing 15 to 30 overlapping eggs in late July to mid-August, depending on location and year, and hatch within 1 to 2 weeks. Newly hatched budworm larvae do not feed but move from the foliage to the interior of the tree, settling in protected crevices on the branches and trunk where they become dormant and remain for the winter.

Larvae emerge in the spring and return to the branch tips in search of buds. Emergence typically occurs 2 to 3 weeks in advance of bud flush. During this period, small budworm larvae feed on old needles, fresh pollen cones, or burrow into expanding buds. As current-year shoots become available, the larvae construct protected shelters. They feed continuously, completing their larval development in July and pupating within their feeding shelters. Adult moths emerge within 2 weeks. The female moth emits a pheromone that attracts males to mate. The warm, humid, atmospheric conditions typical of summers in the boreal forest favour increased moth activity at dusk. Both male and female moths may fly above the forest canopy where they may be transported many kilometres on air currents.

Outbreaks are most frequent in mature forest stands dominated by white spruce and/or balsam fir. The intensity of defoliation is greatest in dense host stands with closed canopies. Once an outbreak becomes extensive, the susceptibility of all host stands increases irrespective of their age or composition.

Weather plays a complex role in outbreak behaviour, determining where budworms can complete their life cycle before seasonal foliage becomes unsuitable, and the extent of moth dispersal. A complex food web of natural enemies ranging from migratory songbirds to diverse insect parasitoids exploit budworm populations. Cumulative changes in forest condition caused by years of damage and the combined effect of natural enemies reduces survival of budworm larvae at the end of outbreaks. Populations are maintained at low densities for several years by less favourable forest conditions resulting from the previous outbreak, reduced mating success, and the action of natural enemies, which are more effective at low budworm densities.

Damage

Mortality of co-dominant trees takes several years of defoliation, but understory trees may be killed within a few years. Where dense, homogenous stands of balsam fir are infested, complete mortality has been recorded. Defoliation reduces radial growth irrespective of the tree species or size. Damage to cones may be significant in managed seed orchards. Economic damage to Christmas tree plantations and field-grown spruce and fir ornamentals also occurs. Secondary effects of defoliation include attacks by wood-boring beetles, infection by pathogens, and increased fire risk, especially in severely damaged stands.

The ecological effects of outbreaks depend on the severity of the damage. At the extreme, the results of mortality of vulnerable, homogeneous stands of balsam fir range from conversion of stands to non-host species to regeneration of new stands of balsam fir. In most cases, changes are more variable with an initial, general shift toward less vulnerable species, including spruce and hardwoods. Overall, there is at least a temporary conversion to a more heterogeneous forest type. Nesting migratory songbirds benefit from budworm outbreaks but there is little evidence of long-term increases in their own population levels or that of other wildlife.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
the value of the host(s) versus the costs of pest management approaches; and
consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

In forest management, one management option against spruce budworm is aerial application of insecticides. Several insecticides have been registered for use and naturally occurring viruses and bio-rational pesticides have been investigated. Priorities for treatment are set according to wood supply and annual monitoring of spruce budworm population levels. Pheromone traps and sampling to estimate density of egg masses or overwintering larvae are well-established methods of forecasting damage.

Historically, forest pest managers have tolerated a few years of defoliation, intervening after outbreaks persist or become more severe. Recently a novel approach called early intervention has been tested. The premise is outbreaks are initiated or amplified by moths dispersing from outbreaks elsewhere. Rather than waiting for these incipient populations to increase to damaging levels, action is taken immediately to maintain populations at a low level where natural enemies may play a more effective role.

Because trees can survive a few years of defoliation, accelerated or re-scheduled harvesting is also a feasible management option, depending on local timber supply and demand. Where mortality has already occurred, salvage harvesting can recover some commercial value.

The relationship between tree vulnerability, stand characteristics, and spruce budworm damage points to silvicultural methods of reducing severity of future outbreaks. Favouring spruce over balsam fir, managing a hardwood component, and maintaining vigorous growth rates and a more open canopy should reduce the negative consequences of infestations.

Spruce budworm is rarely a serious problem for individual ornamental spruce and fir trees on private property. In Christmas tree plantations and field-grown spruce and fir ornamentals where populations are high and the level of acceptable damage is low, control may occasionally be warranted. Pheromones and pesticides are defined as pest control products and are regulated in Canada. Pheromones and pesticides registered for use against spruce budworm under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered products and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be harmful to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Selected references

Eveleigh, E.S.; McCann, K.S.; McCarthy, P.C.; Pollock, S.J.; Lucarotti, C.J.; Morin, B.; McDougall, G.A.; Strongman, D.B.; Huber, J.T.; Umbanhowar, J.; Faria, L.D.B. 2007. Fluctuations in density of an outbreak species drive diversity cascades in food webs. Proceedings of the National Academy of Sciences, 104(43), 16976-16981. https://doi.org/10.1073/pnas.0704301104

Johns, R.C; Bowden, J.J.; Carleton, R.D.; Cooke, B.J.; Edwards, S.; Emilson, E.J.S.; James, P.M.A.; Kneeshaw, D.; MacLean, D.A.; Martel, V.; Moise, E.R.D.; Mott, G.D.; Norfolk, C.J.; Owens, E.; Pureswaran, D.S.; Quiring, D.T.; Régnière, J.; Richard, B.; Stastny, M. 2019. A conceptual framework for the spruce budworm early intervention strategy: can outbreaks be stopped? Forests 10(10): 910. https://doi.org/10.3390/f10100910

Nealis, V.G.; Régnière, J. 2004. Insect-host relationships influencing disturbance of the spruce budworm in a boreal mixedwood forest. Canadian Journal of Forest Research 34(9): 1870–1882. https://doi.org/10.1139/x04-061

Nealis, V.G. 2016. Comparative ecology of conifer-feeding spruce budworms. The Canadian Entomologist 148(S1): S33–S57. https://doi.org/10.4039/tce.2015.15

Pureswaran, D.S.; Johns, R.; Heard, S.B.; Quiring, D. 2016. Paradigms in eastern spruce budworm (Lepidoptera: Tortricidae) population ecology: a century of debate. Environmental Entomology 45(6): 1333–1342. https://doi.org/10.1093/ee/nvw103

Pureswaran, D.S.; Neau, M.; Marchand, M.; De Grandpré, L.; Kneeshaw, D. 2019. Phenological synchrony between eastern spruce budworm and its host trees increases with warmer temperatures in the boreal forest. Ecology and Evolution 9(1): 576–586. https://doi.org/10.1002/ece3.4779

Régnière, J.; Cooke, B.J.; Béchard, A.; Dupont, A.; Therrien, P. 2019. Dynamics and management of rising outbreak spruce budworm populations. Forests 10(9): 748. https://doi.org/10.3390/f10090748

Régnière, J.; Delisle, J.; Sturtevant, B.; Garcia, M.; Saint-Amant, R. 2019. Modeling migratory flight in the spruce budworm: temperature constraints. Forests 10(9): 802. https://doi.org/10.3390/f10090802

Volney, W.J.A.; Fleming, R.A. 2007. Spruce budworm (Choristoneura spp.) biotype reactions to forest and climate characteristics. Global Change Biology 13(8): 1630–1643. https://doi.org/10.1111/j.1365-2486.2007.01402.x

Cite this fact sheet

Nealis, V.G. 2024. Spruce budworm. In J.P. Brandt, B. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis. (editors). Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.