Beet Armyworm

The adult beet armyworm is a small, mottled gray- or dusky-winged moth. The moths are nocturnal (fly mostly at night) but may be seen by taking sweep net samples in the field.

Females deposit pale greenish or pinkish, striated eggs on the upper side of the alfalfa leaves in small or large masses covered with white cottony material. The eggs hatch in a few days, and the tiny caterpillars begin feeding on the plant. Heavy feeding on the tips of plant stems can cause flagging as terminal leaves turn white. The smooth-skinned caterpillars become full grown in about 2 to 3 weeks and are about 1.25 inches long. They range from olive green to almost black in color down the middle of the back with a yellow stripe on each side of the body.

Armyworms are common pests in the Central Valley and desert valleys from June through September. There are at least 5 generations per year in the low desert and four in the Central Valley. The final generation may overwinter as large larvae or pupae.

Armyworms skeletonize foliage, leaving veins largely intact. First- and second- instar larvae tend to feed in clusters around the egg mass from which they hatch. This frequently causes a tattered appearance to the terminals. This whitish appearance caused by the feeding is known as "whitecaps" and can be seen across a field. As the larvae mature and move to more stems, the areas of "whitecaps" tend to coalesce and the entire field takes on a tattered look.

Populations of armyworms are frequently controlled by natural enemies and are more or less cyclic, occurring in large numbers only every few years. Early harvest, border cutting, and biological control are important components of a management program that will prevent damage from armyworms.

Natural enemies can provide good control of armyworms in many fields.

Predators include bigeyed bugs, spiders, minute pirate bugs, damsel bugs, assassin bugs, and lacewings.

The parasitic wasp, Hyposoter exiguae, is the most important of at least 10 parasites attacking this pest. Sample for parasitism by pulling the heads from older caterpillars and squeezing the body contents out toward the head end. Hyposoter larvae are a light, translucent green color.

Viral diseases of armyworms are also important natural control agents under certain conditions of temperature and humidity. Diseased caterpillars first appear yellowish and limp. After death they hang from plants as shapeless forms oozing the disintegrated body contents.

Border-strip harvesting is a useful method for preserving natural enemies because it helps retain parasite larvae in the field. For more details, see BORDER-STRIP HARVESTING. Early cutting will give satisfactory control if the infestation appears late in the cutting cycle.

Biological and cultural controls, as well as sprays of Bacillus thuringiensis, are acceptable for use on an organically certified crop.

In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars and continue through fall. Divide each field into 4 sections and take 5 sweeps per section with a 15-inch diameter sweep net, for a total of 20 sweeps. For information on sampling, see SAMPLING WITH A SWEEP NET.

Combine monitoring of armyworms with monitoring for alfalfa caterpillar as described in ALFALFA CATERPILLAR AND ARMYWORM MONITORING. Count and record the number of healthy and parasitized caterpillars caught in your sweep net on a monitoring form (PDF).

If cutting is not practical or not scheduled soon after monitoring, apply a pesticide if there is an average of:

• 10 or more nonparasitized alfalfa caterpillars per sweep,
• 15 or more nonparasitized armyworms per sweep, or
• 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.