Leatherjackets and chafer grubs

What you are looking at

Patches of the lawn yellow and then brown, and they widen. Take hold of the grass in a patch and pull, and it comes away in your hand. Push a hand under the edge of the turf and it lifts like a loose carpet, because there are no roots underneath holding it to the soil.

Very often the first thing anyone notices is the birds. Starlings, rooks, magpies and crows work the lawn over and tear the surface into shreds, and badgers and foxes will dig it into ridges overnight. That mess is worse than the grubs’ own damage and it is the reason most people investigate.

Roll the turf back and look in the top few centimetres of soil.

Two different grubs, two different nematodes

Leatherjackets are the larvae of crane flies. They are greyish-brown, legless, up to about 4 cm long, tough and rubbery to the touch, with no obvious head. The name comes from the skin. They eat grass roots and chew through the stem bases just below the surface.

Chafer grubs are beetle larvae. They are fat, white, curled into a firm C shape, with three pairs of legs at the front and a distinct orange-brown head capsule, up to about 3 cm. They eat roots.

The distinction matters because the two nematode species sold for them are different. Steinernema feltiae goes after leatherjackets and Heterorhabditis bacteriophora after chafer grubs, and buying the wrong pack wastes the money and the season.

The autumn timing is the whole thing

Crane flies emerge and lay in August and September, into turf that is damp. The larvae hatch and feed through autumn, sit out the coldest part of the winter, and do their most visible damage in early spring when the lawn is trying to grow and they are nearly full size.

Nematodes need warm moist soil and small grubs, which puts the treatment window in September and early October. By the time a gardener sees a ruined lawn in March, the grubs are large, the soil is cold, and nematodes applied then will disappoint. That mismatch between when people notice and when the treatment works is why the products have a mixed reputation.

Chafers run a similar calendar with the adults flying in May and June and the grubs doing their damage from late summer into autumn.

There is no insecticide for lawn pests available to amateur gardeners in Great Britain. The old products have all gone, and nematodes plus a well-fed lawn are what is left.

What to do about it

Count first. Lift a 10 cm square of turf and pick through it. A handful of grubs is normal and needs nothing.

Water nematodes on in September or early October, matched to the right grub, into soil that is warm and damp, and keep the lawn watered for a fortnight afterwards.

Feed and water the lawn so it outgrows what is left.

Aerate, scarify and improve the drainage, since damp turf in late summer is what the crane flies lay into.

Lay black polythene over a watered patch overnight and sweep up what comes to the surface in the morning.

Reseed the bare patches in September.

Leave the birds to it and repair the turf afterwards.

What it gets mistaken for

Red thread bleaches the leaves and leaves the crown and roots untouched, so the turf stays firmly anchored. Grub damage lifts away.

Drought browns the whole lawn at once, worst over buried rubble and pipe runs, and the roots hold.

Dog urine leaves small dead patches ringed with dark green.

Fairy rings show as dark green arcs, often with toadstools.

Vine weevil grubs look almost identical to small chafer grubs and live in plant containers rather than in turf. Anything white and C-shaped in a pot is vine weevil until proved otherwise.

Ant nests raise fine soil through the sward without killing it.