Fall webworm

What is fall webworm?

Fall webworm is a moth whose caterpillars feed in groups inside a silk web at the tip of a branch. As they eat, they pull more leaves into the web, so it keeps growing. Penn State says it feeds on almost 90 kinds of deciduous trees and shrubs, and UMass says nearly every shade, fruit and ornamental tree except conifers can be a host.

Damage is usually limited to a few branches. The caterpillars skeletonize the leaves inside the web. Penn State says defoliation of whole trees is occasional and tree death is rare; the main problem is how the webs look, and old webs can hang on through winter.

UMass says this pest usually doesn't need chemical control. Pruning out small webs you can reach, and letting the many natural enemies do their work, is usually enough.

Signs and damage

• Loose, grayish silk webs wrapped around leaves at the ends of branches in mid to late summer
• Webs that grow larger as the caterpillars feed inside
• Skeletonized brown leaves inside the web
• Old, empty webs hanging on into winter

Fall webworm identification: what to look for

• Adult: White moth, sometimes with black spots, with a wingspread of about 1 1/4 inches (32 mm).
• Larva: Young larvae are pale yellow with two rows of black marks. Grown larvae are about 1 inch long. Penn State says they are usually greenish with a dusky stripe on the back and a yellow side stripe, covered in whitish hairs. UMass describes black-headed (yellowish-white) and red-headed (brown) forms, both with long, silky gray hairs.
• Eggs: Masses of several hundred pale yellow eggs on leaf undersides, covered with white hairs.
• Damage: Silk webs at branch tips with skeletonized leaves inside.
• Timing: Adults first appear mid-June in PA. Small webs show by mid-July, and large webs are most obvious late summer into early fall.

Life cycle

Spends the winter as a pupa in a cocoon in leaf litter, bark cracks or soil. Moths start coming out in mid-June and can keep emerging in small numbers through summer. Eggs hatch in about a week. The caterpillars feed together inside the web for about six weeks, then leave to pupate in or on the soil. Pennsylvania has one or two generations a year depending on location.

When to look: Watch for small webs starting in July. Webs are most noticeable late summer through early fall.

Fall webworm look-alikes

A few things get mixed up with it:

• Eastern tent caterpillar (Malacosoma americanum): Builds its tent in branch forks and crotches in early spring, and the caterpillars leave the tent to feed. Fall webworm webs are at branch tips, wrap the leaves, and appear in summer and fall.
• Spongy moth (Lymantria dispar): Feeds in spring and early summer without a web. Penn State notes fall webworm is later, lighter yellow, and makes grayish webs.

How to control fall webworm

Cultural control

• Prune out and destroy small webs you can reach while they are small, if it won't ruin the plant's shape.
• Don't burn webs on the tree; UMass warns the fire can do more damage than the caterpillars.
• On most shade trees, doing nothing is fine; the damage is mostly cosmetic.

Biological control

• Birds, many predatory insects and parasitic wasps attack the eggs and caterpillars.
• UMass reports more than 50 parasite species and 36 predator species in North America, including the egg parasite Telenomus bifidus.

Chemical control

UMass says chemical control usually isn't needed. If treatment is warranted, Penn State says to apply a registered insecticide when webs and larvae are small, usually in July, and to treat only the webs and nearby leaves, not the whole plant. UMass lists Bacillus thuringiensis subsp. kurstaki (Bt) among options for caterpillars, and notes horticultural oil targets eggs; apply after bloom to protect pollinators. Use only products labeled for this use; the label is the law. For tall trees, use an ISA Certified Arborist or licensed applicator. Muthler Landscaping does not spray or apply pesticides; see our pesticide applicator license guide.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Check favorite hosts such as walnut, hickory, cherry and crabapple in July while webs are still small.

When to call a pro

Rarely needed. Consider an ISA Certified Arborist if webs cover much of a small or young tree year after year, or if webs are too high to prune safely.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about fall webworm

Similar pests and diseases