What is lawn armyworm?
Armyworms are the caterpillars of several moth species, and in Sydney they feed on most common lawns, including buffalo, couch and kikuyu. The moths lay their eggs in clusters, and the young caterpillars feed mostly at night and early in the morning, hiding down in the thatch during the day. That is why the damage seems to appear from nowhere.
They get their name from the way they move: a large group eats its way across a lawn in a front. A lawn that looked fine at the weekend can be patchy and brown a few days later.
When is armyworm season in Sydney?
Armyworm is most active in warm, humid weather, from late spring through to autumn. Outbreaks are often worst from late summer into autumn, especially after a spell of rain and humidity.
The life cycle is quick. In warm weather it takes around 30 to 40 days from egg to moth, so a second wave can follow the first within weeks. That is why one treatment is not always the end of it.
What does armyworm damage look like?
- Brown or straw coloured patches that grow noticeably from one day to the next.
- Grass blades that look chewed, ragged or scalped, as if the lawn had been mown far too low.
- Birds such as ibis, magpies and mynas pecking at the lawn far more than usual.
- Small grey brown moths flying up from the grass when you walk across it around dusk.
- Striped caterpillars in the thatch after dark or early in the morning.
Is it armyworm or something else?
Brown patches have plenty of causes, and treating for the wrong one wastes time while the real problem spreads.
| Cause | What you see | How to check |
|---|---|---|
| Armyworm | Patches that spread by the day in warm, humid weather, with chewed grass blades | A soapy water flush brings caterpillars to the surface |
| Curl grubs | Dying patches where the turf lifts like loose carpet, because the roots have been eaten | Peel back a section of turf and look for C shaped white grubs in the soil |
| Dry patch | Irregular patches that stay brown after rain or watering while the rest of the lawn greens up | Water beads on the surface or runs off instead of soaking in |
| Fungal disease | Rings or patches, often in cool, damp weather or shade, with no insects present | Look for discoloured, spotted or rotting grass blades at the edge of the patch |
| Dog urine | Small round dead spots, often ringed with dark green grass | The spots stay about the same size and match where a dog goes |
Dry patch is common in sandy soil near the coast and over sandstone. A soil wetting agent and deep, less frequent watering usually help.
How do you do the soapy water test for armyworm?
- Mix about 50 millilitres of dishwashing detergent into a bucket holding around 9 litres of water.
- Choose a spot where brown grass meets green. That edge is where the caterpillars are feeding.
- Pour the mixture slowly over about one square metre so it soaks into the thatch.
- Wait 10 to 15 minutes and watch the surface. The detergent irritates the caterpillars and they wriggle up to the top.
- If nothing appears, try another spot along the edge, or repeat the test late in the afternoon.
How do you get rid of armyworm?
Once you have confirmed armyworm, act quickly, because a large outbreak can strip a lawn in a few days.
- Use an insecticide registered for armyworm in home lawns. Garden centres sell them as granules and as liquids.
- Read the label and follow it exactly: the rate, whether to water it in, and how long to keep children and pets off the lawn.
- Apply late in the afternoon, when the caterpillars are about to start feeding.
- Treat the damaged area and a margin of healthy looking lawn around it, since the caterpillars feed ahead of the visible damage.
- Check again a week or two later with another soapy water flush. Eggs that hatch after the first treatment can start a second wave, and a follow up treatment is sometimes needed.
Will the lawn grow back after armyworm?
Usually, yes. Armyworm eats the leaf rather than the roots, so a healthy warm-season lawn such as buffalo, couch or kikuyu generally recovers once the caterpillars are gone, especially while the weather is still warm enough for strong growth.
- Water deeply and less often to encourage new growth.
- Give a light feed to push out new leaf, rather than a heavy dose.
- Raise the mower a notch until the lawn has thickened up again.
- Repair any patches that have not recovered after a few weeks of good growing weather.
If a lawn has thinned out after an outbreak, lawn care covers the feeding, aeration and patch repair that bring it back.
How do you stop armyworm coming back?
- Check the lawn often from late spring to autumn, especially after warm, humid, rainy weeks.
- Keep the lawn healthy with the right mowing height, watering and feeding, so it recovers quickly if caterpillars do arrive.
- Treat moths around outdoor lights at night and birds working the lawn as early warnings.
- Where outbreaks happen every year, consider a preventative product registered for lawn caterpillars, applied before the peak of the season.
