Beet Armyworm

Beet armyworm caterpillars (larvae) vary in color, but are commonly olive green with a yellowish, lengthwise stripe on each side of the body. Often there is a dark spot on each side of the body above the second pair of true legs. The body surface is smooth and almost hairless. There are four pairs of prolegs (leglike appendages, or fleshy stubs) in the middle of the abdomen, on segments 3 to 6. Older larvae may be up to 1.5 inches long.

Adult moths lay their eggs in scale-covered, cottony masses on leaf surfaces. The first- and second-instar larvae tend to feed in groups near the egg mass, scraping the leaf surface, skeletonizing leaves and leaving veins largely intact, or completely consuming leaves. As they grow older, larvae disperse toward stems and the center of the plant and feed more individually.

Beet armyworms increase in numbers as weather warms and are most common on late-summer and fall crops.

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

Use biological and cultural controls to suppress armyworm numbers. Monitor fields and field borders regularly from planting until heading. 

Use the following cultural methods to control beet armyworms:

• 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.

Many natural enemies attack beet armyworms. Common parasites include the wasps Chelonus insularis and Hyposoter exiguae, and the tachinid fly Lespesia archippivora. Consider installing insectary plants to attract natural enemies.  

Entomopathogenic viral diseases may provide some control of beet armyworm. Though these viruses may occur naturally, natural occurrence is not likely to keep this pest below damaging numbers.

Use biological and cultural controls in an organically certified crop. Sprays of Bacillus thuringiensis and the Entrust SC formulation of spinosad are also organically acceptable.

Start monitoring for beet armyworm before seedlings emerge. Frequently monitor seedlings, which are very susceptible to armyworm damage.

1. Check for egg masses and young larvae in pigweeds, lambsquarters, nettleleaf goosefoot, and other weeds surrounding the field. If numbers are high on weeds, watch carefully for infestations on crop seedlings.
2. Consider placing pheromone traps to monitor adults and predict egg laying.
3. Once seedlings emerge or right after transplanting, check plants at least twice per week for armyworm egg masses and young larvae. Sample for armyworms along with loopers and cabbageworms and include them in the total caterpillar count.

An insecticide application may be needed if one second- or third-instar beet armyworm is found for every 10 plants. However, it is usually not necessary to apply insecticide for armyworms on older plants (between thinning and heading). Apply insecticide just before heading if caterpillar numbers threaten to damage the heads.

Beet armyworms are more difficult to control with insecticides than loopers and cabbageworms, so note their presence in monitoring records. Additionally, beet armyworm has developed resistance to carbamate insecticides in some regions, such as the low desert.