Reviewed by William "Willy" Vizcarrondo, NYS DEC Certified Commercial Pesticide Applicator C1825832. Updated September 23, 2026.
Short answer
Humidity. UC IPM notes silverfish are found in basements, laundry rooms and bathrooms, especially shower stalls, and prefer temperatures from 71 to 90 degrees with high humidity. A bathroom with a weak or unused exhaust fan is the most humid room in the house. They're harmless to people; they eat paper, glue, starch and dust, and the fix is drying the room out and sealing the gaps around pipes.
A wingless, fast, tapered insect that looks like a tiny fish. UC IPM describes silverfish as shiny, silver or pearl gray, about 1/2 inch long, with two long slender antennae and a body that tapers from front to rear to three long, thin tail-like appendages. They run in a wriggling motion and hide the instant a light comes on, which is why you see them in the tub or the sink at night or when you move a box in the basement. The firebrat is a close relative, mottled gray or brown, that UC IPM notes lives in very warm dry places around ovens and heating units. Neither bites, stings or carries disease; UC IPM's page has nothing on health risk to people because there isn't one.
Starch, paper and glue. UC IPM lists cereals, flour, pasta, pet food, paper with glue or paste and book bindings, plus household dust, dead insects and certain fungi. In a bathroom that's the paper backing on drywall, the glue on wallpaper, the cardboard box under the vanity, the labels on bottles and the starch in a linen closet. UC IPM notes they scrape rather than bite the surface of paper, leaving irregular holes and page discoloration from the fungi associated with their feeding. A few silverfish in the tub is a humidity sign; silverfish in the closet, the bookshelf and the attic boxes is a bigger population that's been feeding a while.
Dry the room, seal the gaps, trap to confirm. UC IPM's control is to keep areas clean and dry, reduce moisture with dehumidifiers and ventilation from fans, and patch holes or spaces around pipes. In a bathroom that means running the exhaust fan during and for 20 minutes after every shower, or replacing a fan that doesn't actually pull, fixing the drip under the sink and the sweating toilet tank, and caulking the gap where the supply lines and drain come through the wall and floor. UC IPM suggests cockroach sticky traps to monitor, which tell you whether the numbers are dropping. Pyrethrin sprays and dusts are for heavy infestations in wall voids and basements, not for a couple in the tub.
Silverfish are a basement and bathroom complaint across the whole island, but the worst of it is in the damp, older basements of Huntington, Oyster Bay and Bay Shore and in the small, fan-less bathrooms of Levittown and Massapequa capes. They come with humid summers and go quiet in dry winters, then come back. Bobby Bugs treats the harborage areas and points out the moisture problems that keep them coming; general insect work like silverfish is done without a guarantee.
Related
No. They're a sign of a humid one. A spotless bathroom with a bad exhaust fan and a basement with a damp slab will have silverfish; clutter of paper and cardboard gives them more to eat and hide in, but the driver is moisture.
No. UC IPM's description of what they scrape and eat is paper, starch, glue and dust. They don't bite people or pets and they don't carry disease. The damage is to books, photos, wallpaper and stored paper.
Legs. A silverfish is a half-inch teardrop with three tail bristles and six legs you barely notice. A house centipede is an inch or more with many long, striped legs and moves in a fast, rippling run. Both like damp basements and bathrooms, and both are harmless to people; the centipede hunts other insects.
Still not sure? Call (516) 509-6001 and we'll tell you what you're dealing with.