Farmer’ beetle finds suitable host trees by tracing scent of its fungus crop

The unintentional introduction of the alnus ambrosia beetle, Xylosandrus germanus, also known as the black stem borer, from its native East Asia to North America and Europe in the early 20th century by human activity has had significant ecological impacts. X. germanus is categorized as an ambrosia beetle, as it cultivates its own food in the form of a specialized fungal symbiont, which it nurtures within the galleries it excavates in wood. This invasive pest is highly destructive, with a documented ability to attack over 200 tree species from 51 different families, including both broadleaf and conifer trees. Although its preference is to infest dead wood, it can also infest and eventually kill weakened or stressed trees, posing a threat to forest ecosystems and tree health.

Researchers have discovered a potential method for monitoring and controlling insect pests by intercepting and manipulating their communication system. In the case of the alnus ambrosia beetle, it has been observed that female beetles tend to gather on specific trees, indicating that they likely use chemical signals to locate each other and identify suitable host trees. Now, scientists have successfully deciphered this chemical code, as published in the journal Frontiers in Microbiology. This breakthrough could open the door to developing strategies that disrupt the beetles' communication, potentially leading to innovative pest management approaches that could help mitigate the damage caused by these invasive pests.

“Here we show that the alnus ambrosia beetle doesn’t produce its own pheromones during tree-host colonization, but rather uses the volatile compounds of their own fungal symbionts to aggregate en masse and potentially kill trees,” said the study’s senior author Dr Peter Biedermann, a professor at the Forestry Institute of the University of Freiburg in Germany.

First evidence of aggregation pheromones in ambrosia beetles

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

The research team utilized molecular techniques to culture and identify the fungal symbionts carried by female Xylosandrus germanus beetles, along with their potential roles as food sources. The two most common symbionts were identified as Ambrosiella grosmanniae, which is likely the primary food source for the beetles, and an unknown species of Acremonium, which may or may not be consumed by the beetles. In 2020, the researchers captured mated female beetles dispersing near Freiburg, and allowed them to excavate galleries in an artificial medium made of beech sawdust, where they were able to cultivate and rear their offspring while studying the interactions between the beetles and their fungal symbionts. This approach provided valuable insights into the dynamics of the beetle-fungus relationship and sheds light on the complex ecological interactions of ambrosia beetles and their symbionts.

The researchers conducted two-choice experiments using an apparatus called a still-air olfactometer to determine the attraction of Xylosandrus germanus females to different fungal isolates. The beetles were placed in an arena with two dead ends, each containing a different fungal isolate. One dead end contained A. grosmanniae and Acremonium sp., the two symbionts commonly associated with the beetles, while the other dead end contained an unrelated fungus, specifically an uncharacterized Cladosporium species expected to be unattractive to the beetles. The preference of the beetles was assessed based on their relative attraction to the microbial volatile organic compounds (MVOCs) released by the fungi, which provided insights into the beetles' chemical communication system and their response to different fungal cues.

In a variant of the experiment, the researchers presented previously healthy beech branches infected with different fungi as options for choice to the Xylosandrus germanus females. This allowed them to investigate the beetles' preference for specific fungal infections in the context of tree-host colonization. By offering branches infected with different fungi, the researchers were able to assess the beetles' response and attraction towards different types of fungal infections, providing valuable insights into the dynamics of the beetle-fungal symbiotic relationship and its role in tree colonization.

Beetles prefer odor of symbionts and colonized branches

The results of the study revealed that Xylosandrus germanus females were highly attracted to the microbial volatile organic compounds (MVOCs) released by A. grosmanniae, the primary fungal symbiont, compared to MVOCs released by Cladosporium, an unrelated fungus. Moreover, the beetles showed a preference for branches that were already colonized by A. grosmanniae compared to uncolonized branches. Based on these findings, the researchers concluded that X. germanus utilizes the MVOCs released by its two symbiont strains as a "synomone," which is a chemical signal that benefits both the emitter and the receiver, facilitating the beetle-fungal symbiotic relationship during tree colonization.

"Occupied tree branches may serve as a cue for suitable substrate signaling in ambrosia beetles' communication with their fungal symbionts. This delicate balance likely involves specific tree requirements to enable successful farming of the food fungi by the beetles," explained Biedermann, the study's senior author.

“But this isn’t an absolute requirement: if a female can’t find any conspecifics, or detect any MVOCs, she could still independently colonize a new tree branch.”

Promising avenue for biological control

These results immediately suggest a new method for biological control.

"We can potentially develop new trapping lures for ambrosia beetles based on the attractive volatiles produced by the fungi," said Biedermann, expressing the potential for practical applications of the research findings.

"Furthermore, the utilization of specific volatile compounds for trapping could lead to the advancement of innovative and selective methods for targeting specific ambrosia beetle species, thereby reducing the capture of non-target arthropods residing in the same ecosystem," explained Biedermann, highlighting the potential for developing environmentally-friendly and species-specific trapping techniques.