It was named Oak City for a reason. Even a decade ago, the streets of Stellenbosch, South Africa, were lined with towering English oaks several hundred years old. European settlers had brought the species with them centuries ago, and it seemed to thrive.

But now the city’s oaks are dying, destroyed by the polyphagous shot-hole borer, an invasive beetle that infects oaks and at least 600 other plant species. Females drill a toothpick-size hole, lay eggs, and inoculate the wood with a fungus that produces fruiting bodies—food for the beetles after they hatch. The fungus and the beetle tunnels clog the tree’s water conducting channels, sometimes killing it. “We are losing a lot of the urban forest canopy,” says David Richardson, an invasion biologist at Stellenbosch University (SU). But the damage extends far beyond Stellenbosch.

The world’s trees are already under siege from a plethora of invasive insect pests—the emerald ash borer, the hemlock woolly adelgid, the European spruce bark beetle, to mention a few. But as their names suggest, many specialize on a single group of trees. The polyphagous shot-hole borer is an all-purpose threat, infecting many woody plants. What’s more, all it takes is one female to start an infestation, because unfertilized eggs can develop into males, which the female can then mate with.

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