Beet Armyworm (Spodoptera exigua)

Introduction

The beet armyworm is not native to the United States, originating in Asia. It was first found on the West Coast in 1876. In less than 50 years, it had spread to Florida. It is mainly found in the southern United States and further south. It does not overwinter in areas where frost occurs. However, it migrates annually as far north as Maryland, Colorado and northern California. It is usually only considered a pest in the southern states. It feeds on a wide range of crops including cotton, beans, cole crops, tomatoes, lettuce, alfalfa, and many weeds. It can also be a pest in greenhouses.

beet armyworm larvae
beet armyworm adult
Image courtesy of Winfield Sterling (retired, Texas A&M)

Identification

Larva:

  • Young larva light green or yellow with few markings
  • Third instar and older larvae have darker stripes
  • Dark spot may be visible on side of mesothorax (above second true leg)

The beet armyworm may be confused with southern armyworm in areas where both are found. However, southern armyworms have a large dark spot on the side of the first abdominal segment. If a spot is present on a beet armyworm larva, it is found on the side of the mesothorax (above second true leg), not the first abdominal segment.

Adult:

  • Wing span 25 to 30mm (about one inch)
  • Forewings mottled with pale spot near center
  • Hindwings more uniform color with dark line at wing margin

Eggs:

The beet armyworm lay eggs in clusters of up to 80 eggs. Once laid, the female covers the eggs with wing scales, which gives the egg cluster a white hairy appearance.

Biology

The beet armyworm overwinters only in areas where frost does not occur. In cooler climates it overwinters as a pupa in the soil. In warmer climates, it is common to find small numbers of all life stages throughout the winter.

Between 50 to 150 eggs are deposited in clusters and covered with scales from the female moth. A single female may produce upwards of 600 eggs over two weeks. Egg masses are most commonly found on the underside of leaves, near flowers, or on the end of branches. Eggs hatch in 2 to 3 days depending on temperature.

After egg hatch, larvae will feed in mass before dispersing. Typically, the larvae pass through five instars although there are reports of additional instars occurring. Approximately 10 days are required during warm weather for the larvae to complete development. After 10 days the larvae crawl into the soil and pupate. Adults emerge about seven days later. Oviposition occurs 2 to 3 days after emergence. Adults usually survive for 9 to 10 days. The complete life cycle can be completed in 24 to 25 days. Multiple generations occur with 4 to 6 generations occurring in warm areas of the United States.

Damage

Young larvae feed in mass on leaf tissue causing windowpane and skeletonization of leaves. Older larvae feed on squares and bolls and cause damage similar to that of bollworm and budworm.

beet armyworm damage beet armyworm damage beet armyworm damage
Image courtesy of Tom Fuchs: Texas A&M

Control

Beet armyworms are considered a sporadic pest. However, when conditions are right large populations may occur resulting in severe damage. Beet armyworm populations are kept in check in most years through the presence of many predators and parasites. Because of the intense insecticide application schedule, boll weevil eradication can reduce the population of these natural controls resulting in an explosion in the beet armyworm population. Hot dry conditions also favor the development of large beet armyworm populations.Control of beet armyworms can be difficult and resistance is a concern. The newer insecticides may be more effective than older products. Intrepid® insecticide is very effective for beet armyworm control and has a long residual. WideStrike™ Insect Protection cotton has been shown to be very effective against beet armyworm.