Pest

Lawn Armyworm

Spodoptera mauritia

Lawn armyworm caterpillars strip grass blades overnight, leaving lawns looking scalped or brown almost without warning.

Photo: Gilmar R. Ferreira via Wikimedia Commons (CC BY-SA 4.0)

Lawn armyworm is the caterpillar stage of a night-flying moth. The adult moth has a wingspan of around 30-40mm, with mottled greyish-brown forewings and plainer pale grey hindwings. Moths are rarely seen during the day as they hide in thatch or nearby vegetation, becoming active at dusk to feed and lay eggs.

The damaging stage is the larva, or caterpillar. Young larvae are pale green and only a few millimetres long, while mature caterpillars grow to around 30-40mm, with a smooth body ranging from green through brown to almost black, and pale stripes running the length of the body. They curl tightly into a C-shape when disturbed. Caterpillars shelter low in the turf canopy or just under thatch during the day and move up to feed on leaf blades at night, which is how the group gets its common name - large numbers can march across a lawn like an army, stripping grass as they go.

Lawn armyworm is most active in warmer months, from late spring through to autumn, and populations often build rapidly after warm, humid weather following rain. It affects couch, kikuyu, buffalo and other common Australian turf species, and can also attack pasture grasses and some crops.

Moths are strong fliers and can move into an area from some distance, so infestations can appear suddenly in a lawn that looked fine only days before. Regular monitoring through the warmer months, especially after rain, is the best way to catch an outbreak early before significant damage occurs.

How to identify

Ragged, chewed grass blades that appear notched or skeletonised, often noticed first at the edges of a damaged patch.
Sudden browning or thinning of turf that can spread rapidly, sometimes overnight, as larvae numbers increase and feeding intensifies.
Bare or scalped-looking patches in otherwise healthy lawn, since caterpillars feed on leaf tissue rather than roots.
Bird activity such as magpies or plovers probing the lawn can be an early clue, as birds feed on the exposed caterpillars.

To confirm the pest, check the lawn at dusk or after dark with a torch, when caterpillars climb up to feed and are easiest to spot. During the day, part the grass at the edge of a damaged patch and look down into the thatch layer where they shelter. A simple soap flush can also help: mix a few drops of dishwashing detergent in a bucket of water and pour it over a suspect area, then watch for caterpillars surfacing within a few minutes.

Lawn armyworm is sometimes confused with cutworm, which causes similar ragged feeding damage but tends to hide singly in the soil rather than moving in groups, and with lawn grubs (scarab larvae), which are curled, cream-coloured grubs found below the soil surface feeding on roots rather than leaf blades. If you are finding smooth, striped caterpillars in the thatch rather than white grubs in the soil, lawn armyworm is the more likely culprit.