Following the scent of its fungus crop, the farm beetle locates the most suitable host trees.

The alnus ambrosia beetle Xylosandrus germanus, also known as the black trunk borer, was accidentally introduced by humans from native East Asia to North America and Europe at the beginning of the 20th century. X. germanus is what is called a food beetle, which means it grows its own food, a specialized fungal “plant” and tends inside galleries that burrow into wood. The food beetle is a devastating invasive pest, known to attack more than 200 species of 51 species of broadleaf and coniferous trees. While it prefers to colonize dead wood, it can invade weakened or stressed trees and eventually kill them.

A proven method for monitoring or even controlling insect pests is by hijacking their communication system and manipulating it to trap them in their doom trap. Female alnus ambrosia beetles have been known to congregate on individual trees, which indicates that they use chemical cues to find each other and locate suitable trees. Now, scientists have deciphered this chemical symbol. The results are published in Frontiers in Microbiology.

The study’s senior author, Dr. Peter Biedermann, professor at the Institute of Forestry at the University of Freiburg, Germany.

First evidence of aggregation pheromones in ambrosia beetles

First author Antonio Guglioso, Ph.D. “This is the first evidence of aggregation pheromones in ambrosia beetles of the Xyleborini tribe,” added a student in the Department of Agriculture, Food, and Environment at the University of Catania, Italy.

Like all ambrosia beetles, females of X. germanus carry their fungal symbionts in special pockets inside their heads. The study authors used molecular methods to culture and identify the two most common species Ambrosiella grosmanniae, most likely the beetles’ primary food source, and an unknown species of Acremonium that the beetles may or may not eat. In 2020, researchers discovered mating of X. germanus females near Freiburg, and allowed them to dig galleries in an artificial medium based on beech sawdust, cultivate these two fungi, and raise their offspring.

They then used these fungi as bait in two-choice experiments in a device called a still air olfactory meter. Here, X. germanus females were placed in an arena with two dead ends for entry and hiding; For example, one containing isolates of symbionts A. grosmanniae and Acremonium sp. , and another containing an isolate from an unrelated fungus predicted to be unattractive to beetles—here, an uncharacterized species of Cladosporium. The beetles’ preference will depend on their relative attraction to microbial volatile organic compounds (MVOCs) released by the fungi.

In one variation of this experiment, previously healthy beech branches infected with various fungi were presented to the beetles as options for selection.

Beetles prefer the scent of symbionts and colony branches

The results showed that beetles were more attracted to A. grosmanniae MVOCs and less to Cladosporium MVOCs, and more to branches already colonized by A. grosmanniae than to uncolonized branches. The authors conclude that X. germanus uses volatile organic compounds (MVOCs) released by its two symbiont lineages as a “synomone”—a chemical signal that benefits both transmitter and receiver.

“Occupying tree branches may indicate that the beetles are a suitable substrate for the food fungus they depend on,” Biedermann said. “This suitability is probably a delicate balance: the trees need specific requirements to allow the beetles to successfully cultivate the fungus.”

“But this is not an absolute requirement: if the female cannot find any specific elements, or detect any MVOCs, she can still independently colonize a new tree branch.”

Promising means of biological control

These results immediately point to a new method of biological control.

“Now that we know that fungi produce attractive volatiles, we may be able to develop new boxed baits for beetles based on these compounds,” Biederman said.

“Furthermore, the use of specific volatile compounds for trapping purposes could contribute to the development of innovative and selective trapping methods that target specific species of ambrosia, thereby reducing catches from non-target arthropods living in the same environment.”