Silverfish, Booklice and Damp
Two small insects turn up constantly in Ho Chi Minh City homes and generate a lot of unnecessary worry. Neither bites, neither damages a building, and neither carries anything of concern.
What they do is measure humidity. Both depend on it absolutely, which makes them the most reliable moisture indicators available and makes insecticide the wrong first response.
Silverfish
Ten to fifteen millimetres, silver-grey, teardrop shaped, with three tail filaments and two long antennae, moving with a distinctive fish-like wriggle. They are nocturnal and run for cover when a light goes on.
They feed on starch and cellulose: paper, cardboard, book bindings, wallpaper paste, the starch in cotton and linen, and dried food residue. Damage appears as an irregular grazed or thinned patch rather than a clean hole, which distinguishes it from moth or beetle damage.
Booklice
One to two millimetres, pale grey or straw coloured, soft bodied, and fast. They are the insect most often photographed and sent to us as "baby cockroaches", and they are not related to cockroaches or to lice.
They feed on microscopic mould, which is the single most useful fact about them. A booklice population is a mould report, and mould needs sustained humidity above roughly 70 per cent.
What they are telling you
Both require high humidity and neither persists in a dry environment. So finding them consistently in one place identifies a moisture condition in that place — condensation on an external wall, a slow leak behind a cabinet, poor ventilation in a closed room, or damp trapped behind stored cardboard.
That is genuinely valuable information. In a new building, booklice on walls frequently indicate residual construction moisture in plaster and concrete, and the population disappears on its own as the building dries over the first year or two.
Where they concentrate
Bathrooms and the cabinets in them. Behind wardrobes standing against external walls, where condensation forms. In stored paper, cardboard and books. Under sinks and behind washing machines. In storerooms and guest rooms that are closed for weeks at a time.
The pattern is always the same: still air, no light, and moisture.
Why treatment misses the point
A residual applied to a skirting board kills what walks across it and does not change the humidity, so the population re-establishes within weeks. We do treat where there is a heavy silverfish population damaging books or documents, and it is a supporting measure rather than a solution.
The intervention that works is moisture: a dehumidifier, better ventilation, a repaired leak, or a wardrobe pulled a few centimetres off an external wall so air can move behind it.
What to do, in order
Find the moisture. Look for a stain, a musty smell, a cold surface, a wardrobe against an external wall, a bathroom with no extraction. Then improve the airflow in that spot and add a moisture absorber or a small dehumidifier.
Remove the food at the same time: stored cardboard, old newspapers, paper stacked on a floor. Cardboard is both harbourage and food, and clearing it out often halves the population without anything else being done.
Two other small insects belong to the same group and turn up in the same conditions. Springtails are minute and jump when disturbed, found in permanently damp soil, bathroom corners and around overwatered plant pots. Fungus gnats are small dark flies emerging from potted plants, and they mean the pot is being watered more than it needs. Both are moisture reports rather than pest problems, and both disappear when the water does.
Protecting books and documents
For anyone here with a real collection of books, papers or artwork, this is a genuine risk rather than a nuisance. Rigid sealed boxes with silica packs, shelving pulled off external walls, and a dehumidifier in the room during the rainy season are the practical measures.
Silverfish damage to paper is slow, cumulative and irreversible, which is why it is worth acting on a small population in a room where documents are stored even though the same population elsewhere would be ignored.
Related reading
See August damp and wardrobes and fabric pests. For rooms that concentrate the problem, storerooms and laundry rooms. New buildings are covered in new buildings. Our parent company covers general household work in Vietnamese through its residential programme.
Global Pest Control is the English-language arm of Khử Trùng Toàn Cầu, based in Thu Duc City.
Common questions
Tiny pale insects are all over one wall. Is that serious?
Almost certainly booklice, and it is a moisture signal rather than an infestation. Check whether the wall is external, whether furniture stands against it, and whether there is any staining or musty smell. Improving airflow at that spot usually clears it within weeks, and spraying the wall buys a fortnight.
Do silverfish damage clothes?
They damage the starch and cellulose in fabric, so cotton and linen — particularly starched or stored items — can be grazed and thinned. They do not eat wool, so holes in a wool garment point to carpet beetle rather than silverfish. Storing clothes clean and dry in sealed containers prevents most of it.
We just moved into a new building and have booklice everywhere. Is it a defect?
Usually not. New concrete and plaster release moisture for a long time, and booklice feeding on the resulting surface mould are a common feature of the first year or two. Ventilate, run a dehumidifier if it is severe, and expect it to resolve as the structure dries. If it persists into a third year, that is worth investigating properly.
