Silverfish in the Bathroom: Why Autumn 2026 Brings Them Out and How to Dry the Problem Up

There's one call we receive without fail from mid-October onwards, and it almost always begins with the same hesitation: "It's not a cockroach, it's not a bed bug… it's a sort of little grey fish that darts away when I switch on the bathroom light."

This is the silverfish, Lepisma saccharina — or its heat-loving cousin Thermobia domestica, the firebrat. It is probably the most misunderstood household pest in France: it doesn't bite, transmits no disease, builds no visible nest, and yet it causes genuine unease in those who discover it, because it always signals something else going on in the home.

That something is moisture. And autumn — that precise moment when we close the windows, turn the heating back on and dry laundry indoors — is the season that turns an occasional sighting into an established colony.

Bare mattress on a bed with a grey headboard in a bright bedroom with curtains and an air conditioner

Identifying a silverfish: what it is, and what it isn't

A primitive, wingless insect with a tapered body

The adult measures 7 to 12 mm, excluding appendages. Its body is flattened, carrot-shaped or like an elongated teardrop, covered in silvery scales that give it its metallic sheen. It has two long antennae at the front and three tail filaments at the rear — and that last detail is what settles the question in two seconds.

Its movement is distinctive: a rapid, undulating wriggle that genuinely does evoke a fish. It doesn't jump, doesn't fly, and climbs smooth vertical surfaces poorly — which is why you find it trapped in the bottom of enamel bathtubs and sinks first thing in the morning.

It is an apterygote insect, one of the oldest existing insect groups. It undergoes no metamorphosis: the young nymph already looks like a miniature adult, and it keeps moulting throughout its life — up to fifty or sixty times. Its longevity is surprising: a silverfish can live three to eight years, which is considerable for a household insect.

The comparison table that prevents treatment mistakes

The mix-up with the most serious consequences is with young German cockroaches. A cockroach nymph moves in bursts, has two cerci rather than three filaments, and above all signals a far more serious infestation. If in doubt, catch a specimen in a jar and photograph it: our technicians can identify the species from a single image.

Why autumn always triggers their appearance

Three mechanisms that stack up in October and November

Silverfish don't "arrive" in a flat in autumn. They're already there, in small numbers, tucked into a crack in the skirting board or under a bath panel. What changes is the comfort you offer them.

1. The heating comes back on. Warm air meets cold surfaces — corners of external walls, window reveals, uninsulated bathroom walls. The result: condensation. Water vapour settles exactly where the insect lives.
2. The windows stay shut. Daily airing drops to a few minutes, when it doesn't disappear altogether. Relative humidity in the bathroom, which used to fall back within an hour in summer, now takes half a day.
3. Laundry drying indoors. A 5 kg load releases roughly two to three litres of water into the home as it dries. Repeat that three times a week in a poorly ventilated one-bedroom flat and you have a permanently tropical climate.

On top of this, in older Paris-region buildings, there's a structural factor: the vertical service ducts running through the floors. They're warm, dark, damp and never cleaned, and they form the motorway along which a building-wide population redistributes itself from flat to flat.

Field rule of thumb: below 50% relative humidity, a silverfish population declines on its own within a few weeks. Above 75%, it doubles without you seeing a thing.

What they actually feed on

Their diet is dominated by polysaccharides: starch, dextrin, cellulose. In practice, that means:

• The bookbinding glue in books and the starch in paper — hence the nibbled page edges and the dulled covers.
• Wallpaper and its paste, attacked from behind, which causes those unexplained bubbles.
• Natural textiles: cotton, linen, silk, viscose, especially if they carry traces of perspiration or starch finishing.
• Dry food residues: flour, cereals, sugar, crumbs under the skirting boards.
• Silver-halide photographs, stamps, archives, sheet music — a genuine concern for anyone storing documents in a cellar.

They are also capable of surviving several months without food, provided there's moisture available. That's why a home left empty but damp is still infested when the occupants return.

Are they dangerous? The honest answer

No, not in the strict health sense. Silverfish don't bite — their mouthparts are incapable of breaking human skin — they don't sting, and they are not identified as disease vectors in the reference literature, unlike cockroaches or rodents.

Two caveats are worth setting out:

• Allergens. Repeated moults release tropomyosin particles into household dust. In sensitised individuals, this can contribute to background respiratory allergy, in the same way as dust mites. It's marginal, but documented.
• Moisture indicator. This is where the real issue lies. A silverfish is a bio-indicator. Its presence in numbers means the home is persistently above the humidity thresholds that favour mould — and mould does have a proven respiratory impact, as ANSES and Santé publique France regularly point out in their work on indoor air quality.

In other words: you don't treat silverfish to protect yourself from them. You treat them because they reveal a ventilation fault or a water ingress that's better fixed before winter.

The method that works: dry out before you treat

Step 1 — Measure, don't guess

First things first: get objective data. An indoor hygrometer placed in the bathroom and a second one in the coldest room of the home will give you, within 48 hours, a reading worth any number of diagnoses. The benchmarks:

Egg-laying is directly correlated with this parameter: a female lays around a hundred eggs over her lifetime, but the eggs barely hatch at all below 50% relative humidity. It's the most powerful lever available to you, and it's free or nearly so.

Step 2 — Cut off the water sources

• Check the mechanical ventilation. A sheet of paper held against the extraction vent should stay put on its own. If it falls, the unit is clogged or the system is out of service. In a co-owned building, this is a matter to raise with the managing agent: maintenance of the shared ducts falls under common parts.
• Redo the sealant. Blackened silicone joints around the bath, shower tray and sink let water through behind the tiles. This is the main invisible reservoir. A silicone cartridge gun and a fungicidal sanitary sealant will fix in one evening what three insecticide treatments will never resolve.
• Air the home for five to ten minutes twice a day, windows wide open, even in November. Cold air, once warmed, is drier than saturated lukewarm air.
• In rooms with no opening window or effective extraction (internal shower rooms, basement laundry rooms), a refillable moisture absorber brings immediate improvement, and a compact electric dehumidifier becomes worthwhile as soon as you're dealing with a cellar or a garage converted into storage.

Step 3 — Starve, clean, seal

The silverfish is a crack-dwelling insect. Depriving it of shelter achieves more than killing it.

• Vacuum thoroughly along skirting boards, corners, under furniture and behind bookcases, then empty the bag or canister outside.
• Fill cracks in skirting boards and pipe entry points with acrylic filler or silicone.
• Clear cardboard boxes out of the cellar and the bathroom: damp corrugated cardboard is simultaneously shelter, larder and ideal egg-laying site. Replace them with airtight plastic storage boxes.
• Protect valuable documents and books in sealed containers, and let air circulate behind furniture by pulling it 3 to 5 cm away from the wall.

Step 4 — The treatment itself

Once the environment has been corrected, treatment is simple and rarely heavy-handed.

• Sticky silverfish traps placed along skirting boards, at the foot of the bath and behind the washing machine serve two purposes: capturing, and above all measuring. An empty trap after a fortnight marks the end of the episode; a saturated trap indicates a focus still active in the immediate vicinity.
• Food-grade diatomaceous earth, dusted in an invisible layer into gaps, ducts and under furniture, acts mechanically on the insect's cuticle. It only works when perfectly dry — there's no point spreading it in a poorly ventilated shower room before you've dealt with the humidity, and that's the most common mistake.
• Off-the-shelf aerosol sprays kill the visible individuals and never reach the eggs lodged in cracks. They give an impression of results for a week or so, then the population comes back. It's worth recalling that ANSES regularly calls for caution regarding untargeted domestic use of insecticides, including "fogger" aerosols, whose real efficacy against crack-dwelling insects is low relative to the exposure they create for occupants.

When to call in a professional

Three situations justify an intervention:

1. The infestation extends beyond one room. Catches in the kitchen, the bathroom and a bedroom point to a population established in the ducts or wall cavities.
2. You're in a co-owned building and neighbours are affected. Treatment then has to be coordinated and include the common parts, cellars and service risers — otherwise recolonisation is certain.
3. You have valuables to protect: an antique library, archives, textile stock, a document collection. Material damage then becomes the primary concern.

Our protocol combines a room-by-room humidity assessment, a search for water entry points, targeted treatment by gel or dusting in inaccessible areas, and follow-up monitoring with traps at 3 and 6 weeks. In nine cases out of ten, half the work is building maintenance, not insecticide.

What to remember before winter

The silverfish is one of the few pests you can genuinely eliminate without heavy chemistry — provided you accept that it isn't the problem, but the symptom.

• It appears in autumn because heating, closed windows and indoor laundry drying push humidity up.
• It doesn't bite and transmits nothing, but it damages books, wallpaper, textiles and archives.
• Below 50% relative humidity, its eggs don't hatch: ventilation is the underlying treatment.
• Resealed joints, checked ventilation, cardboard cleared out, cracks filled: that's what puts an end to the episode.
• Sticky traps to measure, diatomaceous earth as a dry-conditions supplement, and a professional as soon as several rooms or several homes are affected.

If you spot these silvery insects in the bottom of your bath over the coming weeks, take them for what they are: a free alert about the humidity levels in your home, sent before mould moves in. Our teams are at your disposal for an assessment anywhere in the Île-de-France region.