Cockroaches in Household Appliances: Why the Fridge Motor and the Oven's Circuit Board Are Their Best Hiding Places

There's one sentence we hear on almost every cockroach call-out in a flat, usually about twenty minutes in: "but we emptied everything, cleaned everything, bleached everything under the sink, and they keep coming back."

That's true. And it's perfectly normal. Because in the vast majority of homes we treat, the main focus of the infestation isn't under the sink. It's inside a household appliance, thirty centimetres away, in a space nobody has opened since the day it was bought.

Popular wisdom places cockroaches in damp spots: the trap under the sink, the cupboard beneath the basin, the bin. That holds true for the oriental cockroach (Blatta orientalis), which lives in cellars, crawl spaces and wastewater networks. But the species behind 90% of problems in French flats is the German cockroach (Blattella germanica), and it isn't primarily looking for moisture. It's looking for stable warmth, total darkness and a gap less than two millimetres wide. Three criteria that, in any kitchen, are met by five or six permanently plugged-in appliances.

Façade of a residential apartment building with windows and exterior blinds, seen from below

Why heat governs everything for the German cockroach

A tropical insect stuck in a temperate climate

Despite its name, Blattella germanica originates from South-East Asia. It cannot survive outdoors at our latitudes: a winter in the Paris region kills it. Its presence in France is strictly tied to heated buildings. It is what entomologists call an obligate domiciliary species — it has no wild back-up population.

The direct consequence: its entire biology is tuned to seeking out the warmest available spot. Its developmental optimum lies between 30 and 33 °C. At 30 °C, an ootheca (the capsule containing the eggs) hatches in 17 to 20 days, and a nymph reaches adulthood in six to seven weeks. At 20 °C — the temperature of a kitchen cupboard — the same cycle stretches over four to five months, and a good share of the nymphs die along the way.

In other words: a colony settled in a cupboard stagnates. A colony settled against a refrigerator compressor at 35 °C explodes. The difference between the two isn't a minor detail — it's a factor of five to eight in reproduction speed.

Thigmotaxis: the need for contact on every side

The second behavioural driver, less well known but just as decisive: the German cockroach is positively thigmotactic. It needs to feel a solid surface touching its back and its belly simultaneously in order to feel safe. An insect sitting flat on a shelf is a stressed insect, one that will look for shelter within seconds.

This explains its shape: a flattened body, 12 to 16 mm long but barely 3 mm thick in adults, and less than a millimetre in young nymphs. It slips into cracks a paperclip couldn't fit through.

Put the two criteria together — heat of 30–35 °C and very fine gaps — and you get an exact description of:

• the back of a refrigerator, around the compressor and the condensate collection tray;
• the control board and insulation of a built-in oven;
• the base of an espresso coffee machine;
• the control panel of an induction hob;
• the motor of an extractor hood;
• the back of a dishwasher, between the tub and the casing;
• the housing of a microwave, on the transformer side.

The refrigerator: the number one nest, by a wide margin

What goes on behind the grille

A domestic refrigerator combines everything a cockroach could wish for. The compressor runs several hours a day and maintains a zone at 35–45 °C. The condenser — that black grille at the back — radiates heat continuously. And above all, the condensate evaporation tray — that small reservoir sitting on top of the compressor, collecting defrost water — provides a permanent, warm source of free water.

An adult German cockroach survives around two weeks without food, but only three to five days without water. That tray settles the question for good, and it sits precisely in the warmest zone of the appliance.

Add the polyurethane foam insulation in the casing, often accessible through cable ducts, and you have a nursery: warm, dry, dark, compartmentalised, and impossible for an aerosol to reach.

During an inspection, our first move in the kitchen isn't to open the cupboards. It's to pull the refrigerator away from the wall and light up the compressor area with a torch held at a grazing angle. In a confirmed infestation, we almost invariably find droppings (peppery black specks), translucent moults and empty oothecae.

The tell-tale signs

Even before moving the appliance, several signals point the way:

That last point deserves emphasis. The German cockroach is strictly nocturnal and gregarious. Seeing a single adult during the day means competition for space in the warm shelter is such that individuals have been pushed out. At that stage, the real population is rarely counted in dozens. A visual inspection with a torch — ideally a narrow-beam LED torch that lets you light surfaces at a grazing angle without contorting yourself — gives a far more reliable picture than counting by eye in ambient light.

The oven, the microwave and the coffee machine: secondary shelters

Electronic boards, an ideal thermal refuge

A modern built-in oven contains a control board housed in the upper fascia, separated from the cooking cavity by a layer of insulation. During cooking at 200 °C, this zone climbs to 40–50 °C, then cools down slowly. For an insect, it's a storage heater.

The practical problem is twofold. First, this zone is invisible without dismantling. Second — and this is something many householders discover too late — cockroach droppings are electrically conductive once damp. Appliance repairers know the phenomenon well: dishwasher and oven boards whose tracks are corroded or short-circuited by a build-up of droppings and carcasses. A prolonged infestation ends up costing you an appliance.

Automatic espresso machines are a case apart. They combine residual heat from the thermoblock, moisture from the drip tray, and an exceptional food source: coffee grounds and sugary residues. We have seen entire colonies settled in the base of a machine sitting on an otherwise spotless worktop.

The toaster, a textbook case of infestation transfer

A logistical detail, but one with real consequences in shared buildings and flatshares: small appliances are the main vectors for transfer from one home to another. A toaster, a microwave or a kettle that has been given away, sold second-hand or picked up during a move may contain an ootheca. A single German cockroach ootheca contains 30 to 40 eggs, and the female carries it until hatching — which considerably improves the survival rate compared with other species.

It's the same logic as with second-hand furniture and bed bugs: the object comes in willingly, and nobody inspects it.

Why aerosols systematically fail here

The problem of reach

An insecticide aerosol, whatever its active ingredient, works by direct contact or surface deposit. But cockroaches nesting in a compressor, a control board or foam insulation are neither reached by the mist nor in contact with the treated surfaces. The formulation doesn't penetrate the gaps, and the electronics rule out any direct spraying anyway.

The problem of dispersal

Worse still: the pyrethroids found in most consumer aerosols have a marked repellent effect. Spraying in an infested kitchen kills only a fraction of the individuals exposed and drives the rest to flee into untreated areas — partition walls, service ducts, the rubbish chute, the neighbouring flat. A localised infestation becomes a building-wide one.

ANSES regularly points out, in its opinions on household biocidal insecticide products, that repeated indoor use of aerosols needlessly exposes occupants and encourages the selection of resistance. Resistance to pyrethroids among French populations of Blattella germanica is now widely documented in the European entomological literature, and observed in the field.

What does work: bait gel, properly applied

The effective strategy is the reverse: instead of trying to reach the insect in its shelter, you make it come to a bait that it will carry back to the shelter.

Insecticidal bait gels (active ingredients such as fipronil, indoxacarb or imidacloprid, depending on the authorised formulations) exploit two German cockroach behaviours:

• coprophagy: nymphs feed on adults' droppings;
• necrophagy: individuals consume the carcasses of their own kind.

An adult that ingests the bait goes back to the shelter to die. The nymphs, which almost never venture out to look for food, are poisoned by consuming its droppings and then its carcass. This is what professionals call the domino or cascade effect — and it's the only mechanism that genuinely reaches individuals hidden inside a refrigerator motor.

The application rules are counter-intuitive:

• Many small dots rather than large blobs. Dots the size of a grain of rice, every 30 to 50 cm along corners, furniture edges and appliance rims.
• As close as possible to warm shelters, never on food preparation surfaces or within reach of children and pets.
• No repellent products alongside. No aerosol, no excess bleach, no aggressive cleaning of treated areas: a gel surrounded by repellent will never be eaten.
• Renew every 10 to 15 days, the time needed for nymphs from already-laid oothecae to hatch.

To measure results objectively rather than rely on subjective impressions, monitoring sticky traps laid flat under appliances give you a weekly capture curve. An infestation that is receding sees its captures fall by more than half within three weeks, with a shift towards adults rather than nymphs.

The inspection protocol, appliance by appliance

Before any treatment, you need to know where you stand. This inspection takes forty minutes and is done with appliances unplugged, preferably in the evening.

1. Refrigerator — Unplug, pull away from the wall, tilt slightly. Light up the compressor, the condensate tray and the cable ducts. Wipe the tray with a cloth: black specks = presence.
2. Built-in oven — Slide the oven out of its housing if the connection allows (otherwise, wait for a professional). Inspect the floor of the housing and the back of the control fascia.
3. Dishwasher — Remove the bottom kick plate. This is the most frequently positive spot after the refrigerator: heat + moisture + food residues.
4. Microwave — Inspect the side ventilation grilles with a torch, and the underside of the appliance.
5. Coffee machine — Empty and wash the drip tray and the grounds container, inspect the base.
6. Extractor hood — Remove the metal filters. Grease-clogged filters are a direct food source.

Once this inspection is done, two underlying measures lastingly reduce the pressure:

• Seal the gaps. Pipe penetrations through walls, cracks behind skirting boards, joints between kitchen units and partition walls. A simple silicone sealant gun and a cartridge of sanitary sealant eliminate half the circulation routes between flats in an older building.
• Cut off free water. A dripping tap washer, a permanently full condensate tray, a soaking sponge in the sink. The German cockroach can go without eating, not without drinking.

And for routine cleaning, swap harsh products for an enzymatic degreasing cleaner on worktops and under appliances: it removes greasy residues and traces of aggregation pheromones without leaving a repellent film that would compromise the gels.

When to hand it over to a professional

DIY treatment has a clearly defined scope. It's reasonable in a detached house, with a young infestation (a few individuals, a single appliance affected) and no adjoining neighbours.

It becomes illusory in three situations:

• Multi-occupancy buildings. Cockroaches travel through service ducts, rubbish chutes and pipe penetrations. Treating a single flat simply moves the problem. The right scale is the building as a whole, with inspection of common areas and adjoining flats — vertically as well as horizontally.
• Established infestation. Adults visible during the day, a perceptible smell, presence in several rooms: the population exceeds what a roughly applied gel can reduce.
• Premises open to the public. Restaurants, hotels, nurseries, care homes: the stakes are as much regulatory as sanitary, and a documented control plan is required in the event of a veterinary services inspection.

On the health side, it's worth recalling what Santé publique France and the medical literature document: cockroaches are passive mechanical vectors (enterobacteria, salmonella carried on their legs and cuticle) and, above all, a major source of respiratory allergens. The Bla g 1 and Bla g 2 proteins, present in droppings and moults, are among the best characterised household allergens and are associated with worsening childhood asthma, as noted in the French health insurance service's documentation (Ameli) on environmental asthma factors. A prolonged infestation in a kitchen is therefore not merely a comfort issue.

One final word, because the question always comes up: no, how clean a home is does not determine whether German cockroaches are present. It determines the speed at which the population grows, and how easy the treatment will be. But a spotless kitchen whose refrigerator is pushed flat against the wall, with a damp condensate tray and a service duct opening onto the building's riser, remains prime habitat. The limiting factor isn't dirt: it's heat, water and the gap.

If you're unsure after inspecting your appliances, an on-site diagnosis removes the ambiguity in a single visit — and, in half of cases, spares you an unnecessary treatment.