Moth-damaged rugs: treatment and restoration

What the larva digests: keratin

Keratin is the fibrous protein that wool, silk, fur and feather are made of. Almost no organism can digest it, because it is locked by disulphide bridges. The larva of the common clothes moth can: its gut works at a very high pH, up to 10, and at a very low redox potential, down to -300 mV. Larval enzymes release hydrogen sulphite there, which reduces the disulphide bridges; proteases then finish the job.

Two practical consequences. The first: a synthetic rug is of no interest to it, a wool rug is, and a wool-and-silk rug is doubly attractive. The second, less well known: plant or synthetic fibres soiled with animal matter — perspiration, urine, food grease — are attacked as well. A stained wool rug is therefore markedly more exposed than a clean one. That is what links cleaning directly to prevention.

Three species concern our interiors. The common clothes moth, Tineola bisselliella, is by far the most frequent and the most aggressive on wool: the adult is 5 to 8 mm, uniformly beige to golden, with a small tuft of reddish hairs on top of the head; the larva is cream-white, 10 to 13 mm, and spins silk tubes or sheets over the surface it attacks. The case-bearing clothes moth, Tinea pellionella, is recognised by the silk case the larva carries about with it. The tapestry moth, Trichophaga tapetzella, is the largest — a wingspan of about 19 mm, the wing black towards the head and iridescent white towards the rear — but it has become rare indoors: central heating makes our homes too warm for it, and its larvae develop mainly in nests and in regurgitated pellets.

Only one stage out of four eats the rug

The female lays on average 40 to 50 eggs, the size of a pinhead, on vulnerable textile, spread over two to three weeks. Hatching takes 4 to 10 days in warm weather.

Then comes the larval stage: the only one that eats. Its length varies enormously — from 35 days to two and a half years depending on temperature and available food, with 5 to 45 moults. Pupation then lasts 8 to 10 days in summer, 3 to 4 weeks in winter. The adult that emerges no longer feeds: it only reproduces. Squashing a moth therefore saves no rug.

From egg to egg, reckon generally on 4 to 6 months, that is about two generations a year. English sources take one generation a year as the norm, with up to three cycles in warm conditions; the Flemish Monumentenwacht booklet gives close to a year for the common clothes moth.

Take one thing from this: a treatment that does not kill the eggs settles nothing. The egg is the most resistant stage, and it can produce adults several weeks after a treatment cut short.

Where it settles, and how to recognise it

The moth needs darkness and stillness. Outbreaks start under heavy furniture, under sofas, on the borders and on the reverse of rugs, at the back of wardrobes, in folds and corners. The adults avoid light: the areas that are never disturbed are exactly the ones they choose.

Add to that warmth, which increases the number of generations a year, soiled textiles, and outside sources — birds' nests, the bodies of small animals and droppings in lofts and chimneys keep permanent outbreaks going. The national survey carried out by English Heritage shows higher catches in buildings dating from before 1950, richer in service voids, chimneys and lofts, and in flats with party walls.

The most reliable sign is not the hole. The Flemish booklet puts it exactly: afgegraasd textiel, geen gaatjes — grazed textile, not little holes. The pile is unevenly cropped, as though shorn in patches, until the cotton foundation shows through. Look also for the tubes or sheets of silk spun on the surface, the mobile cases, the frass — minute granules of excrement exactly the colour of the fibre eaten — the larval skins mixed in with those granules, the cream-white larvae in dark areas, and the beige moths that run or flutter rather than fly properly.

Where to look first: the reverse of the rug, the borders, the part under a heavy piece of furniture, under the sofa, along the skirting boards.

The treatments that genuinely kill, eggs included

The effective protocols are those of museum conservation, and they are published with their figures. Cold first. The Canadian Conservation Institute's reference is -20 °C for a week, with more aggressive protocols working between -30 and -40 °C; English Heritage takes -18 °C for at least two weeks. The rate of cooling matters as much as the temperature: the descent must not take more than four hours, failing which some insects produce antifreeze compounds and recover on warming. The rug is therefore spread out flat, never rolled — halving the thickness quarters the cooling time. It is sealed in transparent polyethylene, out of contact with the walls, then brought back to room temperature with the packaging closed, so that all the condensation forms on the outside of the bag. One sufficiently cold treatment is enough: repeated freeze-thaw cycles are no longer judged necessary.

Anoxia next: the oxygen is replaced with nitrogen, or absorbed, inside a sealed barrier enclosure. The threshold is 0.3 % oxygen or less; complete mortality of all stages has been obtained after seven days at that level, between 33 and 75 % relative humidity. To work safely, reckon on around two weeks at 25 °C, appreciably more at 20 °C, and longer again as the temperature falls. At 0.6 % oxygen, considerably longer is needed. Published protocols vary: this is not a recipe, it is a controlled enclosure. The Monumentenwacht Vlaanderen booklet recommends a month minimum with monitoring of the oxygen level. This is the treatment for fine silks, fragile dyes and pieces on which no chemical product is acceptable.

Controlled heat, finally. A short exposure to 55 °C is enough to eradicate every stage; the Belgian reference gives 48 to 55 °C held for 24 hours, with gradual rise and fall and control of humidity. On a rug, the risk is not the temperature in itself, but the change in relative humidity it brings about: that is what has to be managed, and that is what the climate chambers of the reference industrial process do.

And permethrin? It is a synthetic pyrethroid that acts as a stomach poison and not a contact poison: the larva only dies by digesting treated wool. In industrial wool treatment it is locked into the heart of the fibre by an exhaustion process — the cuticle opens, the solution penetrates, the cuticle closes again — and none is left on the surface; European regulation 528/2012 requires the treatment to be declared. That is its place: preventive protection of the fibre, and treatment of the premises in cracks and crevices. Not as a curative solution on a piece of value, and never sprayed directly onto a heritage textile. The clean curative solutions are cold, anoxia or controlled heat.

Two further measures complete the picture. Hand washing flushes out the eggs, the larvae and the frass lodged at the base of the pile; vacuuming remains the first measure and the cheapest. Pheromone traps, for their part, draw the males onto a sticky base: they are a tool for detection and monitoring, never for eradication — useful for knowing whether the outbreak is over, useless for ending it.

After treatment: repiling and reknotting

The treatment kills; it does not give the pile back. Restoration begins afterwards, and on a moth-damaged rug it is generally simpler than people fear: the larva has eaten the wool and left the cotton. Warp and weft are intact, the foundation holds, and most often there is nothing to reweave.

On a grazed area where the pile is cropped without the rug being holed right through, we repile: we fill back in with hand-dyed wool, strand by strand, until the neighbouring height is reached. Where the larva has gone as far as the foundation, we reknot with the original knot — senneh or ghiordes — following the design read off the sound parts, then shear the new pile to exactly the height of the old.

Prevention comes next, and it is straightforward. Vacuum regularly, starting with the reverse, the borders and under the furniture. Move the rugs and turn them round: the moth needs to be left in peace. Never put a dirty rug away — a dirty rolled rug is a larder — and wrap it closed after washing. Deal with organic stains quickly. Check chimneys and lofts. Put down pheromone traps as permanent monitoring, on the floor and under the furniture.

And above all: treat the rug and the room. A rug put back clean into an infested room is recontaminated before the end of the season.