Every summer from about November we get the same call from Raceview, Springfield Lakes and Redbank Plains: "My lawn was fine last week and now there's a brown patch the size of a car." More often than not the answer is lawn grubs. They are the single most-searched lawn problem in Australia, and South East Queensland's warm, wet summers are exactly what they like.
What lawn grubs actually are
"Lawn grub" is a catch-all for a few different pests. In Ipswich the two that matter are:
- Army worm and sod webworm, the caterpillars of small brown moths. They feed on the leaf blade at night, so the lawn looks grazed or scalped and turns brown from the top down. These are the fast ones: a lawn can go from green to brown in a few days.
- Curl grubs (the larvae of African black beetle and cockchafers). They live in the soil and eat the roots, so the grass yellows, loses vigour and can be rolled back like carpet because nothing is holding it down.
Both show up most between December and March, with a second smaller wave in autumn, and both are far easier to deal with early.
The two-minute check
- Soak a patch. Mix a bucket of warm water with a good squirt of dishwashing liquid and pour it over a square metre on the edge of the brown area, where it meets green grass. Army worm and webworm will come to the surface within ten minutes.
- Lift a corner. Push a spade in and lever up a sod. If the roots are gone and the grass peels away, or you can see fat white C-shaped grubs in the top five centimetres, you have curl grubs.
- Watch the birds. Magpies, ibis and butcherbirds pecking the same patch every morning are a reliable early warning.
- Look at the moths. Small brown moths lifting out of the lawn at dusk in December usually mean eggs have already been laid.
What usually works (and what doesn't)
Treat with a registered lawn grub product, applied properly. Granular products for curl grubs need to be watered in so they reach the root zone. Liquid or hose-on products for army worm and webworm need to go on in the late afternoon, because the caterpillars feed at night. Either way, follow the label rate; doubling it does not work faster and can burn the lawn.
Treat the whole lawn, not just the patch. By the time a patch is visible the pests are already working outward into the green grass around it. Spraying only the brown area is why "it came back".
Keep mowing, but lift the height. Scalping a stressed lawn makes it worse. Raise the cut a notch and keep the lawn moist (not soggy) while it recovers.
Feed it afterwards. Once the pests are gone, a light application of a balanced lawn fertiliser plus consistent water will bring buffalo, couch and kikuyu back from leaf damage in two to four weeks. Root damage from curl grubs takes longer and bare patches may need a plug or seed repair.
Skip the home remedies. Detergent brings grubs to the surface for a check, but it does not control them. Neither does petrol, bleach or "just mowing it short".
Prevention for next summer
A thick, healthy lawn is the best defence because it tends to recover faster than the grubs can eat it. Through spring: fertilise, water deeply but less often, and mow regularly at the right height. If you have had grubs two summers running, a preventative application in late November before the moths lay is far cheaper than a lawn repair in February.
Frequently asked questions
How do I know if I have lawn grubs or just dry grass?
Dry grass browns evenly across sunny or high areas and greens up again after rain. Grub damage spreads outward from a patch in days, often with birds pecking at it, and the soapy-water test will bring caterpillars to the surface.
When is lawn grub season in Queensland?
December to March is the main season in South East Queensland, with a smaller wave in autumn. Warm nights after summer rain are when moth activity and egg laying peak.
Will my lawn recover from lawn grubs?
Usually, yes. Leaf damage from army worm recovers in two to four weeks with water and a light feed. Root damage from curl grubs takes longer and bare patches may need plugs or seed.
Can I mow a lawn that has grubs?
Yes, but raise the cutting height and avoid scalping. Mow a day or two after treating so the product has time to work.
