Silverfish, and the second species nobody tells you about
The common silverfish only lays eggs in damp conditions, so drying the room out is usually the fix. Grey silverfish, first found in the UK in 2014, breeds at lower humidity, and on that one drying the room may not be enough.
By The Pest GuyLast updated
The short answer
They are a humidity reading. Mostly.
The common silverfish will only lay eggs in damp conditions. That is the single most useful fact about them, because it means you do not have to kill them to end the problem. Take the damp away and there is no next generation. The same damp is usually what has brought woodlice indoors as well, if you have been finding those in the same rooms.
They do not bite you and they are not known to carry disease. What they eat is paper and starch, so the thing genuinely at risk is a box of documents or books in a damp cupboard, not you and not the building.
The complication, and the reason this page exists, is that there are now two silverfish in British houses. The second one breeds at lower humidity than the first, which means the standard advice does not reliably work on it.
What the common silverfish actually needs
English Heritage publishes David Pinniger's silverfish fact sheet as collections-care guidance, and it is unusually direct about the mechanism. The common silverfish, Lepisma saccharina, is 10 to 15mm long, covered in silver scales, with long antennae and three long whip-like bristles at the back end.
On breeding it says: they "will only lay eggs in damp conditions and the nymphs live in or very close to damp food where they feed and grow". On risk it says: "the key factor for Lepisma silverfish is damp and they will only cause serious damage when collections are kept at high humidity".
The life cycle runs 6 to 12 months depending on temperature, humidity and food. That is slow, and it is good news: a silverfish problem that has been building for a year unwinds over a similar timescale once the damp goes, rather than exploding the way a fly or a flea problem does.
What they eat, which is not what people assume
The list from the same source is specific: paper, books, prints, drawings, photographs and labels. Occasionally silk and some other textiles. They particularly like materials with coatings of starch, gelatine or organic dyes. Flock wallpaper and wallpaper paste are often attacked. They also eat microscopic mould and the greasy surface dirt left by handling, which is the one thing their diet has in common with what woodlice are eating in the same damp corner.
Read that list again and it tells you where to look. Not the middle of the room. The back of a cupboard with paper in it, the underside of wallpaper on a wall that is damp, a box of photographs in a cellar.
The damage has a signature too. They graze the surface first, leaving bare or translucent patches, and only later eat through, leaving holes with irregular thinned edges. If you have found holes, they have been there a while.
Holes in wool knitwear are not this, whatever else is in the same cupboard. That is a moth problem, and the stage doing the eating is not the one you can see.
The second species, and why it breaks the standard advice
Grey silverfish, Ctenolepisma longicaudatum, was first found in the UK in 2014. It is long established across mainland Europe and it is spreading here. Pinniger calls it "a much larger species which has only recently been found in the UK" and notes it "appears to be spreading rapidly".
The distinction that matters is one sentence: "Ctenolepisma seems to be able to breed and cause damage at lower humidity than Lepisma."
That is the whole problem with the advice everyone gives. Dehumidify the bathroom and you have usually dealt with the common silverfish, because you have taken away its ability to lay eggs. Do the same to a grey silverfish population and you may simply have made the room slightly less pleasant for something that can carry on regardless. The published work on its spread in Europe attributes the northward move partly to insulation and central heating producing exactly the conditions it likes, and describes it as troublesome in modern buildings, where it reaches high densities and moves between rooms and flats.
So it is worth knowing which one you have. Grey silverfish is 15 to 20mm rather than 10 to 15mm, grey and slightly blotchy rather than silver, with a fringe of short bristles down the sides of the body, antennae longer than its own body, and longer rear bristles. Side by side it is obvious. On its own, the length and the antennae are the tell.
The two species, side by side
Most UK pages describe one animal. They are two, and the difference decides whether drying the room works.
| The tell | Common silverfish (Lepisma saccharina) | Grey silverfish (Ctenolepisma longicaudata) |
|---|---|---|
| Adult size | 10-15mm | 15-20mm |
| Appearance | Silver scales, long antennae, three whip-like bristles at the rear | Grey, sometimes blotchy scales, a fringe of short bristles on the head and body sides, longer antennae and rear bristles |
| Damp needed | Lays eggs only in damp conditions, and causes serious damage only at high humidity | Breeds and causes damage at lower humidity than the common silverfish |
| In the UK | Long established | First found in 2014, spreading, helped by insulation and central heating |
Sources: David Pinniger for English Heritage, "Silverfish and grey silverfish" fact file; Insects (PMC8471186); BPCA PestWatch. No humidity percentage is given here on purpose: the figures in circulation disagree by source, and the defensible claim is the relationship between the two species rather than a number.
What to do, in order
Deal with the humidity first regardless of species, because it is free, it is usually the whole answer for the common silverfish, and it makes conditions worse for the other one. In a bathroom that usually means the extractor actually running long enough after a shower, and the door shut while it does.
Then move what they are eating. Paper and photographs out of the damp cupboard and into a dry room, in boxes rather than loose. This is the step people skip, and it is the one that protects the thing you would actually mind losing.
Then look, properly, at what you have got. If the animals are 15 to 20mm with antennae longer than their bodies, and they are turning up in dry rooms as well as wet ones, the humidity route on its own is unlikely to finish it, and that is the point at which paying someone who knows the difference is worth the money rather than a waste of it. If you rent, check who is supposed to be paying for it before you do, because sustained damp in a let property is rarely the tenant's bill.
If it is the grey one
On the common silverfish, drying the room out is usually enough and you need buy nothing at all. On grey silverfish, which breeds and damages at lower humidity, that is not enough on its own, and this is the crawling-insect treatment the standard advice leaves out.
What I’d use
Crawling insect spray, 600ml
Zero In Ultra Power Ant & Insect Killer (600ml)
A contact spray for crawling insects, listing ants, cockroaches, earwigs, woodlice and silverfish, with a residual claim.
It is the part the standard advice leaves out. Drying the room usually ends a common silverfish population because it only lays eggs in damp conditions, and that is the advice on every page about them. Grey silverfish breeds and damages at lower humidity, so on that species the dehumidifier is not the answer on its own and a residual crawling-insect treatment is what the usual advice is missing.
- Size
- 600ml
- Actives
- Prallethrin and cyphenothrin
- Listed for
- Ants, cockroaches, earwigs, woodlice and silverfish
- Use
- Indoor only
Watch out for
- Work out which silverfish you have first. On the common one, drying the room is the fix.
- The residual claim carries no stated duration on the listing.
- Indoor only, and not suitable for use around pets.
- Danger: extremely flammable, and very toxic to aquatic life.
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About this resource
The identification features, sizes, breeding conditions, life cycle, damaged materials and damage signs on this page come from David Pinniger's "Silverfish and grey silverfish" fact sheet for English Heritage. The 2014 UK first record and the spread of Ctenolepisma longicaudatum come from the published work on its establishment in Europe and from British Pest Control Association reporting.
The honest limit, and it is deliberate: this page gives you no humidity percentage. The figures in circulation for these two species disagree with each other across sources, and quoting one as if it were settled would be inventing precision. What is well supported is the relationship between them, which is the part that changes what you should do.
Common questions
Are silverfish harmful?
Not to you. They do not bite, and they are not known to carry disease. What they damage is paper and starch: books, prints, photographs, labels, wallpaper paste and flock wallpaper, and occasionally silk and other textiles. So they are a threat to what is in a cupboard, not to you or to the building.
Why are they always in the bathroom?
Because it is the dampest room you have. English Heritage lists the common silverfish as "frequently found in damp areas, such as bathrooms, toilets, kitchens and cellars". The bathroom is not attracting them so much as being the one room where conditions let them breed.
Will drying the room out get rid of them?
For the common silverfish, largely yes: it "will only lay eggs in damp conditions", so removing the damp removes the next generation. For grey silverfish it is not that simple, because that species breeds at lower humidity. Which one you have therefore changes the answer, and it is the question almost nobody asks.
How do I tell the two species apart?
Size and antennae. The common silverfish is 10 to 15mm and covered in silver scales. Grey silverfish is 15 to 20mm, covered in grey scales that can look blotchy, with a fringe of short bristles down the sides of the body, and both its antennae and its rear bristles are noticeably longer. The grey one is the bigger, scruffier, longer-limbed one.
Do silverfish eat clothes?
Not the way clothes moths do. They go for starch, gelatine and organic dyes, so they will attack starched fabric and can occasionally damage silk, but wool knitwear with holes in it is a moth problem, not this one.
What damage should I actually look for?
Surface grazing before holes. English Heritage describes silverfish scratching the surface of paper and books "leaving, bare or translucent patches", and eventually eating "through the paper leaving holes with irregular thinned edges". If you have boxed papers or books in a damp cupboard, that is where to look first.