YOU PROBABLY WOULDN'T enjoy a home made of alcohol-soaked wood. But for ambrosia beetles, it doesn't get any better.
These insects make a living by growing fungal "gardens" in dead, dying, or stressed trees. When trees are stressed, for instance by drought or flood, the plants produce ethanol as a chemical byproduct—which serves as a cue to these fungus-farming beetles that the plant might be ripe for invasion.
The insects first excavate networks of tunnels and galleries within these sick trees, often killing the host plant if it’s not dead already. Inside the tunnel walls, they plant fungal spores carried within their bodies and tend to the fungi, their sole source of food, says Christopher Ranger, a research entomologist with the U.S. Department of Agriculture’s Agricultural Research Service.
The insects are so attracted to ethanol that in days past, there were reports of beetles boring into wooden casks, says Peter Biedermann, an entomologist with the University of Würzburg in Germany who, with Ranger, co-authored a new study about the phenomenon.
The creatures are also attracted to German beer halls, which Biedermann sometimes visits (for research purposes, of course). "Regularly in summer I [find] ambrosia beetles in the beer," he says.
Ambrosia beetles are the world’s first farmers; their ancestors first evolved more than 100 million years ago.
PHOTOGRAPH BY CHRISTOPHER RANGER
It was previously thought that beetles were only attracted to ethanol because it serves as a signal of tree stress. But when Ranger and Biedermann met a few years ago, they began to question this orthodoxy.
Ethanol, after all, is well known for killing many types of microbes, presumably including the symbiotic fungus. "It seemed counterintuitive, counter-productive," Ranger says. Perhaps the chemical helped the beetles in another way?
The team designed several experiments to find out, working mainly with a kind of Ambrosia beetle called the black stem borer (Xylosandrus germanus).
This beetle, native to East Asia, has wreaked havoc in forests, nurseries, and orchards in Europe and the United States, and can attack more than 200 types of trees, Ranger says. It's one of more than 3,000 species of ambrosia beetles found worldwide.
At first, the scientists put ethanol-soaked baits next to healthy trees at a research site in Ohio. Ambrosia beetles would come, tunnel into the tree, and then leave without starting fungal gardens or reproducing.
When the researchers irrigated trees with ethanol, however, and the plant soaked up the chemical, the beetles not only stayed but thrived, producing gardens and offspring.
That suggested the ethanol wasn't solely serving as an attractant, says Ranger, whose study was published April 9 in the Proceedings of the National Academy of Sciences.
A female fruit-tree pinhole borer tunnels through an artificial substrate.
PHOTOGRAPH BY GERNOT KUNZ, INSTITUTE OF ZOOLOGY, UNIVERSITY OF GRAZ
Killing the Competition
Next, the scientists raised black stem borers in the lab, exposing them to a tube full of sawdust. When they put small concentrations of ethanol within the tubes, the beetles raised larger fungal gardens, and more offspring, compared with beetles in tubes without alcohol.
Further work found that ethanol actually promotes the growth of the beetle's symbiotic fungus and kills off undesirable, "weedy" species that the insects don't eat, Biedermann says.
Ulrich Mueller, an entomologist at the University of Texas at Austin who wasn’t involved in the study, describes the beetle's ability to selectively grow its food source as a strategy to eliminate competitors.
He was surprised by its simplicity, and says he would’ve expected such a scheme to be more complex and involve multiple chemicals.
What's more, the researchers found that ethanol promotes the growth of symbiotic fungi preferred by several other types of ambrosia beetles, suggesting the phenomenon is not unique to the black stem borer, Biedermann says.
The researchers hope that understanding how the beetles' symbiotic fungi grow may reveal how to limit the damage they cause to our crops.
As it is now in nurseries and orchards, "you have trees in early stages of stress emitting ethanol, and they can look perfectly healthy, and the growers think they’re fine," Ranger says. "And then the beetles attack."
Lead Image: Adult female ambrosia beetles tend to their fungal gardens. PHOTOGRAPH BY CHRISTOPHER RANGER