Last Review/Updated: March 25, 2004

The forest tent caterpillar (Malacosoma disstria Hubner) is the most serious aspen defoliator in Canada. Forest tent caterpillar outbreaks may last 4 to 6 years and may repeat about every 10 years.

Primary Hosts and Distribution

• Trembling aspen (Populus tremuloides Michx.)

Larger populations tend to occur in the parkland and boreal mixed wood areas. For historical maps, click here.

Life Cycle

The forest tent caterpillar has a one-year life cycle.

Larvae

In the spring, larvae (caterpillars) hatch at the time when the leaves start to unfold. Mature larvae are hairy with distinct broad blue bands, a row of white, keyhole-shaped markings and broken orange-brown lines on the body (4.5-5.5 cm in length). They feed on the leaves until about the middle of June, stop feeding and growing, then search for places to pupate.

Pupae

They normally pupate in silken cocoons spun between two aspen leaves. Under severe outbreak conditions, they may pupate anywhere including on the siding of houses and on outdoor household articles.

Adults and Eggs

Hairy, light brown moths (with a 35-45 mm wingspan) emerge in late June through July, and mate. Females lay eggs (clusters of 100-300 eggs) in bands around twigs. Young larvae develop inside of eggs before the winter but they do not hatch until the following spring.

Detection and Damage

Look for the following signs and symptoms:

• Egg bands in late summer through early spring;
• Defoliation of aspen in April-June;
• Hairy larvae with keyhole-shaped markings feeding in large groups in the trees in April-June;
• Silken cocoons between leaves in late June and early July.

Forest tent caterpillar infestations cause branch diebacks and growth reduction. Several years of severe defoliation may cause tree mortality, especially if other stress factors such as drought are present.

Management

As aspen becomes a valuable commercial timber species in Alberta, management of this pest may become more important. Management of the forest tent caterpillar may include:

• Aerial surveys, flown late spring to early summer. Aspen defoliation is sketch-mapped. For more information, click here;
• Egg band surveys to predict the severity of forest tent caterpillar defoliation expected in the following year;
• Forest tent caterpillar pheromones are currently being field tested to predict defoliation severity expected in the following year;
• Bacillus thuringiensis var kurstaki (B.t.k.) can be successfully used to control forest tent caterpillar infestations;
• Most infestations collapse after 4-6 years due to the action of natural enemies.