Sod webworm
Crambus and Parapediasia spp.
also called Lawn moth
Small buff moths flying up in zigzags ahead of the mower, and their caterpillars grazing the grass off at the crown in ragged brown patches.
How to know it
The adult is a slender buff or dull gray moth 12 mm long that holds its wings tightly wrapped round its body at rest and has a distinct snout — it flies in a low erratic zigzag when disturbed and drops back into the grass. The caterpillar is greenish-gray with dark spots, up to 20 mm, and lives in a silk-lined tunnel in the thatch.
What you'll see
- Small buff moths flying up in zigzags ahead of you as you cross the lawn at dusk.
- Irregular ragged brown patches the size of a dinner plate, spreading through late summer.
- Grass blades chewed off at the crown, so the patch looks scalped rather than dug.
- Silk-lined tunnels and green frass pellets in the thatch when you part the grass.
- Birds working the lawn hard and probing at the patch edges.
Where it comes from
The moths do not feed on grass at all; they scatter eggs over the lawn while flying at dusk. The caterpillars hatch and build silk tunnels in the thatch, coming out at night to graze. Two generations a year in most of the range, with the second doing the visible damage in July and August. They overwinter as caterpillars deep in the thatch.
Hot, dry summers on sunny, thatchy lawns. Drought-stressed turf shows the damage far more, and a well-watered lawn often carries the same number of caterpillars with no visible patches at all.
Dealing with it
- Confirm with a soapy-water flush on a suspect patch; the caterpillars surface within a few minutes.
- Water and feed the lawn — this alone masks and outgrows most damage.
- Apply beneficial nematodes to damp soil in the evening while the caterpillars are small.
- Dethatch, which removes the tunnels they live in.
- Leave the birds to work; they are efficient and the damage is cosmetic.
Keeping it out
- Overseed with endophyte-enhanced ryegrass or tall fescue, which they will not eat.
- Keep thatch under a centimeter.
- Water deeply through dry spells so the turf can carry a population without browning.
- Mow a little higher, which shades the thatch and helps the grass recover.
- Encourage ground beetles and birds rather than treating a lawn preventively.
What it goes for
Lawn grasses of every kind; the caterpillars occasionally graze low bedding plants at the lawn edge.
Who hunts it for you
- Beneficial nematodesSteinernema & Heterorhabditis spp. Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com