How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Loopers arch their backs as they crawl. Cabbage loopers usually have a narrow, white stripe along each side and several narrow lines down the back. Eggs are dome-shaped, and laid on the undersurfaces of older leaves. Adult moths have brown, mottled forewings marked in the center with a small silver figure 8.

DAMAGE

Young larvae feed primarily on the undersides of lower leaves, skeletonizing them. High populations can damage seedlings severely enough to kill them or slow growth enough to inhibit uniform maturing of the crop. Older larvae may burrow into the head from the top.

MANAGEMENT

Biological Control
Loopers have many natural enemies that often keep populations below economic thresholds. Maximize the use of these natural enemies by limiting treatments, especially between thinning and heading. In some areas an important biological control agent is a nuclear polyhedrosis virus that occurs naturally in the field. Bodies of loopers killed by the virus are dark, soft, and shapeless with their body contents often spilling onto the leaves.

Another important natural enemy in southern California, the tiny parasitic wasp, Trichogramma pretiosum, attacks looper eggs and eggs of certain other caterpillars. Eggs parasitized by Trichogramma turn black as the parasite matures inside and are easy to distinguish from normal eggs that remain white except for a black spot that appears just before hatching. Other parasites that attack looper caterpillars include the tachinid fly, Voria ruralis, and three wasps, Hyposoter exiguae, Copidosoma truncatellum, and Microplitis brassicae.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically certified crops, but spinosad is very detrimental to populations of syrphid flies.

Monitoring and Treatment Decisions
Look for signs of looper feeding: plants with holes in outer leaves, feces, caterpillars feeding on edge of hole. Check fields twice a week once seedlings emerge. Loopers, cabbageworms, armyworms, corn earworms, tobacco budworms, cutworms, and other caterpillars that feed on leaves and heads of lettuce can be assessed together.

Check at least 25 plants for caterpillars in each quadrant of a 40- to 80-acre field twice a week. Fields smaller than 40 acres may require fewer samples. In fields where the crop is heading, stop at five different locations in each quadrant and sample five plants at each location.

Treat seedlings or small plants if populations of medium-sized to large loopers are large enough to stunt growth. Treat well established plants only if you find an average of more than one-half larvae per plant. For cabbage looper control, the best time to apply insecticide is in the afternoon.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
Acknowledgment for contributions to the insects and mites section:
N. C. Toscano, Entomology, UC Riverside

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