Fall webworm
Hyphantria cunea
Gray webbing wrapped over the ends of branches in late summer, with the caterpillars feeding inside it. Ugly, late in the season, and almost never worth treating.
How to know it
A pale hairy caterpillar 25–35 mm long, yellow-cream to gray with two rows of black spots and long white hairs. The web is the identification and it is at the branch tip, enclosing the leaves the caterpillars are eating, in late summer — the tent caterpillar builds in a branch fork, in spring, and leaves its tent to feed.
What you'll see
- Loose gray-white webbing over the end of a branch, enlarging as the caterpillars eat outward.
- Skeletonized and brown leaves inside the web, along with frass and cast skins.
- Several webs on one tree, usually on the outer and lower branches.
- Damage appearing from July onward, when the tree has already made its year growth.
- Webs hanging on through the winter, empty and weathered.
Where it comes from
One to four generations a year depending on latitude. Moths lay hair-covered egg masses on the underside of leaves in early summer; the caterpillars web the shoot and feed together inside it for four to six weeks, extending the web as they go, then drop to pupate in bark crevices and litter. They overwinter as pupae.
Roadside and boundary trees, especially walnut, pecan, cherry and mulberry, and worst in years following a warm spring. Because it feeds so late, the leaves it takes have already done most of their work for the tree.
Dealing with it
- Pull the webbed shoot off with a pole pruner and drop it in soapy water, or simply tear the web open so the birds and wasps can get in.
- Do not burn the web in the tree.
- Spray Bt into an opened web if a small ornamental is heavily hit; a closed web sheds any spray.
- On a mature tree, do nothing. This is a cosmetic problem in September.
- Remove the old empty webs in winter if the look bothers you.
Keeping it out
- Check trees in early July and remove the first small webs, which is far easier than the large ones.
- Encourage birds, paper wasps and parasitic wasps, all of which work the webs hard once they are opened.
- Keep young trees watered so a late defoliation is not a stress on top of a drought.
- Accept it on boundary trees; the population collapses on its own.
- Do not spray broadly for it — the collateral damage exceeds the problem several times over.
What it goes for
Walnut, pecan, cherry, apple, mulberry, birch, elderberry, hazel and around a hundred other broad-leaved trees.
Who hunts it for you
- BatsChiroptera A single bat takes hundreds of moths and mosquitoes in a night, and the moths are the parents of your cutworms, corn earworms and hornworms. Put up a bat box on a warm south wall, leave a night-scented corner unlit, and never spray after dark. Grow to recruit it: Evening Primrose,Honeysuckle,Nicotiana,Summer Jasmine
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com