How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Beet armyworms vary in color but are usually a shade of olive green with many fine, wavy, light colored stripes down the back and sides. Often there is a dark spot on the side of the body just above the second true leg. The body surface is smooth and almost hairless. Mature larvae may be up to 1.5 inches long. Adult moths lay their eggs in scale-covered cottony masses on leaf surfaces. When eggs first hatch, the tiny larvae feed in groups near the egg mass, skeletonizing or completely consuming leaves. As they grow older, larvae disperse and move toward the center of the plant. Beet armyworms build up as weather warms and are most common on late summer and fall crops.

DAMAGE

Beet armyworms can destroy seedlings, consume large portions of leaves, or stunt growth by feeding on buds. However, serious economic damage to cole crops is uncommon.

MANAGEMENT

Cultural and biological controls help suppress armyworm populations. Disc fields immediately following harvest to kill larvae and pupae. Destroy weeds along field borders; armyworms often migrate from these areas into newly planted fields. Seedlings are very susceptible to armyworm damage. Fields should be monitored frequently for beet armyworm from planting until heading.

Biological Control
Many natural enemies attack beet armyworms. Among the most common parasites are the wasps Hyposoter exiguae and Chelonus insularis, and the tachinid fly Lespesia archippivora. Viral diseases may also be important.

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
Start monitoring for beet armyworm even before seedlings emerge. Check for egg masses and young larvae in pigweeds, lambsquarters, nettleleaf goosefoot, and other weeds surrounding the field. If populations are high on weeds, watch especially carefully for infestations on crop seedlings. Pheromone traps are also available for monitoring adult flights in order to predict egg laying.

Once seedlings emerge, check them at least twice a week for armyworm egg masses and young larvae. Treat if you find one second or third instar larva for every 10 plants. Sample for armyworms at the same time you are sampling for loopers and cabbageworms and include them in your total counts for caterpillars. It is not usually necessary to treat older plants between thinning and heading. Treat just before heading if caterpillars are in the field. Beet armyworms are more difficult to control with insecticides than loopers and cabbageworms, so be sure to make note of their presence in your monitoring records. There are reports of insecticide resistance to certain materials in certain areas. The more broad-spectrum insecticides (endosulfan, methomyl) adversely affect natural enemies in most cases.

PUBLICATION

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

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