"We sweep up a whole glassful every couple of days at the foot of the upstairs bedroom window. And when you squash one, it leaves a yellow stain that never comes out of the wallpaper." This homeowner in Nogent-sur-Marne is describing, without realising it, a perfectly ordinary and perfectly seasonal phenomenon: the overwintering of Asian lady beetles and attic flies in homes across the Val-de-Marne.
Between mid-October and the first real frosts, two families of insects go looking for a dry, temperate shelter in which to spend the winter in slow motion. They do not bite, they transmit no disease, and they eat neither your roof timbers nor your food stores. But they arrive in their dozens or hundreds, always concentrate in the same rooms — converted loft spaces, top floors, roof-window reveals, roller-shutter boxes — and come back to the same place the following year, because the individuals that managed to overwinter there leave chemical traces behind.
It is a comfort nuisance, not an infestation. So it is dealt with through a gentle removal method and some sealing work, not with insecticide.
Two different insects, often confused
The Asian lady beetle (Harmonia axyridis)
Introduced into Europe in the 1980s and 1990s as a biological control agent against aphids, it has become naturalised and is now one of the commonest ladybirds in the Île-de-France region. It can be told apart from our native ladybirds by:
- a larger size (6 to 8 mm, against 4 to 6 mm for the seven-spot ladybird);
- a highly variable number of spots: from 0 to 19, on a yellow, orange, red or almost black background with two large red patches;
- a white pronotum marked with a black M- or W-shaped pattern — this is the most reliable criterion;
- gregarious behaviour in autumn: it gathers in clusters of several dozen individuals in nooks and crevices.
In the wild, Harmonia axyridis overwinters in cliffs, rock fissures and under bark. In town, light-coloured façades facing south and west, warmed by the afternoon sun, play exactly the same role. The beetles line up along window frames, then slip inside through the smallest gap in the joinery.
Two concrete nuisances:
- Reflex bleeding. When disturbed or crushed, the Asian lady beetle releases a yellow-orange, alkaloid, foul-smelling fluid from its joints that leaves lasting stains on paint, wallpaper, upholstery fabrics and plaster.
- Allergic reactions. Cases of rhinitis, conjunctivitis and contact urticaria linked to massive Harmonia axyridis clusters in homes are documented in the medical literature, particularly in the United States where the species is even more abundant. The risk remains low and mainly concerns very large colonies in attic bedrooms.
The attic fly (Pollenia rudis and related species)
Also known as the cluster fly, this is something else entirely. Slightly smaller and duller than the house fly, it can be recognised by:
- a thorax covered in crinkled golden hairs, visible with a hand lens, giving it a matt, golden appearance;
- wings that overlap at rest across the abdomen, unlike the house fly;
- a slow, heavy, clumsy flight, often buzzing against the window pane;
- mass aggregation behaviour in unheated spaces: attics, unconverted loft spaces, unoccupied rooms, behind pictures and hangings.
Its larvae are parasites of earthworms, so it never breeds inside the house. The individuals that appear in the middle of winter, on mild days or when the heating goes back on, are adults roused from an overwintering period that began in October. Their main drawback is accumulation: piles of dry corpses in loft spaces, which end up attracting other pests — carpet beetles and hide beetles, whose larvae feed on dry animal matter and then go on to attack woollens and carpets.
This point is worth stressing: a population of cluster flies neglected for three or four winters in an unconverted loft is a larder for textile beetles. The link with pantry pests and carpet beetles is direct.
Not to be confused with the brown marmorated stink bug
The third classic overwinterer in our homes is the brown marmorated stink bug (Halyomorpha halys), of a clearly different shape — a mottled brown shield some 15 mm long — and with a characteristic smell. We devote a specific guide to it: stopping it overwintering in the house. The sealing methods described here apply to all three species.
Why your house and not your neighbour's
Professionals see it time and again: on the same street in Champigny-sur-Marne or Saint-Maur-des-Fossés, two adjoining houses can experience completely opposite situations. The determining factors are always the same.
| Factor favouring overwintering | Why |
|---|---|
| Light-coloured façade facing south or west | Heat build-up at the end of the day, an attraction signal |
| Detached house or one bordering a green area | Close to a population reservoir (trees, waste ground, banks of the Marne) |
| Recent roof windows and converted loft spaces | Gaps in the joinery, poorly sealed flashings and aprons |
| Surface-mounted roller-shutter boxes | Hollow space open to the outside, an ideal shelter |
| Unheated but dry attic | Stable temperature between 2 and 10 °C, exactly the optimum |
| Timber cladding, exposed stonework, cracked render | A multitude of micro-crevices |
Add the memory effect: Asian lady beetles emit aggregation pheromones that persist on surfaces. A loft space that overwintered 300 lady beetles in 2025 will, all other things being equal, receive as many or more in 2026. That is why cleaning the aggregation areas is part of the treatment, and not merely housework.
Getting them out without staining everything: the method
What not to do
- Do not squash them. A crushed lady beetle means a persistent yellow stain plus an extra alarm pheromone.
- Do not spray insecticide in the room. It is useless against insects that are already inside and in diapause, and counterproductive: you end up with hundreds of corpses in inaccessible places that will feed carpet beetles. ANSES regularly reminds us that household biocidal products should be used as a last resort, on a precise target, and never as a preventive space treatment.
- Do not use the living-room vacuum cleaner without precautions. The lady beetles die in the bag, the defensive fluid soaks into the filter, and the machine keeps a stubborn smell for weeks.
What does work
1. Vacuuming with an interface. The cleanest method is to slip a piece of nylon stocking or a disposable filter into the tube, held in place with an elastic band at the nozzle: the insects are trapped before reaching the body of the machine and can be released alive. For large volumes (unconverted lofts, a whole attic), a wet-and-dry workshop vacuum with a washable filter saves sacrificing the household vacuum, and its tank can then be rinsed out with a hose.
2. A light trap for cluster flies. Attic flies are strongly phototropic. In a dark loft space, a lamp placed above a basin of soapy water will capture a large proportion of them in a single night. A light trap with a sticky board, positioned high up in the attic, gives the same result without spilled water, and serves as a population indicator from one year to the next.
3. Releasing the lady beetles. Individuals vacuumed up alive can be deposited in a pile of leaves, an insect shelter or a bundle of wood at the bottom of the garden: they will finish their overwintering outdoors. They are a valuable ally against aphids in spring — so let them do their job, outside.
4. Cleaning the aggregation areas. Once the room is cleared, wash the affected frames, window reveals and ceiling corners with hot water and washing-up liquid or white vinegar. That erases the chemical traces. For yellow stains already set into matt paint, an enzymatic fabric cleaner applied by dabbing gives better results than a conventional detergent.
