The beet armyworm (Spodoptera exigua) is a small, nondescript moth, that hides during the day and is active only at night. The forewings are grey-brown suffused with a dark brown or black. The head and thorax are brown and the abdomen grey-brown. The forewings have a yellowish, kidney-shaped mark. The white hindwings have a clearly outlined brown venation.
The beet armyworm (Spodoptera exigua) lays its eggs in packets, sometimes in several layers, and on top of a mass of white, cottony hairs and scales from the moth’s body. The eggs, usually grey but sometimes greenish or pinkish, are laid at night, preferably low in the crop on the underside of leaves, in groups of 10 to 250 eggs.
They hatch after several days, revealing small, drab yellow-green caterpillars. Their colour gradually changes to yellow, green, brown or even black. The colour and pattern of the caterpillars is highly variable and depends partly on the host plant, the stage of development and the climate. A full-grown beet armyworm (Spodoptera exigua) caterpillar can be 25 to 38 mm in length, with dark, wrinkled stripes dorsally and a yellow band running the length of the body on either side, above which there is a black dot on each segment.
The young caterpillars live in groups and spin loose webs over the leaves, under which they remain until the third or fourth instar. Older caterpillars are active walkers and can cover considerable distances. Therefore, caterpillars from the same cluster of eggs can cause damage at various locations within the crop. The beet armyworm (Spodoptera exigua) feeds mainly at night and conceals itself during the day, although the youngest instars will feed during daytime. Full-grown caterpillars move to the ground in preparation for pupation. Although pupation sometimes occurs in the crop, it usually takes place on the surface of the ground in a loosely spun cocoon, consisting of soil particles glued together with a sticky secretion. Pupae are brown and 15 to 20 mm long, and resemble those of other noctuid moths. After five or ten days the adult moth emerges from the pupa.