Species

"Caterpillar" is a catch-all term referring to the larval stage of butterflies and moths belonging to the order Lepidoptera. There are about 21 different families of Lepidoptera common to the Pacific Northwest. They include Noctuidae (cutworms and armyworms) and Geometridae (inchworms and loopers).

Host/Site

Most commonly found on plant leaves and stems. Some caterpillars, such as Codling Moth or the corn earworm, feed on fruits and vegetables. These species can cause much more harm than leaf-eating caterpillars.

Identification

Appearance
Species vary greatly in color, body pattern, and the presence of hair, head horns, spines, knobs, and other identifying features. It is important to identify the species of your caterpillar pest for more effective control.

Their are several tools available online to help you identify the pest you found in your garden or vegetable patch.

The Univesity of Georgia caterpillar indentification site

Texas A&M caterpillar identification site

What's this North American Caterpillar

Indicators
Most caterpillars eat leaves of certain plants. Some, such as armyworms, skeletonize leaves, leaving only veins. Others, such as alfalfa caterpillars, consume the entire leaf, midrib (central vein) included. Some caterpillars fold or roll leaves together with silk for shelter.

Life Cycle

Caterpillars are the larval stage of butterflies and moths. They hatch from eggs into caterpillars and then go through four or five instars (life stages between molts). The pupal stage is known as a chrysalid, where the insect forms a cocoon around itself and often attaches to a tree limb. The insect emerges as a moth or butterfly

Natural Enemies

Natural predators of caterpillars occur in abundance. In a healthy garden they should be able to take care of a caterpillar infestation so that you don't have to take action. Some common natural enemies are rodents, reptiles, bats, birds, spiders, nematodes, beetles, true bugs, and parasitic wasps.

Monitoring

Discover infestations early by inspecting plants every few days for eggs or small larvae. Catching an infestation early makes control much easier.

Salt Marsh Caterpillar eggs on underside of leaf Photo courtesy of Colorado State University

Action Threshold

Injury often appears more serious than it is. Seedlings and small plants are the most vulnerable to caterpillars. Control action should be taken as soon as caterpillars or damage is noted on seedlings. Larger plants, especially trees, can take a substantial amount of damage before fruit yield begins to decrease. Action should also be taken if damage exceeds aesthetic standards.

Physical/Mechanical Controls

Large caterpillars can be picked off of leaves by hand or cut in half with gardening snips. Egg masses can also be hand-picked off the plant. For an infestation of caterpillars that roll into leaves, remove the entire leaf and destroy the caterpillar inside of it.

Many leaf-feeding caterpillars will spend the spring season feeding on weeds before plantings provide a source of food. Weeding in planting areas before your plants begin to grow in early spring may prevent caterpillar populations from developing.

Remove and destroy crop debris after harvest to prevent overwintering.

When possible, keep lights near the garden turned off at night. Lights attract moths that fly at night and may lead them to lay eggs in the garden.

The saltmarsh caterpillar and armyworm can migrate into growing areas in large numbers by crawling across ground. To prevent them from accessing your garden, erect a 6 inch high heavy aluminum foil barrier. Bury at least 1 to 2 inches in the ground for stability, and slant it away from the garden slightly.

Biological Controls

Parasitic Nematodes such as Neoaplectana carpocapsae can be an effective control for caterpillars, in particular species of borers.

Parasitic Wasps can also be effective, especially Trichogramma spp.
T. pretiosum is a parasite of caterpillars that destroy vegetable crops.
T. platneri can be effective against caterpillars of fruit orchards and vineyards in warmer climates.
T. minutum is also effective against caterpillar pests of fruit trees and vineyards.
Trichogramma spp. is most effective when at periods of peak egg laying.

Be sure to identify the species of caterpillar in order to choose an effective biological control. If you purchase biological controls, follow all instructions that are included.

Chemical Controls

Insecticide control can often be counterproductive if the product is non-selective. Applications may cause harm to beneficial insects such as pollinators and insects that prey on caterpillars. The net result can be an increase in caterpillar population.

The microbial insecticide Bacillus thuringiensis kurstaki (Btk) is effective against live caterpillars and leaves natural enemies unharmed. Btk must be applied to the leaves of the plant and the caterpillars must continue to feed. Btk causes feeding to cease within 5 days. Btk is most effective when populations are small, when continued feeding will not damage plants.

Use of horticultural oils can be effective on destroying eggs.

References

"Caterpillars of Pacific Northwest Forests and Woodlands." 2006. United States
Geological Survey. 5 Sept 2008. <http://www.npwrc.usgs.gov/resource/insects/catnw/ecol.htm>.

Flint, Mary Louise. Pests of the Garden and Small Farm: A Grower's Guide to Using
Less Pesticide. University of California, 1999.

Lifton, B. Bug Busters. Garden City Park, NY: Avery Publishing Group, 1991.

Rosetta, Robin. "Fall webworm." 2007. Oregon State University Department of
Horticulture. 1 Oct 2008. <http://oregonstate.edu/dept/nurspest/fall_webworm.htm>.