Where do silverfish come from?
By The Pest GuyLast updated
From somewhere damp, close by, that also has something starchy to eat. Silverfish are not arriving from outside the way most household insects do. They are breeding inside the building fabric, in a void, cupboard or cavity that stays humid, and the ones you see on the bathroom floor at night have come out of it.
The requirement is strict, and it is the useful part of the answer. English Heritage guidance on the common silverfish is that they "will only lay eggs in damp conditions", and that the young "live in or very close to damp food where they feed and grow". So a breeding population needs both things at once: sustained damp, and material to eat.
That pairing is why the plughole theory is usually the wrong end of it. A drain is damp, but it is the food that anchors a population, and their food is paper and starch: books, boxes, labels, wallpaper paste, the starch coatings on packaging. A cupboard of paperwork on a cold external wall is a far better silverfish habitat than a waste pipe.
It also explains why they seem to appear from nowhere. They are nocturnal, they run when disturbed, and the life cycle takes six to twelve months. A population can build for the better part of a year in a void you never open before anything walks across a floor in front of you.
One thing has changed recently, and it changes where they can come from. Grey silverfish, a larger species first found in the UK in 2014, breeds at lower humidity than the common silverfish. Its spread across Europe has been linked to insulation and central heating making modern buildings suitable, and it is reported as spreading between rooms and flats. So a silverfish in a room that is not obviously damp is no longer the contradiction it once was.