Fall webworms will eat the leaves but not harm the woody parts of a tree. (Photo by Dan Gill)

For Release On Or After 06/20/14

By Dan Gill
LSU AgCenter Horticulturist

Caterpillars of various types are common pests in vegetable gardens, flower beds, trees, shrubs and even lawns all through the summer. Fall webworms infest a wide variety of fruit and nut trees as well as ornamental trees and shrubs. The large nests of silk webbing they produce are noticeable and widespread.

First, it’s important to know what caterpillars are. Basically, they are baby moths or butterflies. Moths and butterflies belong to the Lepidoptera family of insects. Lepidoptera means scale wing, referring to the scales that cover the wings of butterflies and moths and give them their color. Members of the Lepidoptera family pass through four distinct phases in their lives: egg, larva (caterpillar), pupa (cocoon or chrysalis) and adult.

If you wonder where the caterpillars feeding on your plants came from, an adult moth or butterfly flew to the plant, laid the eggs and flew off. Female moths and butterflies do not lay eggs on just any plant. They choose only the plants that will properly nourish their caterpillars. Most of these insects feed on a specific group of related host plants. Some, however, have wide-ranging tastes, and females will lay their eggs on many different kinds of plants.

When moths or butterflies are in the egg, pupa and adult stages, they pose no direct threat to our plants. But the larval stage is another matter. We call moths and butterflies in the larval stage caterpillars. You will also hear caterpillars called “worms,” and the word worm appears in the common name of many caterpillars. But don’t confuse them with the true worms, like earthworms

The caterpillar phase is basically the stage when the insect eats large amounts of food to grow and develop. Caterpillars have chewing mouthparts; they bite into tissue and swallow it. Caterpillars of various types feed on virtually every part of the plant. They bore into stems and trunks (peach tree borer, squash vine borer), they eat fruit (tomato fruitworm) and they chew on flowers. But by far, the caterpillars we most often have problems with are those that feed on the foliage.

Fall webworms (Hyphantria cunea) are a common caterpillar we see every year, and the female moths lay eggs on a variety of trees and some shrubs. I’ve gotten reports of them on pecan, persimmon, redbud, Bradford pear and yaupon holly, to name a few.

Despite the common name, fall webworms begin to show up in our gardens in early summer. Several generations extend from early summer into fall. So although you may get rid of those you see now, you may see new infestations later in the season.

Female fall webworm moths lay masses of eggs on the undersides of leaves. When the caterpillars hatch out of the eggs, the siblings are gregarious and stay together. They enclose the foliage they are feeding on in a silken web. As the caterpillars grow, they expand the nest and enclose more and more foliage. The webbing serves to protect the caterpillars from predators like birds and wasps.

When the caterpillars have reached full size, they stop eating and leave the tree. They pupate in leaf litter on the ground or on the tree bark. This is very common. Caterpillars rarely pupate on the plant upon which they fed.

The leaves inside the webbing are killed by the feeding and turn brown. But the caterpillars are just eating the foliage. The branch itself is not attacked and often survives. The webbing can persist long after the caterpillars have gone. There is no need to attempt control if the silken nests are already empty.

Caterpillars that feed on foliage of trees and shrubs, like fall webworms, are not life-threatening to the plants they infest. They could eat every leaf, and the tree or shrub would recover by sending out new growth.

But where controling fall webworms is practical and desirable, you can spray to control the caterpillars. If the tree or shrub is small enough, you can handle it yourself. Use a stick or pole to tear open the web nests because the webbing will otherwise protect the caterpillars from the spray. Make sure caterpillars are actually still in the nest; there’s no use spraying if they’re already gone. You don’t need to spray the entire plant. Focus on the nests and nearby foliage.

Bt (Bacillus thuringiensis) and spinosad are organic insecticides that are the least-toxic options for caterpillar control. They can be used when caterpillars infest food plants like vegetables, pecans and fruit trees as well as on ornamentals.

Chemical controls for caterpillars include carbaryl and permethrin. These are good on ornamentals but are also labeled for food crops – check the label.

When large trees like pecans are infested, control is more difficult. Because home gardeners don’t have the equipment to spray trees that large, you would need to have this done by a commercial tree company.

Stinging caterpillars are also out this summer. The saddleback caterpillar, puss moth caterpillar and IO moth caterpillar can all be found on a variety of trees and shrubs through summer. Children, in particular, should be shown what they look like and cautioned about touching them. They can be controlled with the products already mentioned.

For pictures of these stinging caterpillars, the LSU AgCenter has an excellent publication you can see online by doing an Internet search using “LSU AgCenter stinging caterpillar” and clicking on the link. Or contact your local LSU AgCenter extension office for a hard copy.

Rick Bogren