Introduction: autumn, the season when the building warms up
There's a statistic every managing agent knows without ever writing it down: the volume of "cockroach" complaints doubles between mid-September and the end of November. This is not an invasion from outside. The German cockroach (Blattella germanica), responsible for almost every flat infestation in mainland France, doesn't come in from the garden. It already lives in the building, quietly, and switching the central heating back on gives it exactly what it was waiting for: constant warmth in service ducts, crawl spaces and wastewater risers.
A female carries an ootheca containing an average of 30 to 40 eggs, and can produce four to six of them in her lifetime. At 25–30 °C — the typical temperature of a heating duct or the back of a fridge — the egg-to-adult cycle completes in six to eight weeks. In other words: what you see in September is the result of what happened in July, and what you don't treat in September will be visible everywhere by December.
The point of this guide isn't to convince you that all is lost. It's to explain why the individual response — an insecticide bomb bought in a panic on a Saturday — is precisely how you turn a localised nest into a building-wide infestation.
Identify the species first: France's three cockroaches
The first useful reflex costs nothing: look closely at the insect, its size, its colour. The treatment strategy changes completely depending on the species.
| Species | Size | Appearance | Where it lives | Difficulty |
|---|---|---|---|---|
| German cockroach (Blattella germanica) | 12–15 mm | Light brown, two parallel black stripes on the pronotum | Strictly indoors: kitchen, bathroom, ducts | Very high, rapid reproduction |
| Oriental cockroach (Blatta orientalis) | 20–25 mm | Glossy blackish-brown, classic "cellar cockroach" look | Basements, cellars, pipework, bin rooms | Moderate, climbs up risers |
| American cockroach (Periplaneta americana) | 30–40 mm | Mahogany red, large, flies short distances | Sewer networks, boiler rooms, catering premises | High, but nests more localised |
Nine calls out of ten in multi-unit housing involve the German cockroach. It's the smallest, the most discreet, and unfortunately the only one that reproduces entirely inside your home without ever needing to leave it. If you see small individuals of 3–4 mm, almost black and wingless, those are nymphs, and their presence is proof of reproduction on site. A single adult may be a visitor; nymphs never are.
The signs that come before visible cockroaches
The German cockroach is light-shy: it flees illumination. Seeing one in broad daylight usually means the hiding places are saturated, and therefore that the population is already large. Before that stage, look for:
- droppings shaped like peppercorns or blackish smears, typically in the upper corners of cupboards, behind hinges, on drawer runners;
- empty oothecae: small ridged brown capsules of 7–8 mm, often stuck under furniture or in a warm nook;
- a persistent sweet-rancid smell, described as "damp mould", noticeable once the population exceeds a few hundred individuals;
- translucent moults, often mistaken for scraps of paper.
A proper diagnosis is done with cockroach glue traps left in place for a week: three per at-risk room, against skirting boards, under the sink, behind the dishwasher and the fridge. The counts will tell you where the nest really is — not where you saw the first cockroach, but where the traps fill up.
Why individual treatment fails in multi-unit buildings
This is the central point, and the most widely misunderstood.
The dispersal effect of contact insecticides
"Knockdown" aerosols and foggers sold in supermarkets contain mainly pyrethroids. They kill what they touch, and repel everything else. A cockroach colony hit by a spray doesn't die: it splits and migrates through ducts, pipe penetrations and suspended ceilings into neighbouring flats. Three weeks later, the treated flat looks clean — and two floors up, a tenant who had seen nothing calls a professional.
This phenomenon is well documented in the technical literature of pest control operators and has been highlighted by ANSES in its work on domestic biocide uses: blanket spray treatments in housing are poorly effective against Blattella germanica and encourage dispersal. On top of that comes a health issue: these products leave a persistent film on worktops, toys and textiles, in homes often occupied by children.
Key takeaway: in an apartment building, a cockroach infestation is never a flat problem. It's a building problem that happens to show up in a flat.
Pyrethroid resistance
For the past fifteen years or so, French German cockroach strains have shown high levels of resistance to pyrethroids, the result of repeated, under-dosed exposure — precisely the kind produced by consumer treatments. A product that "doesn't work any more" isn't necessarily a bad product: it's often a population already selected by twenty generations of exposure.
The method that works: bait gel and the domino effect
The current professional standard relies on insecticidal bait gels, applied in micro-dots. The principle is biological, not chemical in the brute-force sense.
- A cockroach consumes a few milligrams of gel.
- It returns to its hiding place, where it dies.
- Its fellow cockroaches consume the carcass and the droppings — the German cockroach is coprophagous and necrophagous.
- The active ingredient thus travels to individuals that never come out, notably gravid females and young nymphs.
This is what technicians call the domino effect, or cascade effect. A single baited individual can lead to the death of several dozen others. It explains why gel succeeds where spray fails: it reaches the invisible portion of the population, which typically represents 80% of the total.
Application rules, often poorly followed
- Tiny dots, and lots of them: a drop the size of a grain of rice every 20 to 30 cm along travel routes, rather than one large visible blob. A syringe applicator lets you reach hinges and runners.
- High up and deep in: behind and beneath furniture, upper corners of cupboards, around pipework, the fridge plinth, under the worktop.
- Never on a surface just cleaned with bleach: detergent residues make the bait repellent.
- No spraying alongside it: mixing gel and aerosol cancels out the gel. This is the most common mistake.
- Renewal: check the dots at day 7 and day 21, and top up any that have been eaten.
A professional-grade syringe cockroach bait gel, combined with sticky cockroach traps to measure the drop in population, forms the backbone of a serious home treatment. Sealed bait stations, for their part, are mainly useful where there are pets or young children, at the cost of reduced accessibility for the insects.
What the gel won't do
It won't compensate for a leak under the sink. The German cockroach survives several weeks without food but rarely more than a week without water. A faulty seal, permanent condensation behind a washing machine, a cracked trap: that's what keeps a colony going. Hunting down moisture is half the job. A compact dehumidifier in a windowless bathroom, or simply replacing a silicone seal, changes the picture more than a second coat of insecticide.
