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Watch out for cabbage loopers in winter vegetables

October 04, 2001|By ERIC T. NATWICK, University of California-Imperial County Cooperative Extension

The cabbage looper, Trichoplusia ni, is one to the most destructive insect pests of lettuce and cole crops; broccoli, cabbage and cauliflower. The looper larvae also feed on tomatoes, cotton, and celery as well as several species of weeds. Alfalfa looper, Autographa californica, is similar in appearance to cabbage looper and also feeds on lettuce, but cabbage looper is the predominate species. The two species need not be distinguished when monitoring vegetable fields as the damage, action thresholds and controls are the same for both species.

Cabbage looper gets its name from the crop that is its primary host, cabbage, and from its looping appearance when walking. A looper larva has three pairs of true legs in the front of its snake-like body and three pairs of fatter unjointed prolegs near the rear of its body. When the larva moves it forms a "loop" by holding on with the front legs, arching the middle section of its body bringing the prolegs in the rear forward, and then extends the front of the body forward by releasing the front legs. This looping motion is repeated over again and again to propel the insect forward; therefore, entomologists and lay people alike call these larvae loopers.

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After feeding for two to four weeks, larvae spin cocoons and pupate while attached to the plant upon which they were feeding. Cabbage looper adults are brown moths with mottled forewings marked in the center with a figure 8. Adult moths emerge in about 10 days in warm weather and are most active at night. Cabbage loopers continue to develop all year in the Imperial Valley but are most abundant in the fall when lettuce and cole crops are being planted.

Female cabbage looper moths prefer to lay their dome-shaped eggs singly on the undersurface of older leaves but will also deposit eggs on seedling plants. Larvae hatching from the eggs feed on the undersides of leaves, feeding between the veins skeletonizing the leaf. Older larvae consume entire sections of leaves, veins and all. On cabbage and head lettuce, these insects can be very destructive as larvae often feed near the base of the heads and burrow into through several leaves at a time into the heads totally ruining these heads for sale. On cauliflower, looper larvae will migrate from leaves to feed on heads, which ruins them for marketing.

Loopers have many natural enemies that help keep them under control: bacterial and viral disease organisms, predacious flies and wasps, and predacious bugs and spiders. Spraying broad spectrum insecticides can kill the loopers' natural enemies. Treatments for many insect pests are needed to establish stands of fall vegetables, but between thinning and heading more loopers and other insects can be tolerated, allowing natural enemies to become established. At the time of head formation it is again necessary to treat for loopers and other pests to prevent the lettuce, cabbage, and cauliflower heads from becoming contaminated. Loopers can be controlled with Bacillus thuringiensis fermentation products. Bt products are biological insecticides that are environmentally friendly, safe to handle, exemption form residue tolerances, and are not disruptive to beneficial insects.

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