Young larvae are nocturnal and feed in clusters from the underside of leaves, often skeletonizing leaves. As they mature they become solitary, and also bore readily into pods. When stressed by a lack of food, they will eat the apical portions of branches and bore into stem tissues. In regions of soybean monocultures, they can develop rapidly and have a high capacity of defoliation. There, they can become an important pest of soybean, causing damage and economic losses.
Damage is caused by the larvae of the southern armyworm, Spodoptera eridania. Adults moths are gray-brown with marked gray forewings and pearly-white translucent hindwings. A bean-shaped spot may be present near the center of the wing. Females lay greenish eggs in groups on the underside of leaves, covered with whitish scales from their body. Larvae have a black body scattered with white specks and a reddish brown head. A faint white line runs dorsally, and yellow stripes along the flank. At later larval stages, their skin become lighter, two rows of black triangles appear along their back and a dark ring on the first segment. Temperatures of 20-25°C are optimum for development, beyond 30°C their life cycle is retarded.
To reduce infestation, promote natural antagonists. For example parasitoid wasps like Cotesia marginiventris, Chelonus insularis, Meteorus autographae, M. laphygmae or Campoletis flavicincta. Other beneficial insects include lacewings and ladybirds. Some birds also feed on adult moths. You can also try infecting the larvae with the fungus Beauveria bassiana. Neem oil has also been used as feeding deterrent for the larvae. However, larvae are difficult to control with botanical insecticides.
Always consider an integrated approach with preventive measures together with biological treatment if available. Use foliar insecticides to control the southern armyworm while the worm is in an early stage. Insecticides vary considerably in their toxicity to larvae. Chemicals of the group of the synthetic pyrethroids have been used against this pest.