Checking for chafer grubs and leatherjackets
A brown patch that ignores everything you do — watering, feeding, raking — is rarely "just" a dry spot. The two most common UK lawn pests both produce that "won't recover" pattern, and both are easy to confirm with a 30-second test.
Why this matters
Chafer grubs (the larvae of chafer beetles) and leatherjackets (the larvae of crane flies — daddy long legs) both feed on grass roots underground. The grass loses its anchor, browns off, and lifts like a carpet when you tug it. Crows, magpies, badgers and foxes know this — they tear up the lawn going after the grubs.
If you mistake pest damage for drought, you'll water for weeks and watch the lawn keep dying. Catching it early lets you intervene with biological control (nematodes) at the right time of year.
When to do it
- Active season is spring through early autumn (Mar–Oct). Adult chafers and crane flies fly in summer; their larvae feed on roots from late summer through the following spring.
- Routine inspection monthly through the active season — 5 minutes is enough.
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Triggered inspection any time you see:
- Brown patches that don't recover with watering
- Birds (especially crows, magpies, starlings) consistently working a patch
- Lawn that lifts like a carpet when you grip it
- Spongy underfoot in a localised area
- Holes in the lawn from foxes or badgers digging at night
When not to skip
If you have any of the trigger symptoms above, don't wait for a routine cycle. The earlier you confirm pests, the better your treatment options.
How to do it
- Pick a suspect patch. A browning area, near where you've seen birds working, or any spot that feels spongy.
- Cut a square of turf — 25 × 25 cm, about 5 cm deep. Use a spade or sharp knife; lift it cleanly.
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Examine the underside and the soil. Look for:
- Chafer grubs: stout, white, C-shaped larvae with light brown heads and three pairs of legs at the head end. Up to about 18 mm long. Curl into a tight C-shape when disturbed.
- Leatherjackets: greyish-brown, leathery, legless tubes with no obvious head. Up to about 30 mm long.
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Look at the picture overall. A small number of either is normal — most soils carry a background population. Worry when:
- You're finding several grubs in every patch you cut, and
- You can see corresponding above-ground symptoms (yellowing patches, lifting carpet, persistent bird-working). The combination is what matters; a one-off "I found one" doesn't justify treatment.
- Replace the turf. Push it back firmly, water it in. It usually re-roots if the area wasn't already damaged.
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If you're confident the pest is doing real damage, plan the treatment. Both pests are managed with biological-control nematodes:
- Chafer grubs — Heterorhabditis bacteriophora nematodes, applied July to September when the grubs are small and the soil is warm and moist.
- Leatherjackets — Steinernema feltiae or S. carpocapsae nematodes, applied September to early October.
- Both need a soil temperature of at least 12 °C and moist (not waterlogged) ground. Outside the right window, they're wasted.
What to expect afterwards
A confirmed pest count doesn't fix the patch — it tells you the cause and the timing window for treatment. The dead grass in damaged areas needs reseeding (Family First for back lawns, Envy for fine turf) once the pests are dealt with.
If your count is below threshold, no action is needed. Re-check periodically; populations can ramp up across a season.
For nematode treatments: results take 4–8 weeks. Damage stops spreading within a fortnight; recovery (reseeding into the patches) starts after the autumn nematode application has done its work.
Common mistakes
- Watering brown patches indefinitely. If the patch isn't drought-stressed it's not coming back with water alone — and the constant moisture sometimes worsens the pest problem.
- Pulling up one grub and panicking. A baseline of grubs in the soil is normal. The decision to treat should rest on visible above-ground symptoms paired with grub presence — not on a single sighting.
- Treating chemically. Most lawn-grub chemicals are now banned for amateur use in the UK; nematodes are the practical option and they actually work.
- Wrong nematode for the wrong pest. Heterorhabditis for chafers; Steinernema for leatherjackets. The packs are clearly labelled — read.
- Applying nematodes to dry or cold soil. They need moisture and warmth (10 °C+). Outside the window, they die before they reach the pests.
Seasonal notes
Spring (Mar–May): if you saw damage last autumn, this is when you'll see the worst of it. Chafer grubs are at full size, eating heavily. Confirm with the test cut; nematodes don't work this time of year (apply in late summer).
Summer: adult flight season. Chafer beetles emerge May–July; crane flies emerge July–September. New eggs go down through summer.
Late summer / early autumn (Aug–Oct): the treatment window. Nematodes applied now catch the larvae while they're small and the soil's still warm.
Winter: too cold for nematodes; grubs are deep down and inactive. Wait.
How do I repair pest-damaged patches?
Confirming the pest is only half the job — the dead, rootless patches need reseeding once the grubs are dealt with. A pre-seed feed plus a hard-wearing seed gets bare areas knitting back together.
MyLawn is our free app: tell it your postcode, grass type and what you’ve already done, and it gives you a plain-English red/amber/green steer on the single best next job — with smart reminders so the timing never slips. Learn more about MyLawn.
Related Mowd guides: Lawn renovation: step-by-step · How to apply lawn seed
Frequently asked questions
How do I know if I have chafer grubs or leatherjackets?
- Look for brown patches that ignore watering, turf that lifts like a carpet, and birds or animals digging at night. Cut a 25 cm square of turf and check the soil for C-shaped white grubs or grey, legless leatherjackets.
How do you treat chafer grubs and leatherjackets?
- Biological-control nematodes are the practical option for UK gardens, applied in late summer to early autumn into warm, moist soil. Most chemical grub controls are no longer available to amateurs.
How do I fix a lawn damaged by grubs?
- Once the pest is under control, rake out the dead grass, loosen the surface, then reseed with a pre-seed feed and a hard-wearing lawn seed and keep it watered.
Disclaimer
This is a general guide for typical UK domestic lawns. Other pests exist (frit fly, lawn cutworms) but are far less common. If your inspection finds something you don't recognise, snap a photo and consult a local lawn care professional before treating.