Silverfish in Winnipeg: a humidity problem wearing an insect costume

What we're dealing with

The common silverfish, Lepisma saccharinum, is a wingless, nocturnal insect in the order Zygentoma — one of the most primitive insect lineages still living. Adults measure 12–19 mm, with a flattened, tapered body covered in silvery scales and three long bristle-like cerci at the rear.¹

They are sometimes called bristletails for those three tail filaments, and they move with a distinctive rapid fish-like wriggle that is diagnostic on its own — nothing else in a Winnipeg basement moves that way.

The close relative you may also encounter is the firebrat (Thermobia domestica), which is mottled grey-brown rather than silver and prefers considerably higher temperatures — around water heaters, boilers and hot water pipes rather than cool damp basements.²

They do not bite, do not sting and do not transmit disease. The damage is to belongings: they chew irregular surface holes and notches in paper, book bindings, wallpaper paste, photographs, and natural-fibre fabric including cotton, linen and silk — particularly starched items.¹

Life stages and cycle length

Silverfish are ametabolous — they undergo no metamorphosis at all. Young resemble adults from hatching and simply grow through successive moults. This is ancient, pre-metamorphic insect development, and it has an unusual consequence: silverfish continue moulting throughout life, even as adults.

Two practical consequences follow from that biology. First, silverfish reproduce slowly compared with cockroaches or fleas — a visible population indicates a long-standing conducive condition, not a recent arrival. Second, they can survive months without food,⁵ so starving them out is not a strategy. Water, not food, is the limiting resource.

Silverfish are also active throughout the year with no true dormancy, and in heated buildings activity stays consistent even through a Manitoba winter.³

Harborage and feeding

In natural habitat silverfish live under rocks, in leaf litter, under tree bark, in caves and even within ant nests.³ Indoors they take the structural equivalent: dark, humid, tight and undisturbed.

Preferred locations are consistent enough to inspect against a checklist: basements, bathrooms, kitchens, attics, closets and pantries; bookcases and closet shelves; behind baseboards; inside wall voids and sub-floors; around sinks and bathtubs.⁴⁶ They hide in cracks, behind baseboards and inside stored boxes, and are usually seen only when a hiding place is disturbed.⁵

Diet is starch and cellulose. They feed voraciously on book bindings, wallpaper paste, clothing and paper products, scraping irregular holes and notches rather than cutting clean through.¹ Damage on stored fabric commonly appears as surface holes a few millimetres across, often along folded edges of bedding.³

That surface-grazing pattern is diagnostically useful. Silverfish abrade the surface; carpet beetle larvae chew through; clothes moth larvae leave a more ragged hole with silk residue. If a stored linen has shallow scuffed patches rather than clean perforations, silverfish are the likely culprit.

Their droppings are small black pepper-like specks — worth knowing, because that distinguishes them at a glance from the rust-red blood spotting of bed bugs.³

Solitary and slow

Silverfish are solitary. There is no colony, no nest, no caste and no aggregation pheromone of the kind that drives cockroach behaviour. Individuals share favourable harbourage because the conditions are right, not because they are attracted to one another.

There is consequently no transfer mechanism — no bait carried home, no secondary kill. Every individual must contact the treatment, be removed mechanically, or be dried out. This is why humidity control does disproportionate work here compared with other pests: it acts on the entire population simultaneously, in a way no product can.

The slow reproductive rate cuts both ways. It means populations build gradually rather than exploding — but it also means an established population has been there a long time, and that the conditions supporting it have been present a long time too.

Behaviour

Silverfish are strictly nocturnal and strongly negatively phototactic. They are typically seen scurrying for cover when lights are turned on suddenly, or when infested materials are moved for cleaning or rearranging.⁴ If you are seeing them regularly in daylight, harbourage is overcrowded.

They are fast and they are excellent at exploiting flat gaps, but they are poor climbers on smooth vertical surfaces — which is why they turn up stranded in bathtubs and sinks. They fell in and could not get out. That is not where they live; it is where they failed.

Nocturnal habits and a preference for sheltered humid microhabitats mean activity remains consistent even during colder months, particularly in heated buildings and indoor storage.³ In domestic settings they may be seen more often during warm humid summers when they forage more actively, though they are present year-round in suitable microhabitats.³

They are harmless to people — no biting mouthparts capable of breaking skin, no venom, no disease transmission.³ The complaint is property damage and, quite reasonably, revulsion.

Where in Winnipeg

Silverfish here are almost entirely a building-condition problem, and the buildings that have them are predictable.

• Older homes with fieldstone or unfinished basements, where there is no vapour barrier, limited ventilation and persistent ground moisture. This is the single most common setting we attend in this city.
• Properties with high water tables or seasonal seepage, particularly in low-lying areas and near the river corridors, where spring melt and summer storms raise basement humidity for weeks at a time.
• Bathrooms and laundry rooms without functioning exhaust ventilation, which sustain the required humidity in an otherwise dry house.
• Homes with cardboard storage on basement floors. Cardboard is simultaneously food, harbourage and a moisture sponge. It is the worst possible storage material in a Winnipeg basement.
• Winter humidification. A Manitoba winter is naturally very dry indoors, which suppresses silverfish. Aggressive humidification to combat static and dry skin can push a basement back into the range they need — an own goal we see regularly.

The prairie climate is genuinely on your side here. Outdoor winter air holds almost no moisture, and heated indoor air is consequently dry. A Winnipeg home with a functioning basement and no water intrusion is a poor silverfish habitat by default. If you have them, something is wet.

Natural control factors

• Low humidity. Overwhelmingly the dominant factor. Below roughly 50% relative humidity, silverfish struggle badly; the preferred range starts around 75%.¹⁴ Dehumidification has been reported as an effective control method in its own right.²
• Cold. Freezing for several days will kill silverfish in infested materials that are not damaged by such treatment² — genuinely practical in this climate for books, papers and textiles.
• Predation. House centipedes and spiders take silverfish readily. A centipede population in your basement is, in part, a silverfish indicator.
• Air movement. Improved circulation lowers local humidity and temperature and reduces the microhabitats they depend on.⁶

This is one of the few pests where the natural control factor is both decisive and completely under your control. A dehumidifier running at 45–55% relative humidity in a basement does more than any product on the market.

DIY: what works, what backfires

Our treatment process

1. Moisture assessment first. We take readings in basements, bathrooms, laundry areas and any suspect space, and we look for the source — seepage, condensation on cold lines, failed ventilation, a slow leak. This is the diagnosis.
2. Harbourage inspection. Baseboards, plumbing penetrations, wall voids, sub-floor, bookcases, closet shelving and stored cardboard.
3. Crack-and-crevice application to harbourage — behind and around baseboards, shelves, bookcases, closets and windows, with attention to wall voids, decorative beams and curtain rods.⁶
4. Insecticide dust in voids and attics, where a long residual is appropriate and a liquid is not.⁶
5. Written humidity guidance, specific to your building — target range, equipment, ventilation changes and storage recommendations. This is the part that determines whether they come back.
6. Return visit on request, though with a slow-breeding insect the more useful measure is whether the humidity has come down.

The chemistry, and how it kills

There is a pleasing logic to desiccant dusts against silverfish specifically. This is an insect that requires 75%+ relative humidity, and the treatment kills by destroying its ability to retain water. Product and environment push in the same direction, which is why the combination outperforms either alone.

Treatment timeline

Physiology and recent thinking

A very old lineage

Zygentoma — formerly grouped as Thysanura — represents one of the oldest surviving insect lineages, predating winged insects entirely. Silverfish are ametabolous, meaning they never evolved metamorphosis at all. What is crawling across a Winnipeg basement floor is functionally a living illustration of what insects looked like before wings and pupation existed.

They have been associated with the indoor biomes of humans for a very long time — long enough that their place of origin is unknown, though they are believed to have originated in a tropical region.⁷ Like the German cockroach, this is a species whose natural range has been effectively erased by its success as a commensal.

Metabolic work

The physiological literature on silverfish is modest but pointed. DeVries and Appel have published on standard metabolic rates of Lepisma saccharina and Thermobia domestica and the effects of temperature and mass, and on the effects of temperature on nutrient self-selection in silverfish.⁸ This work underpins the temperature and humidity preferences quoted throughout this article — the numbers come from measurement, not from folklore.

Museums and archives took it seriously first

Some of the most useful thinking about silverfish comes from conservation science rather than pest control, because libraries, archives and museums face the damage directly. Work on insect damage as a function of climate⁸ reflects a discipline that concluded decades ago that controlling the environment — temperature, humidity and air movement — is more effective than controlling the insect.

The synanthrope question

An interesting recent thread proposes that increasingly synanthropic species — those living in close proximity to human habitation, silverfish among them — may be usable as bioindicators of indoor environmental conditions.⁷ That framing suits what we see on site: a silverfish population is a fairly reliable readout that a building is holding more moisture than it should.

Cultural history

The name. "Silverfish" is one of the more literal common names in entomology — silver scales, tapered body, fish-like locomotion. The species epithet saccharinum refers to sugar and reflects the early belief that starch and sugar were the dietary centre. It is broadly right, and the animal is one of the few household insects whose scientific name tells you what it eats.

Marlatt, 1902. C. L. Marlatt's USDA Division of Entomology circular on the silverfish⁸ is among the earliest dedicated treatments of the species as a household pest, and it is a reminder that this problem was being systematically described more than a century ago.

The librarian's insect. Silverfish occupy a specific cultural niche as the pest of books, archives and old paper — a creature associated with neglected libraries and forgotten boxes. There is something apt about an insect from a pre-winged lineage feeding on the glue of book spines.

The bathtub encounter. Almost everyone's first silverfish is stranded in a bathtub or sink at night. It is such a common experience that it functions as a shared reference point — and, as noted above, it reflects the animal's inability to climb smooth vertical surfaces rather than any preference for plumbing.

The internet era. Silverfish generate less horror content than cockroaches and less sympathy than spiders, occupying a quiet middle ground of "what is that" posts. The most useful modern development is that home hygrometers are now cheap and ubiquitous, which means a homeowner can diagnose their own silverfish problem in about thirty seconds — something that was not practical for most of the century since Marlatt described them.

References

1. Silverfish (Lepisma saccharinum) — morphology, ametabolous development, moult counts, humidity and temperature preferences, feeding on starch and cellulose. species overview
2. LSU AgCenter — Lepisma saccharina, Common Silverfish (Zygentoma: Lepismatidae). Non-chemical management, freezing, dehumidification, boric acid. lsuagcenter.com
3. Silverfish identification, removal and prevention — humidity thresholds, egg hatch period, droppings vs. bed bug spotting, year-round activity. reference guide
4. Joshi, M.J., Prithiv Raj, V., Solanki, C.B. & Birari, V.V. (2020). Silverfish (Lepisma saccharina): an overview and their management. Agriculture & Food e-Newsletter 2(3). Preferred habitat and the humidity-first control argument. researchgate.net
5. Silverfish pest identification — survival without food, harbourage, egg-laying rate, development period and lifespan. pest identification guide
6. University of Missouri Extension G7376 — Silverfish and firebrats: humidity and temperature reduction, air circulation, crack-and-crevice and dust application. extension.missouri.edu/publications/g7376
7. Animal Diversity Web — Lepisma saccharina: worldwide distribution, unknown origin, indoor biome association, 70–95% humidity requirement. animaldiversity.org
8. Phillips, E.F. & Gillett-Kaufman, J.L. (2018). Silverfish Lepisma saccharina Linnaeus. UF/IFAS Extension EENY-705. Selected references include DeVries & Appel (2013, 2014) on metabolic rate and nutrient self-selection; Child (2007) on insect damage as a function of climate; Marlatt (1902). edis.ifas.ufl.edu (PDF)