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.