Last Review/Updated: March 25, 2004

Spruce budworm (Choristoneura fumiferana (Clemens)) is one of the most severe forest defoliators in North America. Spruce budworm caterpillars damage trees by feeding on the current year's needles, conelets and occasionally on mature needles.

Primary Hosts and Distribution

• White spruce (Picea glauca (Moench) Voss)
• Balsam fir (Abies balsamea (L.) Mill.)

The budworm is found throughout Alberta, including cities and towns. Larger outbreaks of the budworm generally occur in the boreal forests. For historical maps, click here.

Life Cycle

The spruce budworm completes its life cycle within a year.

Larval Stage 1

The eggs hatch to produce the first instar (L₁) of the budworm.

Larval Stage 2

The first instar (L₁) caterpillars develop into the second instar (L₂) caterpillars, which move by wind and find sites under bark scales. Here, they will spin silken cocoons for hibernation (overwintering).

Larval Stage 3

In May, L₂ budworm, which are yellowish with dark brown heads, develop to the next instar (L₃), which either bore into needles or buds or spin silken webbing around new shoots and begin to feed on the needles within.

Larval Stages 4, 5, 6

The most damage to the foliage normally occurs early to mid-June when the larvae are in their final stage of development (L₆). Larger budworms with brown bodies (18-24 mm in length), black heads, and two rows of paired whitish spots down the back, can be spotted on shoots in June.

Pupae

After L₆, the budworms stop feeding and develop into brownish pupae, which in turn become moths.

Moths (Adult)

Moths mate and then the female lays eggs.

Eggs

Up to 300 green eggs per female are laid in masses of about 15-50 eggs on the underside of the needles.

Detection and Damage

Look for the following signs and symptoms:

• Defoliation of the current year's growth;
• New greenish egg masses found in the fall on the underside of needles; old whitish egg masses on older needles;
• Silken webbing seen in May/June around needles and shoots.
• Spruce budworm larvae found in late May to June on open shoots;
• Rusty brown tree crowns in July, caused by the dead brown needles and pupal cases becoming entangled in silken webbing.

Epidemic populations of budworm can lead to reduced tree growth and vigour. Four to five consecutive years of severe damage results in death of tree tops, and if it continues for another one to two years, it may cause tree mortality. The spruce budworm affects aesthetic, recreational and economical value of our forests.

Management

In Alberta, integrated spruce budworm management programs are undertaken each year. They include detection, monitoring, and if necessary, control of the budworm. For the spruce budworm management strategy, click here. For more information about aerial or ground survey standards, data or initiatives, click here. Spruce budworm management may include the following:

• Multi-Pher I ® pheromone traps are used to monitor population and dispersal trends of adult moths. They are established throughout the province from mid-June to mid-August.The results are used to predict the likelihood of new budworm outbreaks occurring in the following year.
• In July, aerial surveys are conducted to detect and to sketch-map budworm defoliation.
• Young budworm larvae (L₂) can be surveyed in the fall to monitor population changes of the budworm and predict the defoliation severity expected in the following year. This information is used to identify control areas.
• Spray programs are conducted to reduce budworm populations in moderately or severely defoliated areas. In Alberta, a biological insecticide, Bacillus thuringiensis var kurstaki
• (B.t.k) and a biorational insecticide, tebufenozide (Mimic®), are used for aerial spraying.
• Additional phenology/prespray surveys are conducted in conjunction with spray programs.
• The rescheduling of harvesting of high-risk timber or severely defoliated areas is used as a management tool in Alberta.
• Clear cutting and replanting with non-host tree species recommended for forest stands with recurrent and severe spruce budworm outbreaks;
• Silvicultural treatments are being developed to reduce budworm damage and reduce susceptibility to budworm.