Garden News: Dealing with caterpillars

Caterpillars of various types are common pests in vegetable gardens, flower beds, trees, shrubs and even lawns all through the summer.

We call moths and butterflies in the larval stage caterpillars, but sometimes they’re called “worms,” says LSU AgCenter horticulturist Dan Gill.

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. Various types feed on virtually every part of the plant, including stems, fruit and flowers.

“But by far, the caterpillars we most often have problems with are those that feed on the foliage,” Gill says.

One leading culprit is the fall webworm — a common caterpillar that lays eggs on a variety of trees and some shrubs.

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

When the caterpillars hatch out and grow, they expand the nest and enclose more and more foliage. The webbing protects the caterpillars from predators like birds and wasps.

The leaves inside the webbing are killed by the feeding and turn brown, Gill says.

But the caterpillars are just eating the foliage, not the branch, which often survives. The web can persist long after the caterpillars have gone.

Caterpillars like fall webworms that feed on foliage don’t threaten the lives of the plants they infest. They could eat every leaf, and the plant would recover by sending out new growth, Gill says.

You can spray fall webworms when control is practical and desirable. Bt (Bacillus thuringiensis) and spinosad are organic insecticides that are least-toxic options, Gill says. And they can be used when caterpillars infest food plants like vegetables, pecans and fruit trees as well as on ornamentals.

Other chemical controls for caterpillars include carbaryl and permethrin, which are good on ornamentals but are also labeled for food crops.

“Check the label,” Gill says.

Got a gardening question? Write to GardenNews@agcenter.lsu.edu.