Move a three-seater for the first time in four years and one of two things happens. Either the carpet underneath is a brighter, plumper version of the carpet everywhere else – which is what should happen – or there’s a patch of thin, grazed, sandy-looking pile that stops in a rough outline about where the sofa’s shadow fell. The second one is almost never wear. Nobody walks under a sofa.
I get this call most often in April and May, when people start moving furniture for spring cleaning, and again in September when the heating goes on and the adults start appearing on the walls. The diagnosis takes about ninety seconds on your knees with a torch, and it decides whether you’re spending £180 or £1,400.
Why do bare patches appear under furniture and not in the doorway?
Larvae want three things: keratin to eat, darkness, and to be left alone. The underside of a sofa in a warm sitting room off Fortis Green Road is the best habitat in Greater London. No light, no vacuum cleaner, stable temperature, and a wool carpet directly beneath.
Traffic areas are the safest part of your floor. Feet disturb the larvae, daylight reaches the pile, and the hoover goes over it twice a week. The pattern of damage runs precisely opposite to the pattern of wear, which is the whole reason the two are so easy to separate once you know to look.
The classic map is: under the sofa, under the bed, behind the curtains where the hem touches the floor, inside the wardrobe, and a strip along the skirting board behind furniture. Edges matter more than people expect – the carpet under the gripper rod at the perimeter is undisturbed and often where the population lives while the visible damage sits half a metre away.
What the larvae eat and what they leave alone
Keratin. Wool, silk, cashmere, feather, hair. They cannot digest polypropylene, polyester or nylon, and a solution-dyed polypropylene carpet will never be eaten no matter how long the sofa sits there.
This gives you a diagnostic shortcut on blends. An 80/20 wool-nylon twist that’s been grazed comes back with a ghost of itself still standing – the nylon component survives as a thin, pale, wispy pile while the wool is gone. That is the tell. A worn 80/20 doesn’t do that; it flattens and greys as one thing.
What does moth damage look like up close?
Pile that’s been eaten is cut. Individual fibres have been chewed through at different heights, so you get an uneven stubble field rather than a smooth low patch, and in bad cases the backing is fully exposed with the pile standing tall right up to the edge of the bare area. Wear never gives you that hard boundary.
Size helps too. Feeding sites tend to be patchy and irregular – a dinner plate here, a thumbprint there, sometimes joining into a larger bald area under a bed where nobody has looked since the mattress went down. The shape follows the furniture footprint and the dark, and it has nothing to do with how the room is used. A worn carpet, by contrast, tells you exactly where people walk: kitchen door to kettle, hall to bathroom, the pivot point at the bottom of the stairs where everybody turns.
The backing itself stays intact. Larvae have no interest in a polypropylene primary backing or jute weft, so you’re looking at a bald carpet with a perfectly sound structure – which is why moth-damaged carpets can sometimes be repaired by re-tufting or by patching from a wardrobe offcut, where a worn-out carpet cannot.
Frass and cases: what to look for on your knees
Frass is larval droppings, and it looks like fine sand or ground pepper sitting down at the base of the pile in the same colour as the carpet, because it is the carpet. Run a hand across the bare patch and then look at your palm. Grit that isn’t grit is diagnostic.
Case-bearing larvae build a small silk tube around themselves, camouflaged with carpet fibre, and drag it about. The result looks like a grain of rice in the same shade as your carpet, roughly 8mm long, often stuck against a skirting board or the underside of a sofa’s frame. Webbing moths leave loose silken threads and small tunnels in the pile instead.
Then the adults – buff-gold, about 7mm, poor fliers that tend to run and scuttle rather than flutter at a lamp. Somebody in a flat on Orford Road told me she’d never seen a moth in her life. There were nine of them on the underside of the sofa cushions.
How is wear different?
Wear is a gradient. It starts at the doorway, deepens down the traffic lane, and fades out where people stop walking. There’s no edge to it. Take a torch, get your eye down at pile level, and you’ll see the damage strengthen and weaken smoothly across the room.
Worn pile is also still there. The fibre tips have splayed, fibrillated and gone matt, and the yarn has untwisted so the carpet looks greyer and duller along the route, but you can feel the pile depth with a fingertip and it’s within a millimetre or two of the protected areas. Moth damage takes the pile down to nothing in patches while leaving the neighbouring tufts at full height.
The fibre-tip test you can do with a torch
Pinch a few surviving fibres at the edge of the bare area and look at the ends against a light. Chewed fibres end abruptly, like a cut hair. Worn fibres end in a splayed, feathered mess of fine filaments that catch the light differently.
Inconsistent lengths with clean ends is the tell. If the ends are fuzzy and the lengths are uniform, you have a carpet that has been walked on for eleven years.
Could it be something else entirely?
Sometimes, and I’ve watched people spend money on pest control for problems that were nothing of the sort.
A bay window in a first-floor flat on Rectory Grove had a bleached, weakened strip across the pile that the owner had been told was moth. It was ultraviolet degradation – south-facing glass, no blind, a mid-blue wool carpet that had gone dusty pink in a band the exact shape of the window reveal. The fibres were weak enough to break, which is what convinced her.
Old cat urine does something similar and worse, breaking down wool from the base and taking the dye with it. Plant pots leave circular damage with a hard edge that looks alarmingly like a feeding site. And the underside of a badly adjusted vacuum head will shave a stripe into a wool loop pile over a couple of years, which turns up as a straight line of low pile parallel to the sofa – straight lines are the tell there, because larvae don’t work in straight lines.
Sun fade, urine and the vacuum stripe
The distinguishing question for all three is whether the fibre is missing or altered. Fade and urine change the colour and strength of fibre that is still physically present at full height. Moths remove it. Get down and feel the pile depth with the back of a finger before you accept anybody’s diagnosis, including mine over the phone.
When is treatment worth paying for, and when is it not?
Here’s where I talk myself out of work.
If you find one dead adult on a windowsill in Richmond and no grazing anywhere, no frass, no cases, you have a moth, not an infestation. Vacuum thoroughly under and behind everything, do the wardrobe, and check again in three weeks. Paying for a treatment at that point buys reassurance and nothing else.
If there’s active grazing, treatment is worth every penny, and it needs to include the bits people skip. A spray across the open floor while the sofa stays in place is theatre. The perimeter under the gripper, beneath every heavy item, inside the wardrobe base, and the underlay if the carpet’s coming up anyway – that’s the job. Insecticide treatment kills larvae and adults and does very little to eggs, so a second visit two to three weeks later is part of the method rather than an upsell. Anyone quoting a single visit for an established infestation is quoting for a visit, not a result.
What I won’t sell anyone is a standing quarterly moth treatment on a flat with no evidence of moths. That’s a subscription for a service you don’t need, and the trade sells a great many of them.
My unpopular view: for a ground-floor Victorian flat with suspended timber floors, a resident cat, and an owner who wants a pale wool carpet, the honest advice is to buy a good synthetic instead. I lose the wool cleaning work and the moth work both. The alternative is treating that flat every two years for the rest of the carpet’s life.
What I do before quoting anything
Lift a corner near the worst patch and look at the underlay and the boards beneath. Frass and cases in the gap between floorboards means the population is under the carpet, and no amount of surface treatment reaches it. That changes the quote from a treatment to a lift, treat, clean and refit – and it’s better for everyone if we find that out before I’ve written a number down rather than after.
The sofa on Doughty Street had been in the same position since 2011. Under the left arm, where the carpet met the skirting, there was a patch of bare hessian the size of a paperback and about forty cases along the gripper rod.