Spiders in the house in September–October: what's really going on (and why insecticide is a bad idea)

There's one call we get every year from 12 September onwards, and it almost always starts the same way: "I don't know if you deal with this, but we found a huge one in the hallway last night." Then a second one the next day. Then one in the bathtub. And after four days, the conviction sets in: the house has been invaded.

It hasn't. And this is probably the single most misunderstood situation in our entire line of work — the one where we spend the most time explaining that you should do no treatment at all, or almost none.

What plays out between mid-September and the end of October in French homes is not an invasion. It's a biological calendar: the sexual maturity of the males in two or three perfectly harmless species that have been living in your home for months without you ever noticing them. Understanding this mechanism changes everything: the effective response has nothing to do with an aerosol can.

Brown cockroach on its back, legs curled up and one long antenna, on a grey concrete floor

Why they all show up at the same time

They're not new arrivals

The large spiders you come across in autumn in a hallway or stairwell are, in the vast majority of cases in mainland France, house spiders — the genus Eratigena (formerly Tegenaria), in particular Eratigena atrica, the giant house spider. Long legs, mottled brown body, a fast jerky gait, and a span that can reach 6 to 7 cm including the legs in large males.

These animals spent the previous winter and the whole summer in your home or right next to it: cellar, crawl space, garage, roller-shutter box, a ceiling corner in a rarely used room, behind a washing machine. They grew slowly there, hidden, spinning sheet webs in nooks nobody ever looks at.

At the end of summer, the males reach maturity. And in house spiders, the adult male abandons his web: he sets off on foot, at night, sometimes covering dozens of metres, in search of a sedentary female. He no longer hunts, barely eats, builds nothing. He walks.

That's exactly why you see him: a male crossing a wooden floor at 11 p.m. under the ceiling light is visible, whereas a female tucked away in a garage corner never was.

The numbers don't change, the visibility explodes

We stress this point because it determines the whole strategy. Between August and October, the spider population in a home does not increase. What increases is the proportion of individuals moving about in the open. A house quietly hosting twenty house spiders in July suddenly "shows" three or four a week in October.

Two secondary factors amplify the phenomenon:

• Falling outdoor temperatures. Arthropods that were living on façades, in the ivy, under thresholds, start looking for more stable microclimates. A worn French-door seal becomes a motorway.
• The heating being switched back on, often in early October, keeping rooms at 20 °C and prolonging night-time activity.

Field rule: if you see three different large spiders in a single week in late September, you don't have an infestation — you have three males looking for a mate and, most likely, a sealing defect somewhere.

The species you actually encounter at home

No spider in our regions poses any real medical danger to a healthy adult or child. The cases sometimes mentioned involve the Mediterranean black widow (Latrodectus tredecimguttatus), strictly Mediterranean and confined to dry open habitats, and the tube web spider (Segestria florentina), found in the south of France on exterior walls: in both cases, documented bites are extremely rare and resolve on their own. Vidal and the French poison control centres classify these bites as benign local incidents requiring no antivenom.

What we do see frequently, on the other hand: flea or bed bug bites attributed to a spider. This is a very common misdiagnosis. A spider does not bite "several times in a line", does not bite repeatedly during sleep, and does not produce multiple lesions on the ankles or back. If you have clustered bites, the problem lies elsewhere — and that one can be treated.

What you should absolutely not do

Insecticide sprays: plenty of drawbacks, almost no effect

It's the number one reflex, and it's a bad bet for three specific reasons.

1. Spider anatomy protects them. They walk on the tips of long legs, with the body held well above the substrate. A residual product sprayed on the floor — designed for crawling insects that drag their abdomen along — is barely absorbed. That's why so many people notice a spider "walking away" after being hit with the spray.

2. You're destroying a regulator. An adult house spider eats flies, mosquitoes, gnats, woodlice, sometimes young cockroaches. In a family home, it's a free, permanent ally. Urban ecology studies all point the same way: emptying a home of its spiders mechanically increases fly pressure.

3. You're exposing your family for no benefit. ANSES regularly points out that household biocidal products — aerosols, diffusers, plug-ins — should not be used preventively or systematically, especially around children, pregnant women, asthmatics and pets. Spraying a pyrethroid in a bedroom because of a spider in the hallway is an absurd risk/benefit trade-off.

The "remedies" that don't work

• Horse chestnuts in the corners of rooms: no demonstrated efficacy, no identified volatile repellent substance.
• Ultrasonic devices: spiders have no hearing organ sensitive to those frequencies. ANSES has issued unfavourable opinions on the efficacy of domestic electronic repellents in general.
• White vinegar sprayed everywhere: a genuine cleaning effect, a very short-lived repellent effect (a few hours).

Suffice to say, the money spent there is better invested in a tube of silicone sealant.

What actually works, in order

1. Catch instead of kill

For people with arachnophobia — and there are many; it's a recognised specific phobia, not a whim — the problem isn't the spider: it's the contact. The fastest solution is mechanical. A long-handled spider catcher, with soft bristles and a trigger, lets you grab the animal from 60 cm away and release it out of the window in ten seconds, without squashing it and without getting close.

It's the item that most durably transforms a household's relationship with autumn. We systematically recommend it to families with children: the operation becomes a routine gesture instead of a crisis.

2. Remove the webs, not the individuals

The real lever isn't the visible spider: it's the sheet of silk you don't see.

A house spider will happily reoccupy an existing web or the same chemically marked spot. Vacuum — with a long crevice tool, essential for ceiling corners and the backs of furniture — in every nook and cranny:

• the upper ceiling corners of every room, especially hallways and stairwells;
• behind and under the washing machine, tumble dryer and fridge;
• roller-shutter boxes, garage door frames, the underside of staircases;
• cellar, utility room, bike store, behind shelving units.

Repeat twice, two weeks apart, during September and October. As you go, you'll be removing the egg sacs (white, lens-shaped pouches): a female house spider produces several, each containing 50 to 100 eggs. It's the only action that genuinely reduces the population the following year.

A cordless stick vacuum makes all the difference to whether this advice actually gets followed: nobody unrolls a cylinder vacuum for two webs, but everyone grabs a stick vacuum standing in the cupboard.

3. Close off the entry routes

This is the work that eliminates 70% of the traffic, and it takes half a day.

• Door bottoms: a self-adhesive brush door seal closes the 5 to 10 mm gap under doors leading to the garage, cellar or outside. Incidentally, it's also an effective barrier against juvenile mice.
• Window and sliding door seals: replace hardened seals, clean the tracks.
• Air vents and ventilation grilles: fit fine mesh behind low cellar and crawl-space grilles, without ever blocking the airflow.
• Service penetrations: fill cable holes, water inlets and electrical conduits with sealant or foam.
• Roller-shutter boxes: check the side seals — a major entry point and one that's systematically overlooked.

4. Rethink your outdoor lighting

Spiders don't come for the light: they come for what the light attracts. A cool-white porch light left on all night concentrates moths, gnats and midges into one square metre — and sets up a permanent restaurant right next to your door.

Two effective fixes:

• switch to a warm tone (2200 to 2700 K), which is far less attractive to night-flying insects;
• fit a motion sensor to limit how long the light stays on. An amber outdoor LED bulb on a sensor visibly reduces the webs around the threshold within two weeks.

5. Dry out and clear the surroundings

House spiders like dark, sheltered areas with stable humidity. Around a house, that means: firewood stacked against the façade, dense ivy, cardboard boxes on the garage floor, leaves piled up in a courtyard.

Move the woodpile 40 cm away from the wall, get the moving boxes out of the garage (they also serve as shelter for German cockroaches and nesting material for mice), and clear a clean 30 cm strip at the foot of the walls. In a damp cellar, a compact electric dehumidifier brings down the humidity and makes the place markedly less welcoming to the whole local fauna — spiders included, but also woodlice and silverfish.

When the spider isn't the problem

We've saved this point for the end because it's the most important one operationally.

An abnormal spider density is an indicator, not a disease. An adult spider has to eat. If a home, a shared basement, a restaurant stockroom or a plant room hosts dozens of webs laden with prey, it means there's a substantial population of flying or crawling insects upstream.

This is how we've identified:

• mosquito breeding sites in blocked stormwater inspection chambers;
• fly infestations linked to a dried-out floor drain trap or a poorly managed bin room;
• German cockroach infestations in professional kitchens, flagged by the abundance of well-stocked webs behind the equipment;
• plumbing leaks sustaining a lasting damp-loving fauna.

In these cases, treating the spiders is completely pointless: they'll be back within three weeks, because the larder is still there. It's the source that has to go.

And conversely: if you notice bites on your skin, black droppings 3 to 5 mm long near a cupboard, a sweet smell in a kitchen or gnaw marks, you have a different problem — fleas, bed bugs, rodents, cockroaches — and that one does call for professional diagnosis and treatment.

In summary

• The September–October spider peak is a peak in the visibility of adult males, not an invasion.
• French house spider species are harmless; repeated multiple bites never come from a spider.
• Aerosol insecticide is barely effective against spiders, destroys a useful insect regulator and needlessly exposes the household — ANSES advises against the preventive use of household biocides.
• The three steps that work: vacuuming webs and egg sacs (two rounds two weeks apart), sealing entry points, and correcting outdoor lighting.
• A high spider density should prompt you to look for the insect population feeding them — that's often where the real health problem is hiding.

If you're seeing an unexplained abundance in a shared cellar, a plant room or a professional kitchen, a targeted diagnosis will identify the source in a single visit. Our teams operate 7 days a week across Île-de-France, free quote — including, sometimes, to confirm that there's nothing to treat at all.