What are they?
The three most common types of tent caterpillars in Canada are the Eastern tent, the Western tent, and the Forest tent caterpillar.
- Forest tent caterpillars feed on deciduous trees (trees that lose their leaves seasonally) in many parts of Canada. Outbreaks last two or more years and usually happen at intervals of ten years or more.
Forest tent caterpillars have a similar life cycle to the Eastern and Western tent caterpillars, with one distinct difference: instead of building tent-like webs, they make a type of silken mat on the trunk or branches where they collect to rest, leaving only to feed on the leaves. These caterpillars are a pale blue colour with black, and have a series of white spots on their back. The adult moth is a yellow-brown colour.
- Eastern tent caterpillars are hairy, brownish black with a light stripe down their back. Blue spots and brown-yellow lines are found along the sides of their bodies. Adult moths are usually a reddish-brown colour, but can be yellow-brown as well.
- Western tent caterpillars tend to be reddish brown on top and pale underneath. They have a row of blue spots on their backs, with orange spots mixed in between. The adult moths are orange-brown with yellow lines on the wings.
Tent caterpillars are social creatures. Caterpillars from one egg mass stay together and spin a silken tent in a fork of a tree, while caterpillars from two or more egg masses may unite to form one large colony.
How do I know if I have a problem?
Eastern and Forest tent caterpillar moths lay their eggs in late June or early July, while the Western tent caterpillar moth appears later and lays eggs in August. The eggs are laid in groups of 150 to 350 and encased in a frothy substance that hardens into a shiny Styrofoam-like material, forming a dark brown or gray band that straddles or encircles twigs of susceptible trees.
In the spring, as soon as leaves open, the eggs hatch into young caterpillar larvae that make communal tent webs. The Eastern tent is usually built in tree or branch forks, while the Western tent may enclose the tips of branches. The size of the tent increases as the larvae grow. The caterpillars mature in four to six weeks, reaching a length of about 2 to 3 cm (.8 to 1.2 inches).
In June or July, the caterpillars cover themselves in cocoons. The cocoons may be found on tree trunks, fences, debris, and beneath sheltered areas like raised plant boxes. About 10 days later, the adult moth will come out of its cocoon and mate within 24 hours. The female immediately begins to lay eggs for the next spring, producing only one generation of tent caterpillars every year.
Caterpillars feed on deciduous trees in most parts of southern Canada:
- The Eastern tent caterpillar feeds mainly on cherry, apple, and crab apple trees.
- The Western tent caterpillar chooses willow, poplar, apple, plum, cherry, and oak.
- Forest tent caterpillars seem to prefer trembling aspen and poplars in Canada, but also feed on many other trees like American beech, apple, basswood, cherry, white ash, birch, mountain ash, sugar maple, red oak, white elm, and willow.
Tent caterpillars are known to build silk trails while travelling between their nest and feeding sites on the tree. You may be able to see these silken strands when the sunlight shines through an infested tree.
Should I be concerned?
Eastern Tent caterpillars have been in North America since 1646. Outbreaks happen about every ten years and sometimes last up to two years.
Tent caterpillar outbreaks are periodic, but do not happen on an exact schedule because they depend on several environmental and biological factors. Although they seldom kill the infested tree, these caterpillars can cause severe damage, often nearly defoliating the entire tree.
If damage is minor and the tree is healthy, the tree can bud again later in the summer. However, repeated defoliation can weaken trees and make them more susceptible to other types of stress.
How can I get rid of tent caterpillars?
When deciding if you need to act, keep in mind that tent caterpillars have only one generation a year, and each female lays only a single egg mass. Since tent caterpillars are native to North America, insect parasites and natural predators like birds and rodents control a certain amount of the population. However, some means of control may be needed where infestations are severe.
Destroying cocoons and egg masses
In the summer, look for tough, yellow-to-white cocoons on tree trunks, fences, debris, and sheltered areas. In the fall, look for shiny, dark brown or gray saddle-like cases which straddle or encase twigs of trees that are known to be susceptible to tent caterpillar infestations. Destroy egg cases and cocoons by scraping them off carefully (to avoid damage to the bark), and dispose of them in a sealed bag.
In the spring, if webs are present, colonies of young larvae can be removed by clipping and destroying the tents and caterpillars. This is best done when the caterpillars are at rest in the tent, either in early morning, late evening, or on cool rainy days. A pole pruner can be used to remove the nests in taller trees.
The bacterium B.t. (Bacillus thuringiensis) is a selective biological insecticide. After eating vegetation treated with B.t., caterpillar larvae will stop feeding and die within five days. Thorough coverage of foliage is needed, and spraying should not be done until the first signs of leaf damage appear. Younger larvae are more susceptible to this product.
Insects other than the larvae of moths and butterflies are not affected by B.t. made to be used against caterpillars. Products that contain this bacterium are also non-toxic to mammals, birds, and fish.
If you use a pesticide to control your pest problem, read the label to make sure you are choosing the right product for the right pest. Follow all label directions and warnings carefully. Always look for a Pest Control Products (PCP) number on the label so you know the product has been approved by Health Canada. See Use pesticides safely for more information on using pesticides safely
- If you have an infestation of caterpillars, you can use a dormant oil spray on susceptible trees in late winter to smother the eggs before they hatch in early spring. Dormant oils are thick oils used mainly on fruit trees to control overwintering eggs, mites, scales, and other insects. Dormant oils can cause damage to plants if not used according to directions, so follow all label instructions.
- There are other pesticide products registered for the control of tent caterpillars as well. If the tent is within reach, break it open with a stick and direct the insecticide into it. Spraying is most effective in the evening, as the caterpillars return to the nesting area at night.
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