Controlling Caterpillars

Fall webworms

Fall Webworms

Just as plants have peak seasons, so do pests. Aphids rule the spring, beetles reign in summer, but fall is the season of the caterpillar. Many different types of caterpillars can currently be found munching on tree, shrub, and vegetable leaves in our area. In many cases they can be left alone, causing no lasting damage to plants, but there are some situations where control is needed. If you have a caterpillar outbreak that requires control, numerous insecticides are available that can do the job, including several organic products.

Understanding Caterpillars
Caterpillars, also referred to as worms, are the immature life stage of butterflies and moths. Many gardeners are reluctant to leave caterpillars untreated, perhaps haunted by memories of Eric Carle’s, “The Very Hungry Caterpillar”, and fearful that if left alone caterpillars will soon devour everything in sight. In reality this is highly unlikely since most caterpillars are extremely picky eaters. In most cases, when you find caterpillars on a plant in your garden they will only be able to feed on plants of same type, and you do not have to worry about them eating everything in your yard.

Caterpillar covered in parasitic wasp cocoons.

Parasitic wasp cocoons cover this hornworm and will eventually kill it.

Caterpillars are easy to detect by the ragged holes their feeding activity leaves in plant foliage. For healthy, established perennials, trees, and shrubs losing a portion of their leaves is not a problem. In addition, caterpillars are an excellent food source for migrating birds, which can help provide control naturally. Other natural enemies of caterpillars include wasps, spiders, parasitic wasps, tachinid flies, assassin bugs, and lacewings.

Sometimes caterpillars come in masses, and can strip the foliage from a plant in a few days. This type of feeding damage is more serious and often warrants control, especially on small or recently planted plants. In vegetable gardens, feeding damage is less tolerated since missing leaves mean less produce. Plus, who wants to eat caterpillars with their cabbage?

Control Options

Polyphemus moth caterpillar

While large, this single polyphemus moth caterpillar is not causing much damage. Control is not needed if you only find a few caterpillars here and there on established trees and shrubs.

Controlling caterpillars can be as simple as picking them off the host plant and squishing them, but most gardeners would rather not take such a hands on approach. Most pesticides will kill caterpillars, including several organic products. Advantages of using organic pesticides include that they are less harmful to beneficial insects and break down quickly so do not leave behind residues. This is also their main drawback. Because organic pesticides break down quickly, usually within a few days of being applied, they must be reapplied often to provide ongoing protection. In addition, organic products are best applied when pests are already present, rather than as a preventative.

Organic insecticidesuseful for caterpillar control include those containing B.t. and spinosad, both derived from naturally occurring bacteria. B.t., which is short for Bacillus thuringiensis, only controls caterpillars and is most effective when applied in late afternoon, since it degrades in sunlight. Younger, smaller caterpillars are more susceptible to B.t., so be sure to apply it when you first notice damage. Spinosadis a relatively new organic insecticide made from a rare type of filamentous bacteria. It persists for three to five days and can be used to control caterpillars and Colorado potato beetles.

Black swallowtail caterpillars feeding on parsley.

Caterpillars that occur in prolific groups, such as these black swallowtails on parsley, are more damaging and need to be sprayed to prevent crop loss.

Other organic pesticides are derived from plants. Pyrethrin is extracted from the flowers of certain species of chrysanthemum, though not the type we commonly grow for their fall flowers. Pyrethrin will help control a wide range of insect pests, including caterpillars. Neem is derived from the seeds of the neem tree, a native of southern Asia. It is effective for treating several common pest insects including caterpillars, aphids, true bugs, and some beetles, but is most effective when the insects are small and have not yet reached maturity.

Many of the synthetic pesticides available today are manmade versions of pyrethrin. Known as synthetic pyrethroids, this group of insecticides includes those with the active ingredients bifenthrin, permethrin, cyfluthrin, and esfenvalerate, all of which are effective for caterpillar control. Older insecticides that control caterpillars include Sevin and malathion. To determine the active ingredient in any pesticide product check the label, which will also tell you how long you have to wait after spraying to harvest edible crops. When using any pesticide be sure to read and follow all label directions. If bees are actively working flowering plants in the area wait until late evening to spray, after bees have returned to the hive.

Learn More! 
Read more about, and see pictures of, common caterpillars in landscapes:

Read more about, and see pictures of, common caterpillars in vegetable gardens:

If you have questions about gardening, contact your local Extension office.

  • If you live in Pender County, call 259-1235
  • In New Hanover County, call 798-7660
  • In Brunswick County call 253-2610
  • Or visit http://www.ces.ncsu.edu/ to find your county office
  • Visit http://pender.ces.ncsu.edu/ to post your questions to be answered by Extension’s ‘Ask an Expert’ where you see this symbol

Visit the Pender Gardener blog to stay up to date with all the latest gardening news, http://pendergardener.blogspot.com/.

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Written By

Charlotte GlenExtension Agent, Agriculture - Horticulture (919) 542-8202 (Office) Chatham County

Posted on Aug 31, 2012

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