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Forest Tent Caterpillar

Information on this page is derived from public domain documents published by the federal government of canada, the provincial government of British Columbia and information contributed on electronic discussion groups. Please bear in mind that any pesticides mentioned in these pages may no longer be recommended or registered for the indicated use — check with your local pesticide officer or regional agrologist for current info (you can use the provincial directory on the internet to search for those job titles or call Enquiry bc at 1 800 663-7867 for assistance). It is recommended that you use a search engine using the common name and/or scientific name of the organism(s) below, together with the name of your province, to find biology and management information relevant to your local conditions.

If you choose to use chemical controls remember to
always follow pesticide label instructions!

insects of economic importance in Canada and British Columbia

family: lassiocampidae
species: malacosoma disstria
common names: forest tent caterpillar
hosts: deciduous trees

Forest Tent Caterpillar


Forest Tent Caterpillar         Forest Tent Caterpillars 

Forest Tent Caterpillar            Forest Tent Caterpillars

Agriculture and Agri-Food Canada Insect Identification Sheet No. 33

Forest Tent Caterpillar

Malacosoma disstria (Hubner)

The forest tent caterpillar is a major pest of deciduous trees in many parts of Canada and the United States. A gradual buildup in the population of the pest in an area may lead to an outbreak that lasts 2 years or longer; outbreaks may occur at intervals of 6 to 16 years.

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Although the trembling aspen is the principal host of this pest, forest tent caterpillars will also attack and defoliate most other species of deciduous trees, including balsam, poplar, birch, maple, ash, oak, elm and basswood. A damaged tree will grow new foliage later in the summer, but the growth of a severely defoliated tree will be retarded for 2 years.

Life History:

The adult is a yellow or yellow-brown moth with a wingspread of 2.5 to 4cm. The forewings are marked with two brown stripes. The moths mate within 24 hours after emerging from the pupal stage and the females, whose adult life span is about 5 or 6 days, begin laying their eggs several hours later. Egg-laying takes place in July and early August and the eggs are deposited in masses on twigs of the trees. Each mass of 100 to 300 eggs appears as a narrow, dark brown band around a twig and is protected by a covering of spumaline, a frothy substance secreted by the females. Hatching occurs the next year (May to July, depending on weather conditions and locality) and a new generation of the pest is born. The young caterpillars, or larvae, migrate and as they grow they attack and defoliate trees. When they reach full growth after 5 to 8 weeks, they spin silken cocoons within which they remain in the pupal stage for about 10 days before emerging as adult moths.

Pest Management:

Environmental factors — weather, disease, birds and parasites — contribute to the natural control of the forest tent caterpillar. Common parasites of the species are the flesh fly (Sarcophaga aldrichi) and the ichneumonid wasp (Itoplectis conquisitor). If an outbreak occurs, consult your provincial Agriculture Department for recommendations.

Agriculture and Agri-Food Canada

Name: Gilkeson, Linda A. Title: Integrated Pest Management Coordinator
Company: Ministry of Water, Land and Air Protection
Reference Text: CATERPILLARS

Tent Caterpillars:

These caterpillars feed together in groups on deciduous trees. Tent caterpillars are hairy and have various color markings. They cannot be mistaken for gypsy moth caterpillars, however, which are distinctly marked with 6 pairs of red dots and 5 pairs of blue dots on their backs. Northern tent caterpillars spin large, conspicuous silk nests in crotches of tree branches in spring and early summer; forest tent caterpillars do not spin these silken mats. Tent caterpillar populations ri se and fall in 5-10 year cycles. When their numbers become too high, the caterpillars are eventually killed off by diseases and parasitic insects. For home gardeners, the best control for tent caterpillars is to pull the nests out of trees (wear gloves) or prune them out and drop the caterpillars in a bucket of soapy water. Start inspecting for tents or colonies of small caterpillars in early spring before they cause much damage and while they are easier to remove. On trees with severe or inaccessible infestations, spray with BTK or products containing pyrethrins and insecticidal soap. Before spraying, make sure caterpillars are still present the empty webbing remains in the trees long after the caterpillars are gone. Old webs can be removed with a strong stream of water from a hose. In the winter, scrape or prune away eggs masses, which look like a silvery-brown, hardened band of foam (about 1 cm wide), partially encircling small tree branches. Dormant oil sprays also kill tent caterpillar eggs.

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Subject: Tent Caterpillars

Tent Caterpillars Fact Sheet No. 28, Revised December 1988 Dr. Jay B Karren, Extension Entomologist


Periodically, tent caterpillars cause considerable defoliation of native trees and shrubs in the canyons and mountains of Utah. These outbreaks often extend into residential areas, causing alarming defoliation of ornamentals in the homeowner's yard. Healthy trees can usually withstand heavy infestations for 2-3 years without suffering permanent injury. Trees under other forms of stress or young, recently planted trees could be severely damaged from a single year's infestation. Trees and shrubs with extensive defoliation will have reduced growth and vigor. In addition the caterpillars produce unsightly webs, or tents which detract from the home landscape. The key to eliminating tent caterpillar problems is early detection and use of appropriate cultural and chemical control measures.

Biology, Description, and Habits:

In Utah there are four species and a subspecies that attack various trees and shrubs including maple, poplar, cottonwood, plum, cherry, choke cherry, aspen, alder, shadscale, willow, birch, apple, and others. Conifers are not usually fed upon. After the caterpillars have stripped the trees of their leaves, they feed on wild and ornamental shrubs and even the leaves of cultivated fruits and vegetables. The different tent caterpillar species are not easy to distinguish, but their life cycles and the damage they produce are similar.

Adults are stout-bodied moths about 1/2 inch long with a wingspan of 1 to 1 1/3 inches. Wings are light yellow to reddish-brown in color with a broad, dark strip across the middle third. They are not generally observed and do not feed on leaves.

The egg masses are laid on trees branches in the fall and usually contain from 150 to 200 eggs. The egg are covered with a dark, reddish brown, frothy material that soon hardens. The eggs may be deposited in a band that completely encircles the twig or in a flattened mass on limbs and boles. Within 2-3 weeks, the young caterpillars are fully formed within the eggs, but they do not hatch until spring. In northern Utah, they chew their way out of the eggs during May, about the time new leaves appear. Noticeable defoliation is reached the latter part of June when most of the larvae reach maturity. At arial large groves of trees on the hillsides are completely stripped of their leaves, leaving the mountainside with a gray cast.

The young larvae are gregarious (gather in groups). Some species construct web tents while others merely cluster together on limbs when they aren't feeding. Mature larvae are about 2 inches long with blue, black, and orange markings. Full grown larvae disperse and spin one-inch long, dusty, silken cocoons in folded leaves, under loose bark, or in litter on the soil surface. New moths emerge in about two weeks, mate, and deposit eggs for the following year. Egg masses appear as swellings around bare twigs.

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Cultural control consists of hand removal of the egg masses during winter pruning or removal of the young larvae and tents before significant injury occurs. This is not feasible on larger trees or when a large number of trees are involved. In many cases, however, it provides a simple and safe solution. It should be done whenever possible.

If chemical controls are required, consult your local pest management representative.

Bacillus thuringiensis (BT) causes a bacterial disease of caterpillars. Commercially available pesticides contain the actual bacterial spores. For Bt to be effective, the caterpillar must consume foliar material that the spores have been sprayed or dusted on. Feeding stops shortly after ingestion, but it may take up to five days for the caterpillars to die. This material has the advantage of being very selective; it only affects certain types of caterpillars and will not harm other animals or even other types of insects, when used as directed.

Cultural or chemical control should be applied as soon as the tents or caterpillars are noticed. The caterpillars are easier to kill Usually, the caterpillars foray from the tent to feed on the surrounding foliage. The tent is expanded as the caterpillars grow and as the adjacent foliage is consumed. If all of the foliage is consumed, the caterpillars may migrate to nearby trees or shrubs and continue feeding. The insecticide application should be directed to the tent, and the foliage immediately around it.


Forest Tent Caterpillars (Malacosoma disstria)
L. A. Gilkeson
April 2000

The forest tent caterpillar is mainly found in the interior regions of the province. It prefers trembling aspen, but in outbreak years will feed on a variety of broad leafed trees and shrubs. Despite the name,
forest tent caterpillars do not make webs or "tents". A species that does make a tent, the northern tent caterpillar (M. californicum pluviale), occurs mainly in the southern half of the province and occasionally in northern deciduous forests. Fall webworms (Hyphantria cunea) are often mistaken for tent caterpillars because they also make large silken webs in shrubs and trees. They appear in late summer, however, 6-8 weeks after tent caterpillars.

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Identification and Life Cycle:

Forest tent caterpillars are the larvae of a large, reddish brown moth. They have one generation a year. The adult moth lays its eggs in late summer on twigs and small branches of deciduous trees. The egg masses look like 1-2.5 cm wide masses of hard, silvery brown foam. They remain on the trees all winter and hatch in early spring, when the buds start to swell.

The caterpillars are dark-coloured, with rows of fine brown hairs. They have a row of key-hole shaped white dots down their backs. There are two fine orange lines on either side of the white dots and a wider blue stripe below that along each side. The caterpillars feed on buds and leaves from early spring into June, congregating in large groups on trees. When they have eaten all the leaves on the host tree, they move onto other trees and shrubs, somearial travelling on the ground in large numbers.

When the larvae reach full size in June, they spin cocoons on tree leaves or other protected sites. They pupate for 8-12 days and then the adult moths emerge to lay the next generation of egg masses.

Tent caterpillar populations naturally rise and fall over a cycle of several years. After a couple of years of outbreak, the populations collapse as the caterpillars are wiped out by naturally occurring diseases and predators.

Is Control Necessary?

In peak outbreak years, stands of aspen and other trees can be completely defoliated, but this usually does not cause long-term harm to the trees. Once the caterpillars leave, the trees produce new leaves to fill in the canopy. The bare trees are unsightly, however, and result in complaints from the public to municipal and parks departments. Unfortunately, the caterpillars consume leaves so quickly, and over such a short time, that by the time damage is obvious it is often too late to do much about the current year's infestation.

It is important to realize that, no matter what steps are taken to control tent caterpillars on individual trees, that the overall populations will increase over several years and then drop to low levels naturally as diseases and predators catch up with the population.

For high visibility plantings, in outbreak years, it may be worthwhile to control tent caterpillars. In that case, a monitoring program should be put in place so that treatments can be done at the right time to be effective.


Look for egg masses on dormant branches any time after the leaves drop. In expected outbreak years, monitoring for hatching egg masses should start by mid-April. Before leaves expand, the hatching caterpillars are particularly easy to see.

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Prune out egg masses and branches with colonies of caterpillars. Burn egg masses and destroy caterpillars by crushing them or dropping them in a bucket of soapy water.

When caterpillars are actively feeding in the trees, the microbial pesticide Bacillus thuringiensis kurstaki (BTK) is useful to control the caterpillars without harming other organisms. It must be applied carefully to ensure good coverage of the foliage. It should not be used if rain is expected within the next 24 hours as it can be washed off before the caterpillars have a chance to eat it. Safer's BTK Biological Insecticide is a product sold for home and garden use. BTK products for Commercial use include Dipel (a wettable powder formulation) as well as Thuricide and Foray (both liquid concentrate formulations).

Spraying other pesticides is usually not recommended because they kill the natural enemies that eventually bring the caterpillar populations under control.


Wood, C. S. 1992. Forest Pest Leaflet: Northern Tent Caterpillar. Forestry
Canada. FRDA Publication No. 29-6/17-1992E

see also:

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Copyright © 2007 Conrad Bérubé, site design, concept and scripting. All rights reserved worldwide.
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