Wasps are “squatting” in ancient tree shelters built for ants, adding a hidden threat to forests already battered by deforestation

Inside the hollow stems of young Macaranga trees in Malaysian Borneo, something was wrong. Researchers cutting open these tropical plants expected to find what has lived there for at least 10 million years: ant colonies, tending their host and defending it from leaf-eaters. Instead, they found paralyzed flies being consumed alive by wasp larvae — the chambers repurposed as larders for a predator’s young.

It was an unsettling discovery. And it was happening far more often than anyone had anticipated.

A 10-million-year partnership inside a hollow stem

Macaranga pearsonii trees didn’t accidentally end up hollow. Over millions of years, they evolved those internal chambers specifically to attract ant colonies — a mutualism researchers estimate is at least 10 million years old. The plant sweetens the deal by producing nutritious food bodies, essentially paying its tenants to stay.

Macaranga species are pioneer plants — among the first to colonize land that has been cleared or damaged, which makes them foundational to forest regeneration.

The ants hold up their end by patrolling aggressively, driving off leaf-eating insects like caterpillars before they can do serious damage. It’s a tight arrangement — and it’s made Macaranga one of the most studied ant-plant systems in tropical ecology. That kind of evolutionary stability is genuinely rare. When a relationship persists for tens of millions of years, it suggests both partners are better off together.

Wasps move in — and bring their own agenda

An international research team from Queen Mary University of London, the Royal Botanic Gardens Kew, and partner institutions set out to study this partnership in Malaysian Borneo. What they found when they dissected young Macaranga trees was not what they expected.

A species of predatory wasp had moved into the hollow chambers — not to protect anything, but to hunt. Adult wasps were paralyzing flies and storing them inside the plant cavities as food for developing larvae. Lead author Lestina described the moment plainly: many stems had been hollowed out in an unusual way, and when opened, they were packed with flies being consumed alive by wasp larvae. What evolved as ant nurseries had become wasp pantries.

The wasps aren’t cheating the plant the way a parasite might. They’re simply using a structure for a purpose it was never designed to serve — which means the plant gets nothing in return.

Disturbed landscapes are fueling the takeover

The wasp occupation wasn’t evenly distributed. Plants growing in oil palm plantations were significantly more likely to contain wasps than those found in logged forest, and the pattern was consistent enough to suggest that human-altered habitats are actively enabling the takeover.

Trees occupied by wasps also had much smaller ant colonies, raising the possibility that wasps are displacing ants — though the researchers note that experiments would be needed to confirm this directly. Co-author Dr. Yusah was direct: the spread is clearly linked to disturbed landscapes, not random chance. Whether the wasp is native to the region or was introduced remains unknown, but its preference for degraded habitat fits a familiar pattern. When ecosystems are disrupted, species interactions shift in ways that are difficult to anticipate and often harder to reverse.

Forests that need ants to recover may be left without them

The stakes extend well beyond individual trees. Macaranga species are pioneer plants — among the first to colonize land that has been cleared or damaged, which makes them foundational to forest regeneration. They’re not just survivors of disturbance; they’re part of how forests begin to rebuild.

If wasp occupation weakens ant defense, these plants become more vulnerable to the herbivores they once repelled. Less vigorous pioneer plants mean slower, less robust recovery across the broader landscape. Dr. Yusah put it directly: if the plants lose their ant defenders and become less healthy, it could hinder forest recovery in precisely the places where regeneration is most needed. Disturbance enables the wasp invasion, and the wasp invasion makes it harder to recover from the disturbance.

When exploitation persists, evolution may follow

The most unsettling implication may be the long-term one. Senior author Dr. Fayle warned that when mutualistic benefits break down over time, they can drive evolutionary change. If hollow chambers become liabilities — exploited by wasps rather than defended by ants — plants may eventually stop investing in the very structures that once sustained them. That kind of structural ecological shift is far less visible than a species going extinct. No population crashes, no disappearance from the record. The plant simply, slowly, stops building what no longer helps it survive.

These findings serve as an early warning about how ecosystems can begin unraveling from the inside — a 10-million-year partnership, quietly destabilized not by chainsaws, but by a wasp in a hollow stem. It’s worth asking how many other ancient relationships are shifting in ways we haven’t thought to look for yet.

Learn more about these findings here: Dan Lestina, Mickal Houadria, Jasmine A. Gavin, Arthur Y.C. Chung, Kalsum Mohd Yusah, Michal Rindos, Klára Schlosserová, Heike Feldhaar, Tom M. Fayle. Exploitation of an ant-plant mutualism by a cavity-nesting wasp. PeerJ, 2026; 14: e20984 DOI: 10.7717/peerj.20984