Forest tent caterpillar

Written by J.P. Brandt Revision 2024

French common name: Livrée des forêts
Scientific name: Malacosoma disstria Hubner
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Lasiocampidae

General information and importance

Forest tent caterpillar is a serious pest of many species of deciduous trees in North America. In Canada, outbreaks of this native forest pest are common in all except the most northern forest regions. When outbreaks occur, they cover extensive areas and can completely defoliate suitable host trees.

Outbreaks are cyclical and happen about every 10 years, lasting 2 to 5 years. During outbreaks, forest tent caterpillar larvae will move en masse to new stands in search of food once local trees are completely defoliated. Some people might remember a ruined picnic or camping trip because of forest tent caterpillars, while others have lost control of their car while driving on roads made slick by millions of migrating larvae. Even train locomotives have stalled on tracks that were too slippery because of tent caterpillar outbreaks. After larvae pupate, moth flights can include millions of individuals.

Despite their common name, forest tent caterpillars do not spin a tent. In early larval stages they form cohesive family groups and shelter together on silken mats spun on tree trunks, especially on cool days or in the evening. Other related species in the same genus (Malacosoma) make silken tents but are distinct in appearance from forest tent caterpillar.

Distribution and hosts

Forest tent caterpillar is found in temperate and boreal deciduous forests from the Atlantic Provinces to British Columbia and north into the Yukon and the Northwest Territories. Its range also extends to southern regions of the United States.

Due to its widespread distribution, the main hosts of this insect vary by bioclimatic region. In temperate regions, outbreaks of this insect are common on a wide variety of hardwoods but most commonly on sugar maple (Acer saccharum) and on red oak (Quercus rubra). In boreal regions, outbreaks occur on trembling aspen (Populus tremuloides). Interestingly, forest tent caterpillar does not feed on red maple (Acer rubrum) because several chemical compounds in their leaves deter feeding.

Host parts affected

Leaves

Symptoms and signs

The most conspicuous symptom of forest tent caterpillar is defoliation in late May and June. Egg masses are found on host twigs from late summer to early spring. These masses contain 150 to 250 eggs, are greyish brown, and form foam-like bands that encircle individual twigs.

Newly hatched larvae are about 2 to 3 millimetres long, black, hairy, and feed in groups on emerging leaves following budbreak. Mature larvae are 45 to 55 millimetres long, hairy, have three pairs of thoracic legs, five pairs of abdominal prolegs, and distinct coloration and markings. Larvae are dark brown to black with a broad blue band along each side. The back of the caterpillar is black and marked by a row of creamy-white to white keyhole-shaped spots and very fine longitudinal orange lines. When viewed from the top, blue is the most prominent colour along with the white key-holed shaped spots.

Pupae are usually found in clumps of leaves tied together with silk. However, they will attach their silken cocoons to whatever is convenient.

Adult moths are dark tan to brown in colour, with two oblique stripes on the forewings. Their wingspan is about 35 to 45 millimetres.

Life cycle

Forest tent caterpillar has one generation per year. In the spring, usually late April to early May, eggs hatch and larvae begin feeding on newly emerging leaves. At first, all larvae from one egg mass feed together on one or a few leaves. During the first four to five instars, larvae are gregarious; older larvae may wander in search of more food, especially when populations are high. Feeding continues for several weeks until larvae are full grown, usually mid to late June, depending on the region.

Mature larvae usually require two weeks for pupation, which typically occurs in late June. Adults usually emerge at the end of June to early July. After mating, female moths lay about 150 to 250 eggs in a single egg mass on twigs in the upper crowns of host trees. Embryos become fully developed larvae before cold weather arrives, but do not emerge from eggs until the following spring.

Cold springs or severe late spring frosts are detrimental to forest tent caterpillar because they delay leaf-out or kill larvae directly. Egg mortality resulting from extreme cold winter temperatures influences the long-term changes in forest tent caterpillar populations in more northern climates.

Several natural factors play a role in regulating forest tent caterpillar populations once outbreaks have developed. Parasitoids and natural predators, such as birds that eat larvae and moths, can often be important forms of natural control. The most important parasitoid is the large flesh fly, Sarcophaga aldrichi, which targets forest tent caterpillar pupae. Its populations increase rapidly after the start of a forest tent caterpillar outbreak where it can destroy up to 80% of the pupae within a population. There are also a number of natural diseases such as microsporidia, fungi, nuclear polyhedrosis virus (NPV), or cytoplasmic polyhedrosis virus (CPV), which also can be important regulators of populations.

In severe outbreaks, starvation can kill millions of larvae if they exhaust their local food supply before they reach maturity.

Damage

During outbreaks, millions of hectares of trees can be completely defoliated. However, even severely defoliated trees can recover from short outbreaks relatively well. Trees defoliated for the first time will usually produce another crop of leaves later in the growing season.

After multiple consecutive years of severe defoliation, radial growth loss on sugar maple and trembling aspen can range from 40% to 75%. This can be accompanied by twig and crown dieback. Tree mortality can occur after severe outbreaks, especially when these outbreaks are synchronous with or followed by other stress events, such as drought.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
the value of the host(s) versus the costs of pest management approaches; and
consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

On smaller individual ornamental trees, mechanical approaches to control are effective. Egg bands on twigs in the crown can be removed in the fall after leaves have dropped. In the spring, colonies of young larvae can be removed by hand or destroyed. Pressurized water can be used to dislodge larvae from the foliage. However, if they are not prevented from climbing back up onto the tree, defoliation will likely resume.

Past research has demonstrated that a biological insecticide based on Bacillus thuringiensis is effective against forest tent caterpillar larvae for larger trees or in forested situations. Pesticides registered for use against forest tent caterpillar under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against forest tent caterpillar. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Selected references

Abou-Zaid, M.M.; Helson, B.; Nozzolillo, C.; Thor Arnason, J. 2001. Ethyl m-digallate from red maple, Acer rubrum L., as the major resistance factor to forest tent caterpillar, Malacosoma disstria Hbn. Journal of Chemical Ecology 27: 2517–2527. https://doi.org/10.1023/A:1013683600211

Cerezke, H.F. 1991. Forest tent caterpillar. Forestry Canada, Northwest Region, Northern Forestry Centre. Edmonton, Alberta, Forestry Leaflet 10. https://ostrnrcan-dostrncan.canada.ca/entities/publication/4776a157-41ee-463f-b716-7f8cfa07c147?fromSearchPage=true

Churchill, G.B.; John, H.H.; Duncan, D.P.; Hodson, A.C. 1964. Long-term effects of defoliation of aspen by the forest tent caterpillar. Ecology 45(3): 630–633. https://doi.org/10.2307/1936115

Cooke, B.J.; Roland, J. 2003. The effect of winter temperature on forest tent caterpillar (Lepidoptera: Tortricidae) egg survival and population dynamics in northern climates. Environmental Entomology 32(2): 299–311. https://doi.org/10.1603/0046-225X-32.2.299

Cooke, B.J.; Roland, J. 2007. Trembling aspen responses to drought and defoliation by forest tent caterpillar and reconstruction of recent outbreaks in Ontario. Canadian Journal of Forest Research 37(9): 1586–1598. https://doi.org/10.1139/X07-015

Cooke, B.J.; Fidgen, J.G.; MacQuarrie, C.J.K.; Roe, A.D. 2018. Forest tent caterpillar. Natural Resources Canada, Canadian Forest Service. Great Lakes Forestry Centre. Sault Ste. Marie, Ontario. Frontline Express 83. 2 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/02fe09ab-28f4-45b7-b4a2-98b0dc130585?fromSearchPage=true

Cooke, B.J.; MacQuarrie, C.J.R.; Lorenzetti, F. 2012. The dynamics of forest tent caterpillar outbreaks across east-central Canada. Ecography 35(5): 1–14. https://doi.org/10.1111/j.1600-0587.2011.07083.x

Fitzgerald, T.D. 1995. The tent caterpillars. Cornell University Press. Ithaca, New York.

Gray, D.R.; Ostaff, D.P. 2012. Egg hatch of forest tent caterpillar (Lepidoptera: Lasiocampidae) on two preferred host species. The Canadian Entomologist 144(6): 790–797. https://doi.org/10.4039/tce.2012.73

Gross, H.L. 1991. Dieback and growth loss of sugar maple associated with defoliation by the forest tent caterpillar. The Forestry Chronicle 67(1): 33–42. https://doi.org/10.5558/tfc67033-1

Hogg, E.H.; Brandt, J.P.; Kochtubajda, B. 2005. Factors affecting interannual variation in growth of western Canadian aspen forests during 1951-2000. Canadian Journal of Forest Research 35(3): 610–622. https://doi.org/10.1139/x04-211

Ives, W.G.H.; Wong H.R. 1988. Tree and shrub insects of the prairie provinces. Forestry Canada, Northern Forestry Centre. Edmonton, Alberta. Information Report NOR-X-292. 327 p. https://www.ubcpress.ca/tree-and-shrub-insects-of-the-prairie-provinces

Kouassi, K.C.; Lorenzetti, F.; Guertin, C.; Cabana, J.; Mauffette, Y. 2001. Variation in the susceptibility of the forest tent caterpillar (Lepidoptera: Lasiocampidae) to Bacillus thuringiensis variety kurstaki HD-1: effect of the host plant. Journal of Economic Entomology 94(5): 1135–1141. https://doi.org/10.1603/0022-0493-94.5.1135

Lachance, D. 1995. Forest insect pests in the Quebec region, Chapter 3. Pages 27–40 in J.A. Armstrong and W.G.H. Ives, editors. Forest insect pests in Canada. Natural Resources Canada, Canadian Forest Service, Headquarters, Science and Sustainable Development Directorate. Ottawa, Ontario. 732 p. https://publications.gc.ca/site/eng/9.889231/publication.html

Rose, A.H.; Lindquist, O.H. 1997. Insects of eastern hardwood trees. Natural Resources Canada, Canadian Forest Service, Headquarters, Science Branch. Ottawa, Ontario. 304 p. https://publications.gc.ca/site/eng/9.581113/publication.html

Schowalter, T.D. 2017. Biology and management of the forest tent caterpillar (Lepidoptera: Lasiocampidae). Journal of Integrated Pest Management 8(1): 1‒10.

Wood, C.S. 1992. Forest tent caterpillar. Forestry Canada, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 17. https://ostrnrcan-dostrncan.canada.ca/entities/publication/3f69d43b-e8bb-47e6-9695-21bfd1dc55ef?fromSearchPage=true

Cite this fact sheet

Brandt, J.P. 2024. Forest tent caterpillar. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.