Each adult female of the cotton leafworm (Spodoptera littoralis) can lay up to 3,000 eggs. Each egg is roughly spherical, about 0.6 mm long and whitish yellow. They are laid in clusters of 20 to 500 pieces on lower leaf surfaces, often on the lower parts of the plant. The female covers the eggs with brownish-yellow hairs from her abdomen to make them less conspicuous and to protect them from desiccation.
The cotton leafworm (Spodoptera littoralis) goes through six larval stages. Young larvae are pale green with a brownish head. In the last larval stage, they are 35 to 45 mm long. Coloration varies from grey to reddish or yellowish, with a median dorsal line bordered on either side by two yellowish-red or greyish stripes and small yellow dots on each segment. The underside of the caterpillar is greyish-red or yellowish. The caterpillars are easily distinguished from other Spodoptera species by four black triangular dots on their body. Before pupation, caterpillars crawl to the ground and settle in the surface layers. The pupa is 15 to 20 mm long and reddish brown in colour.
The forewings of the adult cotton leafworm (Spodoptera littoralis) are brownish with blueish overtones and straw yellow along the median vein. The ocellus is marked by two or three oblique whitish stripes. The front of the wingtip has a blackish marking, which is more pronounced in the male. The hindwings are whitish, with a brown leading edge. Adults are only active at night.