Nature in Canada: Forest Tent Caterpillars

The Canadian provinces of Quebec and Ontario have witnessed an explosion of forest tent caterpillars this summer. This type of caterpillar can be found throughout North America, but especially within the eastern region of the continent. These gray, dark brown, or black caterpillars have distinctive white spots down the middle of their bodies that are flanked by length-wise, thin yellow stripes, along with thicker blue stripes. Their bodies are also covered in brown hairs. Forest tent caterpillars are scientifically known as Malacosoma disstria, appearing as only one generation per summer season. These insects hatch in very social groups by the hundreds on trees, with every caterpillar following another in a single file over the same silk strands laid down by those leading the trail. Together, they weave silken mats to rest and molt on, descending the tree with each stage of development (1). Here is a picture of a forest tent caterpillar surveying my backyard from atop one of our patio chairs. Given its independence and full body, it can be assumed that it is ready to cocoon.

Forest tent caterpillars enter life as groups of 100 to 350 eggs. The eggs are deposited as a cylindrical belt around small branches within the upper crown of a tree, totalling one fourth of an inch in diameter and up to half an inch in length. These collections of eggs are stuck together with a glue-like substance that hardens and becomes a lustrous grey or dark brown. Within three weeks of having been laid, the embryos form into larvae within the eggs. Although the eggs are laid in the late summer time, they do not hatch until late spring or early summer of the following year. The larvae then emerge by gnawing off a circular cap at the upper end of the egg shell. Their completely black bodies are a quarter of an inch thick with sparse brown hairs. The larvae typically pass through five stages of molting over five to six weeks, eventually gaining a full body length of two inches. Afterwards, the forest tent caterpillar will begin to spin a silk cocoon, usually within a folded leaf or a crevice of a building or a tree’s bark. The outer cocoon is loosely spun, while the interior is tightly woven, creating a structure similar to parchment paper on the inside. The silk strands originally appear white, but they are coated in a sealant liquid that turns to a yellow powder upon drying. Over the course of ten days, the forest tent caterpillar will transform into a brown, oval-shaped pupae, and then open to reveal a tan-colored moth with feathery antennae and a dark brown stripe across its wings. Male moths appear significantly smaller with broader antennae. These moths live for a rather short time, mating almost immediately and then dying after laying their eggs (1 & 4).

Forest tent caterpillars have been observed in nearly all plant life, such as apple, ash, asparagus, barberry, beech, birch, black tupelo, cherry, clover, elm, ferns, fringe tree, geranium, grape, grass, hawthorn, hickory, honeysuckle, horse chestnut, hydrangea, lilac, linden, maple, oak, peach, pear, plum, poplar, rose, smoke tree, sweetgum, syringa, walnut, and violet (4). The problem with this abundance in range is the forest tent caterpillar’s ability to cause extensive damage by defoliation, capable of reducing a plant’s diameter growth by as much as 90% (1). A study by researchers Cooke and Lorenzetti revealed that forest tent caterpillars have defoliated a surface area of 384, 540 km2 from the year 1938 to 2002 in Quebec, totalling twenty-five percent of the province’s surface area. This damage affects pollination and seed production (2). A forest tent caterpillar outbreak can last from a year up to six years (3). The size of an outbreak depends on climatic perturbations, population cycling, topography, and predators (2). This includes freezing temperatures, flies and wasps depositing parasitic maggots in egg collections, fungal diseases, humans finding and destroying eggs, and predation from ants, beetles, spiders, birds, and small mammals (1). Stable forest tent caterpillar populations pose little threat; however, serious environmental harm can occur during large outbreaks due to humans declining predator population sizes and global warming raising climate temperatures.


  1. Batzer, H. O., & R. C. Morris. (1978). Forest Tent Caterpillar. Washington, D. C.: U. S. Department of Agriculture, Forest Service.
  2. Cooke, B. J., & F. Lorenzetti. (2006). The Dynamics of Forest Tent Caterpillar Outbreaks in Québec, Canada. Forest Ecology and Management, 226(1), pp. 110-121.
  3. Krichel, Sarah. (2018, June 3). Why there are so many forest tent caterpillars right now. Montreal Gazette. Retrieved from:
  4. Weed, C. M. (1899). The Forest Tent Caterpillar. Durham, N. H.: New Hampshire College Agricultural Experiment Station, New Hampshire College of Agriculture and the Mechanic Arts.

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