The eggs are white to yellowish, brownish at hatching. Young caterpillars are pale green, but later instars are very variable in color (yellowish-green to dark brown) and markings. They become up to 40 mm (1.57 inch) long. The adults vary greatly, too; the forewings are yellowish to orange in females and greenish-gray in males, with a slightly darker transversal band in the distal third. The kidney-shaped marking is slightly distinct and smoky. The hind wings are pale gray with a broad, darker marginal band and a small, brown marking near the base spanning 35-40 mm (1.36 inch).
The voracious caterpillars of H. armigera can feed on leaves and stems, but they show a strong preference for reproductive organs such as buds, inflorescences, berries, pods, capsules etc.
They bore into these parts, leaving large, round holes. Older larvae often enter the plant tissue with the anterior part of their bodies only. Young instars, however, may disappear completely inside, so they are sometimes not discovered before the produce (e.g. tomatoes) is processed.
Secondary infections by fungi and bacteria are very common and they lead to rotting of fruits.
Injury to growing tips disturbs normal plant development; maturity may be delayed, and fruits are often dropped. So in cotton for example, attacked blooms will frequently open prematurely and stay fruitless: when the bolls are damaged, some will fall off, and those that remain either fail to produce lint entirely, or they produce lint of inferior quality.
Each female of H. armigera can lay several hundred eggs, distributed on all parts of the plants, flowers and fruit included. At optimal temperature, the larvae can hatch after less than three days. They then pass through four instars over a three to four week period.
The caterpillars are rather aggressive, occasionally carnivorous and, when the opportunity arises, cannibalistic. If disturbed, they let themselves drop from the plant and roll up on the ground. They pupate inside a silken cocoon several centimetres deep in the soil. During this stage, they can overwinter if necessary in seasonal climates, but they cannot resist severe frost. Otherwise, pupation lasts about 2-3 weeks. Under favorable conditions, the whole development cycle can be completed in little more than a month, so numerous generations per season are possible, especially in warmer areas. In the tropics, reproduction continues throughout the year.
Helicoverpa armigera is almost indistinguishable from its near relative H. zea. However, the two species have different areas of distribution. Helicoverpa armigera, also called the „Old World bollworm“, is found in parts of Europe, Asia, Africa and Australasia: Helicoverpa zea, the „New World Bollworm“ in the Americas. Their host ranges are broadly similar.
Both species originate from tropical and subtropical regions, but they will immigrate over long distances into areas with temperate climates each summer. The adult insects are good fliers and are mostly active at night.