Bacillus thuringiensis kurstaki
Bt (kurstaki)- Rate
- Label rate — ½–4 tsp per gallon on home concentrates; varies by product
- Best for
- Small webs, soon after hatch
Pest
Fall webworm
Hyphantria cunea
The short answer
Fall webworm is a native North American caterpillar that builds loose silk webs over the ends of branches from midsummer into autumn. The caterpillars live and feed inside the web, which can grow to cover two or three feet of branch. The webs are ugly, but the damage is mostly cosmetic. Penn State says it rarely kills trees, and Clemson says the feeding is usually limited to a branch or two. On most shade trees the right answer is to prune out the webs you can reach and leave the rest to the birds and wasps. Orchards and small trees are the exceptions.
hero · loose web enclosing a branch tip, caterpillars inside
Hyphantria cunea web over the end of a branch in late summer. Photo: pending.
Identification
The web is the thing to recognise. It is loose, grey-white and dirty with droppings, and it encloses the leaves at the end of a branch rather than sitting in the fork. The caterpillars stay inside it and feed there, extending the web over fresh leaves as they grow. That is the tell that separates it from eastern tent caterpillar, which builds a dense tent in a branch crotch in spring and leaves it to feed.
Young caterpillars are pale yellow with two rows of black marks. Full-grown ones are about 1½ inches long, very hairy, and green, yellow or tan, with a broad dusky stripe down the back and two rows of black dots. There are two races: black-headed caterpillars mostly in the north and red-headed ones mostly in the south, which lay their eggs at different times. The moth is white, sometimes with black spots, with a wingspan of about 1¼ to 1½ inches. The eggs are laid in masses of several hundred, covered in white hairs, on the leaves.
Easily confused with
- Eastern tent caterpillarAlso a pestBuilds a thick tent in the fork or crotch of a branch in spring, and the caterpillars leave it to feed. Fall webworm’s loose, dirty web encloses the branch ends in summer and autumn, with the caterpillars feeding inside.
Damage symptoms
Inside the web, young caterpillars skeletonise leaves, eating the surface and leaving a lacy net. Older ones eat all but the main veins. Even when it strips a tree, it rarely kills it. The losses that count are in pecan and fruit orchards and on small ornamentals, which can be stripped completely.
- Loose grey-white webs enclosing the leaves at the ends of branches, sometimes 2–3 feet across
- Brown, skeletonised leaves inside the web, with droppings caught in the silk
- Hairy caterpillars wriggling inside the web when it is touched
- Webs appearing from midsummer and most obvious in late summer and early autumn
- In the South, webs from spring onwards, with further generations through the summer
Lifecycle & degree days
The number of generations depends on latitude, which is why fall webworm is a late-summer pest in the North and a season-long one in the South. In Pennsylvania the moths start emerging in mid-June and keep appearing in small numbers for most of the summer, so webs of different ages turn up over several weeks.
- Generations / yr
- 1 in Canada and New England; 1–2 in Pennsylvania; 2–5 in the South
- Moths
- From mid-June in Pennsylvania, in small numbers all summer
- Egg hatch
- About a week after laying
- Caterpillar stage
- 4–8 weeks, up to 11 instars, all inside the web
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Host crops
Almost any deciduous tree. The counts vary by source and region: almost 90 deciduous trees in Penn State's fact sheet, more than 300 hardwoods in Clemson's fact sheet, and more than 400 forest and shade trees in the 2017 national review. Pecan, hickory, walnut, persimmon, maple and sweetgum are the common hosts in the East; Penn State adds birch, cherry and crabapple.
Monitoring & action thresholds
Action threshold
None on most shade trees
Penn State says fall webworm rarely kills trees and mainly reduces their ornamental value. Clemson says that in most cases defoliation is limited to a branch or two. We found no published home-garden threshold. Treatment is worth considering only on small trees that could be stripped completely, and in pecan and fruit orchards, where the 2017 review records serious losses.
From midsummer, look at the ends of branches for new webs, which are small and easy to cut out in the first week or two. In the South, start in spring. A dense tent in a branch fork in spring is not this insect; it is eastern tent caterpillar.
Beneficial
Biological control
Fall webworm is native, and a crowd of natural enemies keeps it in check. The 2017 national review counts at least 50 species of parasitic flies and wasps, plus 36 species of predators and parasites, and birds, wasps and diseases all take a share. Tearing a web open lets them in. The control you can buy is Btk, which works if it goes on while the colonies are young.
Cultural controls
Most webs can be removed by hand faster than they can be sprayed.
- Prune out webbed branch tips while the webs are small
- Tear larger webs open with a rake or pole so birds and wasps can reach the caterpillars
- Remove leaves carrying egg masses before they hatch, where you can reach them
- Leave webs high in large trees alone — on a big shade tree the damage is cosmetic
Chemical, last resort
The web shields the caterpillars — spray only small, young coloniesCheck any product before it goes on. Open the compatibility matrix →
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
Schowalter & Ring, Biology and Management of the Fall Webworm, J. Integr. Pest Mgmt 8(1):7 (2017)
Penn State Extension, Fall Webworm (Hoover, 2001)
Clemson HGIC, Fall Webworm (Elferdink & Coyle, 2022)
UNH Extension, Fall webworm & Eastern Tent Caterpillar fact sheet
Bonide Thuricide concentrate label, EPA Reg. 4-226 (2016)
Monterey B.t. label, EPA Reg. 70051-106-54705 (2014)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Fall webworm by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.