Cabbage looper on tomatoes

The cabbage looper is primarily a pest of brassica crops, but will sometimes expand its smorgasbord to include everything from cantaloupe to tomatoes.

Looper caterpillars are easily recognized because they crawl by arching their backs. Looper eggs are laid singly on the undersurface of mature leaves. They are similar to fruitworm eggs, but flatter, and have finer ridges radiating from the top.

Cabbage loopers feed only on foliage, not on fruit. Damage is usually insufficient to require control measures so treatment is rarely recommended.

Distribution

The cabbage looper is found throughout Canada, Mexico, and the United States wherever crucifers are cultivated, and on any other continents. However, overwintering in the United States apparently occurs only in the southernmost states. It is somewhat erratic in occurrence, typically very abundant one year, and then scarce for two to three years. This is likely due to a nuclear polyhedrosis virus. The cabbage looper is highly dispersive, and adults have sometimes found at high altitudes and far from shore. Flight ranges of approximately 200 km have been estimated.

Damage

Cabbage loopers are leaf feeders, and in the first three instars they confine their feeding to the lower leaf surface, leaving the upper surface intact. The fourth and fifth instars chew large holes, and usually do not feed at the leaf margin. In the case of cabbage, however, they feed not only on the wrapper leaves, but also may bore into the developing head. Larvae consume three times their weight in plant material daily. Feeding sites are marked by large accumulations of sticky, wet fecal material. Despite their voracious appetite, larvae are not always as destructive as presumed.

Prevention

• Grow healthy organic plants. Strong plants can handle some damage from cabbage loopers and worms better than weak, struggling plants. Make sure that your crops are getting enough sunlight and water, and that the soil is well-drained and rich in nutrients and organic matter.
• Remove or till in old plants. Because pupae of both these caterpillars spend the winter on dead plants and garden litter, it is important to remove that stuff from the field and compost it or till it into the soil. This works best in fall, and must be done before adult moths and butterflies emerge in spring. [top]
• Use row covers. Keep the moths and butterflies from finding your crop by covering your plants with a lightweight “floating” row cover such as Reemay. These materials (as opposed to plastic or heavier fabrics) allow water, air, and sunlight to get through. You can get them at garden supply stores or order them from seed catalogs. The covers can lie right on the plants (the plants will lift the cover as they grow), or you can support the covers with wire hoops. Broccoli, cabbage, and other Brassica crops do not need pollination by flying insects to produce, so you can leave the covers on all season.
• Some other prevention methods are:

• Keep the garden free of weeds, especially those preferred by cabbage loopers – wild mustard, peppergrass, and wild cabbage.

• Monitor susceptible plants for cabbage looper eggs and crush them before they hatch.

• Check the undersides of leaves for young looper larvae. Hand pick and destroy them by dropping the caterpillars in soapy water.

• Use floating row covers as a barrier to moths. Be sure to anchor all sides of the row covers.

• Collect diseased caterpillars and make your own cabbage looper remedy. Cabbage looper larvae are susceptible to a virus that kills them. Infected caterpillars will look yellow or white, and swollen. Blend these sick caterpillars with water and spray it on plants to infect other larvae.

• Apply Bacillus thuringensis when larvae are young.

Management and treatment

Although common in tomato fields, cabbage looper populations are generally kept below damaging levels by naturally occurring parasites and a viral disease. Moderate numbers of loopers are considered more beneficial than harmful because they serve as alternative hosts for parasitic wasps that also attack tomato fruitworms and other pest caterpillars. Treatment is only necessary if feeding is extensive enough that sunburn of the fruit is a concern. Consider nondisruptive insecticides such as Bacillus thuringiensis to protect natural enemies.

Biological control
There are several important naturally occurring parasites that help control loopers in tomatoes. One of these is Hyposoter exiguae, which also attacks tomato fruitworms and armyworms. Another parasitic wasp, Copidosoma truncatellum, commonly kills cabbage looper and other larvae by attacking the overwintering pupae. Looper eggs are often killed by Trichogramma released for tomato fruitworm control often parasitize cabbage looper eggs as well. Alfalfa and cabbage loopers are also subject to disease caused by a nuclear polyhedrosis virus. Conserve these parasites by not treating with disruptive pesticides, particularly early in the season.

Organically acceptable methods
Biological control and sprays of Bacillus thuringiensis or the Entrust formulation of spinosad are acceptable for use on an organically certified crop.

Monitoring and treatment decisions
Determine the extent of damage in the field. If damage is severe enough to expose fruit to sunburn, treat mid- to late-season foliage to maintain the plant canopy. Spot treat only severely infested areas.