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IdentificationLow risk

How Do I Know If I Have Armyworm?

Written by PestMan Editorial TeamTechnical review pendingPublished Updated

Quick answer

Armyworm damage appears as brown, straw-like patches that spread quickly across the lawn, often in late summer or autumn after rain. Look for chewed blades, small droppings in the thatch, and birds feeding on the lawn. To confirm, pour a bucket of soapy water over the edge of a patch; armyworm caterpillars wriggle to the surface within minutes.

Why this happens

Lawn armyworm caterpillars hide in the thatch during the day and feed at night, so you rarely see them without testing.

Moths lay eggs in large batches, so many caterpillars hatch at once and move across the lawn as a front, leaving damaged grass behind them.

What you may be seeing

  • Green to brown caterpillars with stripes, up to about 40 mm, that curl up when disturbed.
  • A clear line between brown and green grass that moves over days.
  • Moths flying up from the lawn at dusk.

What you can do

  • Do the soapy-water test at the edge of the damaged area, where caterpillars are actively feeding.
  • If caterpillars appear, treat promptly with a product registered for armyworm in lawns, following the label (often applied late in the day).
  • Water and fertilise lightly afterwards to help warm-season grass recover.
  • Re-check a week or two later; a second generation can follow.

What not to do

  • Do not wait to see whether damage stops by itself; armyworm can strip a lawn within days.
Using pest products safely

If you use any insecticide, bait or trap, read the label first and follow it exactly. Australian law requires products to be used only as the label directs, including around food, children and pets.

When professional help may be needed

For large lawns or sports turf, or repeated outbreaks, a lawn care or pest professional can treat and advise on timing.

Lawn Pests

Caterpillars and beetle larvae that eat grass leaves or roots, leaving brown, dying patches. Armyworm and curl grubs cause most lawn damage in eastern Australia, especially in warm, humid months.

Species mentioned

Lawn Armyworm

Spodoptera mauritia

A moth caterpillar that feeds on grass at night and can turn lawns brown within days, especially in Queensland and northern NSW in late summer and autumn.

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Sources & further reading

PestMan provides general information only and does not replace licensed pest-management, medical, veterinary or structural advice. Always read and follow product labels. Read our disclaimer.