Wasps and hornets in late September: why colonies turn aggressive just before they die

There is one very specific moment in the year when the phone calls change subject. Between 10 and 30 September, calls about bed bugs and rodents carry on as usual, but another category abruptly takes over: "we can't eat outside any more", "there are about thirty of them around the bin", "my daughter was stung twice in three days and she wasn't even moving".

The sentence that comes up every single time is: "they're far more aggressive than they were this summer".

And it's accurate. It isn't just an impression born of end-of-season fatigue: the behaviour of worker wasps genuinely changes from late August onwards, for perfectly well-documented biological reasons. Understanding this shift means understanding why half the nests reported in September warrant no intervention at all — and why the other half warrant one, fast.

Grey rat nibbling a piece of food against a pink background, next to decorative baubles

What is happening inside a wasp nest in September

The population peak comes at the end, not in the middle

A colony of common wasp (Vespula vulgaris) or German wasp (Vespula germanica) starts in spring with a single foundress. She builds a few dozen cells, lays eggs and feeds her first generation of workers single-handedly. In May, the nest fits in the palm of your hand. By July, it holds a few hundred individuals.

The maximum is only reached between mid-August and the end of September: 2,000 to 6,000 workers for a well-established Vespula colony, sometimes more in a sheltered space such as a roller-shutter box or a roof void. In other words, the nest you discover "out of nowhere" in September was already there in June — it was simply discreet.

This build-up explains the first part of the phenomenon: there are quite simply ten times more wasps in flight than in June, in the very same garden.

The change of diet

The second part is more interesting. Throughout the growth phase, workers hunt prey (flies, caterpillars, aphids) to feed the larvae. In exchange, the larvae secrete a sugary droplet that the workers consume — this is known as trophallaxis. A colony in full activity is therefore largely self-sufficient in sugars.

At the end of the summer, the queen stops laying workers and produces the reproductives: future queens and males. The last larvae emerge, and the internal source of sugar abruptly dries up. Thousands of workers then find themselves:

• with no larvae to feed, and therefore no social role;
• with no internal sugar supply, and therefore starving;
• with a remaining lifespan of just a few weeks.

They set off to look for sugar outside. Fallen fruit, grapes, jam, cans of fizzy drink, ice creams, bins, barbecues. That is the mechanism that brings them to your table — not some form of "aggression". Grape pickers know this phenomenon well: this is exactly the period when ripe bunches become a wasp magnet across every French vineyard.

Why a sting becomes more likely

A starving wasp, at the end of its life, with its colony breaking apart, has a far lower defensive trigger threshold. It lands on the food, you swat it away, it stings. It slips into a can, you take a drink, it stings inside your mouth — the most dangerous scenario of the season.

A useful figure to put things in perspective: according to pharmacovigilance and allergy data reported by Ameli and Vidal, the vast majority of hymenopteran stings cause nothing more than a benign local reaction. The real risk concerns multiple stings, stings in the mouth, throat or nose, and people allergic to the venom — roughly 0.3 to 3% of the population according to European studies.

Wasp, European hornet, Asian hornet: three different cases

The most common mistake in September is treating all three the same way. These are three distinct situations.

The European hornet is the most unfairly loathed

Vespa crabro is imposing — it is the largest social hymenopteran in France — but it is also the least inclined to sting without reason. It hunts alone, including at dusk, and is strongly drawn to sources of light. Hence those thuds against lit windows in the evening, which alarm a great many households without the slightest danger being involved.

It is not protected by any national decree, but several regions and naturalist associations explicitly recommend not destroying nests located away from dwellings: it is an effective predator of flies and caterpillars. A nest in an old oak twenty metres from the house, discovered in September, will die out on its own in November. Destroying it achieves nothing.

If the only problem is the night-time attraction, simply changing the outdoor lighting is often enough: a warm-light outdoor LED bulb emits far less in the wavelengths that attract nocturnal insects than cool white lighting.

The Asian hornet: a listed invasive alien species

Vespa velutina nigrithorax, which arrived in France in 2004, is now present in almost the whole of mainland France, including the Île-de-France region. It is classified as an invasive alien species of concern and is monitored by the Muséum national d'Histoire naturelle (INPN), as well as through departmental control plans.

Two concrete points for September:

1. This is the month when nests become visible — not because they appear, but because they have reached their maximum size (up to 80 cm) and the falling leaves are starting to reveal them.
2. It is also the month when future foundresses are produced. A mature colony releases several hundred young queens that will go on to overwinter. Destroying a nest in late September therefore still serves a purpose; destroying it in December, in a bare tree, serves none at all: the nest is empty and the foundresses have left.

Its nest defence is markedly sharper than that of the European hornet. A lawnmower or hedge trimmer passing within five metres of a low nest can trigger a group attack. It's a classic autumn-maintenance accident.

Brown rat perched on a damp tree trunk, surrounded by ivy and dead leaves

What to do — and what never to do

The measures that genuinely reduce encounters

September logic isn't the same as July logic: it's no longer about avoiding the nest, it's about making your living space uninteresting to workers on the hunt for sugar.

• Pick up fallen fruit every day beneath apple, plum and fig trees and vines. This is measure number one, and by far the most effective.
• Keep bins closed, rinse sugary packaging, don't leave open cans lying around. A container with a tight-fitting lid completely changes how busy a terrace gets.
• Serve drinks in glasses, never in cans, and use insect-proof glass covers during outdoor meals — the risk of a sting in the mouth disappears.
• Don't swat at a wasp with your hand. Sudden movements and breath (CO₂) are threat signals. Move it away with a slow gesture, or step aside.
• Protect fruit on the tree with netting if the orchard is close to the terrace.
• Avoid sweet perfumes and floral-patterned clothing when eating outdoors — a cliché, but genuinely documented.

For end-of-season meals, a folding table food net or a food dome remains the simplest solution: it solves the problem without killing a single insect, and with no chemicals on the food.

Traps: beware of false good ideas

Bottle wasp traps filled with beer and syrup are everywhere in September. Their effect on a colony is close to nil: catching a few hundred workers from a colony of 4,000 individuals changes nothing about the pressure, and the bait also attracts passing insects that would never have visited your terrace.

Worse, non-selective traps kill huge numbers of flies, butterflies and solitary bees. ANSES and several research programmes on Vespa velutina insist on selectivity: if you do trap, it must be with a selective wasp trap fitted with escape holes for small insects, and only in spring for Asian hornet foundresses, not in the middle of the season.

What you must never do to a nest

Let's be blunt: nest destruction attempted by a member of the public is the leading cause of serious multiple stings in France.

• No water, neither hosed nor by the bucket: it soaks the structure and triggers a mass exodus.
• No fire, no petrol, no smoke. Every year, loft fires start this way.
• No aerosol spray from a ladder. Balancing three metres off the ground with two thousand workers pouring out is the textbook accident scenario.
• Never block the entrance hole: wasps chew through plaster, insulation and soft wood, and often re-emerge inside the home.

A nest in a roller shutter, in a loft, inside a wall, or at head height where people pass calls for professional intervention with a full protective suit, a suitable insecticide and, if needed, a telescopic pole. It's quick, properly regulated, and generally costs between €80 and €200 in the Île-de-France region depending on accessibility.

Stings: telling the benign from the serious

Normal reaction

Immediate sharp pain, redness, local swelling of a few centimetres, itching for 24 to 48 hours. What to do, in line with Ameli's recommendations:

1. Remove rings and bracelets if the sting is on the arm or hand, before swelling sets in.
2. Bring a heat source close (the tip of an extinguished cigarette, a hairdryer held at a distance) for a few seconds: the venom is heat-labile. Then apply cold.
3. Disinfect.
4. Oral antihistamine or soothing cream if itching is significant.

An outdoor first-aid kit containing antiseptic, dressings and fine tweezers remains basic equipment for a garden, a hike or a worksite.

Warning signs — call emergency services immediately

You should call the emergency medical service without delay in the event of:

• a sting in the mouth, throat or on the neck;
• multiple stings (more than about ten in an adult, fewer in a child);
• widespread hives, swelling of the face or eyelids, breathing difficulty, wheezing;
• faintness, dizziness, a drop in blood pressure, vomiting.

People already known to be allergic to hymenopteran venom must carry their prescribed adrenaline auto-injector at all times from May to October — and an insulated pouch for a medical kit prevents it from cooking in a car parked in full sun, a situation that genuinely degrades the product.

What will happen in October and November

The good news is that all of this stops by itself. With the first serious frosts, the colony collapses: the workers and males die, the old queen dies, and only the young mated foundresses survive, hidden under bark, in a log pile or in a gap in a building.

Two practical consequences:

• A nest is never reused the following year. Not by wasps, and not by hornets. An empty nest in a loft in January is an inert object: you can remove it to stop it attracting carpet beetles and other scavenging insects, but there is no urgency whatsoever.
• The real prevention window is March–April. That is when foundresses are looking for a nesting site. Screening roof vents, loft air inlets and roller-shutter box openings with a fine-mesh stainless steel insect screen eliminates a good proportion of the following summer's nests. The same visit lets you check the loft, which usefully overlaps with autumn rodent prevention.

September is therefore an uncomfortable but transitional month. The goal isn't to eliminate wasps from the neighbourhood — that's impossible and pointless — but to get through six weeks without an avoidable sting, and to deal only with the nests that pose a genuine risk: those on a thoroughfare, within an inhabited structure, or within reach of children.

If you're unsure about the species or the exact location of a nest, a photo taken from a distance and a phone call are worth more than a ladder and an aerosol can. Our teams operate across the Île-de-France region on wasp, European hornet and Asian hornet nests, with a preliminary assessment that sometimes concludes, quite honestly, that there is nothing to be done.