Bare patches in lawn: grubs and larvae - prevention and repair

Why do you get bare patches in your lawn?

Bare spots and brown patches on your lawn are rarely accidents. Most bare patches are caused by larvae living under the grass: leatherjackets (larvae of crane flies, Tipula spp.) and chafer grubs (larvae of the cockchafer, genus Melolontha, and the garden chafer, Phyllopertha horticola). They eat the roots of the grass until whole sections simply die. In August, September and October you see this most because the larvae are then larger and eating harder. You recognize it by brown or yellow spots that keep getting bigger, and when you pull at the grass, it comes loose easily because the roots are gone.

The good news: this is preventable and in many situations even repairable. The key is recognizing larvae in time, knowing when they occur, and keeping your lawn strong enough to withstand damage. Crows, magpies and pheasants suddenly pecking in your lawn? That is the signal they are hunting larvae. Use that knowledge: you now know what is underneath. This article guides you through identification, timing, biological control with nematodes, and repair techniques.

Recognizing larvae before they cause damage

Leatherjackets are pale, maggot-like larvae about 5 to 8 millimetres long, with a pointed head. They live just below the grass surface and eat grass roots. You see them in May, June and July if you carefully dig in the soil, or you hear it from the bird noise above your lawn: crows and magpies can perfectly feel where they are. Chafer grubs are larger, white to yellowish, and have a thick head with mouthparts. They live deeper in the soil (up to 20 centimetres) and are particularly active from August to October. Chafer grubs can be identified by their white colour and C-shape. They cause even worse damage because they are larger.

If you see bird activity or brown patches appearing, then it is time to check the soil yourself. Grab a spade and carefully lift a piece of turf, look under the root zone. See white larvae? Then you must take action. The more larvae, the worse the damage will become. Early intervention saves your lawn.

Crows and magpies as your early warning system

Birds know well before you do that your lawn is full of larvae. Magpies, crows and pheasants peck holes in your lawn looking for food. This is no coincidence: they can feel where larvae are. Birds suddenly pecking holes in your lawn? Make that your alarm bell. You still have a few weeks before the larvae are big enough to cause major damage. So start now with prevention and control.

Crows usually peck holes about 3 centimetres in the grass, not always in neat lines. Magpies are more careful, they make much smaller holes. Both can remove dozens of insects in one morning, so they are actually helping you along. Let them work, but know: their arrival is a message that you must take action.

Nematodes in September: the biological approach

Nematodes are microscopic worms that attack and kill larvae. They are not toxic to you, your pets or beneficial insects, but they are deadly to chafer grubs and leatherjackets. How do nematodes work? They penetrate the larva and carry bacteria with them. The larva becomes infected and dies. This can happen within 2 to 3 weeks.

The best time is September to October, when the soil temperature is between 12 and 20 degrees Celsius. Too warm? Nematodes do not work well. Too cold? They become inactive. September and October in the Netherlands and Belgium are ideal. You buy nematodes online: Steinernema feltiae against leatherjackets, Heterorhabditis bacteriophora against chafer grubs. You spread them on your wet lawn. The lawn must stay moist for at least 2 weeks so nematodes can move around. This is a patient process, but it works.

Keeping lawn strong and healthy as prevention

A strong lawn with deep roots can withstand larvae much better than a weak lawn. The stronger your grass, the more damage it can handle. This starts as early as May, June and July when the larvae are still small. So look after your lawn well: mowing once a week (not shorter than 4 centimetres), regular water (if it is dry), and feeding gives your grass the strength to withstand larval damage.

In July and August, feeding with extra potassium is useful. Potassium strengthens the cell walls of the grass, so it becomes more resilient. Avoid nitrogen feeding in July and August as that makes the grass softer and more attractive to larvae. A healthy lawn recovers faster from damage and grows out over bare patches. Lawn aeration (in March or August) also helps: it encourages roots to grow deeper and stronger.

Drought and damage go together

Larvae feel drought. This causes leatherjackets to go deeper into the soil or die. But conversely: dry lawns suffer more severely from larvae because the grass itself is already weakened by moisture deficit. Grass that is already thirsty becomes extra vulnerable in August when larvae attack. So: ensure regular feeding and sufficient moisture in dry periods. This does not need to be much: once a week in dry spells is enough. Your lawn holds up better.

Recovery: reseeding bare patches

Is the damage already done? Do not panic. Bare patches recover easily with reseeding. You wait until October-November when larvae are less active, or you treat with nematodes and wait until December. Then proceed as follows: loosen the soil with a rake or spade, remove dead grass and moss. Scatter grass seed (standard repair mix) and ensure contact between seed and soil. Gently press with your foot or back of rake. Sprinkle thin seed sand over it.

Keep the seed bed moist for 3 to 4 weeks by light sprinkling. By February-March the seeds germinate and the grass grows. Once the new grass is 5 centimetres high, you can mow carefully. This is the cheapest and most sustainable repair.

Chemical control: when necessary

Chemical products against leatherjackets and chafer grubs, such as those based on imidacloprid, are no longer sold to home gardeners in the Netherlands and Belgium. They harm bees and other beneficial soil life, which is why these retail garden products were withdrawn from sale years ago. For you as a home gardener, nematode treatment is therefore not just the better option, it is in practice the only one that is still allowed. If in doubt, ask a garden centre what is currently permitted, but do not count on a quick chemical fix.

Step by step

Step 1: Check your lawn

Grab a spade, lift a small piece of turf. See white larvae? Note how many. This determines whether you take preventive action or must act quickly.

Step 2: Ensure adequate feeding and water

From now until September: feed with potassium (no nitrogen in July-August) and water regularly if it is dry. This makes your lawn stronger.

Step 3: Observe bird activity

Are crows and magpies suddenly making holes in your lawn? Then you know the larvae are present and the time for nematodes is approaching.

Step 4: Apply nematodes in September-October and repeat if needed

Order nematodes online. Spread them on a moist lawn and make sure it stays wet for 2 to 3 weeks. Check again in October: if you still see many larvae, repeat the treatment.

Step 5: Reseed bare patches

Once the larvae are controlled (October-November), reseed bare patches with grass seed. Keep it moist.

Step 6: Prevent recurrence

A strong lawn is your best insurance. Look after it well: regular mowing, feeding and water. Next year larvae will be much less of a problem.

Frequently asked questions

Can I apply nematodes already in August?

Not ideal. Nematodes work best when soil temperature is between 12 and 20 degrees. August is often warmer. Wait until September, October and even November (until the soil does not drop below 5 degrees). Then you have optimal conditions.

How long does it take for the lawn to recover?

That depends on the damage. Small brown spots grow out on their own in May-June if you feed the lawn well. Larger bare patches where you see soil must be reseeded. This takes 6 to 8 weeks from sowing to playable lawn.

Are crows and magpies bad for my lawn?

No, they actually help you. They remove larvae, so they reduce the problem. Yes, they make holes, but that is far less damage than what larvae would cause. Let them work.

Can I combine nematode treatment with lawn feeding?

Yes, absolutely. Feeding helps your grass become stronger, which together with nematodes ensures faster recovery. Just make sure you spread the nematodes well and that the lawn stays moist enough.

Recognition at a glance

Leatherjackets are pale and maggot-like, about 5-8 millimetres. Chafer grubs are larger, white to yellowish, C-shaped. Both eat grass roots. Bare patches, brown spots and bird activity are your signals. The faster you respond, the less damage.

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