End of wasp and hornet season: why October is the riskiest month, and what to do with the empty nest

There's one sentence you hear every year at the same time, and it's wrong.

"Let's just wait, wasps are done anyway, it's getting cold."

It's a logical assumption. It's also the reason October accounts for a large share of the year's serious stings, and why call-outs for nests don't drop off once the school year starts — they go up.

Because a wasp or hornet nest doesn't follow the temperature curve. It follows its own biological cycle. And that cycle reaches its demographic peak between late September and late October, at exactly the moment when everyone lets their guard down: trimming hedges, picking apples, the last mow of the year, tidying the garden shed, stacking logs under the lean-to.

Two beige mice next to a glass jar lying on its side against a black background

The real calendar of a colony: why October is a peak, not an ending

A colony of common wasps (Vespula vulgaris, Vespula germanica) or European hornets (Vespa crabro) is annual. It starts from scratch every spring.

• March–April: a single founding queen, just out of hibernation, builds a primary nest the size of a ping-pong ball. At this stage, dealing with it takes ten minutes.
• May–June: the first workers take over. The colony numbers a few dozen individuals. Nobody notices it.
• July–August: exponential growth. The nest becomes visible, the comings and goings noisy. This is when most people call.
• September–October: the peak. A mature wasp nest can house several thousand workers, an Asian hornet nest (Vespa velutina) up to 2,000 to 3,000 individuals according to monitoring by the Muséum national d'Histoire naturelle. The colony then produces reproductives: future queens and males.
• November–December: collapse. The workers die with the first frosts, and the young mated queens leave the nest to overwinter alone. The nest is abandoned for good.

That's the key point: between 15 September and 5 November, a nest holds both the largest number of individuals it will ever contain and the generation that will carry the species into the following year. Its biological value is at its maximum — and so is its defence.

Why they become distinctly more aggressive

It isn't your imagination. Three mechanisms stack up in autumn.

1. The diet changes. All summer long, workers hunt protein (insects, larvae) to feed the brood. At the end of the season, the queen stops laying: no more larvae to feed, so no more need for protein. The workers switch massively to sugars — fallen fruit, juice, beer, jam, grape harvests, bins. Hence their sudden presence on terraces and in kitchens.

2. They no longer have a "job". A worker with no brood to feed is available for defence. The threshold that triggers an attack drops.

3. Many are at the end of their lives. A worker lives three to four weeks. The ones flying in October are disoriented, slower, and more often in accidental contact with people — under a bare foot, in a glass, in a fold of clothing, on a picked-up apple.

Rule of thumb from the field: in August, a wasp goes around you. In October, it disputes the ground with you.

Then there's the vibration factor. A nest inside a roller-shutter box, under a roof tile or in a laurel hedge doesn't react to your presence but to vibration: hedge trimmer, mower, drill, ladder propped against the façade. That's the number one cause of multiple stings at this time of year.

Asian hornet: what makes October different in the Île-de-France region

Vespa velutina, present in France since 2004 and now established across almost all of mainland France, including the Île-de-France, changes the picture in two ways.

First, where the nest is. The secondary nest is very often perched 10, 15, sometimes 20 metres up a tree — plane, oak, cedar, lime. It stays invisible as long as the tree is in leaf. Then it appears all at once in late October or November, when the leaves fall. Hence the huge wave of sightings in autumn: the nests haven't just arrived, they've just been seen.

Second, the defence radius. The Asian hornet defends a zone of roughly 5 metres around the nest, with patrol flights. It does not chase you for hundreds of metres, contrary to what you read, but a group attack at close range is formidable: the sting, being barbless, allows repeated stings.

The Asian hornet is no more venomous than the European hornet, sting for sting — it's the number of stings and their location that determine severity. On this point, the recommendations from ANSES and Santé publique France are consistent: the main risk is multiple stings and allergic reaction, not the toxicity of a single sting.

Should an Asian hornet nest be destroyed in October?

This is the most frequent question. The honest answer comes in two parts.

The argument that "destroying it in October limits next year's population" needs qualifying: young queens start leaving the nest gradually from late October onwards. A late destruction in November has an almost nil effect on the species' dynamics. On the other hand, early destruction (before mid-October) of a nest close to a home has real, immediate public-health value.

The five mistakes that send people to the emergency room

They come back every autumn, in the same order.

1. Off-the-shelf aerosol on a mature nest. A "wasp and hornet" aerosol has a stated range of 4 to 6 metres and an active-ingredient volume calibrated for a primary nest. On a colony of 2,000 individuals, it stirs up the swarm without neutralising it. The person spraying is exposed, off balance, and often on a ladder.

2. Water, fire, petrol. A pressure-washer jet on a nest under the roof, a blowtorch, a soaked rag: every year this produces burns, roof-frame fires and mass attacks. Absolutely never do this.

3. Blocking the entrance hole. On a nest in a shutter box, a chimney or a wall, sealing the access kills no one: the insects look for another way out, often into the house.

4. Intervening in broad daylight. Two-thirds of the colony are out foraging at midday. They come back. Intervention is done at dusk or early in the morning, when every individual is at the nest.

5. Underestimating mechanical vibration. Before any hedge trimming, any pruning or any ladder work in September–October, spend two full minutes watching the façade and the hedge. A nest's traffic is visible: a steady stream of insects entering and leaving at the same point.

What a homeowner can do alone — and where the line is

There is a reasonable zone of self-management, provided you're clear-eyed about what you're attempting.

You can reasonably handle yourself a spring primary nest, the size of a golf ball to a small orange, reachable without a ladder, in an open spot, when you're not allergic.

You do not handle yourself a mature autumn nest, a nest up high, a nest in a cavity (wall, shutter box, chimney, under the roof), a nest in a shared building or a public-access premises, and never if you have a history of allergy.

For the rest of the season, a few items genuinely make life easier without pretending to replace professional intervention:

• A selective wasp trap placed at the edge of the terrace, not next to the table: the idea is to divert traffic, not to draw it towards you. Models with calibrated entry holes limit the capture of bees and butterflies.
• A magnetic window screen on the kitchen and bedroom, especially on the ground floor near a compost heap or a pear tree.
• A no-contact insect vacuum, very useful for the isolated, disoriented individuals that come in during the evening, rather than the swatter and the instinct to crush by hand.
• A telescopic cleaning brush to remove the debris of abandoned nests under an awning without a ladder — in winter only.
• And for those who want to understand what they're looking at, a decent insect identification guide avoids the classic mistake that leads to destroying a colony of wild bees or a perfectly harmless Osmia nest.

Stings: what to do, without dramatising or dismissing

The benchmarks from the Ameli and Vidal fact sheets are stable and worth knowing.

Single sting, non-allergic person:

1. Remove rings and bracelets if the sting is on a limb (swelling comes fast).
2. Bring a heat source close (an extinguished cigarette tip 1 cm away, hot water) for a few seconds: the venom is heat-labile.
3. Disinfect, apply cold for 15 minutes, elevate the limb.
4. Monitor for 24 hours. A painful red area of 5 to 10 cm is a normal local reaction.

Call emergency services immediately (15 or 112):

• Sting in the mouth, throat or neck — risk of airway swelling;
• Multiple stings (as a guide, more than 20 in an adult, far fewer in a child);
• Systemic signs: widespread hives, facial swelling, difficulty breathing, faintness, vomiting. This is suspected anaphylactic shock.

Anyone already identified as allergic must have their prescribed adrenaline auto-injector, in date, and keep it within reach — not at the back of a medicine cabinet upstairs. Desensitisation to hymenoptera venom exists and should be discussed with an allergist: it is one of the few immunotherapy protocols whose effectiveness is extremely well documented.

The empty winter nest: the only genuinely good news of the season

Now for the question that arrives in January.

"We've found a huge nest in the cherry tree / under the roof. What should we do?"

Nothing urgent. A wasp or hornet nest is never reoccupied the following year. Young queens don't hibernate in the maternal nest: they shelter alone under bark, in a woodpile, a crack in a wall, a garden shed. In spring, each one builds a new nest elsewhere.

An abandoned nest is therefore an inert papier-mâché object that breaks down in the rain. You can let it fall on its own, or remove it for aesthetic reasons — but only after one to two weeks of frost or sustained low temperatures, and never "just to be safe" in October.

Two caveats all the same:

• A nest in a cavity (loft, shutter box, wall) left in place contains organic residue that can attract moths, carpet beetles and other stored-product insects. In that case, removal is worthwhile.
• The presence of a nest signals a favourable environment: dead wood, a wall cavity, a shifted roof tile, an unscreened vent. That's the underlying cause to address in winter.

The real window for action: November to March

If you never want to have this October conversation again, the work gets done in winter, when not a single insect is active.

• Stainless steel mesh with 5 mm apertures on every vent, eaves opening and mechanical ventilation outlet.
• Repairing shifted roof tiles and ridge joints.
• Inspecting and sealing roller-shutter boxes — the number one nesting site in urban areas around Paris.
• Moving the woodpile and compost away from the façade, and above all away from doors and windows.
• Spotting primary nests in March–April: a lone queen, a tiny nest, often under an awning, in a garden shed, under an upturned patio table. Ten seconds of work at that stage is worth half a day in October.

The paradox of this season is simple: the month when we think the subject is behind us is the one that concentrates the most risk, and the month when we don't think about it at all is the one that would let us avoid everything.

If you're in any doubt about a nest up high, in a cavity or close to a home, have the situation identified before touching it. A diagnosis always costs less than a fall from a ladder or a trip to A&E.