First the good news: For the most part, fall webworms (Hyphantria cunea) don't kill or permanently injure the trees they infest, and are harmless to people, pets, and other living things. The bad news? Their webs are gross and temporarily disfigure ornamental trees and the overall landscape.
In late summer and early fall they spin large webs at the tips of branches of trees or shrubs and communally eat the leaves within. The webs become laden with bits of leaves and black frass (excrement).
Image courtesy of Lacy L. Hyche, Auburn University, Bugwood.org
They attack over 400 plant species, including birch, crabapple, cherry, hickory, walnut, chokecherry, maple, cottonwood and willow.
Fall webworm is native to North America, but has been introduced throughout the northern hemisphere, so that it can now be found in Europe and Russia, North Africa, and parts of Asia.
The moth itself is pretty, as moths go, with snowy white wings, sometimes with black spots. They are active at night.
Image courtesy of Lacy L. Hyche, Auburn University, Bugwood.org
Fall webworms overwinter in a cocoon in the soil or leaf litter. Adults emerge in late spring and early summer. The female lays rows of pale yellow eggs on the undersides of leaves. The eggs hatch in about a week.
Young caterpillars start out pale yellow and turn greenish as they grow, developing black or red spots that become hairy. They work together to spin webs over the leaves they feed on, protecting themselves through the six or so weeks it takes to molt. The web grows to encompass more leaves as the hungry larvae mature. Then the 1 ½ inch long caterpillars drop from the tree and search for a sheltered place to overwinter.
Large infestations are rare in most of the country, and mature trees recover. The exception is young or newly planted small trees without dense foliage. An infestation can weaken these trees, making them more vulnerable to disease, drought, or more destructive insects. In the north there is usually only one webworm generation per year, but in southern states there can be multiple generations, and become a problem in fruit and nut orchards.
Image courtesy of Pennsylvania Department of Conservation and Natural Resources - Forestry, Bugwood.org
Because fall webworms look worse than they are, and the destruction they do comes right as trees are getting ready to drop their leaves anyway, controlling them with chemicals can do more damage than the caterpillars themselves. Besides birds and small animals, many other insects eat them, including spiders, parasitic wasps and tachinid flies, helping to keep their populations under control.
If you can't stand looking at the webs, break them up using a stick or pole. This exposes the caterpillars to predators. Or prune off the branches you can reach and burn them. Do not attempt to burn them off the tree itself.
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By Dan Heims, President, Terra Nova Nurseries, Inc.
Photographs courtesy of Terra Nova Nurseries, Inc.
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