Silverfish — Lepisma saccharinum

The insect is the gauge, not the fault

There is a version of this page that would open by describing what silverfish chew and how quickly they multiply. It would be accurate and it would send you in the wrong direction.

Start instead with the number, because the number governs everything else. Ohio State’s extension fact sheet places silverfish in a band of 75 to 95 percent relative humidity. Texas A&M’s field guide puts what they need to survive at 75 to 90 percent and describes them as sensitive to moisture. UC IPM’s Pest Notes give a preferred range of 71 to 90 degrees with relative humidity above 75 percent at the warmer end of that.

Three land-grant sources, one message: this is an animal that lives inside a narrow envelope of wet air, and outside it the animal is not viable.

Whatcom County is a machine for producing that envelope. Roughly thirty-five inches of rain falls across a long mild season with nothing in it resembling a hot dry spell, and the older housing here sits over bare-earth crawlspaces that give up ground water continuously into the floor system above. The building does not have to leak. It only has to be built the way most buildings here were built.

So the honest framing is that a silverfish population is a reading. It says the air in some part of your house has been sitting in the seventies or higher for long enough to raise a slow-breeding insect to visible numbers. That reading is worth more than the insect is, and it is the reason this page spends its length on hygrometers rather than on products.

What you have in front of you

Shape. Flat, tapering from a broad head end to a narrow tail, which people describe as teardrop or carrot-shaped. Wingless.

Size. UC IPM gives about half an inch, occasionally reaching three quarters. The PNW handbook, writing for this region, gives about three eighths of an inch, which is closer to what turns up in a Bellingham bathroom.

Color. Silver to pearl gray, from a coat of fine detachable scales. Rub one and the scales come off on your fingers, which is a reliable field check and also why a silverfish that has been through a dusty void can look duller than the photographs.

Appendages. Two long antennae forward, three long bristles trailing behind. Nothing else in a Northwest bathroom carries that particular arrangement.

Movement. A rapid side-to-side wriggle in short bursts, then stillness. It reads as fish-like, which is where the common name came from.

One genuinely odd thing about the group, worth knowing because it explains their longevity: they never stop molting. Most insects reach adulthood and stop shedding. These do not. Ohio State records a firebrat observed through 41 molts, and the Florida literature puts the lifetime range at somewhere between 25 and 66. An insect that keeps renewing its cuticle is an insect that keeps going for years, and that has practical consequences further down this page.

A note on the name

The binomial has moved, and it is worth stating plainly because the two spellings sit side by side in the reference literature and look like a mistake.

The genus Lepisma was reassigned to neuter gender, which under the rules of zoological nomenclature changes the ending of the species name. A 2024 worldwide revision of the synanthropic silverfish in the Journal of Insect Science uses Lepisma saccharinum. A great many extension fact sheets, including several cited on this page, still print saccharina. Both refer to the same animal.

The same paper is a useful corrective on a second point. It recognizes nine species of silverfish and firebrat living commensally with people worldwide, and it documents Ctenolepisma longicaudatum — the gray silverfish — turning up for the first time in country after country, including across northern Europe. That species tolerates a wider humidity range than the common silverfish does. We are not going to claim it as a Whatcom County species, because we have no basis for that claim. We mention it because a page built entirely around one humidity band should say out loud that the group is not uniform.

The humidity range, and what dropping it achieves

This is the operative section.

The band the sources describe — 75 to 95 percent, or 75 to 90 at the tighter end — is not a comfort preference. It is a water-balance problem. These insects take up moisture from humid air across the body surface and lose it continuously the same way, and they have no capacity to make up a deficit by drinking the way a larger animal does. Drop the ambient humidity and the loss side of that exchange wins.

The target to aim at is below 50 percent relative humidity, held steadily. That figure is not a biological threshold anybody has published as a hard line; it is the practical operating number that puts a room a long way outside the range every source describes. A basement running at 45 percent through a Bellingham February is a basement that has stopped being habitat.

What that achieves, in order:

Recruitment stops first. Eggs and small nymphs are the most vulnerable stage to dry air. Long before the adults you can see have gone, the supply behind them has.

The adults leave or die. They move toward whatever damp remains, which is diagnostically useful — if you dry the basement and the sightings shift to the one bathroom, that bathroom is now the whole reservoir and it just identified itself.

The count falls slowly. Ohio State puts adult life at two to eight years, and maturity at anywhere from 3 to 24 months. Texas A&M gives up to six years. An animal with that lifespan does not vanish from a room in a fortnight. Expect the visible decline to run across a season.

UC IPM is direct about the tools. Dehumidifiers in closed spaces discourage silverfish, and ventilation from fans can bring relative humidity down to levels they cannot tolerate. NPIC’s guidance runs the same way: ventilate closed rooms, basements and attics; consider a dehumidifier in bathrooms, basements, garages and attics; repair the leaking pipes and standing water first.

Note what is missing from both lists. Neither leads with a product.

Buy the hygrometer before anything else

An inexpensive digital hygrometer costs less than a single service visit and it settles arguments that otherwise run for years.

Put one in the room where you see them and one in the space below or behind it — crawlspace, basement, the cupboard against the exterior wall. Read them in February and read them again in August. What you are looking for is not a single number but a floor: the level the space never gets below.

A room that touches 80 percent after a shower and settles to 45 percent an hour later is fine. A crawlspace that never reads under 75 percent from October to April is the reservoir, and no amount of treatment at a baseboard upstairs changes what that space is doing.

We would rather a homeowner owned that instrument than took a technician’s word for the same conclusion.

Where the water is coming from

The sources of the humidity are boringly consistent across this county, and none of them is exotic.

Bare crawlspace soil. Ground moisture rising into an unlined space is the largest single contributor under most older homes here, and it runs constantly without any leak to point at. The guide to why crawlspaces here stay wet sets out what the ground cover is supposed to be and what the code actually requires.

A bath fan discharging into the attic rather than through the roof or the wall. It moves the moisture out of the bathroom and deposits it on cold sheathing, which is why attics turn up on the list of silverfish locations more often than people expect.

A fan that is undersized, or that stops with the light switch. The moisture from a shower needs to keep moving after the room is empty.

Downspouts discharging at the foundation, and grade that falls toward the building. Both put water into the soil immediately beside the space you are trying to dry.

Slow plumbing failures under a sink or behind a tub panel, which support a population indefinitely on a volume of water nobody would notice on a bill.

Stored cardboard on a concrete floor. Concrete moves moisture continuously. The PNW handbook makes the point directly: do not store cardboard boxes on concrete, because the underside stays damp. That is food, shelter and a humid microclimate in one object.

What they eat, and where the loss is real

Not the house. Starch, and the things starch is in.

Paper. They scrape the surface rather than biting through it, which produces a characteristic grazed, irregular patch and small ragged holes rather than the clean round bore an insect that chews would leave. Glazed and coated papers are taken preferentially.

Book bindings and the glue in them. Ohio State lists bindings specifically, and the damage pattern on a shelf in a damp basement is a notched, frayed edge along the spine.

Wallpaper paste, which in the older city housing here means original paper lifting at seams because what is behind it is being eaten.

Photographs and documents. The archival loss, and the one that is not recoverable.

Starched and sized natural fabric — cotton, linen, silk, rayon — with anything put away damp or starched taken first. Not wool; that is a different group of insects.

Dry goods in unsealed packaging: flour, cereal, oats, pet food.

For most households this is a nuisance and nothing more. For a household with boxes of family papers in an unconditioned basement, or a working library, or a textile collection, the loss is cumulative, silent and permanent, and it is worth acting on properly rather than waiting to see how bad it gets.

That is the distinction this page is built around, and it is what makes the verdict what it is.

The bathtub is a bucket

Worth its own note, because it produces a badly distorted impression of numbers.

Silverfish cannot grip a glazed vertical surface. They are drawn into the humid room, end up on the rim or the floor of a tub or a basin, go over the edge, and cannot get out. Everything that wandered through overnight is still there at seven in the morning, arranged at the bottom where you cannot miss it.

So the tub is a passive trap that has been collecting for eight hours. Three in it does not mean three in the house, and it also does not mean thirty. What it does mean is that there is a humid harborage within a short walk of that bathroom — most often a wall void around plumbing, sometimes the crawlspace directly beneath.

The related and equally common misreading is the drain. They did not come up it. Traps hold water and the sewer is not the source; the humidity is.

Firebrats read the opposite way

A firebrat is the same body plan pointed at the other end of the thermometer, and the distinction changes where you look rather than what you do.

UC IPM gives them very warm, dry places, relative humidity as low as 30 percent, temperatures above 90 degrees, and an optimum of 98 to 102. Ohio State puts them above 90 and near heat sources. They are mottled gray-brown rather than silver.

That sends the search to water heaters, furnaces, boiler rooms, the runs beside heating ducts, insulation next to a warm flue, and in commercial buildings to ovens and mechanical plant. Looking for a firebrat in a cold damp crawlspace is the most common reason one of these jobs stalls, and the color of the insect is what tells you which search you are running.

The food and the damage are the same in both cases. Only the place changes.

Why the verdict here is “depends”

Two of the three species in this humidity group get a plainer verdict than this one. This page does not, and the reasoning is worth setting out.

Where there is nothing starchy at stake and the space can be dried, treatment is not warranted. A few silverfish in a bathroom, a hygrometer reading in the sixties, a fan that needs re-ducting — that is a ventilation job and we will say so. Nothing we apply improves on fixing the fan.

Where the reservoir is a wet crawlspace, the moisture work is the job. A ground cover, cleared and screened vents and corrected drainage change the humidity of the entire floor system above, and they resolve a good deal more than the insect that prompted the call. That is crawlspace restoration rather than pest control, and the sequence matters more than the products do.

Where there is a genuine collection at risk and the drying will take a season, treatment earns its place as a holding action. Books, archives, artwork, stored textiles — material that is being eaten now and cannot be un-eaten later. Targeted work into the voids and along the plumbing runs buys time while the environmental fix takes effect, and in that specific case we think it is justified.

Where the population is large and long established in an occupied space, the same applies with less urgency. These are slow animals; a big count means a long history, and a long history means the conditions have been in place for years.

The line, then: treat when something is being lost while the building dries. Otherwise dry the building. That is a genuinely conditional answer, which is why the verdict at the top of this page is the conditional one.

Storage, which does more than treatment for the things you care about

If the loss is paper or fabric, this section is worth more to you than anything else here.

Move it out of the wet space. A box of documents in an unconditioned basement is inside the reservoir. Relocating it to a heated room removes it from the humidity band and from the population at the same time, and costs nothing.

Lidded plastic, not cardboard. A sealed plastic container excludes them outright. Cardboard is simultaneously food, harborage and a moisture sponge.

Get boxes off the concrete. Shelving or pallets. The PNW handbook flags this one specifically and it is the step people skip.

Watch the seams of original wallpaper in the older city stock. Paper lifting in a damp room means the paste is going.

Store textiles laundered and dry. Starch and sizing are the attractant, so an item put away clean is considerably less interesting than one put away starched.

For genuinely valuable material the answer is a conditioned space with stable humidity, full stop. A recurring treatment schedule protecting boxes that should simply be somewhere else is an expense with no end date attached.

What a silverfish finding says about the building

This is the part that matters after the insects are gone.

Air held at 75 percent and above through a Northwest winter is not only supporting silverfish. Sustained moisture in a crawlspace or a wall cavity is what allows decay fungi to work on framing, and decay is the process that actually costs money in a house here. Wet insulation stops insulating. The species that require genuinely wet wood follow the same water into the same members.

So a silverfish call is frequently the cheapest early warning a building produces. It costs nothing, it arrives years before a soft sill plate does, and it points at a specific space.

That is why a visit for this species tends to involve a light, a hygrometer and twenty minutes under the floor. It is not scope creep. It is reading the instrument the house handed you.

The company they keep

A space wet enough for silverfish supports a small community, and recognizing it saves a second call.

House centipedes are the one people notice, and they are worth understanding rather than treating: they are predators, and silverfish are among their preferred prey. A centipede population appearing after a silverfish population is a food-chain event, not a second infestation. Centipede and millipede work is the same moisture conversation from the other end.

Springtails in large numbers, sowbugs, pillbugs and millipedes all sit in the same envelope. So do the wood-destroying organisms, which is the part with a budget attached.

When in the year to look

The seasonal shape here is shallow, because the reservoir is indoors and largely buffered from the weather. But there is one timing point that is genuinely useful and almost nobody uses it.

Through the wet months the whole building runs damp, crawlspace humidity peaks, and almost any enclosed space will read high. That tells you very little, because in a Whatcom County January most of them do.

Late summer is the diagnostic window. By August the rest of the structure has finally given up some of its winter water. Whatever is still sitting in the seventies then is not responding to weather at all — it has a source. If you want to know where your moisture problem actually is, that is the month to walk the house with a hygrometer, and a silverfish sighting in August is worth several in February.

The other timing note is the one people misread. Sightings often climb in late winter and early spring as heating patterns change and the animals move more. That is a change in activity rather than in numbers, and treating a spike that was always going to flatten is how a seasonal pattern gets mistaken for a successful treatment.

What it is not

A short list, because the wrong identification sends the whole job somewhere else.

Not a firebrat, if it is silver rather than mottled brown — and that distinction moves the search from the crawlspace to the water heater.

Not a house centipede. Long legs, much faster, and a predator of the very insect on this page. Finding both means you have a food chain rather than two problems.

Not a booklouse, which is far smaller, paler and softer, turns up in the same damp cupboards and on the same paper, and is grazing mold rather than starch.

Not a young earwig, which people report because both are pale and quick in a torch beam. The forceps at the tail end settle it in a second.

If you are unsure, photograph it beside a coin. Identification takes moments and it decides everything that follows.

What we will not sell you

A treatment schedule instead of a dehumidifier. If the honest answer to your call is a machine and a working extractor fan, that is what you will be told, on the phone, before anyone books anything.

A fast result. These animals live for years and molt for all of it. Anyone describing a rapid clearance for this species has either misidentified it or is selling repeat visits.

Interior fogging. It reaches insects in the open and none of the ones inside the voids and under the floor where the population actually lives.

Alarm. They do not bite, they carry nothing, and they will not weaken a joist. The expensive thing in this story is the water.

Doing it yourself, in order

One: measure. Hygrometer in the affected room and in the space below it. Read it across a wet month.

Two: fix the ducting. Bath and kitchen fans discharging outside, not into an attic or a soffit, and running long enough after use. A timer switch does this without anybody remembering to.

Three: cover the ground. A complete, lapped polyethylene ground cover in the crawlspace is the highest-value item under most houses here and it is not an expensive material.

Four: move the water outside. Downspouts discharging well clear of the building, gutters clear, grade falling away, and mulch kept off the siding.

Five: dehumidify what is left, in a basement or a closed utility space, and empty it.

Six: seal at the threshold and the penetrations. Door sweeps, plumbing penetrations and the gaps at baseboards in wet rooms. That is exclusion and repair work and it reduces several problems at once.

Seven: move the paper. Out of the damp space, off the concrete, into lidded plastic.

Done in that order, most households never need step eight. Where step eight is warranted, the first seven are what make it hold.