How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Looper caterpillars can be distinguished from most other common caterpillars in cole crops by their distinctive looping movement in which they arch the middle portion of their body to bring the prolegs or hind legs forward to meet the front legs. Loopers are green, usually with a narrow white stripe along each side and several narrow lines down the back. Loopers are smooth-skinned with only a few long bristles down the back; they may grow up to 1.5 inches long. Mature larvae spin silken cocoons and pupate, usually attached to leaves. Adults are brownish moths with a distinctive silvery figure-8 on the front wings. Eggs are ridged and dome-shaped and usually laid singly on the undersurface of leaves. Loopers may have numerous generations and continue to develop all year long in cole crops growing areas of California with the highest populations usually occurring in fall.

DAMAGE

Although seedlings are occasionally damaged, most injury occurs after heading. Loopers eat ragged holes into leaves, bore through heads and contaminate heads and leaves with their bodies and frass. Young plants between seedling stage and heading can tolerate substantial leaf damage without loss of yield.

MANAGEMENT

Cabbage loopers have many natural enemies that frequently keep loopers below economic levels, at least until heading, if they are not killed by insecticide treatments for other pests. Monitor to determine population levels of loopers and natural enemies and to determine the need for treatment following heading. If treatment is needed, use a selective material such as Bacillus thuringiensis.

Biological Control
Important parasites include the egg parasite Trichogramma pretiosum, the larval parasites Hyposoter exiguae, Copidosoma truncatellum, and Microplitis brassicae, and the parasitic tachinid fly Voria ruralis. A nuclear polyhedrosis virus disease is also important under certain circumstances; the bodies of diseased caterpillars turn into shapeless sacks of dark liquid and can often be spotted hanging from leaves. Be sure to monitor for natural enemies; if looper populations are close to treatment thresholds but you find a significant percentage of parasitized or disease-killed individuals, delay treatment for a few days to see if these natural controls will bring populations down on their own. If treatment is necessary, use of Bacillus thuringiensis insecticide will minimize injury to natural enemies.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable management tools.

Monitoring and Treatment Decisions
Check 25 plants selected randomly throughout the field. Look for eggs and small larvae on the underside of lower leaves. If you find holes, search the general area for the caterpillar, opening damaged heads as necessary. Although damage can give you a general idea of where loopers may be and the seriousness of the infestation, do not base treatment on damage levels. Base treatment on numbers of healthy larvae present (include imported cabbageworms in counts, too, if they are also present). Treat seedlings or small plants if populations of medium-sized to large caterpillars are large enough to stunt growth. Before heading, well-established plants do not need to be treated unless you find more than 9 small- to medium-sized larvae per plant. Treat just before heading or at Brussels sprouts formation if counts show more than one looper or other caterpillar in 25 plants.

Where possible, use a selective insecticide to avoid adverse impacts on natural enemies. Bacillus thuringiensis and most other selective insecticides are very effective against cabbage loopers, especially when applied to early-instar caterpillars (i.e., very young). Cabbage loopers are also controlled with the more toxic materials recommended for use against other lepidopterous (caterpillar) pests. If significant numbers of other caterpillars (armyworms or diamondback moths) are present, the use of a carbamate or pyrethroid may be warranted.

PUBLICATION

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgments for contributions to Insects and Mites:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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