Introduction: the little silver insect that says a lot about your home
It's 11 p.m., you switch on the bathroom light, and something darts towards the seal around the bathtub: three centimetres long, silvery, flattened, wriggling like a fish out of water. You have just met a silverfish (Lepisma saccharina).
The first reaction is almost always the same: reach for an insecticide. That is precisely the wrong approach. Silverfish don't bite, don't transmit any disease and don't get into food the way cockroaches do. Their presence, on the other hand, is an indicator of humidity as reliable as a hygrometer: wherever they thrive, the air stays above 70% relative humidity, and that humidity causes far more problems than the insect itself.
Every autumn, as the rain returns, windows stay shut and the heating comes back on, reports shoot up. This guide explains what is really going on behind your skirting boards, how to tell apart the two species that are constantly confused, and above all how to tackle the cause rather than the symptom.
What exactly is a silverfish?
The silverfish belongs to the thysanurans, an order of primitive insects that appeared more than 300 million years ago — long before cockroaches, long before ants. It never developed wings and undergoes no metamorphosis: the juvenile that hatches from the egg is a miniature copy of the adult.
A few useful characteristics to know:
- Size: 8 to 12 mm as an adult, with a flattened, carrot-shaped body.
- Colour: silvery grey, due to fine scales that rub off on contact (hence the powdery mark on a pale wall).
- Appendages: two long antennae at the front, three tail filaments at the rear — an unmistakable signature.
- Behaviour: strictly nocturnal and light-shy, it flees illumination within a fraction of a second.
- Lifespan: two to eight years depending on conditions, which is remarkable for a household insect. A female may moult more than fifty times over her lifetime.
Its slow reproduction rate is good news: a female lays around a hundred eggs over her entire life, compared with several hundred in a few months for a German cockroach. A silverfish infestation never builds up in three weeks. By the time the population becomes visible, the humidity problem has been there for months, if not years.
The cousin not to confuse it with: the firebrat
A second thysanuran is common in our homes: the firebrat (Thermobia domestica). It is brownish and mottled, and above all it seeks out the exact opposite of what the silverfish wants.
| Criterion | Silverfish (Lepisma saccharina) | Firebrat (Thermobia domestica) |
|---|
| Colour | Uniform silvery grey | Mottled brown-grey |
| Preferred temperature | 20 to 25 °C | 32 to 41 °C |
| Preferred humidity | 75 to 95% | Tolerates drier air |
| Typical location | Bathroom, toilet, laundry room, cellar | Around boilers, ventilation ducts, ovens, bakeries |
| Treating the cause | Drying out, ventilation | Thermal insulation, cleaning hot zones |
Confusing the two leads to a classic mistake: dehumidifying a room while the colony is actually living against the boiler flue in the utility cupboard. Take three seconds to photograph the insect before chasing it away.
Why autumn is their season
Three phenomena overlap between September and November in the average home.
1. Indoor humidity rises. Windows get closed, laundry is dried indoors, showers get hotter. One load of washing dried indoors releases between 2 and 3 litres of water into the surrounding air. A family of four commonly produces 10 to 15 litres of water vapour a day through breathing, cooking, showers and laundry.
2. Cold spots appear. As soon as the heating restarts, the gap between warm, vapour-laden air and cold surfaces (north-facing wall corners, window sills, the backs of furniture against external walls) causes condensation. This is where the micro-zones at 90% humidity that silverfish look for are formed.
3. Food is available. Silverfish digest cellulose thanks to specialised gut flora. Their menu: wallpaper paste, book bindings, storage boxes, newspaper, starch in textiles, but also hair, skin flakes, crumbs and mould. Cardboard boxes in the cellar, old magazines in the cupboard under the sink: all so many larders.
France's national health and safety agency (ANSES) points out in its work on indoor air quality that excessive humidity in homes is associated with the proliferation of mould and dust mites, and with worsening respiratory symptoms. In other words: the silverfish is not the problem, it is the warning light on the dashboard.
Are they dangerous? What they actually damage
There is no direct health risk: no bite, no sting, no identified disease vector. A few cases of allergic sensitisation to the proteins in their moults have been described in the literature, but they remain marginal.
The damage is material and sometimes costly:
- Wallpaper peeled away or nibbled at the surface, especially along the seams.
- Old books, archives, identity documents, photographs: small irregular erosions along page edges.
- Textiles rich in starch or natural fibres: starched cotton, linen, silk, particularly in storage trunks.
- Labels, stamps, food cartons.
For documents and books you care about, the simple reflex is to take them out of damp areas and store them in airtight storage boxes rather than in cardboard boxes sitting directly on a cellar floor. This is a conservation measure before it is a pest-control one.
Diagnose before you treat
Step 1: measure, don't guess
The human eye is a very poor hygrometer. A digital thermo-hygrometer left for a few days in the bathroom, the laundry room and the bedroom gives an objective picture. Useful benchmarks:
| Measured relative humidity | Interpretation | Silverfish risk |
|---|
| 35 to 50% | Ideal comfort | None |
| 50 to 60% | Acceptable | Low |
| 60 to 70% | Watch zone | Colonisation possible |
| Above 70% | Abnormal, cause to be identified | Population settling in |
Measure at skirting-board height rather than at head height: the layer of cold air near the floor is often 10 points more humid.
Step 2: locate the hotspots
Do a night-time round with a torch, an hour after the lights go out. Inspect:
- Bath and shower seals, the basin trap, around the toilet.
- Underneath the washing machine and the dishwasher.
- The utility cupboard housing the ventilation unit or the boiler.
- Skirting boards along external walls, on the north side.
- The bin store, the cellar, the crawl space if accessible.
One reliable empirical test: place a slightly dampened piece of bread or a concertina-folded piece of cardboard in a dark corner and check it in the morning. Silverfish readily gather there, which confirms the main hotspot.
Treating the cause: dry out, ventilate, seal up
This is where 80% of the result is decided. Insecticide without drying out is a bucket emptied under a running tap.
Ventilate effectively
- Air the place for 10 minutes morning and evening, windows wide open, including in winter: this renews the air without cooling the walls.
- Check the ventilation vents: a clogged vent loses half its airflow. A simple test is to hold a sheet of toilet paper against the grille — it should stay up on its own. If it falls, the system needs cleaning or has broken down.
- Never block the air inlet grilles on windows, even when they whistle.
- Run the extraction for 20 to 30 minutes after a shower, with the door closed.
Reduce vapour production
- Put lids on pans while cooking.
- Avoid drying laundry in a bedroom or a bathroom with no extraction.
- Wipe down shower walls after use: this removes up to half a litre of water a day.
Dry out actively
In a laundry room, a cellar or a windowless shower room, an electric dehumidifier rated at 10 to 20 litres a day will bring humidity below 55% within a few days and make the environment quite simply uninhabitable for silverfish. For a small cupboard or a walk-in wardrobe, refillable moisture absorbers are often enough.
Seal off the hiding places
Silverfish live in cracks. Redoing degraded bath seals, pipe entries under the sink and gaps in skirting boards with sanitary silicone sealant removes dozens of hiding spots in a single afternoon. It is the most cost-effective step in the whole process.
The additional treatments that really work
Once humidity is under control, the population declines naturally over a few weeks to a few months — the adults' longevity simply slows their disappearance. To speed things up:
Diatomaceous earth. A mineral powder of fossil origin, it acts mechanically by abrading the insect's cuticle, which then dehydrates. No resistance is possible, and there is no neurotoxic agent involved. Apply it as a very fine film (a thick layer is simply walked around) behind the washing machine, along skirting boards and into crevices, using a brush or a powder duster. It only works when dry: reapply after cleaning. Take care not to breathe in the dust while spreading it.
Sticky traps. Glue traps for crawling insects placed along skirting boards and behind furniture mainly serve as a monitoring tool: counting the catches each week tells you whether the situation is improving, rather than relying on an impression.
Baiting. Gel baits designed for crawling insects can complement the setup, but their usefulness remains limited against an insect that can find food everywhere in a home.
What doesn't work, or barely: "knockdown" aerosol bombs, which kill the visible individuals without reaching the eggs laid deep in cracks; lavender or cedar essential oils, repellent for a few hours at best; ultrasonic devices, whose effectiveness against crawling insects has never been demonstrated by any serious study.
An important reminder: since the widely reported poisonings linked to the use of banned products against bed bugs, it bears repeating — no insecticide that is not approved for domestic use should ever be used in a home. Always check for a marketing authorisation number on the packaging and observe the stated airing times.
Special cases: rented homes, apartment blocks, new builds
In rented accommodation. The presence of silverfish linked to structural humidity (faulty ventilation, water ingress, no extraction in a windowless shower room) falls under the landlord's obligation to provide decent housing, in the sense of the French decree of 30 January 2002. Report it in writing, with photos and dated humidity readings. Conversely, humidity caused by inappropriate use (laundry dried indoors, blocked air vents, no airing at all) remains the occupant's responsibility.
In an apartment block. A broken-down communal ventilation system, a damp service duct or a flooded cellar fall under the common areas. Our analyses of pests in apartment blocks and rented homes set out how costs are shared; the principle is the same here: shared cause, collective treatment.
In new builds. Recent, highly airtight homes concentrate humidity if the ventilation is poorly adjusted or if construction moisture (slab, renders) has not dried out. It is not unusual to find silverfish in a two-year-old flat. That is not a hygiene failing, it is a failing in air renewal.
When to call in a professional
A typical domestic case can be sorted out on your own. Call in a pest control company certified Certibiocide if:
- The population persists after three months of documented drying out.
- You are simultaneously seeing other moisture-loving species (woodlice, silverfish, extensive black mould): a building diagnosis then takes priority over pest treatment.
- You keep archives, antique books or paper artworks that are at risk.
- You suspect a concealed leak: a damp survey, with thermal camera and moisture meter, generally costs between €200 and €500, and avoids repeated, pointless treatments.
For a targeted intervention against crawling insects, expect an order of magnitude comparable to other light pest treatments. Our indicative intervention prices give up-to-date ranges, and our selection of tested products lists useful drying and trapping devices.
The seven-day action plan
- Day 1: place a thermo-hygrometer in every suspect room.
- Day 2: night-time round, map the hotspots, take a photo to identify the species.
- Day 3: check and clean all ventilation vents and air inlet grilles.
- Day 4: take cardboard boxes, newspapers and textiles out of damp areas and rehouse them in sealed containers.
- Day 5: redo degraded silicone seals and fill pipe entry gaps.
- Day 6: install a dehumidifier or moisture absorbers depending on the room.
- Day 7: apply a fine film of diatomaceous earth in crevices and set out two or three monitoring traps.
Record the catches every week for a month. A falling curve confirms that the cause has been dealt with. A flat curve signals that a damp hotspot has escaped you — most often behind a built-in bathtub, under a walk-in shower or inside a service duct.
Key takeaways
The silverfish is one of the few household pests that can be eliminated almost entirely without chemicals. It doesn't live in your home because it is dirty: it lives there because it is damp. Restore humidity to between 45 and 55%, get rid of cracks and cardboard boxes on the floor, and the colony disappears by itself.
And if the humidity persists despite proper ventilation, it is no longer an insect problem: it is a building problem. The silverfish will simply have done you the service of warning you before the mould moves in.
