By August, a lawn with irregular brown patches has usually been blamed on both of these already, often by two different people standing in the same garden. The confusion is reasonable. From standing height the damage is genuinely similar: patches with no clean edge, spreading through the hottest weeks, in grass that was fine in June.
The reason it matters is that the two live in different places. A grub treatment has to be watered down into the soil to reach anything. A chinch bug treatment has to stay up on the surface. Apply either one to the wrong problem and you have spent the money, lost two or three weeks of the growing season, and still have the insect.
What each one is actually doing
Grubs are beetle larvae — fat, white, curled into a C, living in the top few centimetres of soil and eating grass roots. The blades themselves are untouched. This is why grub damage is so often noticed late: the grass looks fine right up until it has nothing holding it down, and then it goes in a matter of days.
Chinch bugs are small sap-feeders that work above the soil, at the base of the blades, in the hottest and driest and sunniest part of the lawn. They drain the plant directly. The grass browns off where it stands, with its root system intact.
So the two produce the same colour by opposite routes, and the difference is not in the colour at all. It is in whether the grass is still attached.
The differences you can see without a test
| Grubs | Chinch bugs | |
|---|---|---|
| Where they feed | Roots, below the soil | Blades and crowns, above it |
| Pull the turf | Lifts away, spongy, like loose carpet | Stays rooted and firm |
| Where it starts | Anywhere; often shaded or irrigated ground | The hottest, sunniest, driest strip — along paving, by a south wall |
| How it spreads | Patches enlarge in place | Damage moves outward from a starting point |
| Early colour | Wilting first, then brown | A purplish tinge, then yellow, then brown |
| Other animals | Birds, skunks and raccoons tearing the turf up | Nothing digs for them |
The animal sign is worth knowing because it is free. If something has been pulling your lawn apart overnight, it was not doing that for chinch bugs — nothing digs for a sap-feeder. That is a strong hint, though it is a hint about what is present, not proof of what caused the patch you are looking at.
Tug the grass in the middle of a brown patch, firmly. If it comes away in a sheet and you can roll it back like a rug, the roots are gone and you are looking at grubs. If it holds and the blades tear instead, the roots are intact — which rules grubs out, and puts chinch bugs at the top of the list.
Then confirm the one you landed on:
For grubs, cut three sides of a 30 cm square and peel it back like a flap. Count the white C-shaped larvae underneath. More than about five under that square is a real infestation worth treating; fewer than that is a lawn that will outgrow them if you feed and water it.
For chinch bugs, cut both ends off a tin, push it a few centimetres into the soil at the edge of the damage — not the dead middle, where they have already left — fill it with water, and wait five minutes. Chinch bugs float up.
Why you take the sample at the edge
This catches people out with both insects, and it is the difference between a test that works and one that tells you nothing.
The dead centre of a patch has no food left in it. Whatever was feeding there has already moved on to the living grass around it. Sample the middle and you will find an empty square of soil and conclude, wrongly, that you have neither problem. The edge — where green meets brown — is where they are now.
If both tests come back empty
That happens, and it is worth taking seriously rather than testing a third time.
Irregular brown patches in high summer also come from heat dormancy, from underwatering, from a fungus, from a mower blade set too low, and from soil compacted hard enough that roots cannot get through it. Chinch bug damage in particular is mistaken for drought stress constantly, and the mistake runs both ways: plenty of lawns get treated for insects that were simply not being watered deeply enough.
If the turf is firmly rooted and the tin can produced nothing, the honest next question is whether the patch is in the driest and sunniest part of the lawn — and whether it has had about 2.5 cm of water a week, in one or two soakings rather than a daily sprinkle.
What to do once you know
Grubs. Confirm with the peel-back test before treating anything. Over about five per square foot, treat; under that, feed and water and let the lawn outgrow them. Whatever you apply has to be watered in — a grub product that stays on the surface never reaches the larvae.
Chinch bugs. Treat the damaged edge and roughly a metre of healthy grass beyond it, which is where they are heading next. Then reduce the thatch layer, because that is where they overwinter, and a lawn with a thick mat of dead stems will have them again next year.