Silverfish and Firebrats: Identification and Prevention

Silverfish are wingless, flattened insects with a tapering, carrot-shaped body, silvery scales, two long antennae and three long tail filaments. If you find one in a bathtub, under a sink, in a basement box of old papers or darting behind a bookshelf at night, it is most likely a silverfish or its heat-loving relative, the firebrat.

Extension entomologists consider them generally harmless to people, and they do not bite or sting. They can nibble paper, wallpaper, book bindings, starched fabrics and dry food, and a large population usually means the house has had them for a long time.

What silverfish and firebrats are

Silverfish and firebrats are an ancient group of wingless insects. They belong to the order Zygentoma, formerly called Thysanura, and are sometimes called bristletails. They never develop wings, and the young look like small versions of the adults. They do not go through metamorphosis at all; see our page on simple and incomplete metamorphosis for how that compares with other insects, and the orders of insects for where Zygentoma sits.

One unusual trait: they keep molting after they reach adulthood, many dozens of times over a life that can last several years.

How to identify them

Silverfish and firebrats share a body plan:

• Flattened, broad near the head and tapering toward the rear, giving a fish-like outline
• Covered in fine, dusty scales that rub off easily
• Two long, slender antennae
• Three long, thin tail filaments at the end of the abdomen
• No wings
• Fast, darting runs with short pauses

Adults are usually about 13 to 19 mm (1/2 to 3/4 inch) long, not counting antennae and tails. Newly hatched nymphs are tiny, about 1.5 mm (1/16 inch), and whitish, and they darken with each molt.

Common silverfish

The common silverfish (Lepisma saccharina) is uniformly silver or pearl gray with a metallic sheen and reaches about 13 mm (1/2 inch). It prefers damp, warm spots, roughly 71 to 90°F (22 to 32°C), and Ohio State University Extension puts its preferred relative humidity at 75 to 95 percent. Look for it in basements, bathrooms, laundry rooms, kitchens and lower floors.

Firebrat

The firebrat (Thermobia domestica) is about the same size but duller, with a mottled gray and brown pattern and dark patches on the back. It needs heat rather than moisture: it prefers temperatures above 90°F (32°C), with an optimum near 98 to 102°F (37 to 39°C), and tolerates relative humidity as low as about 30 percent. Look for it near furnaces, boilers, water heaters, ovens, fireplaces and hot attics.

Gray and four-lined silverfish

University of Missouri Extension describes two more household species. The gray silverfish (Ctenolepisma longicaudata) is uniformly gray and reaches about 19 mm (3/4 inch). The four-lined silverfish (Ctenolepisma quadriseriata) has four dark lines running down its back. Both tolerate a wider range of humidity than the common silverfish and can turn up anywhere from the basement to the attic.

Look-alikes

People sometimes confuse silverfish with house centipedes (which have long legs all along the body), earwigs (which have pincers rather than tail filaments) or young cockroaches (which lack scales and the three long tail filaments). Our bugs in the bathroom guide can help narrow it down.

What silverfish eat

Silverfish and firebrats prefer starchy, carbohydrate-rich materials, along with some protein. Reported foods include:

• Paper with glue or sizing, including books, magazines, photographs and wallpaper
• Wallpaper paste, book bindings and glues
• Clothing starch and starched linens
• Cotton, linen, rayon and silk
• Flour, cereals, pasta, dry pet food and other dry foods
• Dead insects, household dust and certain molds

They can go months without food, which is one reason they persist in storage areas.

The damage they cause

Silverfish have weak jaws, so they scrape at surfaces rather than chew through them. Feeding often begins in one spot and spreads as unconnected, irregular holes or thin, grazed patches on paper and fabric. University of California IPM notes that mold associated with their feeding can also discolor book pages. Scales and debris near damaged items are a useful clue.

Things at the most risk:

• Books, especially older ones with glued bindings
• Stored papers, documents, photographs and cardboard boxes
• Wallpaper in damp rooms
• Starched or stored linens and clothing
• Opened dry food in pantries

Most damage is slow and cosmetic. Rare books, archives and family papers are where it matters most.

Life cycle and life span

Female silverfish lay eggs in cracks and crevices near food, in clutches that average about 50 eggs, according to University of California IPM. University of California IPM puts hatching at about 14 days for firebrats and 19 to 32 days for silverfish.

Development is slow and depends strongly on temperature. University of California IPM reports that in warm conditions firebrats can reach adulthood in as little as 40 days and silverfish in about 60 days, while in the cold it can take up to 300 and 500 days respectively. University of California IPM gives life spans of about three years for silverfish and two years for firebrats, and other extension sources report longer.

Because they grow slowly and lay relatively few eggs, populations build slowly. A large silverfish infestation usually means a long-standing one.

Why they are in your house

Silverfish usually arrive inside, rather than walking in. Eggs, nymphs and adults travel in cardboard boxes, old books and papers, furniture and stored items moved from an infested space. Outdoors, they live under bark, under stones and in animal nests, and some enter homes that way, especially houses with wood shingles.

Once inside, they settle where they find:

• Moisture for silverfish: bathrooms, basements, laundry rooms, leaky plumbing
• Heat for firebrats: furnace rooms, water heaters, ovens, attics in summer
• Hiding places such as wall voids, insulation, bookshelves, closets, stacked boxes and gaps around pipes
• Food in the form of paper, glue, starch, crumbs and dust

They often turn up in sinks and bathtubs because they fall in while looking for moisture and cannot climb the slick sides, which can make it look as if they came up the drain.

How to prevent silverfish

Prevention is about making the house drier, less cluttered and less inviting:

• Reduce humidity. Run a dehumidifier in damp basements, use bathroom exhaust fans, and ventilate closets and crawl spaces.
• Fix leaks. Repair dripping pipes, faucets and condensation problems under sinks and in laundry areas.
• Cool down hot spots. For firebrats, insulate hot water pipes and improve air circulation around water heaters and furnaces.
• Cut the clutter. Get rid of stacks of old newspapers, magazines and unneeded cardboard boxes.
• Store paper and linens in sealed plastic bins, not cardboard.
• Store dry food in tight containers. See our pantry pests storage tips.
• Vacuum regularly, including along baseboards, shelves and closet floors.
• Seal cracks around pipes, baseboards, windows and doors.
• Inspect boxes and secondhand items before bringing them in or moving them into living areas.

How to control silverfish

Confirm and monitor

Place sticky traps (the kind sold for cockroaches) or small glass jars wrapped in tape on the outside, so silverfish can climb in but not out, in corners and along walls where you have seen them. Check them every few days. Traps show where the insects are concentrated and help you judge whether things are improving.

Fix the conditions

Drying out and decluttering are the main control steps, and for light infestations they are often enough. Silverfish may linger for a while because they can go months without food, so keep monitoring for several weeks after you make changes.

Targeted treatments

For heavy or persistent infestations, insecticides labeled for silverfish can be applied to the cracks, crevices and voids where they hide: behind baseboards, around pipes, in closets and attics. Ohio State University Extension notes that control can take 10 to 14 days because the insects must contact the treated surface. Desiccant dusts such as silica gel or diatomaceous earth work in dry, hidden voids. Baits are of limited use, and University of California IPM does not recommend foggers. Read and follow every product label.

When to get help

If silverfish keep coming back after you have dried out and cleaned up, or you cannot find where they are living, a licensed pest professional can locate hiding places in wall voids and attics and treat them.