Carpet moths: prevention and treatment

Three species, a single food

Three moths invite themselves into our homes, and it is the larvae that eat. The webbing clothes moth, Tineola bisselliella, is by far the most common and the most destructive on wool: the adult is 5 to 8 mm, uniformly beige to gold, with a tuft of reddish hairs on top of the head; the larva is cream-white, 10 to 13 mm, and spins silk tubes or patches over whatever it attacks. The case-bearing clothes moth, Tinea pellionella, can be told apart without a magnifier: its larva carries a case of silk and debris into which it withdraws.

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. It has become rare in homes, which central heating makes too warm and too dry for it: it is found mainly in lofts and chimneys, where its larvae live in birds' nests. If it is this one, look high up before looking under the sofa.

All three are treated in the same way, because they eat the same thing: keratin, the fibrous protein of wool, silk and feather, locked together by disulphide bridges that almost no organism can break. The larva's gut works at a very high pH, up to 10, and under strongly reducing conditions: there it releases hydrogen sulphite, which breaks those bridges.

The consequence is decisive: on a hand-knotted rug, the larva eats the wool pile and leaves the cotton warp and weft intact. That is why a grazed rug keeps its structure and can almost always be repaired — see moth-damaged rug. Conversely, a plant or synthetic fibre soiled with animal matter — perspiration, urine, greases — is attacked as well. Cleanliness is the first prevention.

Only one stage in four feeds

The female lays 40 to 50 eggs on average, the size of a pinhead, 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, and its length is extraordinarily variable: from five weeks to more than two years depending on temperature and food. A larva with little to eat stays a larva, and waits. Pupation then lasts 8 to 10 days in summer, three to four weeks in winter.

The adult that emerges no longer feeds: it only reproduces. Swatting a moth therefore saves no rug. From egg to egg, allow four to six months, that is up to two generations a year in a heated home, where the English sources take one generation a year as the norm. Above all remember this: the egg is the most resistant stage, and a treatment that spares it produces adults several weeks later, just when everything seemed settled.

Where it hides, and why there

The moth seeks darkness and stillness. It does not settle in the middle of the sitting room: it settles where you never look. That is why an infestation is found late.

• Under heavy furniture: the part of the rug trapped beneath a bookcase, a bed, a sofa
• Along the skirting boards and in the corners, off the rug as well as on it
• On the back of the rug, where the wool is accessible and where nothing is ever disturbed
• On the borders and the selvedges, thicker and less often brushed
• At the back of wardrobes and chests, and in rolled rugs that are never checked

Two factors make everything worse: heat, which increases the number of generations a year, and the permanent infestations in lofts and chimneys, which re-seed the house every year.

The early signs, before the hole

The hole is a late sign. The first useful sign is elsewhere: the pile becomes unevenly short, as though mown in patches. The Flemish Monumentenwacht leaflet puts it in a phrase — afgegraasd textiel, geen gaatjes: grazed textile, not little holes. An area that is shorter under a piece of furniture is not wear: nothing wears where nobody walks.

Then look for the frass: minute granules of droppings, exactly the colour of the fibre eaten, lodged at the base of the pile — ivory on ivory wool, blue on blue wool, which makes them invisible from a distance. With them come the larval skins, the patches of silk spun on the surface, the cream-white larvae in the dark areas, and beige moths that run and flutter rather than fly.

The check takes a minute, twice a year: turn back a corner of the rug, run your hand against the pile under the heaviest piece of furniture, follow the skirting boards with your eye.

What works, in order

Vacuuming is the first measure, the cheapest, and the one that gets neglected. It mechanically removes eggs, larvae, cocoons and frass. Methodical means: both faces of the rug, slowly, with crossed passes; the borders and the fringes; the floor underneath; then the skirting boards, the corners, the backs of heavy furniture. Empty the bag outside, straight away.

Freezing is the mainstay treatment for wool. The conservation protocols give the figures: -20 °C for one week at the Canadian Conservation Institute, -18 °C for at least two weeks in the English reference. Two details matter as much as the temperature. The drop must be fast, four hours at most, failing which some insects produce antifreeze compounds and recover on warming. And the rug must be laid out flat, never rolled: halving the thickness quarters the cooling time. It is sealed under polythene and left to come back to room temperature with the bag closed.

Anoxia is the treatment for pieces to which nothing may be done: fine silk, fragile dyes, antique rugs. Oxygen is brought down to 0.3% or less, under nitrogen or with absorbers, in a barrier enclosure. Below 0.3% oxygen, the conservation literature puts the treatment at around two weeks at 25 °C, and considerably longer if the oxygen level rises again. Published protocols vary: this is not a recipe, it is a controlled enclosure.

Controlled heat is the fast route: 48 to 55 °C held for twenty-four hours, with gradual ramps up and down. In a conditioned chamber, with no free water and no rubbing, wool withstands that level. This does not contradict the 40 °C rule: felting requires heat, moisture and agitation together. What threatens the rug is not the temperature, it is the swing in relative humidity it brings with it, and controlling that calls for dedicated equipment. Washing with water, finally, is part of the treatment: it flushes out the eggs, the larvae and the frass lodged at the base of the pile — moth treatment.

What does not work

Mothballs. They act only by fumigation: what is needed is a saturating vapour concentration, held for several weeks, in a genuinely airtight container. In a half-open wardrobe, and still less on a rug lying on the floor, it is never reached. All that is left is the smell, which lingers in the wool.

Lavender, cedar, scented sachets. At best they repel adults. But the adult eats nothing: repelling it protects no wool and reaches neither the larva already settled nor the egg already laid. In a sound wardrobe a sachet does no harm; on an established infestation it gives the feeling of acting while the larvae carry on.

Surface insecticide. Sprayed on a pile, it does not reach the base of the tuft, where the eggs and the larvae are, and its residual action does not cover a hatch spread over several weeks. Pyrethroids have a place, but a narrow one: in the cracks and crevices of the home, never on an antique or valuable textile, and never on clothing.

Pheromone traps are not a treatment either, but they do not claim to be: they are measuring instruments. Put them on the floor and under the furniture, in every room that holds wool, and leave them there permanently. Catches that stop and then start again a few weeks later signal surviving eggs.

Where what you can do at home stops

Most of the prevention is yours: methodical vacuuming, moving the furniture and the rugs regularly, checking the loft and the chimney, setting traps, refusing to store a dirty textile. What you cannot do at home is hold -20 °C on a large piece, bring the temperature down fast enough, or maintain 0.3% oxygen for two weeks. And once the larva has grazed whole areas bare, no treatment brings the pile back: it kills, it does not make wool grow again. What follows is repiling and reknotting. One rule holds for all the rest — treat the rug and the room together: a cleaned rug put back in an infested room is recontaminated before the end of the season.