Forest Tent Caterpillar

Forest tent caterpillar, Malacosoma disstria, a native tree pest, is the most widespread leaf eating insect in North America. The caterpillar stage feeds voraciously on the leaves of deciduous trees. Outbreaks typically occur every 10 to 12 years and normally last three to six years. The last outbreak recorded in Manitoba was 2000/2001.

Long-term damage to trees is generally not severe, although repeated years of heavy attack can weaken trees and slow the growth of aspen forests in the Prairie Provinces. Typically, forest tent caterpillars are more of a nuisance to homeowners, cottagers, and outdoor recreationalists, as they can completely defoliate trees, cluster in large groups and sometimes form cocoons on the sides of buildings.


The primary hosts of the forest tent caterpillar are deciduous trees. In Manitoba, trembling aspen is the preferred host. However, under outbreak conditions the larvae will feed on other deciduous trees.

Life cycle

The moth form of the forest tent caterpillar lays eggs from July to early August in bands around small twigs. They develop into caterpillars within three weeks. These caterpillars over-winter in the eggs and emerge in spring. Emergence coincides with bud development on trees. The tiny caterpillars feed openly in colonies on the unfolding leaves. Contrary to their name, they do not construct tents of webbing. Instead, they spin silken threads along which they travel. The larvae often cluster together while resting or during moulting to the next caterpillar stage.

Feeding lasts from June to mid-July, with most of the feeding occurring in the fifth caterpillar stage. In this stage the caterpillars migrate in search of new food supplies or suitable cocooning sites. After five to eight weeks of feeding the caterpillars spin silken cocoons.

The pupal stage normally lasts 10 days. Adult moths emerge in mid to late July, mate, and lay eggs. The adult moths are active fliers and can be carried for several hundred kilometres on air currents. The adult stage lives for five to 10 days. There is one generation per year.

forest tent caterpillars

Forest Tent Caterpillars
(Herbert A. ‘Joe’ Pase IIII, None,

moth form of forest tent caterpillar

Adult moth
(Scott Tunnock, USDA Forest Service,

forest tent caterpillar egg mass and moth

Egg mass and moth
(Scott Tunnock, USDA Forest Service,

forest tent caterpillar cocoon

(Herbert A. ‘Joe’ Pase IIII, None,


Feeding damage ranges from a light thinning of the tree top to the complete stripping of tree leaves. This damage can occur to trees over large areas. Usually the trees do not die, and a second set of leaves emerges later in the season.

Following two or more years of severe feeding there is a general decline in tree health, including twig and branch dieback. After three or four consecutive years of being stripped of their leaves trees can be weakened and may be more susceptible to insects, such as wood borers, and stem disease. Following forest tent caterpillar infestations the incidence of hypoxylon canker, a fungal stem disease, may increase and cause extensive damage in aspen forests.

forest tent caterpillars feeding

Caterpillars feeding on leaves
(USDA Forest Service, Region 8, Southern Archive,

damage to trees by forest tent caterpillars

Severe feeding damage
(William M. Ciesla, Forest Health Management International,

Population control and management

Since the forest tent caterpillar is a native cyclical pest, the Manitoba government does not actively manage outbreaks. Forest tent caterpillar populations are controlled by a variety of natural factors:

  • High population levels and late spring frosts (which destroy foliage) can cause mass starvation.
  • Excessively high temperatures during the moth stage of the lifecycle can cause substantial adult mortality (death) and reduce egg viability.
  • Numerous parasites also play an important role in population control. The most effective parasite is the flesh fly. Adult flies lay live maggots on forest tent caterpillar cocoons. The maggots burrow through the silken case to feed on the pupa.
  • Many insects, birds, and small mammals can feed on forest tent caterpillars at different life stages. This not only can help reduce the forest tent caterpillars population but can lead to population peaks by opportunistic feeders.

To decide whether active management methods are necessary, first inspect trees to find out whether caterpillars are still present. The damage is often not noticed until after they have finished feeding and left the tree.

  • The caterpillars can be removed from branches and from the stem by hand or a broom, or they can be hosed off of the tree. Dispose of the caterpillars afterwards.
  • Twigs with egg bands can be removed in late summer to reduce next year’s population.
  • There are chemical controls for this pest. Please see your local retailer, nursery centre, or arborist for more information on currently registered products.

Additional information and resources

Province of Manitoba pamphlet on forest tent caterpillar:

Agriculture and Agri-food Canada:

Natural Resources Canada:

United States Department of Agriculture:

Forestry Branch contacts: