Invasive Ambrosia Beetles cultivate fungi and fear among arborists

If you're an arborist, you're likely familiar with the term Ambrosia Beetle. These beetles were named by entomologists for their unique relationship with fungi—unlike most insects, they don’t feed directly on wood but rather cultivate and consume fungal gardens within it. These fungi are transported by the beetles using specialized structures called mycangia.

There are over 3,500 known species of ambrosia beetles worldwide, and even more bark beetle species—some of which also feed on fungi, while others consume the vascular tissues of trees. While this intricate symbiosis is fascinating from a biodiversity standpoint, it can have deadly consequences for host trees, especially when beetle populations boom or invasive species are introduced to regions where trees lack natural defenses.

Classic examples include the Elm Bark Beetle (Scolytus scolytus), which vectors Dutch Elm Disease (Ophiostoma ulmi), and the Mediterranean Oak Borer (Xyleborus monographus), which carries the wilt-inducing fungus Raffaelea montetyi. These partnerships can lead to devastating outcomes, both ecologically and economically.

Here in California, we’re witnessing the arrival of several invasive ambrosia beetle species that are already causing substantial damage in both natural and landscaped settings. Two of the most concerning is the Polyphagous Shothole Borer (Euwallacea fornicatus and related species) and the Mediterran Oak Borer (Xyleborus mongraphus), the former being established in Southern California and later affecting the bay area and Sacramento area. PSB is highly polyphagous, attacking hundreds of tree species and severely affecting over 100 of them. The full extent of its ecological impact is still unfolding, but early signs point to serious, long-term consequences for both urban forests and native woodlands. MOB mainly has been affecting our native white oak group, Valley Oaks and Blue Oaks in partcular, which means their impact may reach far from urban areas.

For arborists working in affected areas, this presents both a challenge and an opportunity, an urgent call for collaboration and innovation. Research is on controlling these species is still in an infancy and there have been minor experiments with biological control strategies. In the future hopefully more exploration of using fungi that may suppress either the fungal symbionts (like Fusarium species) or the beetles themselves could yield positive results. With creative solutions and shared knowledge, arborists, mycologists, and land managers have the potential to mitigate some of the damage and help safeguard California’s trees.