Beet Armyworm

Beet armyworm adults are 1/2 to 3/4 inch, nondescript brown moths. Adult moths lay 20 to 400 eggs in mass that they cover with their own light-colored scales, giving the masses a distinctive cottony appearance. When eggs first hatch, the pale green 1/16 inch (1.5 mm) long larvae feed for first few days in groups near the egg mass, skeletonizing or completely consuming leaves.

As the larvae age, they disperse and move toward the center of the plant and feed singly. Older larvae vary in color but are usually a shade of olive green to black with many fine, wavy, light-colored broad stripes down the back on each side and may be up to 1.5 inches long. 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.

The larval stages last from 2.5 to 3 weeks at 70° to 80°F. Large larvae generally shelter in the center of the plant or underground during daylight hours. After five to six instars, the larvae burrow into the soil to pupate. The pupal stage persists 7 to 10 days but may be much longer when temperatures are cooler. Although newly emerged adults can be found throughout the year in central and southern California, major emergences and migration flights are usually observed in the spring through late summer. Beet armyworms numbers grow as weather warms and are most common on late summer and fall crops.

Beet armyworms can destroy seedlings, consume large portions of leaves, and stunt growth by feeding on developing buds. Young worms feed on leaves but rarely cause substantial damage. However, larger larvae feed on petioles and can cause significant crop loss. Large larvae are quite mobile and have been observed to travel over 10 feet per night, feeding on several plants. Beet armyworms also cause damage by contaminating cilantro or parsley with their bodies and frass, reducing crop marketability. In the San Joaquin Valley, fall populations are more damaging than spring populations.

Cultural and biological controls help suppress armyworm numbers.

Disc fields immediately following harvest to remove the food source for any remaining larvae. Some larvae and pupae will be directly killed by discing as well. Destroy weeds along field borders as armyworms often migrate from these areas into newly planted fields.

Naturally present predators and parasitoids can maintain armyworm numbers at low levels. Among the most common parasites are the wasps Hyposoter exiguae and Chelonus insularis, and the tachinid fly Lespesia archippivora. Armyworms can easily be checked for the presence of Hyposoter exiguae by pulling the larva apart and looking for the parasite larvae of light, translucent green color. Naturally occurring viral diseases can also play an important role. Diseased caterpillars first appear yellowish and limp. After death they hang from plants as shapeless, dark tubes oozing the disintegrated body contents.

Biological control and applications of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable management tools. Bacillus thuringiensis is most effective when applied on first and second instar larvae. Spinosad can be detrimental to natural enemies like syrphid flies, which are predators of aphids and other soft-bodied insects.

Seedlings are susceptible to armyworm damage. Monitor fields frequently for beet armyworm from planting until harvest. Beet armyworm eggs and larvae are often easier to find on weeds in and near the field than in cilantro or parsley. Weeds can be pulled and readily examined whereas cilantro and parsley are often brittle and easily damaged during the early morning and late evening hours when larvae are active. Chenopodium species (e.g., lambsquarters, goosefoot) are particularly attractive to beet armyworm larvae.

Begin monitoring for beet armyworm twice per week even before seedlings emerge.

1. Check at least 25 plants in each quadrant of a 40- to 80-acre field. Fields smaller than 40 acres may require fewer samples.
2. Check for egg masses and young larvae on pigweeds, lambsquarters, nettleleaf goosefoot, and other nearby weeds. If numbers are high on weeds, watch carefully for infestations on crop seedlings.
3. Consider placing out pheromone traps (delta, wing, or bucket type) along field edges to monitor adult flights and predict egg laying.
4. 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 cabbage loopers and include them in the total caterpillar count on your monitoring record.

Most insecticides are more effective against young larvae than against eggs, so wait until the majority of eggs have hatched before treating. Treat plants when numbers of small larvae are large enough to impact plant growth. Beet armyworms are more difficult to control with insecticides than loopers. Make treatments when larvae are small; large larvae are more difficult to kill with compounds such as Bacillus thuringiensis. The best time for an insecticide treatment is in the evening hours because larvae become active at dusk, and sunlight degrades many pesticides.

Broad-spectrum insecticides (e.g., methomyl) adversely affect natural enemies in most cases.

Watch out for reports of insecticide resistance to certain pesticides (e.g. spinosad, methoxyfenozide, and others) in your area. To manage insecticide resistance in beet armyworm, limit the total number of sprays of each insecticide. The best way to do this is to practice the basic principles of IPM:

• Monitor pests and maximize the use of biological and cultural controls.
• Spray only when pest numbers warrant an application.
• Use the most selective insecticides first to conserve natural enemy populations so they help with the control of pests.
• Do not use insecticides with the same mode-of-action group number on successive beet armyworm generations.