Webworm caterpillars are seldom seen as they come above ground level only when conditions are cool and damp and usually at night. They may be located in their web-lined tunnels from which plant parts may be seen protruding. The caterpillars are pale to deep brown with a tinge of the green gut contents showing through. The head appears black or dark brown. Fully grown caterpillars are about 15 mm long.
Caterpillars hatch from eggs laid amongst grass in autumn and feed throughout the winter. Spring and summer are passed in the tunnels as resting stage caterpillars. After this, the insects proceed through the pupal stage and emerge as adult moths, which are about 10 mm long and may be seen flying in large numbers on autumn nights. By day they hide in dry grass, the colour of which they closely resemble.
Large areas of emerging wheat or barley crops may be destroyed by the continual chewing damage of a heavy webworm infestation. The caterpillars sever leaves or whole plants which they scatter on the ground or pull into holes near the plants. In pasture, the grass component may be removed from large areas.