The stem of Macaranga pearsonii, a tree of Borneo’s forests, is hollow. Ants of the genus Crematogaster live inside. The plant gives them shelter and food, small nutrient bodies it makes for the purpose. The ants defend it against insects that eat its leaves. It is one of the best-studied partnerships between plants and ants, long used as a model of mutualism, a relationship in which both sides gain.
A third tenant has moved into the stem. It is a wasp, Dasyproctus agilis, described by the entomologist Frederick Smith in 1858. It hollows out the stem cavity and fills it with adult flies, which feed its larvae.
The study is by Dan Leština of the Biology Centre of the Czech Academy of Sciences and Tom Fayle of Queen Mary University of London, with seven co-authors from Czech, Malaysian, British and German institutions. It appeared in PeerJ on 15 April 2026. ScienceDaily ran it on 5 October, almost six months later.
The work took place in the Malaysian state of Sabah. The authors dissected 213 Macaranga pearsonii trees across the habitats where the wasp occurs, to gauge how widespread it is. Then they compared two landscapes, heavily logged forest and oil palm plantation.

The hypothesis had two parts. The wasp, which elsewhere favours badly disturbed habitats, should be commoner in plantations. Where wasps are many, ant colonies should be smaller, because the two tenants compete for the same space. A shrunken colony, in turn, might defend the plant less well against herbivores.
The first prediction held. The wasp colonises stems more often in plantations than in logged forest, and uses several Macaranga species over a wide area. The second held in part. Plants with more wasps have smaller ant colonies, but the authors write that the direction of the relationship is unclear. Wasps may reduce the ants, or colonies that are already small may let wasps in. The third prediction failed. The authors found no sign of weaker defence against herbivores.
The aggregator’s headline says wasps are taking over the ants’ homes. The paper is more careful. It measures a higher presence where people have changed the landscape most, and a link with smaller colonies, without settling who displaces whom. The authors conclude that habitat change can open the door to a species that exploits the partnership, and could potentially weaken it, because the wasp gets first to the space that plant and ants trade.

The point reaches beyond one Bornean tree. A mutualism holds as long as the benefits stay with those who pay the price. In plantations, the study finds, the intruder has more stems to take. The study was funded by the Czech Science Foundation, the Bavarian-Czech Academic Agency BTHA and a Czech research programme co-funded by the European Union. The article is open access under a CC BY licence.

References
Leština D., Houadria M., Gavin J.A., Chung A.Y.C., Mohd Yusah K., Rindoš M., Schlosserová K., Feldhaar H., Fayle T.M., "Exploitation of an ant-plant mutualism by a cavity-nesting wasp", PeerJ, 14, e20984, 15 April 2026, open access (CC BY 4.0). doi: 10.7717/peerj.20984
Queen Mary University of London, press release via ScienceDaily, "Tropical Trees Built Homes for Ants. Wasps Are Taking Them Over", 5 October 2026 (used only as a lead).

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