Article
Caterpillars on a tree: tents, webs and bare branches, and when to leave them
What a silk tent, a web at a branch tip, bags on an evergreen or oaks bare in June mean, why most need nothing, and when and how to step in without spraying.
On this page
What each sign meansWhy most caterpillars are worth leavingWhen to step in, and howThe year after a tree is strippedWhere to startCommon questionsSourcesMost caterpillars on a healthy leafy tree can be left alone. Birds eat them, wasps and flies raise their young inside them, outbreaks collapse within a few years, and a tree stripped in spring usually grows new leaves by late July. Bagworms are the exception, because on a red cedar or an arborvitae they can kill the plant. The other tree worth protecting is a young or weak one facing a second year of stripping, and that work starts by hand.
What each sign means
The rows run in the order the signs appear, from spring into fall:
| What you see | What it is | When | Trees it favors | What to do |
|---|---|---|---|---|
| A silk tent in the fork of a branch | Eastern tent caterpillar | Leaf-out to early summer | Black cherry, chokecherry, crabapples and plums | Leave it, or pull the tent out at dusk |
| Caterpillars massed on a silk mat on the trunk | Forest tent caterpillar, which spins a mat, not a tent | May and June | Aspen, maples, basswood and water tupelo | Leave it: the tree leafs out again |
| Oaks bare in June, with no tent or web | Spongy moth, formerly called gypsy moth, brought from Europe in the 1860s | May to late June | White oak, chestnut oak, other oaks and birches | Band the trunk with burlap, and protect a young tree |
| Brown bags of silk and needles hanging from twigs | Bagworm | Feeds June into fall, and the bags stay all winter | Eastern red cedar, northern white cedar, other junipers and arborvitae | Act: pick off the bags before the eggs hatch |
| Leaves eaten to lace between the veins | Japanese beetle, not a caterpillar | July and August | American basswood, crabapples, cherries and birches | Knock the beetles into soapy water |
| A loose web over the end of a branch | Fall webworm | Mostly July into fall | Hickories, walnuts, birches and crabapples | Leave it, or cut out a web within reach |
A tent caterpillar carries a white stripe down its back, a forest tent caterpillar a row of white keyhole spots, and a spongy moth caterpillar pairs of blue and red dots. We do not grow aspen.
Why most caterpillars are worth leaving
The trees these insects favor are the ones that host the most kinds of caterpillars. In the National Wildlife Federation’s list of keystone plants for the eastern forests, oaks host 436 kinds of caterpillars and cherries and plums 340, the highest counts by genus. Birches host 284 and crabapples 237. Blue Whale Trees grows every tree linked in the table from seed, the cherries and oaks among them. Ninety-six percent of land birds rely on insects that such plants support, and birds are among the tent caterpillar’s many enemies.
The two tent caterpillars are natives, kept in check by birds, predatory insects, diseases, and wasps and flies that develop inside them. A stripped tree leafs out again the same summer, as a northern catalpa does after catalpa sphinx caterpillars strip it. Fall webworms feed late, after the leaves have made most of the year’s food, so their damage is mostly cosmetic:
“Tents that are out of reach near the tops of taller trees can be left alone since any defoliation they cause will not affect the overall health of an established tree.” (The Morton Arboretum, Fall Webworm)
We advise against spraying a whole tree for caterpillars. Bt, the bacterial spray sold for them, spares birds and bees but kills butterfly and moth caterpillars along with the pest. Other insecticides put beneficial insects and birds at risk.
When to step in, and how
Step in for bagworms on an evergreen, for a young tree, and for any tree facing a second stripping in a row. Work by hand first:
- Tents. Pull a tent out with a stick at dusk or early in the morning, when the caterpillars are inside, and destroy it. In winter, prune off the shiny egg masses that ring the twigs.
- Bagworms. Pick or snip off the bags before the eggs inside hatch in late spring, and drop them in soapy water. On a hedge, check the plants beside the first one hit, since the damage spreads along the row.
- Spongy moth. Tie a strip of burlap 18 inches wide around the trunk with a string at its middle, making a two-layered skirt. Caterpillars collect in it, and you scrape them into soapy water.
- Japanese beetles. Knock them into soapy water in the cool of the morning or evening, and skip the traps, which draw more beetles than they catch.
A spray belongs only on a young or weak tree, or one facing a second year of stripping, and only while the caterpillars are small. Bt works best within one to two weeks of hatching, sprayed on the leaves, since a caterpillar must eat it. It does little against older caterpillars.
Never spray a tree in bloom, when bees are working it. A web within reach is cut out with its whole colony inside, so it needs no spray. Never burn a tent either:
“Besides potential danger to nearby structures, the fire will cause more damage to the tree than the caterpillars could.” (Penn State Extension, Eastern Tent Caterpillar)
The year after a tree is stripped
A healthy tree takes one stripping well, and often two or three in a row. Hardwoods stripped in June usually leaf out again in mid to late July, though a heavy year of feeding slows their growth.
The danger is a run of years, or a stripping on a tree already weak. Heavy stripping four or more years in a row can kill a tree, and so can stripping on a tree stressed by drought. For oaks, fewer years can be enough. A white oak stripped by caterpillars can die when a root fungus and the two-lined chestnut borer follow, and two or three years of stripping in a row have killed many black oaks.
Water a stripped tree at least once a week through a dry spell, as an established tree in drought is watered. Then watch its crown for dieback, which repeated stripping can bring.
Where to start
Match the sign to the table first, and leave tents, mats and webs on a healthy tree to the birds. Then:
- Describe anything the table does not match on our support page; a photograph can follow by email once we reply.
- Plant for birds: the tree finder, set to wildlife value, picks the trees and shrubs we grow that feed or shelter them, by zone, size, sun and soil.
- Read why an oak is the best tree for birds in native trees for birds.
Common questions
Should I spray caterpillars on my tree?
Not on a healthy tree, which grows new leaves the same summer while birds and parasitic wasps end the outbreak. Pull out a tent within reach at dusk, and save a spray for a young tree facing a second stripping.
Why did my oak lose its leaves in June?
Likely spongy moth or forest tent caterpillars, which strip oaks from May to late June without building a tent. The tree usually leafs out again by late July, so water it weekly in a dry spell.
How do I get rid of bagworms on an evergreen?
Pick or snip the bags off before the eggs hatch in late spring and drop them in soapy water. Bagworms can kill an arborvitae or a red cedar, so they are the caterpillar to act on.
What are the webs at the ends of tree branches?
Fall webworms, caterpillars that feed inside a loose web over the end of a branch from about July into fall. Their damage comes late in the season and is mostly cosmetic, so cut out a low web and leave a high one.
Sources
(9 references)
- Eastern Tent Caterpillar: Penn State Extension (Michael J. Skvarla, 2025): tents in the forks of branches in spring and early summer, unlike the fall webworm’s at branch tips, cherry and crabapple as hosts, the white stripe down the back, a native insect with birds, predators and parasitoids among its enemies, caterpillars that shelter in the tent and feed away from it, shiny egg masses on the twigs pruned out in winter, tents removed with a stick and never burned, and Bt for young caterpillars.
- ENTFACT-424: Tent Caterpillars: University of Kentucky Entomology (Joe Collins): the eastern tent caterpillar’s plums, the forest tent caterpillar’s mat on the trunk, its keyhole spots and its maples and basswood, the fall webworm’s hickories and its tent at the ends of branches, tents destroyed at dusk or early morning, and slowed growth after a heavy year of feeding.
- Forest tent caterpillars: University of Minnesota Extension: aspen, basswood and oaks as hosts, larvae from May to the end of June, the silken mat on trunks and branches, viral disease and the wasps and flies that develop inside the caterpillars, a second set of leaves the same season, healthy trees that take two or three years of heavy defoliation, trees killed by four or more years of it or by drought, and Bt that spares birds and beneficial insects.
- Spongy Moth: UMass Extension Landscape, Nursery and Urban Forestry Program: the insect brought from France in the late 1860s, oaks and birches as favored hosts, oaks that often suffer the most defoliation, no webs or tents, feeding from May until late June, blue and red dots, healthy trees that withstand a single year of defoliation and repeated years that cause dieback or death, Bt that kills other moth and butterfly caterpillars and fails on older ones, and sprays kept until after bloom.
- Spongy moths are waning, but keep an eye on your trees: Michigan Department of Natural Resources (2025): the spongy moth’s former name, gypsy moth, the burlap trunk trap, Bt sprayed on the leaves within one to two weeks of hatching, young and vulnerable trees protected first, watering at least once a week, outbreaks that last up to three years and collapse, the native forest tent caterpillar, and hardwoods that leaf out again in mid to late July.
- Japanese beetles in yards and gardens: University of Minnesota Extension: adults that feed in July and August and skeletonize leaves between the veins, linden, crabapple, cherry and birch among their favorites, beetles knocked into soapy water in the cool morning or evening, traps that draw more beetles than they catch, and insecticides that put beneficial insects and birds at risk.
- Bagworm: UMass Extension Landscape, Nursery and Urban Forestry Program: arborvitae, juniper and eastern red cedar as hosts, bags of silk and plant bits, eggs that winter in the bag and hatch in June, feeding until pupation in fall, plant death after severe or repeated defoliation, damage that spreads along a hedge, and bags picked before the eggs hatch and drowned in soapy water.
- Fall Webworm: The Morton Arboretum: feeding in July and August on walnut, birch and crabapple among about 120 trees, damage that comes after the leaves have made most of the season’s food, the whole colony inside the web, webs within reach cut out, and high webs left alone.
- Keystone Native Plants: Eastern Temperate Forests, Ecoregion 8: National Wildlife Federation, from Douglas Tallamy’s records: caterpillar species by genus, with oaks at 436, cherries and plums at 340, birches at 284 and crabapples at 237, and the 96 percent of land birds that rely on insects.