Introduction: the pest you never hear coming
The pests that send French households into a panic are noisy, visible or itchy: a rat in the cellar, bed bugs in a mattress, a wasp nest under a roof tile. The ones that cost the most, by contrast, barely announce themselves at all. Wood-boring insects — those that feed on timber or tunnel galleries through it — work away out of sight for years. By the time an owner discovers the problem, it is usually because a roof purlin has sagged, a door frame gives way under thumb pressure, or a tradesman has pushed a screwdriver into a beam that no longer has a core.
Autumn is the right season for this check. Lofts are accessible before winter storage goes in, timber has finished its summer drying cycle, adult insects have stopped flying, and the year's traces are still fresh: frass, exit holes, mud tubes. It is also the moment when carpenter ants look for somewhere heated to overwinter, and when their colonies become detectable by something other than a fleeting nuptial flight.
This article is not a guide to chemical treatment — that field belongs to certified professionals. It is a domestic diagnostic protocol, room by room, so you know what you have, at what stage, and whether you need to call someone this week or simply keep watch until spring.
Who eats what: the four families to tell apart
Mixing up these four culprits leads to expensive, pointless decisions. The treatments, the legal obligations and the urgency differ radically.
1. Carpenter ants (Camponotus spp.)
Contrary to a persistent belief, they do not eat wood. They hollow it out to nest in it, pushing coarse sawdust mixed with insect fragments out of the entrance. In France, Camponotus ligniperda and Camponotus herculeanus are the species most often involved: ants of 6 to 15 mm, frequently two-toned (russet thorax, black abdomen), with an evenly rounded thorax seen in profile.
Their target: timber that is already damp or partly decayed. A lintel behind a leaking gutter, a joist under a poorly ventilated deck, a window frame where water has been sitting for three winters. A carpenter ant colony is therefore, above all, the symptom of a water problem. Treating the ants without fixing the leak guarantees a repeat performance.
Signs to look for:
- small heaps of pale, fibrous, "shaving-like" sawdust at the foot of a wall or on a skirting board;
- a dry rustling sound audible at night with your ear against the partition;
- large workers moving around alone, with no visible feeding trail in the kitchen;
- in late spring, winged individuals indoors — a near-certain sign of a colony inside the building, not outside.
2. The house longhorn beetle (Hylotrupes bajulus)
The most destructive of the structural wood borers in mainland France. It attacks the sapwood of softwoods (fir, spruce, pine): roof structures, rafters, sarking boards, floors. Its larva tunnels for 3 to 10 years, sometimes longer, before emerging through an oval hole 6 to 10 mm across.
Signs:
- oval exit holes, not round, with clean edges;
- very fine frass, almost like talcum powder, packed into small cylinders;
- a timber surface that blisters and gives way under finger pressure, the galleries running just beneath the skin;
- an irregular gnawing sound audible in warm weather in a silent loft.
3. Woodworm (common and deathwatch)
The common furniture beetle (Anobium punctatum) leaves round holes of 1 to 3 mm and a dust as fine as sand; it attacks furniture, floorboards and staircases, in both softwood and hardwood. The deathwatch beetle (Xestobium rufovillosum) prefers old, damp timber associated with fungal decay: holes of 3 to 4 mm and deep damage in the beams of historic buildings. Its famous springtime "tick-tock" mating call has fed generations of haunted-house legends.
4. Termites (Reticulitermes spp.)
A category of their own, because they are regulated. Subterranean termites live in the soil, climb up through the foundations and leave almost no visible holes: they hollow out from the inside, leaving a thin surface skin intact. Their clues are the tubes of compacted soil on walls, in cellars and along pipework, and timber that sounds hollow while looking untouched.
Termites are covered by a specific legal framework: Article L. 131-3 of the French construction and housing code requires any confirmed presence to be reported to the town hall, and the sale of a property located in a zone defined by prefectural order requires a termite presence report less than six months old. More than 50 French départements are covered by such orders, mainly along the Atlantic coast, in the South-West, the Rhône valley and the Île-de-France region.
The autumn inspection protocol, room by room
Allow two hours for a detached house. The kit fits in a bucket: a powerful torch with an adjustable beam, a thin screwdriver or awl, a tape measure, a telescopic inspection mirror, a pencil, and your phone to photograph and date everything.
Step 1 — The loft and roof structure (the most important)
Inspect with a grazing beam, never head-on: oblique light reveals the holes and surface sagging that direct lighting washes out.
- Work along principal rafters, struts, purlins and common rafters, looking for holes, frass and blistering.
- Test every bearing point with the screwdriver: purlin bearings on walls, joints, the feet of king posts. That is where moisture lingers and attacks begin.
- Check the sarking boards under the tiles around the perimeter, a classic condensation zone.
- Look for frass on the floor, on joists and on strips of insulation: it is often the first clue, more visible than the hole itself.
A screwdriver that sinks more than a centimetre into dry timber without effort signals a loss of structural section. Photograph it, measure it, and do not scrape any further.
Step 2 — External joinery and outdoor timber
Wooden windows, shutters, cladding, garage doors, decking, pergolas, garden sheds and firewood stacks. Here the target is twofold: wood borers and carpenter ants.
- Any piece of timber in direct contact with the ground or with a damp wall is suspect on principle.
- Firewood stored against the façade is a classic mistake: it acts as a reservoir for longhorn beetles, ants and woodlice twenty centimetres from your insulation. Move it away from the house and onto a raised storage rack that lets air circulate underneath.
- Check gutters, downpipes and flashings: every leak lays the groundwork for a future colony.
Step 3 — Cellar, crawl space, base of walls
Look for mud tubes (termites), damp patches, and ground-floor joists that sound hollow. In a crawl space, a telescopic mirror and a torch are enough to inspect without crawling. Note the relative humidity: sustained readings above 70% make timber attractive to the deathwatch beetle and to ants. An indoor hygrometer left in place for a week beats a vague impression.
Step 4 — Floors, staircases, antique furniture
Woodworm gives itself away through small heaps of powder under a piece of furniture that reappear after cleaning. That is the decisive test: frass that comes back means an active infestation. Mark a hole you have spotted in pencil, wipe the surface, and come back in three weeks.
