Addressing Forest Tent Caterpillars

It’s been a couple of years since I last posted an article, for which I am sorry. It feels like a bit of small talk might be in order, if you’ll permit it. What have I been doing that keeps me from writing and posting? Well, the answer is simple. I’m still adjusting to a new-ish job (5 years now) and I’ve been upskilling, in the form of more education. It takes energy and focus (and a ready topic) to push me to write. Well, today, all of those came together.

It looks like 2026 is our year for a Forest Tent Caterpillar outbreak, so I figured I’d share a bit of what I’ve seen and learned about it.

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On average, approximately every 10 years (sometimes more/less) frequently, there is an outbreak of the Forest Tent Caterpillar. This voracious, foliage-eating caterpillar can cause quite a bit of damage to trees in the forested regions. It can also show up in rural and urban plantings, which can be unnerving for residents. This pest (and similar caterpillars) can definitely dial up the “ick factor” to an 11 (on a scale of 5).

The adult of this species is a moth. Unless you are an entomologist with a particular interest in Lepidoptera (the order of insects that we’re dealing with – butterflies and moths, and therefore, caterpillars), you likely won’t see or worry to much about the moth. They don’t feed on their host trees like the larvae, but after mating in the mid-summer (late June to early August), they lay their eggs (up to about 300 per female) in small little bands on twigs and stems on the outsides of the trees. Moths can be carried several hundred kilometres, so they get around. Unless you happen to stumble across them, or be looking for them, you might mistake the egg bands for a node or a normal swelling on the branch (see Figure 1). The egg bands have a bit of a metallic gleam or sheen to them. If you put them under a microscope, they actually look like a bunch of squat barrels under a cargo net (see Figure 2). It’s fascinating to look at.

Figure 2 – Forest Tent Caterpillar egg band – magnified under dissecting microscope

Photo by Dr. Ken Fry

In the spring, the eggs hatch, and the real monster is unleashed. This consists of a whole bunch little black fuzzy looking caterpillars (see Figures 3 & 4). As they get bigger, the larvae start to show their true colours, which is what will differentiate them from other, similar species. They have broad, grey-blue bands along the sides of their bodies and have a distinctive white keyhole-shaped mark in a row down their backs (see Figure 5). The larvae are also hairy and can get up to about 2 inches (50mm) long at maturity.

Larvae tend to stay grouped together most of the time, with strands of webbing that they follow or that they cling to (see Figure 6). The groups or masses tend to move up and down the tree, collecting on trunks or in branch crotches to shelter, or to move nearer to food sources (see Figure 7). These munchers can eat their way through a significant amount of foliage in a short period of time. I’ve heard people say that you can hear them feeding at night when an outbreak is occurring, and I’m inclined to believe them. Larvae feed for 4-6 weeks before pupating in a cocoon. They emerge as adults to start the cycle all over again. There is only one full generation per season.

I’ll note that while these critters can eat a tremendous amount of foliage and expell a great deal of frass (a.k.a. poop), the amount of damage that they do is most aesthetic and cosmetic. Trees will regrow leaves. It takes several consecutive years of complete defoliation to kill a big tree, so, for the most part, trees that are healthy won’t be killed by this pest. 

Figure 4 – Groups of Forest Tent Caterpillar larvae resisting moisture

Photo by Robert Spencer

Figure 6 – Lines of Forest Tent Caterpillar larvae moving up and down the tree trunk along a silk strand

Photo by Robert Spencer

Figure 8 – Mass of Forest Tent Caterpillar larvae part way through being removed physically – note  how the mass is tied together with silk

Photo by Robert Spencer

Host Species

Forest tent caterpillars attack a wide range of hardwood broadleaf deciduous tree species, including aspens, birch, ash, and maple, but can also be found on other deciduous trees.

Management?

Managing forest tent caterpillars isn’t entirely obvious but isn’t impossible in most situations. The masses of larvae actually provide the overall group with some protection from attack, as sprays can hit and kill outer layers, but leave inner larvae unscathed. The webby masses also repel water and resists removal. Avoid handling these caterpillars without gloves as they can cause a bit of a reaction in some people, as their hairs are an irritant.

If you have a small population of FTC that you can reach, you can sweep them off the trunk and dispose of them in a bucket of water and dish soap, with a glug of bleach thrown in for effect. The soap changes the surface tension of the water, so they sink, and the bleach makes it more toxic. You’ll find that the masses that are on tree trunks might resist removal, as they are tied together with silk. If you find masses in a bit of a nest on a branch, you can trim that off rather than trying to sweep them. Even if you miss a few, you will be reducing the population more than doing nothing.

There is a good biological control available for managing FTC (and other caterpillars) in the form of a bacteria-based product called Bacillus thurigiensis kurstaki (BTK, for short). This bacterium produces a tummy-paralysing toxin that stops feeding and then kills the caterpillar from the inside out. It is ONLY toxic to Lepidoptera, or butterflies and moths, AND ONLY to the larvae, no other stage. The catch? The larvae have to eat it. So, that means that you have to douse the leaves where the larvae are feeding. Hosing down the mass won’t cut it.

There are other insecticides out there that you can use, although it is a little bit different from the BTK. For homeowners, you can’t access much more than Insecticidal Soap, with or without pyrethrin or permethrin. These products have very limited residual effect, so you need to get these either right on the caterpillars or on leaves that they are going to immediately consume. Masses will resist the product, as the outer layers protect the inner layers.

If you are able to use a commercial product (legally), you might have some more residual options and might have the ability to reach them systemically. It’ll depend on the product.

For all sprays, pay attention to the weather. Imminent rain is going to wash off products, rendering them ineffective. Also, masses of larvae will adjust their location based on the weather, so be aware, and adjust accordingly.

Outbreaks can last two to four years, with natural predators, viruses, pathogens, and other environmental factors dropping the population back to a more stable and normal level. Some environmental conditions at certain times of year (e.g. cool, wet springs during larval emergence) can knock the population down quite a bit.

Other Tent Caterpillars?

There are a number of other, similar looking, tent caterpillars out there. Some of them have different primary host plants, but there is a fair bit of crossover in the populations, it seems.

The species may differ in how the egg bands or masses are distributed. For example, Prairie tent caterpillars and Northern tent caterpillars lay eggs about 1 foot (30cm) from the base of the tree, not in an encircling band. Eastern tent caterpillars lay their eggs similar to Forest Tent Caterpillars.

Each caterpillar looks a bit different and may have more distinctive markings. Prairie tent caterpillars (see Figure 9) are black when young, but develop to have black backs, with a white-coloured mid-back strip. The sides of the larvae are light blue. They are hairy. This might seem quite similar to FTC, but the FTC have bands of blue on the side and the distinctive white keyhole pattern.

Northern (formerly Western) tent caterpillars (see Figure 10) are also black when young but eventually have a broken white stripe along their back and broad orange band enclosing blue/white and black markings. Larvae are hairy, like the other ones.

Eastern tent caterpillars (Figure 11) start off like the other ones, black and sparsely hairy, but have a white stripe along the back and narrow orange stripes parallel on the mid back, with white and blue markings on the sides.

Ugly nest caterpillars (see Figure 12) are a bit different from the tent caterpillars but do form webby nests that might be mistaken for a tent caterpillar mass at first glass. Beyond that, they are quite different.

Figure 10 – Masses of Northern Tent  (formerly Western) Caterpillar larvae

Photo by Whitney Cranshaw, Colorado State University, Bugwood.org

Figure 12 – Ugly Nest Caterpillar nest

Photo by Robert Spencer

Is management different for these different tent caterpillars?

In a word, No. The management is basically the same, although based on their habits, their locations, and the hosts, you might see some different behavior and need to adjust how you manage them.