Beetles invented fungus farming many times over

Ambrosia beetles bore tunnels in wood, but they usually do not feed on the wood itself. On the walls of their galleries they establish a fungus garden, and the mycelium and spores are food for the larvae and the adults. To start a new colony, a beetle carries its partner in specialised body cavities called mycangia. On entering a new tree it inoculates the fresh tunnel with it.

The word "ambrosia" covers many beetle lineages, and their farming does not stem from a single invention. Evolutionary analyses indicate that strict fungus cultivation arose independently many times, as different groups of bark beetles and weevils moved from feeding on plant tissue to depending on a fungal garden. Mycangia also differ in structure and position, which fits multiple origins rather than one inherited organ.

The arrangement can benefit both sides. The fungus gains transport to a new substrate and an environment maintained by the insect, while the beetle gains food that processes compounds from wood and sap. It is not, however, a peaceful monoculture. Competing microorganisms appear in the galleries, so the insects clean them, remove infected sections and work with bacteria or compounds that suppress foreign moulds.

Some ambrosia species become serious invasive pests. Moved without their natural constraints, they may attack new trees, and their symbiotic fungi contribute to the wilting or death of the host. That does not mean every beetle with a garden kills healthy trees; many colonise weakened or dead material. The phenomenon matters precisely because it joins the evolution of agriculture, an insect's microbiome and forest health in one repeatedly reinvented arrangement. When such a partnership crosses continents in packaging timber, the insect, the fungus and the susceptibility of local trees all have to be monitored at once. This farming can therefore become a phytosanitary problem.

What the evidence actually shows

“Ambrosia beetles” describe a way of life, not one branch of the beetle family tree. A phylogeny built from six genes showed that obligate fungus gardens evolved repeatedly among bark and ambrosia beetles, in independent lineages over tens of millions of years. A female excavates a gallery in wood, carries fungal propagules—often in specialized structures called mycangia—and inoculates the tunnel walls. Adults and larvae feed on the growing crop, which extracts nutrients from wood. “Invented” does not mean every lineage cultivates the same fungus or that the partnership is harmless to the tree; some systems involve pathogenic partners and can kill hosts. The inferred number of independent origins changes with taxon sampling and with how fungiculture is defined. “At least several times” is therefore more durable than presenting one fixed count as a timeless fact.