Introduction: the pest that doesn't sting, but costs the most
There is an unspoken hierarchy in the way people rank household pests. At the top, the ones that bite or sting: tiger mosquitoes, bed bugs, hornets. In the middle, the ones that disgust: cockroaches, rats, flies. And right at the bottom, almost invisible, the ones that eat the house itself.
Yet it is this last category that generates the heaviest bills. A bed bug infestation can be dealt with for a few hundred to a few thousand euros. A roof structure weakened by twenty years of house longhorn beetle activity runs into tens of thousands of euros of structural repair — before you even count the scaffolding and stripping the roof covering.
The end of summer is the ideal moment for this survey. Lofts are dry and accessible, the low raking light of August and September picks out surface relief on beams, and above all: carpenter ant swarming and the emergence of adult wood-borers have just taken place. The evidence is fresh and still readable. In three months' time, dust and damp will have blurred everything.
The four suspects: know what's eating your timber
Before any treatment, you have to identify the culprit. Strategies differ radically from one species to another, and aiming at the wrong target means spending money for nothing.
| Pest | What it attacks | Characteristic sign | Speed of damage |
|---|---|---|---|
| Carpenter ant (Camponotus spp.) | Wood that is already damp or decayed | Coarse sawdust in piles, mixed with insect corpses, rustling at night | Slow but continuous |
| House longhorn beetle (Hylotrupes bajulus) | Softwoods, sapwood in new to moderately old roof timbers | Oval holes 6–10 mm, audible gnawing sound | Very severe, invisible for a long time |
| Common furniture beetle / woodworm (Anobium punctatum) | Hardwoods and softwoods, furniture, floorboards | Round holes 1–3 mm, frass as fine as talc | Moderate |
| Subterranean termite (Reticulitermes spp.) | All timber, paper, plasterboard | Earth tubes, hollowed-out wood with no surface holes | Fast and discreet |
The carpenter ant: a widespread misunderstanding
Contrary to popular belief, the carpenter ant does not eat wood. It excavates it to build its nesting galleries, pushing the shavings out. That is a fundamental difference: it almost never attacks sound, dry timber.
Its presence is therefore first and foremost a symptom. Wherever it settles, there is moisture: an old roof leak, an overflowing gutter, a post base in contact with damp masonry, a poorly protected window lintel. Treating the ants without treating the damp is mopping the floor without turning off the tap.
Camponotus is the largest ant in mainland France: 6 to 15 mm depending on the caste, black or two-tone black and red depending on the species. It is most often noticed in spring and summer during swarming: dozens of winged individuals suddenly appear indoors, often near a window. Winged ants coming out from inside the house — rather than getting in from outside — signal a nest established within the structure.
The house longhorn beetle: the most formidable
If one single insect warrants a serious inspection of your roof structure, this is it. The larva of Hylotrupes bajulus develops over three to ten years in the sapwood of softwoods, boring galleries that can reduce a beam to a shell while the surface remains perfectly smooth.
The adult only emerges at the end of its cycle, cutting an oval exit hole 6 to 10 mm across. In other words: by the day you see the holes, the attack is already several years old. The earliest sign is acoustic: in warm weather, in the silence of a loft, you can distinctly hear a steady rasping, like a fingernail on cardboard. A wide-beam head torch and a moment of absolute silence are often enough to confirm it.
Woodworm: discreet but persistent
Anobium punctatum leaves round holes 1 to 3 mm across and extremely fine, almost floury frass. It readily attacks old floorboards, furniture and panelling. The death watch beetle (Xestobium rufovillosum), rarer, prefers hardwoods already attacked by fungus and produces a curious tapping courtship signal in spring.
Beware the classic trap: holes do not mean an active infestation. Timber attacked sixty years ago still shows its holes. The sign of activity is fresh, light-coloured frass that reappears after you have cleaned the area.
Termites: the only case governed by law
Subterranean termites are subject to a specific regulatory regime. The French Construction and Housing Code requires, in zones defined by prefectural order, a termite presence report when a built property is sold, valid for six months. Declaring any confirmed presence to the town hall is also compulsory. The départements concerned are listed by the Observatoire national termite and by the prefectures; the South-West, the Garonne valley, the Loire valley and part of the Atlantic seaboard are particularly exposed.
Crucial point: termites do not open holes at the surface. They consume the wood from the inside, leaving a thin outer skin intact. Timber that sounds hollow when tapped with a hammer, a skirting board that a screwdriver pierces without resistance, earth tubes running along a cellar wall: these three signs should prompt an immediate call to a certified professional.
The inspection protocol: two hours, a torch and a screwdriver
Here is the method used by surveyors, adapted for a methodical homeowner.
1. Get your kit together. A powerful head torch with an adjustable beam is tool number one: it frees both hands and, held to cast raking light, reveals relief that is invisible head-on. Add a sturdy flat-head screwdriver, a light mallet, a tape measure and a notebook.
2. Start with the damp areas. The underside of the roof covering around chimneys and valleys, post bases, wall plates in contact with masonry, dormer surrounds. Statistically, these are the dominant entry points for carpenter ants and for wood-decaying fungi.
3. Sound the timber. Strike the beam with the mallet every 30 cm. A dull, muffled sound, different from the clear ring of sound timber, indicates loss of material. Then push in the tip of the screwdriver: if it penetrates more than 5 mm effortlessly into dry softwood, there is a problem.
4. Look for fresh frass. Vacuum beneath the suspect beams, note the date, and come back a fortnight later. New sawdust = active attack. A pin-type wood moisture meter is a useful complement to the reading: above 20 % moisture content, wood becomes attractive to ants and vulnerable to fungi.
5. Photograph and date everything. Every suspect area, with an object for scale. This file will serve you when dealing with a contractor, an insurer or a seller.
