Life cycle

Egg: Eggs are laid in clusters of 50 to 150 eggs per mass. Normal egg production is about 300 to 600 per female. Eggs are usually deposited on the lower surface of the leaf, and often near blossoms and the tip of the branch. The individual eggs are circular when viewed from above, but when examined from the side the egg is slightly peaked, tapering to a point. The eggs are greenish to white in colour, and covered with a layer of whitish scales that gives the egg mass a fuzzy or cottony appearance. Eggs hatch in two to three days during warm weather.

Larva: There normally are five instars. The larvae are pale green or yellow in colour during the first and second instars, but acquire pale stripes during the third instars. During the fourth instars, larvae are darker dorsally, and possess a dark lateral stripe. Larvae during the fifth instars are quite variable in appearance, tending to be green dorsally with pink or yellow colour ventrally and a white stripe laterally. A series of dark spots or dashes is often present dorsally and dorsolaterally. Sometimes larvae are very dark in colour, even black. The spiracles are white with a narrow black border.

Pupa: Pupation occurs in the soil. The chamber is constructed from sand and soil particles held together with an oral secretion that hardens when it dries. The pupa is light brown in colour and measures about 15 to 20 mm in length. Duration of the pupal stage is six to seven days during warm weather.

Adult: The moths are moderately sized, the wing span measuring 25 to 30 mm. The forewings are mottled gray and brown, and normally with an irregular banding pattern and a light colour bean-shaped spot. The hind wings are a more uniform gray or white colour, and trimmed with a dark line at the margin. Mating occurs soon after emergence of the moths, and oviposition begins within two to three days. Oviposition extends over a three to seven day period, and the moths usually perish within 9 to 10 days of emergence.

Nature of damage

Beet armyworm (Spodoptera exigua) is one of the most destructive polyphagous pests. Young beet armyworm larvae feed on the under-surface of leaves. Fully-grown larvae devour foliage completely, leaving only major veins.

Newly hatched larvae are gregarious and migratory skeletonisers. The larvae feed on the tender leaves. They scrape out the green matter in the leaf leaving the epidermis alone.

The second and third instar larvae feed by making small holes, late instars feed on entire lamina, petiole, even the twigs on the terminal shoots of plants. Later, the larvae spread in the field causing defoliation. The larvae also feed on the developing pods. During fruiting stage, the larva scrapes the pods and feed on it.