It takes 3-4 weeks for the larvae to reach maturity. When mature, the larvae construct pupal chambers or cells in a few centimetres below the surface of the soil. The pupae are about 20 mm long and reddish-brown in colour. Pupation may last about 2 weeks. Larval populations of true armyworms seem most abundant in July when the larvae are full-grown, their feeding is greatest, and damage is most apparent.
True armyworms do most of their feeding damage during their last few days as nearly-mature larvae.
In Manitoba, the damage is usually most apparent during July. Larvae feed on the leaves, stripping the leaf margins and move up the plants to feed on the panicles and flowers, stripping off the awns and kernels. Dense, lodged cereal crops are more susceptible to damage from armyworm larvae. In corn, armyworms will strip the leaves and feed on the developing ears. Armyworms are often controlled naturally by various fungal diseases.