How to Get Rid of Silverfish on the Central Coast

1What silverfish are and why coastal homes get them

Silverfish (Lepisma saccharina) are wingless insects covered in fine silver scales, tapered toward the back like a carrot. Adults run about half an inch long, and some reach three-quarters of an inch. Two long antennae come off the head and three long filaments trail from the tail, which is the quickest way to separate them from anything else in the house. They feed at night and hide by day, so most people meet one when it darts across a bathroom floor and disappears under the vanity. UC IPM places them in damp, warm rooms such as a laundry, at 71 to 90 degrees Fahrenheit. Above that range they need relative humidity over 75 percent to survive.

Coastal Monterey and Santa Cruz county homes sit under marine fog for much of the year, and that ambient humidity keeps indoor moisture high without a single leak anywhere in the house. In the homes we inspect from Pacific Grove to Aptos, the silverfish calls trace back to the same three places. The first is a bathroom with a fan that vents into the attic instead of outside. The second is a crawl space with a torn vapor barrier over bare soil. The third is an uninsulated attic where stored boxes sit against a cold roof deck. Inland, in Salinas and Hollister, the pressure is usually seasonal and traces back to one specific leak. UC IPM also notes that silverfish and firebrats move indoors in large numbers as the wild and landscaped areas around a house dry out over the summer. That is when many of these calls arrive.

2How to tell a silverfish from a firebrat

Silverfish and firebrats look nearly identical at a glance. Both are wingless and scaled, both taper toward the back, and both carry two long antennae and three long filaments at the tail. Color is the first split: silverfish are shiny silver to pearl gray, and firebrats (Thermobia domestica) are mottled gray and brown.

The room is the more reliable tell. Silverfish want damp and moderate. Firebrats want hot and dry, and UC IPM records them living above 90 degrees Fahrenheit, best between 98 and 102, at humidity as low as 30 percent. That describes a water heater closet, a furnace room, or an attic in late summer, not a bathroom. Getting the identification right tells you which room to correct.

• Silverfish: silver to pearl gray, found in bathrooms, laundry rooms, crawl spaces, and cool storage. UC IPM gives their damp range as 71 to 90 degrees Fahrenheit, with humidity above 75 percent needed past that.
• Firebrats: mottled gray and brown, found near water heaters, furnaces, and hot attics, at 90 degrees and up and humidity as low as 30 percent.

3What silverfish damage looks like

Silverfish scrape at a surface rather than bite through it, so the damage reads differently from a rodent chew or a moth hole. UC IPM describes three marks. Large dark spots are where they fed. Gray patches with letters missing are where they grazed the printed surface off the page. Dark brown spots are fungal discoloration from the moisture that let the insects be there at all. Damage tends to start in one place on a page or a garment and spread outward as irregular holes that do not connect to each other.

• Paper and books: notched edges, holes scattered through a stack, and ragged markings along the glued spine of a binding, where the paste is the attraction.
• Wallpaper and paste: grazed patches where the paper meets the wall, most often in a bathroom or a back hallway, since the paste is starch and the wall behind it holds moisture.
• Stored textiles: cotton, linen, and starched or sized fabrics in a box or a back closet, with cast skins, silver scales, and dark fecal specks left on the cloth and on the bottom of the carton.

4Where to look during a self-inspection

Silverfish follow water, so a useful walkthrough follows water too. Work at night with a flashlight, when the insects are active, and start where the humidity is highest rather than where you happened to see one.

1. 1

   Start in the bathroom and the laundry room

   Check the shower stall, the floor under the vanity, the base of the toilet, and the gap where supply pipes pass through the wall. Silverfish that scatter when the light hits them are living within a few feet of where you saw them.

2. 2

   Open the crawl space hatch

   Look for standing water, damp soil showing through a torn or missing vapor barrier, condensation on ducts, and wet framing near a supply line. A crawl space under a coastal home holds humidity year round and feeds it up into the rooms above through the floor.

3. 3

   Go through storage that has not moved in a year

   Pull stacked magazines, boxed books, and closet bins away from the wall and check the bottom carton first. Silver scales, dark specks, and grazed paper point to an established population rather than a stray insect. Set cockroach sticky traps, or small glass jars wrapped on the outside with masking tape so the insects can climb in but not out, and read them once a month.

5Why moisture control is the real fix

Lowering the humidity does more than any spray, because it takes away the condition that lets a population hold. UC IPM treats the removal of moisture, food, and hiding places as the foundation of silverfish management, with product as a secondary measure for severe cases. No university source names a single target humidity number for silverfish. UC IPM gives a threshold instead: above roughly 75 percent relative humidity in a warm room, they thrive. Keep a room well under that and the population loses its footing.

A remodel is its own case. UC IPM notes that fresh concrete and green, unseasoned lumber both give off moisture, and that wallpaper paste supplies the food. A newly finished bathroom or addition can run damp for months on that alone. Silverfish also travel in on delivered lumber and wallboard. If sightings start within a year of new work, check the drying materials before you go hunting for a leak.

• Ventilation: run the bathroom fan during and after every shower, and confirm it exhausts outside rather than into the attic. EPA gives the same guidance for bathroom and kitchen exhaust and recommends a dehumidifier or air conditioning where humidity stays high. Dry the shower stall, and keep air moving in closed rooms such as a laundry or a back closet.
• Crawl space: an intact 6-mil to 10-mil polyethylene vapor barrier over the whole soil floor cuts the moisture rising into the house, and clear foundation vents carry off the rest. Fix standing water and drainage faults first. Otherwise the barrier traps the water underneath it.

6What does not get rid of silverfish

Three approaches absorb most of the effort homeowners spend on silverfish, and none of them ends an infestation on its own.

• Foggers and bug bombs. UC IPM does not recommend them for silverfish or firebrats. The insects spend the day deep in cracks, crevices, and wall voids, and fogger mist settles on the open surfaces they are not on.
• Killing what you see. UC IPM reports that silverfish and firebrats can go several months without food and molt continuously through 45 to 60 instars over their lives. The ones crossing the floor are a fraction of what sits behind the baseboard.
• Waiting them out. UC IPM puts development from egg to adult at as little as 60 days above 75 degrees Fahrenheit, and as long as 500 days below 50. A female lays about 50 eggs at a time, in a range of 1 to 200.