Tree Caterpillars: Tent Caterpillars, Webworms, and Bagworms

You walk out in late April to find a fist-sized ball of silk in the fork of your cherry tree, with little dark caterpillars crawling in and out. Or it’s August and the tips of the walnut branches are wrapped in dirty gray webbing. Or worse, the arborvitae hedge that looked fine last summer has gone half-brown, and on closer look every brown spot is a little cone hanging from the foliage. Three different pests, three different timelines, three different fixes, and one good piece of news: most of them can be controlled with a single bacterial spray that’s safe enough to use near a vegetable garden.

The trick is figuring out which caterpillar you’re actually looking at before you spray anything. Our tree pest guide covers the full pest picture, but this article is the chewing-caterpillar deep dive: tent caterpillars, fall webworms, and bagworms, plus the BT timing that decides whether your spray works or wastes a Saturday.

How to tell which caterpillar you’re looking at

Start with three questions: where is the silk, what tree are you on, and what month is it. Those three pieces of information identify the pest about 90 percent of the time without ever looking at the caterpillar itself.

Silk in the fork of a branch, spring, on cherry or apple. Eastern tent caterpillar. The tent grows from a tight little pouch in early April to a softball-sized mass by mid-May, always built where two branches meet.

Silk wrapped around the branch tips, late summer, on almost any deciduous tree. Fall webworm. Their webs are messier, looser, and built at the ends of branches rather than in the forks. You’ll see them from July through September.

Tiny spindle-shaped cones hanging from arborvitae, juniper, cedar, or spruce, looking like little ornaments. Bagworm. Each cone is a single caterpillar’s portable house. By the time you spot them clearly, the larvae have been feeding for weeks.

No silk at all, caterpillars marching up a trunk or across a sidewalk in big groups, May or June. Forest tent caterpillar. The name is misleading because they don’t build tents. They mass-migrate.

Caterpillars on California oaks in spring, no obvious tent, light defoliation. Western tent caterpillar or one of the native oak moths. The western tent caterpillar (Malacosoma californicum) does build a small silk mat, but it’s flatter and less obvious than the eastern species.

Color and markings help confirm. Eastern tent caterpillars have a white stripe down the back with blue and yellow side markings. Forest tent caterpillars have a row of cream-colored keyhole or footprint shapes down the spine instead of a stripe. Fall webworms are pale yellow or greenish with long white hairs and a darker line down each side. Bagworms are rarely seen outside their bag, but if you tear one open early in summer you’ll find a small dark brown caterpillar with a pale head.

Eastern tent caterpillars

Eastern tent caterpillars (Malacosoma americanum) are the spring tent species across the eastern and central US. The female moth lays a glossy black egg collar around a thin twig in early summer of the previous year. Those eggs sit through the winter and hatch right around the time wild cherry and crabapple leaves are unfurling, which in most zones means late March to early April.

The newly hatched larvae spin a small silk pad in the nearest branch fork and start expanding it as a group. They leave the tent in the morning, feed on nearby leaves, and return to the tent at night and during bad weather. By mid-May the tent is bigger than your fist, the caterpillars are an inch long, and the host tree looks half-eaten.

Primary hosts are wild cherry, ornamental cherry, apple, crabapple, and sometimes plum. Cherry is the favorite. If you have a flowering cherry that gets early-spring webs every year, that’s almost always eastern tent caterpillar, not something else. Our cherry tree diseases and pests guide covers the rest of what hits cherries; tent caterpillar is the most common spring issue on this host.

The damage looks alarming and usually isn’t. A healthy cherry that gets defoliated in April has plenty of summer ahead to push out a second flush of leaves. One year of moderate tent caterpillar feeding will not kill an established tree. The problem is repeated years. A tree that loses 40 percent of its leaves three springs in a row burns through stored carbohydrates faster than it can rebuild them, and you start seeing branch dieback, smaller fruit, and reduced overall vigor.

Young trees and trees already stressed by drought, root damage, or disease are the exception. A two-year-old cherry that loses most of its first flush can die outright. Same for any tree planted within the last three years or recovering from a hard prune.

Forest tent caterpillars and western tent caterpillars

Forest tent caterpillars (Malacosoma disstria) are the species that makes the news. Every five to fifteen years a population explodes across a region, and suddenly there are caterpillars on every sidewalk, every driveway, every wall. They don’t build a real tent. They build a silk mat on the trunk or a large branch and use it as a resting platform between feeding trips.

Forest tent caterpillars hit a much wider host range than the eastern species: sugar maple, aspen, oak, gum, and a long list of other hardwoods. In outbreak years they can defoliate thousands of acres of forest, and the USDA Forest Service tracks population cycles by region at fs.usda.gov. For most homeowners the issue is the migration phase, not the feeding. Once they’re done with the host tree, mature larvae crawl in all directions looking for a place to pupate, and that’s when you find them on siding, fences, and pool covers.

Western tent caterpillars (Malacosoma californicum) are the Pacific Coast version. They show up on California oaks, alders, willows, and fruit trees, with a small silk mat similar to the eastern species but usually less obvious. Defoliation is rarely severe on mature oaks. If you have repeat infestations on a young oak, see our oak tree diseases and pest guide for the broader pest picture, since multiple pests stack on stressed oaks.

The key practical difference: an eastern tent caterpillar problem is a single tree with an obvious silk ball, and you can prune it out. A forest tent caterpillar problem is a regional outbreak with no tent to target, so the only treatment is BT applied to the foliage while the larvae are still small.

Fall webworms

Fall webworms (Hyphantria cunea) are the late-summer counterpart to tent caterpillars, and they confuse a lot of homeowners because the webbing looks similar. The differences are timing, location, and host range.

Fall webworms hit in July, August, and September. Their webs wrap the tips of branches and the foliage inside, growing as the larvae feed. A mature web on a walnut or pecan can be two or three feet long, hanging off the end of a branch like a dirty net. Inside, the caterpillars eat the leaves and leave their droppings behind, which is why the web looks gray and grubby rather than clean white silk.

The host range is enormous. University of Minnesota Extension lists more than 100 species, and you can find more host data at extension.umn.edu. Walnut, pecan, hickory, persimmon, mulberry, sweetgum, and most fruit trees are favorites. American sycamore, ash, and a wide range of ornamentals also get hit.

Here’s the good news on fall webworm: most of the time, you can ignore them. They show up in late summer when the tree has already done its growing for the year and is about to drop leaves anyway. A healthy mature tree that loses 20 percent of its late-summer foliage to webworm has no measurable problem the following spring. The damage looks bad. The biological impact is small.

Young trees, recently transplanted trees, and trees under other stress get a different recommendation. If a five-year-old walnut has webs on a third of its branches in August, that’s enough leaf loss to matter the next year, and treatment is worth the trouble. Same if webworms hit the same tree two or three years running.

Bagworms

Bagworms (Thyridopteryx ephemeraeformis) are the species that earns the alarm. They’re not a common problem in California, but across the Midwest, mid-Atlantic, and southeast they’re one of the most economically damaging tree pests for homeowners. A heavy bagworm infestation can defoliate a mature arborvitae in a single summer, and an arborvitae that loses most of its needles often does not refoliate. The plant dies branch by branch over the following year.

The bag is the giveaway. Each larva builds a spindle-shaped cone out of silk and bits of the host plant’s foliage, and it carries the cone with it as it feeds. On arborvitae and juniper the bags look like little tan or brown cones, an inch or two long, hanging from the branches. They blend in with normal cone-like foliage so well that homeowners walk past them for months. By the time the bags look obvious, the larvae have been feeding for six to eight weeks.

Eggs hatch in late May to early June across most of the US. The first-instar larvae are the only stage BT reliably kills, because the bag is still small and the spray reaches the caterpillar through the open top. Once the bag grows past about half an inch, the larva spends most of its time inside, and contact sprays bounce off the silk.

Hosts are arborvitae, juniper, cedar, leyland cypress, and to a lesser extent spruce, pine, and many deciduous trees. Penn State Extension publishes detailed bagworm timing for the mid-Atlantic at extension.psu.edu, and the eastern timing is broadly similar across the bagworm range. If you live in a state with bagworms, mark the calendar for the first week of June and walk every conifer in the yard with a flashlight.

The economic damage comes from how arborvitae responds to defoliation. A deciduous tree leafs back out the following spring. An arborvitae that lost 70 percent of its foliage to bagworms in July does not push new growth from the bare wood; that section stays bare permanently, and the plant either dies or has to be cut down to a stump and regrown. A row of arborvitae used as a privacy hedge is a $400-to-$2,000 replacement cost, depending on size and length. This is the pest where prevention matters most.

Damage assessment, or when defoliation actually matters

A healthy mature deciduous tree tolerates one season of moderate defoliation without lasting harm. That’s the baseline rule, and it’s why most homeowners can ignore most caterpillar damage on most years. The tree pushes a second flush of leaves, burns some stored sugar, and gets through the year.

The exceptions:

• Conifers. Arborvitae, juniper, cedar, and spruce do not refoliate the same way deciduous trees do. Bagworm damage on these species is closer to permanent. Treat early, before defoliation hits 30 percent.
• Young trees. Anything planted in the last three years has small carbohydrate reserves. Even moderate defoliation can kill a recently planted tree. Treat any visible caterpillar damage on a young tree.
• Repeated years. Two consecutive years of 40 percent or higher defoliation will weaken any tree. Three years in a row will start causing branch dieback even on a mature healthy tree. Break the cycle by treating the second-year infestation, not the third.
• Already-stressed trees. Drought-stressed, root-damaged, recently transplanted, or recently heavy-pruned trees have less in the tank. Treat any caterpillar damage on a tree that’s already struggling.

For everything else, the right answer is usually: leave it alone, count it as feeding for the local birds, and check next year. Parasitic wasps and predatory birds eat a huge percentage of the population before the next generation, and most caterpillar outbreaks self-correct without any spraying at all.

Treatment options

The default treatment for tree caterpillars is BT, short for Bacillus thuringiensis kurstaki (sometimes labeled Btk). BT is a soil bacterium that produces a protein toxic to lepidoptera larvae. When a caterpillar eats sprayed foliage, the protein binds to its gut lining and the larva stops feeding within hours, then dies in two to four days. BT does not affect mammals, birds, fish, bees, beetles, or anything that isn’t a moth or butterfly larva, which is why it’s the standard organic recommendation for everything from cabbage worms to tree caterpillars.

The catch is timing. BT only works on actively-feeding larvae. The caterpillar has to eat sprayed leaves, so:

• The larvae have to be out of the egg.
• The leaves have to be coated where the larvae are feeding.
• The spray has to be fresh, because BT breaks down in sunlight within two to three days.

This is also why BT does nothing to sawflies, which look like caterpillars but are actually wasp larvae with a different gut chemistry. If you spray BT on sawflies and nothing dies, you don’t have a bad batch; you have the wrong species. UC IPM has a good comparison at ipm.ucanr.edu for telling sawfly larvae from real caterpillars before you reach for the sprayer.

The spray timing for the three groups in this article:

• Eastern tent caterpillars. Spray in early to mid-April when the tent is the size of a golf ball and the larvae are out feeding in the morning. Hit the foliage around the tent, not the tent itself. The caterpillars will not feed through the silk.
• Fall webworms. Spray when the web is the size of a softball, which is usually mid to late July. Earlier is better. Once the web is bigger than a basketball, BT still works on the leaves outside the web, but you’ll have to repeat the application.
• Bagworms. Spray during the first two weeks of June, when first-instar larvae have emerged and bags are still under half an inch long. This is the only window where BT reaches the larva. Miss it by two weeks and you’ll be hand-picking bags in October.

For larger caterpillars, BT loses some of its effectiveness because the larva eats less per day as it nears pupation. Spinosad is a stronger alternative that works on a wider age range, though it has more bee impact and should be applied in the evening when bees are off the foliage. Bonide Neem Oil Concentrate is another option for younger larvae, working as a contact spray and feeding deterrent rather than a stomach poison, which means it’s useful for caterpillars that have stopped feeding heavily but are still on the leaves.

Physical removal works on tent caterpillars and bagworms where the spray window has already closed. For eastern tent caterpillar tents, the best time is early morning when the larvae are still inside. A pair of Felco F2 bypass pruners on a small tree, or a pole pruner on a taller cherry, clips out the infested branch tip. Drop the whole thing into a bucket of soapy water; do not just throw the tent on the ground, because the larvae will crawl back up the trunk. For fall webworms higher in a tree, a pole pruner cuts out the branch tip with the web attached. For bagworms, hand-pick every visible bag from October through April. Each bag may contain 500 to 1,000 overwintering eggs, and hand-picking 50 bags off an arborvitae now prevents tens of thousands of larvae next June.

Beneficial insects do most of the long-term population control. Parasitic braconid and ichneumonid wasps lay eggs inside caterpillars; the wasp larvae eat the host from the inside and emerge as adults. You can encourage these wasps by leaving a few flowering plants near the trees they need to protect, since adult parasitic wasps feed on nectar. Broad-spectrum insecticides wipe out the parasitic wasps, which is part of why caterpillar problems often get worse on heavily-sprayed properties. Stick to BT when you can.

One safety note worth knowing: BT is one of the only pesticides considered safe enough that it’s used on commercial organic vegetable crops up to the day of harvest. If you spray BT in the morning, kids and pets can be back in the yard that afternoon. This is the same logic mklibrary covers in their notes on garden cleanup and reducing chemical use in spaces where kids and pets play. If you’ve avoided treating caterpillars because you were worried about a stronger pesticide near the vegetable garden, BT removes that objection.

Prevention

Prevention pays off the most on bagworms and the least on fall webworms, in that order, with eastern tent caterpillars in the middle.

Dormant oil for eggs. Eastern tent caterpillar eggs are laid in glossy black collars wrapped around pencil-thick twigs on cherry, apple, and crabapple. They’re easy to spot in winter once the leaves are off. A horticultural oil spray in late February or early March suffocates the eggs before they hatch. Same approach works on a few other caterpillar species that overwinter as eggs on twigs.

Hand-pick bagworm cones in winter. From October through April, every bagworm cone you see hanging from arborvitae or juniper is an overwintering female full of eggs. Snip them off, drop them in soapy water, and you’ve prevented the next year’s population in about an hour of work. This is the single most effective bagworm control because it skips the timing problem altogether.

Sticky barrier bands for trunks. Young first-instar bagworms can balloon on silk threads to nearby plants, but they also crawl up the trunk of the host plant after dropping. A sticky band wrapped around the trunk in late May catches a meaningful percentage. This works on tent caterpillars and forest tent caterpillars too. It does not work on fall webworms because they emerge from pupae in the soil and fly to the host as moths.

Don’t plant solid arborvitae hedges in bagworm country. This is the harder advice. If you live in the Midwest, mid-Atlantic, or southeast, and bagworms hit arborvitae in your neighborhood every few years, planting a 30-tree privacy hedge of arborvitae is signing up for a bagworm problem. Mix in non-host species: holly, boxwood, viburnum, or mixed deciduous understory. A mixed hedge survives a bagworm outbreak. A solid arborvitae row often doesn’t.

Spring monitoring. The general spring tree care calendar covers when to walk the yard and check for pest activity. See our spring tree care tips for the broader month-by-month list. For caterpillars specifically, mark April for cherry tents, late May to early June for bagworms, and mid-July for fall webworms.

A side note for homeowners who already deal with aphids and other sucking pests: the chewing-pest treatment plan is completely different. BT does nothing to aphids, scale, mites, or whiteflies. Different gut, different feeding behavior, different chemistry. Don’t try to use one spray for everything.

When to call an arborist

Most tent caterpillar and webworm problems are DIY scope. The cases where it’s worth bringing in a certified arborist:

• A mature shade tree with high webworm nests you can’t reach with a pole pruner. Renting a lift to remove webs from a 40-foot pecan costs more than hiring an arborist for half a day.
• A long arborvitae hedge with established bagworm infestation. Treating 60 arborvitae with timed BT and follow-up hand-picking is a multi-day project that an arborist can knock out in a morning.
• Three or more consecutive years of heavy defoliation on any tree. At that point you’re past the threshold where the tree can recover on its own, and you want a professional assessment before you lose the tree to secondary pests or disease.
• Bagworm on tall blue spruce or leyland cypress in a property line hedge. These are harder to treat because the upper foliage often holds the worst infestation and a homeowner sprayer can’t reach.

Expect $300 to $800 for a single targeted spray treatment from a licensed arborist, more for a multi-tree property. Worth it for a $2,000 hedge.

FAQ

Do tent caterpillars kill trees? Almost never on their own. A healthy mature deciduous tree tolerates one year of moderate tent caterpillar defoliation without lasting damage. The risk shows up with repeated annual infestations on the same tree, on young recently-planted trees, and on trees already stressed by drought or root damage. Bagworms are the exception: they can kill an arborvitae in one or two seasons because conifers don’t refoliate the way deciduous trees do.

What is the best spray for caterpillars on trees? BT (Bacillus thuringiensis kurstaki) is the standard treatment for tent caterpillars, fall webworms, and bagworms. It’s a bacterial toxin that affects only moth and butterfly larvae, is safe for mammals, birds, and bees, and works as a stomach poison after the caterpillar eats sprayed foliage. Timing matters: spray when larvae are small and actively feeding. Spinosad is a stronger alternative for larger larvae, and neem oil works as a backup for younger stages.

Why don’t beetle sprays work on bagworms? Bagworms are moth larvae, not beetles, and the protective silk bag blocks contact sprays from reaching the caterpillar. Beetle treatments that work on contact, like pyrethrins, can’t penetrate the bag once it’s grown past about half an inch. The only effective spray window for bagworms is early June, when first-instar larvae have just emerged and bags are still small enough that BT reaches them through the open top.

When should I spray BT for caterpillars? For eastern tent caterpillars, spray in early to mid-April when tents are the size of a golf ball and larvae are out feeding. For fall webworms, spray in mid to late July when webs are the size of a softball. For bagworms, spray during the first two weeks of June, when first-instar larvae have emerged but bags are under half an inch. BT only works on caterpillars that actively eat sprayed leaves, so timing the application to active feeding is the entire game.

Are fall webworms the same as tent caterpillars? No. Eastern tent caterpillars build silk tents in the forks of cherry and apple branches in spring, from late March through May. Fall webworms wrap the tips of branches in dirty gray webbing in July through September, on more than 100 different host species. Different life cycle, different damage pattern, different month, often different tree. The treatment (BT) is the same, but the timing is months apart.

Sources: UC IPM Pest Management Guidelines, Penn State Extension on bagworm timing, University of Minnesota Extension on fall webworm host range, and the USDA Forest Service for forest tent caterpillar outbreak data.