The redbay ambrosia beetle (Xyleborus glabratus) and the polyphagous shot hole borer (Euwallacea aff. fornicatus) are invasive beetles whose impact in the United States came as a surprise, but a closer look at their effect on forests in their native Asian regions suggests their damaging potential could have been predicted. (Photo credit: Jiri Hulcr, Ph.D.)

By Jiri Hulcr, Ph.D.

Ambrosia beetles include some of the scariest invasive bugs. These tiny relatives of bark beetles travel the globe in wooden packaging used on commercial ships. They take their food with them: Instead of eating the wooden crates, ambrosia beetles grow symbiotic fungi inside the timber just like little farmers. As a result, dozens of ambrosia beetle species have spread to multiple continents beyond their native habitat.

Most of the ambrosia travelers are harmless, which is why the group has not often been studied historically. But a few species have reproduced wildly in their newly colonized lands and gained a reputation as some of the most damaging tree pests. Two examples are the redbay ambrosia beetle (Xyleborus glabratus) and the polyphagous shot hole borer (Euwallacea aff. fornicatus). The redbay ambrosia beetle has devastated lauraceous trees such as avocados throughout the American Southeast by inoculating them with its pathogenic symbiont fungus Raffaela lauricola. The shot hole borer has wreaked havoc on the other side of the continent in California. Euwallacea aff. fornicatus has destroyed trees in cities, orchards, and forests. Its fungus is not particularly pathogenic, but the beetles mass-colonize branches of trees until they eventually break off.

Invasions and damage by ambrosia beetles have come as a surprise to most entomologists. After all, venerable forest entomology texts teach us that these ambrosia beetles are not pests in their native regions. But has anyone actually checked?

The University of Florida’s Forest Entomology team collaborated with the National Chung Hsing University in Taiwan and the Hong Kong University on expeditions to the native regions of these insects. As published recently in Florida Entomologist, the assumption that these pests are harmless was a delusion. Both our examples, the redbay ambrosia beetle and the polyphagous shot hole borer, cause plenty of damage in their home jungles. But the only people who were aware were a few Chinese entomologists. Their humble reports had never been translated into English, and Western foresters seldom wandered into southern China. This is unfortunate. Had we studied these insects in their native habitats earlier, we would have recognized their potential for damage and had a chance to combat them before they got out of control. (In the case of the newly discovered beetle attacking American sweetgum trees in China, we hope research on its potential economic impact should it arrive in the U.S. will help to guide decisions on how to prevent it.)

University of Florida student You Li, originally from southern China, puzzles over the attacks of the redbay ambrosia beetle (Xyleborus glabratus) on an apparently healthy Machilus pauhoi in Hong Kong. The arrow points to the sawdust noodles produced by the wood-boring beetles. (Photo credit: Jiri Hulcr, Ph.D.)

In their native Taiwan, the redbay ambrosia beetle and its pathogenic symbiont readily colonize native laurel trees whenever the trees are stressed or injured. A squirrel bite is no reason for a tree to die, but the live wound is an open door for the beetle, and the introduced ambrosial pathogen triggers wilt throughout the rest of the tree.

A mystery gap has appeared in the forest in the Tai Po Kau Nature Reserve in Hong Kong. The gap is expanding due to trees wilting at its edges (in circles), likely caused by the aggressive symbionts Xyleborus glabratus and Raffaela lauricola. (Photo credit: Jiri Hulcr, Ph.D.)

Strangely, in Hong Kong where the redbay ambrosia beetle is also presumed native, damage has been quietly spreading. There is actually so much mortality of Lauraceae trees that it may be affecting the dynamics of the entire tropical forest system. Attacks on Machilus, a genus of Lauraceae trees that is common in South China, triggered mass wilt and resulted in forests gaps the size of soccer fields in Hong Kong.

A branch of Litsea monopetala (Lauraceae) in Hong Kong was severed from its mother tree by a maze of tunnels of Euwallacea aff. fornicatus, also known as the polyphagous shot hole borer. (Photo credit: Jiri Hulcr, Ph.D.)

In the same forest in Hong Kong, E. fornicatus behaves exactly the same way is it does in the invaded California. Beetle families riddle sections of branches with their fungus-laden tunnels until summer storms send hundreds of them cracking down to the forest floor.

A forest edge in Hong Kong is littered by branches broken off by the tunnels of Euwallacea aff. fornicatus, also known as the polyphagous shot hole borer. Some of the branches still have green leaves. (Photo credit: Jiri Hulcr, Ph.D.)

These discoveries open up as many new questions as they provide answers: How does the tropical forest sustain the onslaught of laurel wilt? How do the branch-borers accumulate in particular spots? And the most urgent question: What other overseas beetles bore into live trees and are poised to become the next big invasive pest on continents where neither trees nor people are prepared for them?

Jiri Hulcr, Ph.D., is an assistant professor at the University of Florida School of Forest Resources and Conservation in Gainesville, Florida. Learn more about his lab’s work at www.ambrosiasymbiosis.org. Email: hulcr@ufl.edu