There are several known methods of controlling tree webworms. Some techniques use pesticides and other pest controls are available by organic means. Whichever method of control is used, tree webworms should be contained to avoid devastation of tree vegetation and fruit and nut production. Actual damage can occur on fruit and nut trees as defoliation affects tree vigor, yield, and nut quality.
Preferred Hosts of the Tree Webworm
The fall webworm, Hyphantria cunea, has been recorded on at least 88 species of shade, fruit, and ornamental trees in the United States. The preferred hosts vary from one area to another. The most common hosts in the southern United States are pecan, persimmon, black walnut, and hickory. In areas of heavy infestation the webworm has been reported to infest sycamore, birch, cottonwood, American elm, bald cypress, and redbud trees.
Damage is caused by the larvae (webworm) feeding on the leaf of the tree. Infestation rarely defoliates the tree, except for young pecan or persimmon trees. The insect is just ugly and is more of a nuisance than a threat to the health of the tree.
Lifecycle of the Webworm
In early spring, moths come out of cocoons hidden in the bark of tree trunks and in ground debris. The adult moths are about two inches from wing to wing and are white or white spotted with brown. The eggs are laid in clusters on the leaves of suitable host plants. The eggs hatch within a week and the first generation arrives as tiny caterpillars. The first infestation is usually mild and unnoticed.
The caterpillars eat their fill by early summer, form cocoons and pupate. By midsummer they re-emerge as moths. A second generation follows, but this time larger and more destructive. In warmer climates there can be as many as four generations in one season. The adult female lays 400 to 500 eggs in masses on the undersides of leaves.
Controlling Webworms with Pesticides
- Bacillus thuringinsis )B.t.)- Selective to caterpillars (also butterfly) only and not harmful to other beneficial insects.
- Tebufenozide (Confirm) – Selective to caterpillars (also butterfly) only and not harmful to other beneficial insects.
- Acephate (Orthene)
- Carbaryl (Sevine)
- Chlorpyrifes (Dursbana, Lorsban)
- Diazion
- Malathion
Insecticide label clearances are subject to change and may occur since this publication. The user is always responsible for the effects of pesticides on his or her property, as well as drift to other properties. Always read and follow carefully the instructions on the label. Professional lawn and tree companies can spray tall trees.
Controlling Webworms the Organic Way
- Remove the webs from the tree with a rake, long stick, or other pole that will reach the web. Even if you cannot completely remove the web, opening the web up will allow beneficial birds, wasps, and other insects to feed on the caterpillars.
- Remove the infested branch and soak the web in a bucket of soapy water to kill the caterpillars.
- Using a long pole, wire wrap a rag around the end, ignite the rag and torch the webworm nest. Wear protective headgear and glasses to avoid falling debris. A burning permit from the local fire department may be necessary for this procedure.
- Since they are just ugly, the best organic method is to leave them alone.
Preventative Measures
Actions can be taken to prevent severe infestations. In the winter or early spring, remove fallen leaves, ground debris and mulch, which may harbor overwintering webworm pupae. Inspect susceptible plants for the greenish egg masses, typically laid on the undersides of leaves and protected by a white woolly or scaly covering. Remove any affected leaves and destroy them. This tactic is impractical for tall trees.
The trick to preventing unchecked rampages of fall webworms is attacking the pest when it is most vulnerable. A little work in winter, spring and summer goes a long way toward reducing the damaging infestations of autumn.
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