Tent Caterpillar | HOTEL LAKE ADVISORY

Northern Tent Caterpillar

(Malacosoma californicum pluviale)

and

Fall Webworm

(Hyphantria cunea)

From early spring to early fall we might look up and notice small gauzy tents enclosing the leafy branches of trees. These milky white or light brown shapes signal the emergence of large numbers of caterpillars. Where we see one tent, how many more are nearby? What’s going on and what kind of damage will the tree sustain?

Part of the explanation is that in the early spring we are seeing the beginning of the life cycle of tent caterpillars. The three most common types of tent caterpillars in Canada are the Eastern tent, the Forest tent and the Western tent caterpillar. The Western tent caterpillar has 6 subspecies and one of those subspecies, the Northern Tent Caterpillar (Malacosoma californicum pluviale) is very prevalent on the Sunshine Coast and is the main focus of this webpage.

But there is another possibility; in July-August you might look up and see similar “gauzy tents”. These tents are the work of the Fall Webworm.

This webpage provides information about the Northern Tent Caterpillar and the Fall Webworm and highlights the differences between these two insects.

Northern Tent Caterpillars

Life Cycle

The Northern Tent Caterpillar goes through the stages of egg, larva, pupa and adult in the period of one year, and only one generation of this caterpillar is produced each year. The following information outlines a year in the life of the Northern Tent Caterpillar; a life cycle that we see in the area around Hotel Lake.

The Larval Stage: Mid April to early June is the time when this social insect is a caterpillar; for ’larvae’ you could read ‘caterpillars’. Within the egg, during the winter, tiny caterpillars hatch and accomplish a first moult. Then, perfectly coinciding with the appearance of new leaves on the trees, these masses of tiny (3mm), dark coloured caterpillars emerge from the egg. The caterpillars feed on the new leaves and establish colonies in silky tents. During this stage, the caterpillars will moult 5 or 6 times and grow to 45 to 55mm. At maturity you will see a 2 inch caterpillar with blue sides, orange stripes, and a series of keyhole-shaped white or light blue markings on its back. After four to five weeks the larvae begin to vacate the tent and become more solitary feeders. While away from the tent they start to search for a sheltered place to pupate.

The Pupa Stage: Late May through to early July is when a sheltered place for the pupa stage is often found on a branch of a tree or shrub. But it could also be under the eaves of buildings, on fences, in bark crevices or beneath litter. The mature larvae weave silken cocoons, which they attach to this sheltered spot with a few thick strands of silk. The larvae then envelop themselves in the cocoons to pupate. What does the pupa look like? It is usually a dark reddish-brown, 15-20 mm in length and it rests inside a pale oval silken cocoon that is dusted with a yellowish powder. The pupa will remain in this cocoon for 12 to 18 days and then emerge as an adult moth.

Adult Moth: Late Summer to Early Fall is when you may see swarms of moths fluttering around the infested area. They vary in colour from pale yellow to buff brown or dark reddish brown, and have thick, short, furry bodies. Light coloured bands run across wings that span 25-37 mm (1-1.5 inch). The adults do not feed and have a short life span. Their purpose is to complete the life cycle by mating and laying eggs. Following mating, the female lays a single dark brown or grey mass of eggs around a twig or branch. These ‘egg bands’ that partially encircle the twig or branch are about 15 mm in width and contain between 150 and 350 eggs. The eggs are held in this ring-like structure by a foamy silver- grey substance called spumaline. This substance bonds the eggs to the branch and serves to protect them from freezing or drying up. Within 3 to 4 weeks, larvae have formed in the eggs. These larvae will overwinter inside the eggs until the following spring.

Tents

A tent caterpillar has a spinneret on its head that uses the liquid silk produced by its silk glands to create a strong thin thread. The spinnerets guide these threads to weave a silky tent.

Northern tent caterpillars build silk tents for communal living and, except for when they are engaged in feeding, they spend all their time in the tent. Tents can be enlarged to accommodate the larvae from another egg mass or the increasing size of the present inhabitants. Tents provide shelter from harsh weather, and protection from birds and other predators. Since the larvae are going to moult 5 or 6 times in this phase, the tent is also a safe place for this vulnerable process.

The tent consists of layers of silk with spaces between these layers. It has been discovered that the temperature varies from the innermost to the outermost space. Because the tent is strategically placed to face the sun, the caterpillars will move to the outermost layer where they are warmed and their digestions are able to function. When the temperature is hot, they move to the innermost layer to stay cool.

Diet

Northern Tent Caterpillars eat new young, tender leaves. On the Sunshine Coast these are usually the leaves of alder, willow, birch, poplar and fruit trees.

There is pressure to complete the larval stage before leaves become tough, old and difficult to digest. Below 15 degrees the Northern Tent Caterpillar cannot process food, so warmth is another necessity for their digestive systems. The adult moth lacks the needed mouthparts and does not eat.

The larvae leave the tent to feed and as they move to food sources they lay down silk trails and pheromone signals. These “trails”ensure that many larvae will find a good food source and the entire colony will find their way back to the tent.

You will enjoy this short documentary on the Western Forest Tent Caterpillar populations on Saturna Island, British Columbia, Canada. Starring population ecologist Judy Myers and disease ecologist Jenny Cory. Copyright: Iain Myers-Smith, 2021 Music by Bensound

Population Numbers

Populations of the Northern Tent Caterpillar follow a ‘boom and bust’ cycle. When numbers are low, they will gradually rise over several years culminating in a peak outbreak. During an outbreak, tents are large and numerous, treed areas may be stripped of leaves, and millions of moths take flight. Outbreaks are common in south coastal areas, including Vancouver Island, the Gulf Islands, the Fraser Valley, and the Sunshine Coast.

Favourable conditions such as lack of rain can contribute to an outbreak. However, as the numbers of tents rise, so do the populations of natural predators and diseases. These natural enemies, especially viruses, take over and cause the caterpillar population to decline, ending the outbreak.

It is suggested that outbreaks occur every 8 to 11 years and can last for a few years. According to Judy Myers, a zoologist who has been studying tent caterpillars in the Pacific Northwest for 48 years, there was an outbreak in 2012, one in 2023 and the next one is expected around 2032.

The chart below is from the 2025 paper: "Long-term population dynamics of western tent caterpillars: History, trends and causes of cycles” found in our library, or click here

Damage and Control

Northern tent caterpillars are voracious eaters, consuming tender young leaves and leaving only the centre rib and veins. If trees are severely defoliated they may be weakened and more susceptible to drought and pests. If the leaves of fruit trees are reduced in numbers, the process of photosynthesis is inhibited resulting in smaller and less fruit.

But what if we are concerned about our favourite fruit trees in the backyard? What can be done about controlling and/or preventing tent caterpillars?

Monitor the health of trees;
Water regularly to strengthen them, helping them better resist and recover from pest damage.

Be aware of natural predators:
Birds such as chickadees, nuthatches, blue jays and robins will feed on caterpillars. Other predators include:

insects such as parasitic wasps and flies, beetles, stink bugs, ants and some spiders
small mammals such as mice, squirrels and bats
black bears, particularly in late spring before other sources of food such as berries are available.

Remove egg bands:

In the fall and winter, find and scrape off the shiny, reddish-brown egg bands that the moths lay on tree branches. Removing the egg masses before they hatch is one of the best ways to prevent Tent Caterpillars.

Physically remove tents and caterpillars:
For small infestations, pull or scrape the tents and caterpillars from the branches. Drop the collected tents and caterpillars into a bucket of hot, soapy water or put them in a biodegradeable plastic bag and then dispose of them.

Prune:
For tents within the reach of cutters, loppers or pole pruners, prune off the infested branches and dispose of them immediately in a plastic bag or by submerging them in soapy water.

Use Bacillus thuringiensis subsp. kurstaki (Btk):
This is a biological insecticide that is toxic to tent caterpillars but safe for other animals, including mammals, birds, and fish. Follow instructions carefully and spray it on the foliage when caterpillars are small and have just begun to feed.

Fall Webworm

And now lets look at our second "Tent Maker”, Fall Webworm. Later in the summer, new and different tents may appear on trees and these belong to another insect, the Fall Webworm. The following information and the table below allow the reader to recognize the similarities and differences between these two tent-makers.

Judy Myers, our previously quoted authority commented in September of 2025: "I have been hearing reports that they are common this year. Like the Western Tent Caterpillars, Fall Webworms have population outbreaks but their cycles are not as regular as WTC.”

Life Cycle of the Fall Webworm

Adult moths emerge from their cocoons in late spring or early summer and mate. Eggs (200-300) are laid on the undersides of tree leaves. Larvae (caterpillars) emerge in one to two weeks. These caterpillars have a noticeable dense covering of long white silky hairs and can be black-headed or red-headed. They spin a silken tent and begin eating the leaves that have been enclosed within the tent.

Pupa: In 6 to 8 weeks the fully grown caterpillars leave the webbing, drop to the ground, and enter the pupal stage. They form a cocoon, often under leaf litter or in the soil, where they spend the winter. The cycle repeats again in late June or early July when pupae shed their cocoons, and adult moths emerge to mate and lay eggs.

The Chart below highlights similarities and differences between NTC and FW.

This short video created in August of 2025 shows Fall Webworms in action.

Ecological Impact

During outbreaks, severe defoliation can weaken trees and make them more susceptible to drought and pests. However most healthy trees will recover and even bud again later in the summer. Overall, Northern Tent Caterpillars and Fall Webworms rarely cause lasting damage to trees. A certain level of caterpillar activity can be tolerated without serious harm.

On the positive side, during these periodic outbreaks:

More sunlight reaches the forest floor because the canopy of leaves has been thinned. This allows smaller trees and plants to grow in the understory area.

Some birds and insects thrive because the increased numbers of caterpillars represents an increased food source

The droppings (frass) of caterpillars which are rich in nutrients act as a fertilizer for the ground below.

The Northern Tent Caterpillar and the Fall Webworm are native to BC, part of the natural ecosystem and not considered an invasive species. Their cycles are a part of forest ecology; a natural and expected driver of change in the ecosystem.

This is a whimsical conversation between a tent caterpillar and a human that contains the main information and would probably be a fun thing to share with kids.

https://labbenchtoparkbench.wordpress.com/2015/12/25/interview-with-a-western-tent-caterpillar/