Forest tent caterpillar eating a leaf

What’s with all the caterpillars?

You arrive at your campsite on a beautiful spring weekend for some early-season camping and begin to set up your site. You’ve already noticed that the trees on the way to the park look a bit thin, like they do at the beginning of spring.

Then one of the kids notices that one of the trees has a big clump of blackish stuff that’s moving…

It’s an army of caterpillars!

But before you pack everything back up and drive home, take a closer look.  No, really…caterpillars are cool!

Close up of a poplar tree in the woods with a mass of caterpillars on the trunk and on a branch.
Forest Tent Caterpillars mass together in the open instead of hiding in web “tents”

Back at home, you may have seen tent caterpillars and not been all too happy about it. They can defoliate trees at an amazing rate, crawl around everywhere looking for their next victim/tree, and generally cause havoc.

Here in the park, they can also defoliate trees, crawl around everywhere, and even poop on your head!  But they’re an important part of nature’s cycle of life.

Tent caterpillar 101

Sometimes known as tent worms or army worms, there are two species of tent caterpillar.

  • Forest Tent Caterpillarthis species doesn’t make tents but rather huddles together on tree trunks for safety. In one big mass, they don’t look like single caterpillars that many predators like to eat. They can be identified by a series of dots down their back and are the most widespread defoliator of hardwoods in North America (Trembling Aspen and White Birch are two of their favourites)
Four caterpillars with a series of dots down their backs, climbing on barely visible silk threads - green background
These Forest Tent Caterpillars (they have dots instead of a line on their backs) climb down silk threads on their way to their next tree/meal
  • Eastern Tent Caterpillar they make large “tents” from the silk they spin, tying leaves together to make the shelter they occupy at night for safety. They can be distinguished by a white line down their backs. These caterpillars especially like the leaves of Black Cherry and plants in the rose family
Single caterpillar on wood...fuzzy with blue dots near the bottom, with orange and white striping along the length of the caterpillar
Eastern Tent Caterpillar (distinguishable by the white line)

Fun fact: caterpillar poo is called “frass.” Sometimes there are so many caterpillars that the sound of frass is a bit like the sound of a light rain.

Metamorphosis is very cool

Each summer, small brown moths lay eggs in the forest (approximately 100 in bands on branches) that are covered with a protective coating. These eggs cling to the branch all winter. Eggs hatch the following spring when the leaves are new and tender, and once out of the egg, the tiny caterpillar eats and eats and eats.

The tent caterpillar sheds its skin a number of times so it can keep growing. Once it’s large enough, it starts to wander and looks for a good spot to spin its web and make a cocoon made from silk from spinnerets near its mouth (“silkworms” are actually caterpillars that were used to make silk in China centuries ago, and still do today).

A white web like thing spun on a tree with caterpillars inside
Eastern Tent Caterpillars make web tents like this, while Forest Tent Caterpillars do not

Inside the cocoon, magical things are happening. The caterpillar’s body sort of liquefies. It reorganizes its cells and structures, and reforms from a soft-bodied caterpillar into a moth with hard body parts, six long legs, and wings!

You’ll see caterpillars in May and June, and the moths from the previous year hatching out of their cocoons in June and July (look for them hiding on the sides of comfort stations and other park buildings).

They have one generation per year, but the moths you may see are from the year before — they aren’t the caterpillars you saw in the spring.

Why are there so many this year?

Every ten to twelve years, we get an outbreak of these guys that lasts from three to six years. The outbreak we are experiencing now is the Forest Tent Caterpillar — the one that doesn’t make tents.

Small brown moth on what looks to be concrete
As an adult, Forest Tent Caterpillars are small brown moths, and hide during the day – here under a ledge on a park comfort station, which are a great place to look for moths

What happens is, the moths lay many eggs and the caterpillar population grows bit by bit each year until it reaches a point where the population explodes because predators can’t keep up — they don’t increase in numbers as fast as the caterpillars do.

When the predator population catches up, this causes a sudden and drastic decline of the caterpillars and it is many years before they recover and then explode again.

Many plants and animals go through “boom and bust” cycles.  Snowshoe Hare populations can explode when conditions are good, but the Canada Lynx, which is its main predator, also increases in numbers, controlling the population boom.

Just like with tent caterpillars.

The effects

Tent caterpillars are voracious and eat all the leaves off the trees in some cases.

When they have stripped a tree bare, they move off in large groups like an army (safety in numbers), and in this way can defoliate a number of trees in a short time.

Caterpillars on the move can be so numerous they cause traffic problems — park roads have actually become slippery with squashed caterpillars.

Not all that bad

But really, it’s not all that bad. Once the caterpillars are large enough and begin to cocoon, the trees send out new leaves.

Forest tent caterpillar eating a leaf
Forest Tent Caterpillar with some good “chew”

The temporary defoliation caused by the caterpillars in the spring allows the sun to reach some of the plants on the forest floor that usually struggle to get enough sunlight. This allows them to grow and increase their chances of survival.

And the frass has actually fertilized the forest floor, enhancing the soil.

Natural predators

Many types of caterpillars are covered with bristly hairs. These hairs make eating them unpleasant for many animals. There are birds that will eat them though – they can regurgitate the irritating hairs.

Jay sized light brown bird with a black bill and black eyes with red around the eye, perched on a small branch.
The Black-billed Cuckoo is uncommon and rarely seen

The Black-billed Cuckoo is one of these birds (at least 60 bird species have been observed eating tent caterpillars) and during breeding season tent caterpillars are one of their favourite foods.

Black-billed Cuckoos are uncommon and rarely seen, but can be identified by their calls — cuckoos sound a bit like a cross between a goose and a muffled flute and their name comes from the sound of their call…“cu-cu-cu-cu… cu-cu-cu.

Jay sized bird in a next in thick foliage with something in it's mouth
A Black-billed Cuckoo on the nest, with a mouthful of caterpillar

When the caterpillars emerge from their cocoons as adult moths, another flying predator awaits…moths are one of the bat’s favourite foods — fat and juicy and full of nutrients.

Many other animals eat tent caterpillars too – frogs, mice and squirrels all include them in their spring diet.

Researchers in Minnesota poked through a day’s worth of Black Bear poop, and found that the bear had eaten around 25,000 caterpillars! The same researchers found that there were over 100 insect species that parasitized (laid their eggs or larvae on) tent caterpillars.

Red squirrel, with tail up, on a stump in the forest
Another of the tent caterpillars natural predators, the Red Squirrel

The Friendly Fly is another of the caterpillar’s natural enemies, and one of those that parasitizes them. Native to Ontario, the Friendly Fly is distinctive because it’s larger, slower and hairier than other flies.

They seem “friendly” as they will land on you, needing to be brushed off rather than fly away at the slightest move like other flies, and are at times very numerous. The Friendly Fly lays live maggots on tent caterpillars in their pupal stage. These maggots feed on and thus kill the developing moth.

Black fly with red eyes on flora
The Friendly Fly, also known as a Flesh Fly, is a parasite of tent caterpillars

When these flies appear, this is a signal that the tent caterpillar infestation will soon be over.

Tent caterpillars, even outbreaks like this, are part of nature. In the long-run, forests stay healthier because of them.

So what should I do?

These caterpillars pose no threat to people and are just a part of spending time in the forest. Tent caterpillars appear in May and June during outbreak years.

Read more about the Forest Tent Caterpillar!