June 27th, the hottest day ever recorded in Berlin, where I live, began with a whimper. My apartment does not have air-conditioning, and neither does a friend’s garden cottage where I took refuge, so my poor, panting greyhound was audibly distressed. I walked her to a park to cool off. At the entrance, I passed a sign that warned of the presence of Eichenprozessionsspinner, caterpillars with stinging hairs that German news outlets have called a “citywide plague.” The government was hiring workers in protective suits to vacuum them up. I wasn’t too worried, though. Germans often overreact to animal nuisances with complicated schemes. Who else would try to clean nature with a Hoover?

The oak processionary caterpillar, as it is known in English, is a squiggle of dark skin covered in wiry white hairs that look like the whiskers on a witch’s warts. But you never see just one caterpillar. Oak processionary caterpillars live with hundreds or thousands of their siblings, and together they look more like a wizard’s beard. The caterpillars move in a single-file line—a procession—and often defoliate oak trees completely. I saw no bare branches or caterpillars in the wooded area where my dog and I found shelter. We cowered in the shade as the temperature rose to a hundred and four degrees.

As I soon learned, you don’t need to touch these caterpillars to experience their sting. The next morning, a galaxy of itchy red bumps and blotches stretched from my shoulder to my hip. Oak processionary caterpillars have not only tufts of visible hairs but also eight hundred thousand microscopic hairs that, according to one study, can be “projected in the air as a fine powder” when a caterpillar is threatened. These hairs are toxic, and they can travel several hundred metres, survive for years on surfaces, and pass through clothing to impale skin. The entomologist Justin O. Schmidt once developed a scale for the pain inflicted by insects, describing a particularly excruciating wasp sting like so: “Torture. You are chained in the flow of an active volcano.” On a scale for itchiness, my Eichenprozessionsspinner rash would outrank all the mosquitoes, spiders, fleas, and midges that had ever bitten into my flesh. I was reminded of bedbug bites that I once suffered on a forearm, after going to a movie theatre in New York City—but the caterpillar stings covered half my body.

The heat wave broke, but the itching lasted another week. I slathered myself in ointments, even after reading a study that said treatment “shows scarce efficacy” for my condition. The main consolation I took from the scientific literature was that it could have been worse. A caterpillar hair in the eyeball can create a crusty yellow nodule that requires professional removal. In some cases, inhaling the hairs may make it difficult to breathe. Guests at a nineteenth-century dinner party suffered “a disgusting, unbearable itching and scratching in the throat” after they feasted on roasted cuckoos that had preyed on Eichenprozessionsspinner. Worst of all was the plight of a forty-six-year-old Italian farm worker who apparently masturbated after handling processionary-caterpillar nests, as described in an old dermatological case study. The scarring from his injuries was so severe that he needed to be circumcised.

Now that oak processionary caterpillars had my attention, I realized that they were not a parochial German pest. Britain has had them since 2006, when London imported infested oaks from the Continent. Lately, the species has been spilling from Germany into Denmark for the first time. The Netherlands, where they have long been present, recorded a doubling of infested oak trees compared with last year, and Switzerland’s forest-research institute called 2026 “an exceptionally strong Eichenprozessionsspinner infestation year.” France has been beset not only with these caterpillars (processionnaire du chêne) but also with a closely related species that has spread northward from the Mediterranean: the processionnaire du pin, or pine processionary caterpillar. A friend of mine, whose university campus in Nuremberg was infested with Eichenprozessionsspinner, told me, “I think nature is trying to destroy us.”

Toxic caterpillars, like deadly heat waves, are becoming a regular part of European summers. Eichenprozessionsspinner love warm, dry, and sunny springs, and the spring of 2026 was the sunniest in Berlin’s history. Heat and solar radiation not only help oak processionary caterpillars survive after they hatch but also make them trigger-happy with their hairs in summer. “The overall trend will likely lean toward an increasing probability of mass outbreaks,” Thomas Sobczyk, a forest engineer who studies the caterpillars and the moths they turn into, told me in an e-mail. “From my perspective, there’s no alternative to coming to terms with the oak processionary moth’s presence in residential areas.”

After I recovered, I visited Volkspark Jungfernheide, one of Berlin’s largest parks and the site of one of the worst outbreaks in the area. It was nearly empty of people. Workers had been vacuuming up caterpillars for weeks. They wore ventilated face masks and, on each hand, multiple gloves that were duct-taped to their sleeves. I no longer viewed these men and women as caricatures of German overzealousness but as frontline heroes who should be applauded from balconies and open windows. Their vacuum cleaners had originally been designed to suck asbestos out of old buildings; a vacuum bag could hold forty pounds of caterpillars and their nests, and a worker could fill three bags a day.

In June, the city’s twelve districts asked the Berlin Senate, which had been slashing city services for years, to declare the caterpillars a public-health threat, which would have unlocked extra funding and made it easier to use regulated pesticides. The request was denied. The state secretary for the environment said that no one could have predicted the extent of this year’s outbreak, even though the numbers of Eichenprozessionsspinner had climbed every year since 2023. As a result, the districts could do little else but remove caterpillar nests—commonly called “tents”—from trees. The district that encompasses the park, Charlottenburg-Wilmersdorf, had closed playgrounds and sports facilities, including a swimming pool. Caterpillar hairs floated on the water’s surface, leaving swimmers with a necklace of red stings.

At a little biergarten that featured an outdoor theatre, I met an arborist named Gernot von Lyskowski. His firm, GvL, was one of three companies sucking up the caterpillar tents. “We haven’t had it this bad before,” he told me. “We’ve been fighting here in Jungfernheide for four or five years now.” Von Lyskowski originally became an arborist because he loved climbing trees when he was a kid. “We weren’t even aware of what was causing the itching back then,” he said. The caterpillars are not unknown to Germans: in the fifties, for example, hundreds of people attending a festival broke out in rashes after they hid from a downpour under infested trees. Yet the species was uncommon enough that, in 1993, conservationists classified the species as “threatened with extinction” in Berlin. Perhaps today’s Eichenprozessionsspinner outbreak is less like a foreign invasion than like a revolution, in which a lowly element rises up to dominate ecosystems of which it has always been a part.

Between 2003 and 2010, as humans heedlessly dumped unprecedented amounts of carbon dioxide into the atmosphere, the amount of sunshine during the caterpillar’s larval and moth phases was eleven per cent higher in Berlin than it had been two decades prior. During that same time, the caterpillars became commonplace in every district of the city. “It really came on quite suddenly fifteen years ago,” von Lyskowski said. When he started removing Eichenprozessionsspinner, he climbed trees with a twenty-metre-long hose that was connected to a vacuum on the ground. Now he owns three trucks with pneumatic outriggers and aerial lifts which cost two hundred thousand euros each. The caterpillars had been good for business, but he didn’t love the work. “We should be earning much more money from this danger,” he said. A single old oak could contain dozens or hundreds of tents, and each tent could contain hundreds or thousands of caterpillars, which translates into tens of billions of stinging hairs.

Oak processionary caterpillars are nocturnal, and they spin tents out of silk in part so they have a place to digest their food. By early summer, the tents sag like dirty diapers from the undersides of boughs. During my visit to Volkspark Jungfernheide, the caterpillars were already beginning to pupate in the soiled tents. In three to five weeks, they would emerge from their cocoons as brown moths, which could fly several kilometres to lay eggs. When caterpillars first hatch, in the spring, they need a few weeks to develop their stinging hairs. That is the time to hit them with insecticides.

Some parts of Germany are already spraying oak processionary caterpillars with chemicals that are strictly regulated under environmental-protection laws. But there are more than three hundred and sixty species of caterpillar that live on oak trees in Germany, and these insecticides can kill many of them too. Meanwhile, a 2013 study found that the two most common agents “were practically ineffective” against oak processionary caterpillars; the authors speculated that intense solar radiation, the very thing the caterpillars loved, reduced their effectiveness. A more recent British study found that sprayed areas were infested again after one or two years.

Von Lyskowski and his workers sprayed some trees with nematodes, a parasitic worm that kills caterpillars and reproduces inside their carcasses. They cause far less collateral damage to other insects, but they are themselves living, sensitive creatures. They can only be applied on dry, windless nights, and two applications are needed to make a dent in caterpillar populations. “You won’t fully control it with nematodes,” von Lyskowski said. “But at least you get the population down somewhat.” The expert consensus seems to be that we cannot defeat processionary caterpillars. We can only learn to keep our distance from them. “There’s a mistaken belief that we can sustainably influence the population using technical methods—plant protection products or biocides, for instance,” Sobczyk, the forest engineer, said in his e-mail to me. “In the long run, natural factors play a far greater role in population fluctuations.”

If the caterpillars are here to stay, is it possible to learn to love them, at least a little? The nineteenth-century German lepidopterist Jacob Hübner named their genus Thaumetopoea, from the ancient Greek words for “wonder” and “create,” perhaps because he admired their single-file processions and silken nests. I could muster a little wonder, too, when I read that caterpillars evolved stinging hairs more than twenty million years ago and radiated across the Eastern Hemisphere. In the Middle East, processionary caterpillars feed on pistachio trees. In Australia, they specialize in acacias and eucalyptuses; in the Himalayas, Alnus nepalensis and rhododendron leaves. Caterpillars cause health problems there, too. A seasonal blindness in Nepal has been linked to caterpillar hairs.

Sometimes I told myself that caterpillars deserve a chance to defend themselves. Nature targets few other organisms with such cruelty. Charles Darwin said that he could no longer believe in God after he watched a parasitic wasp inject its eggs into a living caterpillar. When they hatched, the wasp larvae slowly ate the caterpillar from the inside, leaving the vital organs for last to keep the caterpillar alive for as long as possible. Maybe caterpillars need to make entire forests uninhabitable just to find a little peace. (Caterpillar-eating predators have evolved defenses of their own—cuckoos and tits, for example, have strategies for neutralizing the stings—but neither bird has checked the caterpillars’ populations.)

In theory, humanity’s carbon emissions could warm the planet too much even for oak processionary caterpillars. Paula Halbig, a scientist affiliated with the Forest Research Institute in Baden-Württemberg, said that if oak processionary caterpillars began hatching too early in the spring, there would be no oak leaves on the trees for them to eat. On the other hand, oak leaves have also started sprouting earlier, because of climate change. And, on the other other hand, warmer winters could accelerate the northward spread of pine processionary caterpillars, which are already invading southern Germany. “It is difficult to find another insect which is so specialized in exploiting climatic conditions,” Andrea Battisti, an Italian entomologist who has been studying pine processionary caterpillars for more than forty years, told me. The ancient Romans apparently ground their hairs into an excruciating poison. Roman soldiers have been said to have tortured prisoners by tossing them into caterpillar pits.

In the nineteenth century, Jean-Henri Fabre, a French entomologist, conducted a famous experiment with pine processionary caterpillars. He coaxed a group of caterpillars into a procession around the rim of a flowerpot, so that each caterpillar both led and followed the others. He expected them to break off at some point in search of food, but, in the end, they mindlessly circled the flowerpot for seven days and then dropped dead from starvation. Fabre took his experiment as proof that scientists would never find “the origin of reason in the dregs of the animal kingdom.” Later, though, his experiment became a metaphor for the dangers of groupthink. As a species that spews toxins into the air by burning fossil fuels, perhaps we humans should feel a certain kinship with these creatures. We might never find the origins of reason in caterpillars, but we may recognize the dregs of the animal kingdom in ourselves. ♦