Beet Armyworm

Beet armyworm, The caterpillars (larvae) of the beet armyworm are stages most commonly observed when they can occur in high numbers.

Caterpillars are overall green or marked in shades of green with stripes, grows up to about 1 1/4 inches long, and can best be identified by a characteristic small black spot on each side of the second body (thoracic) segment behind the head. Adult moths have a wingspan of 1 to 1 1/4 inches. However, wings are held over the back when at rest The forewings are grayish-brown with two pale spots near the center.

Symptoms: Chewing on leaves from margins inward, ragged appearance on leaves. More pronounced in dry years and on Spanish peanut varietie

Habitat and Food : Caterpillars have chewing mouthparts. Adults have siphoning mouths. Young larval stages feed close together, often grazing on the outer (epidermal) layer of cells on the underside of leaves. They may spin a light web over the foliage. Older, larger caterpillars feed alone and consume leaf tissue that can result in complete defoliation of host plants.

In cotton fields, they also feed on developing squares and bolls. Alfalfa, citrus, corn, grasses,legumes, onion, ornamental plants, pea, pepper, potato, soybeans, sugar beets, sunflower, tomato, vegetables and weeds such as plantain and pigweed also serve as hosts for this species.

Life Cycle :Egg clusters are usually deposited on the underside of leaves. Females normally deposit 300-600 eggs during their lifetime. Eggs hatch in 2-3 days during warm weather. Early instar larvae are gregarious, feeding as a group and skeletonizing leaves.

Larvae are primarily foliage feeders during the first two instars which require about 4 days. Third instar larvae disperse and will attack fruit but can complete development on foliage in the absence of fruit. Normally, larvae develop through 5 instars in 9-10 days. Larvae reach a maximum size of about 22.5 mm. Pupation occurs in the soil and the pupal stage generally lasts 6-7 days. Total generation time is about three weeks.

Seasonal distribution: Beet armyworm generally does not overwinter in Georgia but can migrate readily from Florida. While the potential for significant infestations is more likely in the fall, this pest can be a problem in the spring production season as well.

This pest has generally been considered a secondary pest, with significant infestations usually occurring only after repeated use of broad spectrum insecticides which decimate its parasites and have little impact on the beet armyworm because of resistance to most older insecticide chemistries. However, in recent years this pest has frequently established in fields prior to heavy insecticide use.

Damage :The first two instar larvae are gregarious and feed in groups on foliage. The clumped skeletonizing of foliage is known as a beet armyworm ‘hit’ in many crops. Third and later instar larvae disperse and may continue feeding on foliage but will readily bore into fruit.

Larvae feed on both foliage and fruit. In Florida it is regarded as a serious defoliator of flower crops and cotton, though much of the injury is induced by insecticide use that interferes with natural enemy activity. Young larvae feed gregariously and skeletonize foliage. As they mature, larvae become solitary and eat large irregular holes in foliage.

They also burrow into the crown or center of the head on lettuce, or on the buds of cole crops. As a leaf feeder, beet armyworm consumes much more cabbage tissue than the diamondback moth, Plutella xylostella (Linnaeus), but is less damaging than the cabbage looper.

Management: Beet armyworm moths can be monitored with pheromone traps, but adult abundance does not always correlate with subsequent larval problems. Scouting for beet armyworms generally involves inspection of foliage for egg masses, larvae, and ‘hits.’ Egg masses can be difficult to locate because of their clumped nature.

In fruiting vegetables, insecticide applications based on the detection of ‘hits’ generally provides ample protection as the early instars do not attack fruit and ‘hits’ can be detected prior to fruit loss. The lowest level of beet armyworm that can be tolerated without significant yield loss is an average of 1 larvae per 20 plants. Beet armyworm should be managed to keep the larval population from exceeding this level in the field.

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