Silverfish: What They Are, Why You Have Them, and How to Get Rid of Them

What are silverfish?

Silverfish are small, wingless insects with a flat, carrot-shaped body covered in silver scales. The common one in homes is Lepisma saccharinum. You can tell it by three things: the silver sheen, two long antennae, and three thin tails at the back end. Adults are usually 1/4 to 1/2 inch (6 to 12 mm) long.

They belong to the order Zygentoma, once called Thysanura, one of the most primitive groups of insects alive. Silverfish hatch looking like small adults and never change shape, so there is no larva or pupa. They hide by day and come out at night to feed. Turn on a light and one runs for cover with a fish-like wiggle, which is where the name comes from.

How big are silverfish?

Most adult silverfish are 1/4 to 1/2 inch (6 to 12 mm) long, not counting the tails. A few old ones reach 3/4 inch (19 mm). That is smaller than many pages claim. WebMD gives “about 3/4 inch” as the usual size, and Terminix says around 3/4 inch, but the university measurements put the typical adult closer to 1/3 inch.

Measurements exclude antennae and tails. Our reading: the 3/4 inch figure is the top of the range.

So what if yours is clearly bigger, or darker? Then it may be another species. They grow all their lives, so a long-lived adult can be large. But a big, grey bug with very long tails is often the gray silverfish, and a brown, speckled one near the furnace is usually a firebrat. Both are covered next.

Silverfish or something else? Bugs that look like silverfish

Check the tail end and the colour first. Three thin tails and a silver body mean a silverfish or a close relative. Pincers mean an earwig. A body with 15 pairs of long legs means a house centipede. Then look at where you found it, because a firebrat lives near heat and its silver cousin lives near damp.

Firebrat details from UC IPM and Penn State. Gray silverfish from Wikipedia and Kulma et al. (2021). Earwig details from UC IPM. House centipede details from Penn State Extension.

People also mix them up with young cockroaches, which have no tails and much longer legs; see our German cockroach identification guide if that sounds closer. Tiny pale specks in damp rooms are more often booklice or springtails, covered in our tiny white bugs guide. Still unsure? A clear photo through the free insect identifier will name it.

Firebrats vs silverfish

A firebrat (Thermobia domestica) is a close relative, about the same size, but mottled grey and brown instead of silver. The bigger difference is heat. UC IPM says silverfish prefer 71 to 90°F (22 to 32°C) with high humidity. Firebrats do best at 98 to 102°F (37 to 39°C), and they cope with humidity as low as 30%.

So the place you find it is the best clue. Firebrats turn up near furnaces, ovens, boilers, hot water pipes, fireplaces and hot attics in summer. A brown “silverfish” by the water heater is very likely a firebrat. The control is much the same, with one exception: UC IPM found that several boric acid dusts do not work on firebrats.

The gray silverfish, and why the usual advice may not work

The gray silverfish (Ctenolepisma longicaudatum), also called the long-tailed, paper or giant silverfish, is the species most pages never mention. Wikipedia names it and Ctenolepisma quadriseriatum as the two other silverfish common in North America. It is light to dark grey rather than bright silver, with long tails.

It matters because it breaks the normal rules. A 2021 study by Kulma and colleagues found that it tolerates low indoor humidity and long starvation, and that the standard strategy fails against it. That fits what people report on Reddit: an infestation in a new, dry apartment, and a dehumidifier that did nothing.

On the other hand, the gray silverfish does take bait, which the common silverfish largely ignores. A 2021 study by Rukke and colleagues, in the lab and in two infested buildings, cleared populations with an indoxacarb gel bait placed as many small droplets along every edge of a room. If you have big, grey, long-tailed ones in a dry home, that is the case to bring to a licensed pest professional, who can choose a bait labeled for silverfish.

Do silverfish bite? Are they dangerous?

No, they do not bite people or pets. Their jaws are so weak that UC IPM says even a 1/2 inch adult scrapes the surface of paper rather than biting it. They have no venom and spread no disease. Texas A&M sums them up as medically harmless. If you woke up with bites, look for a different bug.

Red marks after a night in bed come from something else. Our bug bite identification guide covers the insects that do bite indoors. A photo of a bite cannot tell you which bug made it, so look for the insect itself.

Can silverfish cause allergies?

Possibly, in some people, but the evidence is thin. They shed scales and droppings that end up in house dust. A 2008 study of 45 children with asthma or rhinitis found antibodies to its proteins in their blood, and those proteins cross-reacted with dust mite and cockroach allergens.

However, the same authors wrote that a role in actual disease “still remains to be proved”. So the insect is a possible allergen, not a proven one. If someone at home has asthma or allergies that get worse indoors, ask a doctor or allergist about testing, and clean up the moisture and dust either way.

Are silverfish harmful to cats and dogs?

No. A cat or dog that eats one is fine, since the insect has no venom and nothing toxic in it. Pets often chase them, because a fast, wiggling bug is irresistible. The real risk to pets is the treatment. Keep dusts and sprays out of reach, and follow the label on anything you use.

What do silverfish eat? Damage and signs

Silverfish eat starch, sugar and protein. That includes flour, cereal, pasta, pet food, paper, book bindings, wallpaper paste, glue, and cotton, linen, rayon and silk, especially when starched. They also eat dust and dead insects. Penn State says that, despite their reputation, they prefer protein, and survive more than 300 days without food if they have water.

They can also digest cellulose, the main part of paper, using an enzyme made in their own gut. That is why old books and boxes of papers suffer most. They are hard to starve out, too: the University of Florida says they can go up to a year without food.

What silverfish damage looks like

Silverfish damage looks like grazing. UC IPM describes grey, scraped patches where the surface of paper is gone, missing letters, and small holes with ragged edges that start in one spot and spread. Fungi that grow where they feed can leave dark brown spots.

• Grazed paper on book pages, labels, photos and wallpaper.
• Irregular holes in paper or fabric, with ragged rather than round edges.
• Stains on clothing, cardboard or paper. Texas A&M notes they can stain clothing.
• Silvery scales under or near damaged items. UC IPM calls this a good sign of who did it, since the scales are dust-like and stick to surfaces.
• Tiny dark droppings and shed skins in drawers, on shelves and in boxes.

They rarely do serious harm to clothes. Penn State says they do not feed on wool, which separates their damage from clothes moths and carpet beetles. If wool sweaters or a wool rug have holes, read about carpet beetle larvae in our small black bugs in the house guide instead.

Why do I have silverfish? What attracts them

You have silverfish because part of your home is damp. These insects need high humidity. Wikipedia puts their needs at 75 to 95% relative humidity, and UC IPM says that above 75% is needed at warm temperatures. Food is easy for them to find, so moisture is what decides whether they settle in.

• A damp room. Bathrooms, basements, laundry rooms and crawl spaces with poor airflow.
• A leak. Look for a dripping pipe, a sweating toilet tank or a wet cabinet base.
• A new home. UC IPM notes that fresh concrete and new lumber hold moisture, and wallpaper paste provides food.
• Boxes, books and lumber brought in. UC IPM says they also arrive on lumber, wallboard and similar products.
• Neighbours. In apartments they can move between units through gaps around pipes, walls and floors.

They do not come up the drain. UC IPM explains that one in the tub or sink fell in and cannot climb the slippery sides to get out. Pouring bleach down the drain will not touch them.

I saw one silverfish. Is it an infestation?

One silverfish every few weeks is not an infestation. Many homes have a few, and you rarely see them because they hide by day. What matters is how often you see them and where. Sightings most nights, of different sizes, mean a group is breeding close by.

Our rule of thumb, built from the biology above. No source sets an official count for an infestation.

Silverfish in the bathroom, the bed and the books

Where you see them points to what they are after. They come out of hiding to find food and water, so the room tells you which one to fix.

A silverfish in the bed is unsettling, but it is not a bed bug. They do not feed on people and do not live in mattresses. If you find small reddish-brown bugs and blood spots instead, see our bed bugs guide.

Silverfish eggs, babies and life cycle

Silverfish grow slowly and live a long time. Eggs hatch in 19 to 43 days, and the young take from a few months up to 3 years to become adults, depending on warmth and food. Adults usually live 2 to 3 years, and sometimes far longer. Unlike most insects, they keep molting all their lives.

Egg and nymph sizes from UC IPM and Wikipedia. Hatching time from Penn State. Development and lifespan from UC IPM, Penn State, the University of Florida and Wikipedia.

Baby silverfish are the white, scale-free nymphs. They get their silver colour as they molt, and Penn State says the scales only appear after several molts. So a tiny white one is simply a young one, and seeing young and adults together means they are breeding in your home.

The eggs are very hard to find. A female lays them a few at a time in cracks and crevices, and Wikipedia says she usually lays fewer than 100 in her life. You will almost never see the eggs, and you do not need to. Drying the area and sealing cracks deals with them.

One popular figure is wrong. Texas A&M and WebMD say the young become adults “within 4 to 6 weeks”. UC IPM gives at least 60 days in a warm house, above 75°F (24°C), and up to 500 days below 50°F (10°C). The University of Florida says it typically takes a year. That slow growth is one reason a problem builds up quietly.

How to get rid of silverfish

To get rid of silverfish, dry the air, remove their food and hiding places, and seal the cracks they live in. Bring indoor humidity below 60%, store dry food and paper in sealed containers, vacuum, and caulk gaps. Trap them to measure progress over 2 to 3 weeks. Use a dust or spray only if numbers stay high.

The order below starts with what costs nothing. Penn State Extension is clear that no single method gets rid of them on its own, so do several at once. And UC IPM adds that insecticides will not work while the moisture, food and hiding places are still there.

1. Dry the air (cheapest, most important)

Moisture is the one thing they cannot do without, so drying the air does more than any spray. Run a dehumidifier in the basement or a closed room, and the bathroom fan during and after every shower. UC IPM notes that fans alone can bring humidity low enough that they cannot stand it.

Aim for EPA’s advice for any home: below 60% relative humidity, ideally 30 to 50%. A humidity meter costs about $10 to $50 and tells you whether a room is there yet. Outside, keep gutters clear and make sure the ground slopes water away from the walls.

2. Clean and remove their food (free)

Vacuum floors, baseboards, closet corners and shelves, since dust and crumbs feed them. Move flour, cereal, pasta and pet food into glass or plastic containers with tight lids. Clear out piles of newspapers, magazines and cardboard from damp rooms. UC IPM also suggests moving the books on your shelves around from time to time.

3. Seal cracks and hiding places (cheap)

They spend the day in tight, dark gaps. Penn State points to cracks under and behind baseboards, window and door trim, and holes where pipes go through walls and floors. Fill them with caulk, spackle or expanding foam. Then swap cardboard storage boxes in the basement and attic for sealed plastic tubs.

4. Protect books, papers and photos

For items you care about, storage matters more than spraying. Keep them in a dry room, in sealed plastic boxes, off basement floors. The University of Florida mentions freezing infested objects as an option some people use. For a valuable book or archive, ask a conservator first, because freezing can harm some materials.

5. Desiccant dusts in cracks

Diatomaceous earth and silica gel dry insects out. NPIC, the National Pesticide Information Center, explains that diatomaceous earth absorbs the oils and fats from the insect’s outer layer, and it only works while it stays dry. Penn State recommends applying these dusts into cracks and crevices or wall voids. Keep them off open floors, where they get disturbed and do little.

Use a bulb duster to leave a thin, barely visible film. UC IPM advises a fine layer, because a thick one makes the surface slippery. Choose an EPA-registered product that lists silverfish on the label. NPIC notes that people are exposed by breathing the dust or getting it in their eyes, so apply it gently and keep your face away.

6. Sprays, last and only if needed

If drying, cleaning and sealing are working, you do not need an insecticide. UC IPM says light infestations do not need one at all. For a large, stubborn problem, sprays with pyrethrins or pyrethroids applied into cracks and crevices kill them on contact and keep working for a while.

Spray into baseboard gaps, door and window casings, closets, bookcases and pipe openings, not across open surfaces. Keep spray out of sinks, drains, food and water. UC IPM also warns that oil-based sprays should not be used near pilot lights or electric motors.

What does not work, or is not proven

Several popular fixes do little. Foggers, most baits, and scents such as cedar or cinnamon all get recommended, but the evidence behind them is weak or negative. UC IPM rules out 3 of them by name: foggers, baits on common silverfish, and boric acid dusts on firebrats. Here is what the sources actually say.

• Myth 1

  “A bug bomb will clear them out.”

  Truth: UC IPM says foggers are not recommended for silverfish. The mist does not reach the cracks and wall voids where they hide.

• Myth 2

  “Any silverfish bait will work.”

  Truth: UC IPM found baits are not very successful on common silverfish, because they tend not to eat them. The exception is the gray silverfish, which does take bait.

• Myth 3

  “Boric acid kills all of them.”

  Truth: UC IPM reports that several boric acid dusts are ineffective against firebrats. UC IPM adds that some dusts containing pyrethrin, including silica gel products, are very effective when applied directly on the insects.

• Myth 4

  “Cedar, cinnamon or lavender keeps them away.”

  Truth: We found no home testing behind these. The University of Florida cites a 2006 lab study in which Japanese cedar oil (Cryptomeria japonica) repelled and killed silverfish on treated filter paper. That is a concentrated oil in a lab, not cedar blocks in a closet.

• Myth 5

  “They come up through the drain.”

  Truth: UC IPM says the ones found in a tub or sink fell in and could not climb out. Pouring bleach down the drain does nothing.

• Myth 6

  “A dehumidifier always fixes it.”

  Truth: It works for the common silverfish. The gray silverfish tolerates low indoor humidity, according to Kulma et al. (2021), so drying alone may not be enough.

• Myth 7

  “One silverfish means hundreds in the walls.”

  Truth: One every few weeks is a stray. Regular sightings of different sizes are what point to a breeding group.

How to keep silverfish away

The best way to keep silverfish away is a dry house. Keep humidity below 60%, run bathroom and laundry fans, and fix leaks quickly. Then make food and paper hard to reach, and give them fewer cracks to hide in. These steps also keep many other damp-loving pests away.

• Run the bathroom fan during and after every shower, and wipe the shower dry.
• Use a dehumidifier in basements and closed rooms, and empty it often.
• Store papers, photos and clothes in sealed plastic tubs instead of cardboard.
• Check boxes and used books from damp places before they go on the shelf, since stored paper and cardboard can carry them in.
• Keep dry food sealed, including pet food.
• Vacuum corners and baseboards every week or two.
• Leave the spiders and house centipedes if you can live with them. They eat silverfish.

The same damp corners often bring other pests. Fuzzy moth-like flies on the bathroom wall are covered in our guide to drain flies in the bathroom, and the common cobweb spiders that hunt silverfish are in our house spider guide.

When to call someone

Most silverfish problems are a do-it-yourself job. Call a licensed pest professional if traps still fill after 3 weeks of drying, cleaning and sealing, if they are damaging valuable books or papers, or if you have large grey, long-tailed silverfish in a dry home. That last case may need baits applied the right way.

• A pest professional for heavy or long-running infestations, wall voids you cannot reach, or gray silverfish.
• A plumber or contractor if the damp comes from a hidden leak, a wet crawl space or condensation inside walls.
• Your landlord if you rent and the cause is a leak, poor ventilation or neighbouring units you cannot treat.
• A conservator for damaged rare books, documents, art on paper or family archives.
• A doctor if indoor allergy or asthma symptoms get worse, so the real trigger can be tested.

Not sure what you have at all? Take a sharp photo, ideally from above, and run it through our bug identifier by picture before you buy anything.

Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.