Beet Armyworm
(Spodoptera exigua (Hubner), Noctuidae, LEPIDOPTERA


DESCRIPTION

Adult - The beet armyworm moth has a wingspan of 25 to 32 mm. Its forewings are mottled gray or brown with a pale spot near the center of each wing. Its hind wings are white with dark veins and have a fringelike border.

Egg - The white to pink, ribbed egg is roughly spherical in shape and slightly peaked on top. Scales and hairs from the moth give the egg cluster a gray, fuzzy appearance.

Larva - This green or black caterpillar has a dark head, five pairs of prolegs, and sometimes three light colored stripes running the length of the body. On the second segment behind the head, there is a small, black spot on each side of the body. This spot usually becomes visible to the field observer when the caterpillar reaches 7 or 8 mm long; however, the spot may be difficult to see on a dark caterpillar. About 1 mm long when newly emerged, a larva may be 25 to 30 mm long when fully grown. Color plate.

Pupa - About 15 to 20 mm long, the pupa is light brown with dark brown margins along the abdominal segments.


BIOLOGY

Distribution - Native to the Orient, the beet armyworm is practically a cosmopolitan species. It is common throughout the southern and western United States and occurs northward into Montana. As a pest of soybeans, it is most damaging from the Mississippi delta area eastward to Florida and northward to North Carolina and southern Virginia. This insect is a sporadic pest of North Carolina soybeans in both time and space.

Host Plants - The beet armyworm infests many weeds, trees, grasses, legumes, truck crops and field crops. It is of economic concern upon cotton, corn, soybean, tobacco, alfalfa, table and sugar beets, pepper, tomato, potato, onion, pea, sunflower and citrus. In addition, plantain, lambsquarters and redroot pigweed are attractive wild hosts.

Damage - Early instar beet armyworms most frequently damage the young terminal growth of seedling soybeans. Skeletonization and, often, a profuse silk webbing which gives the plants a shiny appearance are characteristic of this species. Although soybean plants can compensate for much foliage loss before bloom, severe beet armyworm damage will retard plant growth. Later instars do not feed gregariously and the production of webbing is discontinued.

Life History - In warm areas, such as Florida and California, the beet armyworm moths may be found year round. In less tropical areas, these insects can survive the winter as pupae in the upper 6 cm of the soil. The extent of overwintering distribution, however, has not been adequately studied. At any rate, this insect is not believed to overwinter as far north as Kentucky or North Carolina. Most states apparently become infested by migrating moths. In the spring, soon after mating, fertilized females begin laying eggs in clusters of about 80. Approximately 600 eggs per female are deposited over a 3 to 7 day period. Moths die 4 to 10 days after emerging from pupae.

Eggs hatch in 2 to 3 days. The newly emerged larvae spin loose webs around themselves, feed gregariously on the remains of the egg mass, and then attack plant foliage. They eventually scatter to different parts of the plant. After feeding for 1 to 3 weeks, larvae (fifth instars) pupate within loose cocoons composed of soil particles, leaf fragments and trash. About 1 week later moths emerge. The entire life cycle requires 4 to 5 weeks. Several generations occur each year.


CONTROL

The beet armyworm has few effective parasites, diseases, or predators to lower its population and is resistant to some insecticides. Therefore selection of the proper material and rate is very important when chemical control is considered. When defoliation reaches the economic threshold of 35 percent foliage loss before bloom or 15 percent foliage loss after bloom, consult the current North Carolina Agricultural Chemical Manual.