Thu. Feb 29th, 2024

You arrive at your campground on a beautiful early-season spring camping weekend and begin setting up your site. You may have already noticed that the trees on the path to the park look a little thin, like in early spring.

Then one of the children notices that one of the trees has a big pile of blackish things that move…

It’s an army of caterpillars!

But before you repack everything and head home, take a closer look. No, really… caterpillars are cool!

Closeup of a poplar tree in the forest with a mass of caterpillars on the trunk and on a branch. Forest tent caterpillars congregate outdoors rather than hiding in web “tents.”

At home, you may have seen tent caterpillars and not been very happy about it. They can defoliate trees at a surprising rate, crawl around looking for their next victim/tree, and generally wreak havoc.

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

Tent Caterpillar 101

Sometimes known as tent worms or armyworms, there are two species of tent caterpillars.

  • Forest tent caterpillarThis species does not make tents, but rather huddles in tree trunks for safety. In a large mass, they do not look like individual caterpillars that many predators like to eat. They can be identified by a series of spots on their back and are the most widespread hardwood defoliator in North America (Trembling Aspen and White Birch are two favorites).

Four caterpillars with a series of spots on their back, climbing on barely visible silk threads - green backgroundThese forest tent caterpillars (they have spots instead of a line on their back) climb silk threads on their way to their next tree/meal.

  • Eastern tent caterpillar They make large “tents” with the silk they weave, tying leaves together to form the shelter they occupy at night for safety. They can be distinguished by a white line that runs along their back. These caterpillars especially like black cherry leaves and plants in the rose family.

Single caterpillar on wood...blurred with blue dots near the bottom, with orange and white stripes along the caterpillarEastern tent caterpillar (distinguishable by white line)

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

The metamorphosis is very good.

Each summer, small brown moths lay eggs in the forest (about 100 in bands on branches) that are covered with a protective layer. These eggs adhere to the branch throughout the winter. The eggs hatch the following spring, when the leaves are new and tender, and once out of the egg, the little caterpillar eats, eats, eats.

The tent caterpillar sheds its skin several times in order to continue growing. Once it is big enough, it begins to wander around and looks for a good place to weave its web and make a cocoon made of silk with spinnerets near its mouth (“silkworms” are actually caterpillars that were used to make silk in China centuries ago, and I still do it today).

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A white spider web woven into a tree with caterpillars inside.Eastern Tent Caterpillars makes tents like this, while Forest Tent Caterpillars does not.

Magical things happen inside the cocoon. The caterpillar’s body liquefies. It reorganizes its cells and structures, and changes from a soft-bodied caterpillar to a moth with hard body parts, six long legs, and wings.

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

They have one generation per year, but the moths you can see are from the previous year; They are not the caterpillars you saw in the spring.

Why are there so many this year?

Every ten or twelve years, we have an outbreak of these types that lasts three to six years. The outbreak we are experiencing now is the forest tent caterpillar, the one that does not make tents.

Small brown moth on what appears to be concreteAs adults, woodland tent caterpillars are small brown moths and they hide during the day, here under a ledge at a park comfort station, which is a great place to look for moths.

What happens is that the moths lay many eggs and the caterpillar population grows little by little each year until it reaches a point where the population explodes because the predators cannot keep up; They do not increase in number as quickly as caterpillars do.

When the predator population catches up, this causes a sudden and drastic decline in the caterpillars and it takes 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 their main predator, also increases in numbers, checking the population boom.

As with tent caterpillars.

The effects

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

When they have defoliated a tree, they move in large groups like an army (security is unity), and in this way they can defoliate several trees in a short time.

Moving caterpillars can be so numerous that they cause traffic problems. In fact, the park’s paths have become slippery due to crushed caterpillars.

It’s not that bad

But really, it’s not that bad. Once the caterpillars are large enough and begin to form cocoons, the trees emit new leaves.

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Forest tent caterpillar eating a leafForest tent caterpillar with a good “chew”

Temporary defoliation caused by caterpillars in spring allows the sun to reach some of the forest floor plants that normally struggle to receive enough sunlight. This allows them to grow and increase their chances of survival.

And the excrement has actually fertilized the forest floor, improving the soil.

Natural predators

Many types of caterpillars are covered in bristly hairs. These hairs make eating them unpleasant for many animals. However, there are birds that eat them: they can regurgitate the irritating hairs.

Light brown bird the size of a jay, with a black beak 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 species of birds have been observed eating tent caterpillars) and during the breeding season, tent caterpillars are one of its favorite 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 thick foliage with something in its mouthA black-billed cuckoo in the nest, with its mouth full of caterpillar

When the caterpillars emerge from their cocoons as adult moths, another flying predator awaits… moths are one of bats’ favorite foods: fatty, juicy and full of nutrients.

Many other animals also eat tent caterpillars: frogs, mice, and squirrels include them in their spring diet.

Researchers in Minnesota examined a day’s worth of black bear poop and found that the bear had eaten about 25,000 caterpillars. The same researchers discovered that there were more than 100 species of insects that parasitized (laid their eggs or larvae) on tent caterpillars.

Red squirrel, tail up, on a stump in the forestAnother of the natural predators of tent caterpillars, the Red Squirrel.

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

They seem “friendly” in that they will land on you and need to be shaken instead of flying away at the slightest movement like other flies, and sometimes they are. very numerous. The friendly fly deposits live maggots on tent caterpillars in their pupal state. These worms feed on the developing moth and thus kill it.

Black fly with red eyes on flora.The friendly fly, also known as the flesh fly, is a parasite of tent caterpillars.

When these flies appear, it is a sign that the caterpillar infestation will soon end.

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

Then what should I do?

These caterpillars do not pose a threat to people and are only part of the time they spend in the forest. Tent caterpillars appear in May and June during outbreak years.