Forest Tent Caterpillars Are Back (Here’s How to Protect Your Trees and Shrubs)

Uh-oh, the tents are going up again. We’re not talking about camping. Forest tent caterpillars are making their spring return across the prairies, and while they don’t build tents like their name suggests, they do know how to eat their way through your trees.

Here’s how to spot them early, understand the risks, and take action before they defoliate your favourite tree.

What are forest tent caterpillars?

Forest tent caterpillars are the larvae of a native moth species (Malacosoma disstria) found throughout much of Canada and the northern United States. Despite their name, they don’t build the telltale silk “tents” that eastern tent caterpillars do. Instead, they travel in large groups and spin thin silk trails on branches or tree trunks as they move.

The eggs hatch in spring, and the caterpillars feed for 5–6 weeks, then pupate and emerge as tan moths in midsummer. Females lay eggs in shiny brown masses on twigs that overwinter and hatch the following spring.

Once hatched, forest tent caterpillars weave silken mats on tree trunks or branches, which they use as communal resting spots. They leave these mats to feed, but typically return after eating. Because they’re super social, caterpillars from a single egg mass often stick together, forming these large, dense colonies that can quickly take over a tree.

Their victim? Primarily deciduous trees like trembling aspen, poplar, oak, birch, and maple.

What do forest tent caterpillars look like?

They’re fuzzy, dark or tan coloured caterpillars with pale blue sides and distinctive white “footprint” or keyhole-shaped spots along their backs.

What damage do they cause?

Forest tent caterpillars are voracious leaf-eaters. While one or two might not do much harm, large colonies can completely strip trees of their foliage, especially during outbreak years when populations surge.

Here’s what their feeding frenzy can lead to:

• Complete Defoliation. In severe infestations, they can defoliate entire trees, leaving them leafless by early summer.
• Tree Stress. While healthy trees can typically regrow leaves by mid-summer, repeated defoliation year after year weakens their defenses. This makes them more vulnerable to drought, disease, and other pests.
• Growth Suppression. Repeated attacks can stunt a tree’s growth and reduce its ability to store nutrients for winter.
• Aesthetic Damage. In yards and urban settings, bare branches and caterpillar clusters can make trees look unhealthy and unsightly, especially ornamental trees that are meant to be beautiful.
• Frass (Caterpillar Droppings). Infested areas often experience a noticeable “rain” of caterpillar poop falling from the canopy, which can cover patios, vehicles, and lawns.

Can it kill your tree?

In most cases, a single defoliation event won’t kill a mature tree, but younger, stressed, or repeatedly infested trees may suffer lasting damage or even die without intervention.

Signs of an infestation

Photo credits to unsplash.com

Spotting an infestation early can help you take action before major damage is done. Here’s what to look for:

• Clusters of Caterpillars. Look for dark, fuzzy caterpillars with pale blue sides and white, footprint-shaped markings along their backs. They often gather in large groups on trunks or branches.
• Silken Mats and Trails. Unlike their eastern cousins, forest tent caterpillars don’t build tents, but they do spin silky mats on tree bark where they rest and fine threads along their travel routes.
• Chewed Leaves and Defoliation. Ragged or missing leaves, especially in the upper canopy, are a key sign. In heavy infestations, entire trees may appear bare by early summer.
• Frass (Droppings). Small black pellets beneath trees (on sidewalks, patios, or leaves) are a sure sign that caterpillars are feeding above.
• Egg Masses. In fall or early spring, look for shiny, dark brown egg masses wrapped tightly around small twigs and branches.

How to protect your trees and shrubs from Forest Tent Caterpillars

1. Manual Removal (Best for Small Trees and Shrubs)

If you’ve only got a few trees and you’re not squeamish, manual removal may work, especially before the caterpillars hatch. It’s a great first step if you’re catching the infestation early.

• If you have the time, remove egg masses from trees before they hatch in the spring.
• Knock caterpillars into a bucket of soapy water.
• Prune out heavily infested limbs if practical. This is best done in fall or in early spring when there are no leaves on the trees and the egg masses are most visible.

2. Biological or Non-Chemical Treatments for Forest Tent Caterpillars

Looking for an eco-friendly way to deal with caterpillars? Biological treatments and natural predators can help reduce populations without harming pollinators, pets, or the planet. Timing is everything—these work best when the larvae are still small.

• Bacillus thuringiensis kurstaki (Btk) is a natural bacteria that targets caterpillars without harming pollinators or pets. Apply when larvae are small to get the best result.
• Encourage birds, predatory beetles, and parasitic flies that feed on caterpillars. You can promote biodiversity by planting native species or installing birdhouses.

3. Chemical Insecticide

Sometimes, infestations are too large for manual or biological methods alone. That’s when targeted chemical treatments can help. The key is using the right product, at the right time, in the right way, and only when necessary.

• An application of targeted insecticides when caterpillars are still small (1–2 cm).
• If you’re heading down the DIY route, ensure all products are registered with Health Canada (look for a PCP number on the label).
• For tall or mature trees, it’s always best to consult a certified arborist for safe application.

Note: Tree banding with sticky substances like Tanglefoot™ isn’t effective against forest tent caterpillars, as adult moths can fly directly to the branches to lay eggs.

4. Integrated Pest Management (IPM)

Integrated Pest Management (IPM) is a smart, science-based approach that combines several strategies to control pests while minimizing harm to people, pets, and the environment. At Green Drop, we prioritize IPM because it focuses on long-term solutions, not just quick fixes, making it one of the safest and most effective ways to manage forest tent caterpillars.

When it comes to these caterpillars, IPM might involve a mix of carefully timed pesticide use, monitoring population levels, encouraging natural predators, and boosting overall tree health through pruning and proper care. The goal is to reduce damage without relying too much on chemicals.

Spotted a caterpillar in June that’s already 1–2 inches long? Chances are, they’ve finished feeding—but that doesn’t mean your job is done. You can still take action to remove them and prevent future infestations.

5. Long-Term Health Boosting

Even if your trees recover this season, repeat defoliation can wear them down over time. Think of these steps as “tree therapy”. They boost resilience and help your landscape bounce back stronger:

• Water trees deeply during drought.
• Fertilize stressed trees in the fall to encourage recovery.
• Mulch around the base of trees to retain moisture and improve soil.

When to get an arborist involved

If caterpillars are overwhelming your trees or you’re dealing with large mature trees, it’s time to bring in the pros. Certified arborists can apply safe and effective treatments at the right time to prevent long-term damage.

Final thoughts…we can help!

Forest tent caterpillars are part of our natural ecosystem, sure, but that doesn’t mean you have to let them take over your trees. Early action and a few smart strategies can help you keep your trees leafy, healthy, and thriving all season long.

Need help managing caterpillars? Contact Green Drop for a FREE tree health assessment and customized pest management plans.