Cutworms

Cutworms are the larval (caterpillar) stage of several night-flying moths in the genus Agrotis, including the black cutworm and common brown cutworm. The adult moths are unremarkable, mottled grey-brown, with a wingspan of around 3.5–4.5 cm, and are strictly nocturnal fliers with no direct feeding impact on plants themselves.

The caterpillars are the damaging stage. They are plump, smooth-skinned grubs up to 4–5 cm long when fully grown, ranging in colour from grey and brown through to almost black, sometimes with faint longitudinal stripes. A key identifying habit is that they curl tightly into a C-shape when disturbed. During the day they hide just below the soil surface or under debris close to the base of plants, emerging at night to feed.

Cutworms are found across most of Australia and are most active in the warmer months, though some species overwinter as larvae and cause damage into early spring. They favour recently cultivated or newly sown ground, home vegetable patches, and turf areas that have been recently renovated or reseeded, since bare or thin soil gives them easy access to tender young growth.

Populations tend to build where weeds or a previous crop have provided food before a new planting goes in, so the moths have already laid eggs in the soil. Good weed control, tilling before planting, and avoiding heavy thatch or mulch build-up around seedlings all help reduce shelter and food sources. In lawns, keeping grass healthy and not over-watering in the evening (which encourages surface activity) can reduce the risk of a damaging outbreak.