Why Are Wasps More Aggressive in Late Summer? The Colony Cycle

What happens inside a wasp nest in late summer?

The yellow-and-black wasps that visit picnic tables in Europe are usually common wasps or their close relatives, German wasps. Their colonies last only one year, and late summer is the turning point in that year. The timing below is typical for Britain and northern Europe; it shifts earlier or later with the weather and latitude.

At the peak, the colony switches from making workers to making the next generation of queens and males. Fewer worker larvae are being raised, and as the last broods mature, the nest contains fewer and fewer larvae to feed.

Why workers suddenly crave sugar

Adult wasps are fueled by sugar. They catch insects all summer, but they do not eat most of that meat themselves; they chew it up and feed it to the larvae. In return, the larvae give the workers droplets of clear, sugary saliva, a two-way food exchange called trophallaxis. For much of the summer, this saliva is a convenient source of energy close to home.

When the colony stops rearing workers and the number of larvae falls, this supply dries up. Thousands of workers, many of them no longer needed for hunting or nursing, still need sugar. They turn to whatever sweet food they can find outside: nectar, honeydew from aphids, ripe and fallen fruit, and the jam, soft drinks, ice cream and sweet scraps that people leave around.

More wasps, more fruit, more people

Several things come together in late summer. Wasp numbers are at their highest for the year, because colonies have spent months growing. Fruit is ripening and falling in gardens and orchards, giving wasps easy sugar. And people are outside eating and drinking, with plates, cans and bins full of exactly what the wasps are looking for.

The result is far more meetings between people and wasps than earlier in the summer. In June, most workers are busy hunting caterpillars and flies for the growing brood and are rarely seen at the picnic table. In late August and September, many of them are searching for sweet food, and they can be persistent, landing on food again and again.

Are wasps really more aggressive?

Not in the sense of attacking people for no reason. A foraging wasp away from the nest is looking for food, and it usually stings only when it is trapped, squeezed, stepped on or swatted. Many late-summer stings happen when a wasp crawls into a drink can or onto a sandwich and is then pressed against the lips or hand.

Things are different near a nest. Workers defend their colony vigorously, and the nest is at its largest in late summer, so accidentally disturbing one while gardening or mowing can lead to many stings. Wasp venom also contains alarm pheromones, chemical signals that excite other workers nearby, so swatting or crushing a wasp close to others can make things worse. A single wasp can sting more than once; can a wasp sting more than once? explains how.

Do wasps get drunk on fermented fruit?

Many people believe that late-summer wasps are drunk on alcohol from rotting fruit, and that this makes them clumsy and bad-tempered. Fermenting fruit does contain some alcohol, and wasps do feed on it. However, there is little scientific evidence that drunkenness is a real or common cause of wasp behavior, and none that it makes them sting more.

There are simpler explanations for slow, clumsy wasps in autumn. Workers live only a few weeks, so many are old and worn by the end of the season. Cold mornings slow insects down, and as the colony collapses, many workers are starving. These wasps are easier to provoke into stinging because they are easier to trap or step on, not because they are intoxicated.

When does it end?

Wasp activity usually falls off quickly after the first cold nights of autumn. The old queen dies, the colony breaks down, and the remaining workers and males die of cold and hunger. The only survivors are the young mated queens, which leave the nest and hibernate in sheltered places until spring; see what happens to wasps in winter? In warm autumns, wasps may stay active into October or later.

Staying calm around late-summer wasps

The best way to avoid stings is to respect wasps and give them no reason to sting:

• Keep sweet food and drinks covered outdoors, and look into cups, cans and bottles before drinking.
• Do not swat or flap at wasps. Move away slowly, or simply wait for the wasp to leave.
• Stay well away from nest entrances, which are often holes in the ground or gaps in walls and roofs.
• Take care when picking up fallen fruit or walking barefoot under fruit trees.

Most stings cause only pain and swelling, but a small number of people are allergic. If someone stung has trouble breathing, swelling of the face or throat, or feels faint, seek emergency medical help immediately: call 911 in the US, 112 in the EU or 999 in the UK. For the useful side of these insects, see what is the point of wasps?