Ants in October: why they come indoors when everyone thinks the season is over

There's one category of call you don't expect to get in October, and yet it comes back every year.

"Ants. In the middle of autumn. We had peace all summer, and now they're in the spice cupboard, on the worktop, behind the kitchen skirting board. I thought this was a spring problem."

It's one of the most persistent misconceptions about household pests. We associate ants with the month of May, with the nuptial flights of July, with the patio and the forgotten jar of jam. And we assume that once temperatures drop, everything returns to normal.

In a garden, that's broadly true. In a heated home, it's exactly the opposite: autumn is the moment an outdoor colony decides to become an indoor colony. And once it has settled behind a partition wall or under a floor screed, it has no reason left to follow the seasons.

Modern bedroom with a large bed, grey headboard, white dressing table and pendant lights

What actually happens inside a colony between September and November

An ant nest doesn't hibernate like a marmot. It enters diapause, a gradual metabolic slowdown triggered by day length and falling soil temperatures. In Lasius niger, the black garden ant that is the most common species in mainland France, this slowdown begins around mid-October and ends when temperatures rise again in March.

But in between, there's a critical window — and that's the one we're seeing right now.

• Late August – September: the colony has finished rearing its reproductives. The nuptial flights are over. The queen reduces egg-laying, and the workers switch diet: less protein (which fed the brood), far more carbohydrates, which they store as fat reserves to get through the winter.
• October: outdoor resources collapse. Aphid honeydew, the main sugar source for Lasius niger, vanishes along with the aphid colonies. Fallen fruit rots and ferments. Workers widen their foraging radius.
• November: if the colony has stayed outside, it moves down to a depth of 50–80 cm and becomes immobile. If it has found heated shelter, it doesn't move down at all.

That's the key point. A nest lodged beneath a heated slab, in a crawl space above a boiler room, in the cavity of a south-facing wall or in the insulation of an underfloor heating system never experiences winter. The temperature there stays between 18 and 24 °C all year round. The queen keeps laying. And instead of a colony that starts from scratch each spring, you inherit a colony in continuous growth.

That's precisely why ant infestations diagnosed in October and November are statistically harder to treat than spring ones: by the time they're discovered, the colony has long been established.

Three species, three completely different problems

The costliest mistake is treating "ants" without knowing which ones. The products, the placement and even the strategy change entirely.

The black garden ant: the most common, the most manageable

This accounts for 70 to 80 % of cases in detached houses. It lives outdoors, comes in to eat, and goes back out. You spot it by its trail: an almost straight line running along a skirting board, a tile joint, a cable. That trail is a chemical pheromone track, and it is both the ant's greatest strength and its weakness.

The fact that it comes indoors in October doesn't mean it nests inside. In most cases the nest is less than three metres from the entry point — under a patio, in a bank of earth, against the foundations.

The pharaoh ant: the real issue in apartment blocks

In flats, and especially in heated multi-occupancy buildings in the Paris region, the picture changes. Monomorium pharaonis is a tropical species that cannot survive outdoors at our latitudes — it lives only inside buildings, in ducts, wall cavities, and around hot water risers.

Two features make it formidable:

1. It is polygynous: a colony has dozens, sometimes hundreds of queens.
2. It practises budding. Faced with stress — typically an insecticide spray — a group of queens and workers breaks away and founds a colony elsewhere.

In other words: treating pharaoh ants with an aerosol doesn't reduce the infestation, it multiplies it. A flat with one focus becomes a building with five. It's the textbook case of a treatment that makes the problem worse.

The pharaoh ant is also a recognised mechanical vector in hospital settings (it travels through dressings and IV lines), which earns it specific monitoring in healthcare facilities. In homes, the health risk is low, but food contamination is a genuine issue.

Following the trail: method before product

Before buying anything, you need to know where they're coming from. The good news is that the ants will tell you themselves, for free.

The indicator bait test. On a small square of aluminium foil, place — in two different spots near an observed trail:

• a drop of honey or sugar syrup (carbohydrate test);
• a dab of tuna in oil or pâté (protein-fat test).

Leave it undisturbed for 30 to 45 minutes, with the lights off if possible. Then see which of the two has been recruited to, and above all which direction the laden workers head back in. In October, nine times out of ten the sweet bait wins — and that's the information you need to choose your treatment bait.

Then follow the line backwards. The classic crossing points:

• the skirting board / tile junction in the kitchen;
• pipe runs under the sink or behind the dishwasher;
• patio door thresholds and roller shutter tracks;
• ventilation housings, recessed spotlights, electrical boxes;
• cracks in the expansion joint of an adjoining patio.

A narrow-beam torch and a small inspection mirror dramatically improve the quality of this survey: most entry points are under a base unit or behind a built-in appliance, where the eye never goes. A USB inspection endoscope, of the type plumbers use for pipework, also lets you see inside a service duct without dismantling anything.

Why spray is the worst instinct

The first reaction of 90 % of people: the aerosol insecticide aimed at the line of ants. The immediate result is very satisfying — a hundred or so dead workers in thirty seconds.

Except that a mature Lasius niger colony numbers 5,000 to 15,000 workers, and a visible line represents less than 2 % of them. You've just killed 2 % of a workforce that the queen rebuilds within days. In exchange, you have:

• destroyed the pheromone trail, thereby scattering the remaining workers in other directions;
• contaminated your kitchen surfaces with a pyrethroid;
• and, with a pharaoh ant, triggered budding.

ANSES regularly points out, in its opinions on domestic biocidal products, that ambient insecticide aerosols are over-used by the general public, with residential exposure (airways, food surfaces) that is disproportionate to their real effectiveness against social insects.

The logic to remember is simple: against a social insect, you don't kill what you see — you use what you see as a means of transport.

The strategy that works: delayed-transfer bait

The whole principle of an anti-ant gel or liquid bait is slowness. The active ingredient (fipronil, imidacloprid, boric acid depending on the formulation) is dosed so as not to kill the worker immediately. She has time to return to the nest and regurgitate the food to other workers, to the larvae and to the queen through trophallaxis. The colony poisons itself from the inside.

In practice:

1. Clean first, but without residual products. Plain diluted white vinegar erases pheromone trails without leaving a repellent barrier that would make the ants avoid the bait.
2. Don't spray anything in the treated area — not before, not during, not after. A repellent near a bait cancels out the bait.
3. Place small doses, and lots of them, directly on the trail or at the entry point: a drop of gel the size of a grain of rice every 30 to 50 cm, rather than one large blob. Enclosed ant bait stations are preferable as soon as there are children or pets in the home.
4. Accept the surge effect. Within 24 to 72 hours you'll see more ants than before. That's the sign it's working: recruitment is under way. This is the point where most people panic and reach for the spray — and lose everything.
5. Top up for as long as the gel is being consumed. Wiping out an average colony takes 7 to 21 days.
6. Match the bait to the season: in October, a sugar-based formulation. In spring, when the queen resumes laying, colonies switch to proteins.

For diffuse infestations in apartment blocks, this is supplemented with a liquid bait reservoir, which lasts several weeks without drying out — a detail that matters in hot service ducts where a gel dries up in five days.

Sealing the house: the part that prevents a repeat

Treatment eliminates the colony that's there. It doesn't prevent the next one. Autumn is precisely the right time to go round all the openings, at the same time as your rodent-proofing checks.

• Redo the seals around pipe runs, cables and electrical conduits. Acrylic filler or sanitary silicone sealant is enough for fine cracks; beyond 5 mm, use expanding foam then render.
• Check the base of the façade: an open expansion joint between a patio and the wall is black garden ants' motorway number one.
• Move woodpiles away from the wall. A stack of logs against a façade is a satellite nest waiting to happen — and, incidentally, a shelter for woodlice, earwigs and rodents.
• Prune branches and climbing plants that touch the roof or the gutters: carpenter ants (Camponotus) use them to reach roof timbers.
• Airtight containers for sugar, flour, cereals and pet food. A set of airtight food storage containers solves in one go the problem of opened packets closed with a clip, which are the leading indoor food source.
• The pet bowl: don't leave kibble out overnight, or stand the bowl in a saucer of water.

A line of diatomaceous earth along an outdoor skirting or in a crawl space can complete the set-up: it's an abrasive mechanical barrier, non-toxic if ingested, that doesn't interfere with baits — provided it stays perfectly dry, which makes it useless outdoors where it's exposed to rain.

Carpenter ants: the case you mustn't miss

One signal deserves particular attention: small piles of fine sawdust (looking like pencil shavings dust) at the foot of a beam, a door frame or a wooden floor, combined with ants noticeably larger than average — 6 to 15 mm, often black or two-toned.

In that case you're probably dealing with Camponotus, the carpenter ant. It doesn't eat wood (unlike termites), but it excavates galleries in it, and it preferentially chooses wood that is already damp: under a roof leak, around a poorly sealed window frame, in a wall plate in contact with damp masonry.

Here, chemical treatment is secondary. The real treatment is drying out: finding and repairing the water ingress. An indoor hygrometer or a wood moisture meter provides objective confirmation of what you suspect — above 20 % moisture in the timber, the problem is structural before it is entomological.

When to call in a professional

DIY baiting has a genuine scope of effectiveness: an outdoor black garden ant colony coming into a kitchen. Beyond that, failure is the rule.

Bring in a Certibiocide-certified applicator if:

• you identify a pharaoh ant (small, translucent yellow, in a heated flat, never seen outdoors);
• the infestation affects several dwellings in the same building — treatment must then be coordinated across communal areas and ducts, otherwise the colony simply moves from one floor to another;
• the ants are emerging from a partition wall, underfloor heating or a suspended ceiling, which indicates an inaccessible indoor nest;
• you find wood dust and suspect carpenter ants;
• after three weeks of properly conducted baiting, activity hasn't decreased.

A serious diagnosis always starts with species identification, not with a spraying quote. At ProDeratisation, a typical intervention combines identification, treatment with baits matched to the species and the season, and a survey of entry points to be sealed — with a three-week follow-up check to confirm the colony really has been wiped out.

The essentials in five lines

• October ants aren't stragglers: they're looking for sugar and warmth ahead of diapause.
• A colony established in a heated building never stops and grows year on year.
• Identify the species first: black garden ants and pharaoh ants have nothing in common.
• Aerosols are counterproductive; delayed-transfer bait is the only approach that reaches the queen.
• A surge in ant numbers in the 48 hours after placing a bait is a good sign, not a failure.

Free quotes and call-outs 7 days a week across the Paris region: better a diagnosis in October than a three-year-old colony behind a skirting board next spring.