Interaction among ambrosia beetles, host tree species, and their microbiomes

Ambrosia beetles are wood-boring insects that excavate tunnels into the xylem of trees. They transport mutualistic fungi in specialized organs known as mycetangia and actively cultivate these symbiotic fungi in "ambrosia gardens" located on the walls of their tunnels, which serve as their primary food source. This unique relationship has made ambrosia beetles a focal point for research into insect-fungus symbiosis. Interest in ambrosia beetles has further surged in recent decades following the growing number of species introductions and interceptions beyond their native ranges, as well as the significant economic and ecological damage caused by certain species in newly invaded areas. Notably, several ambrosia beetle species are associated with stressed trees that produce and emit ethanol, which acts as an olfactory cue for locating these vulnerable hosts. Given the anticipated impacts of climate change and the imminent increase in natural disasters such as plant-stressing floodings, the risk of increased ambrosia beetle attacks is heightened. Considering these factors, research into the biology, ecology, and effective strategies for monitoring and managing ambrosia beetles is of paramount importance. This thesis addresses existing knowledge gaps concerning the interactions of ambrosia beetles with other organisms. The first section focuses on the parasitic symbiosis between the beetles and their host plants, and includes two studies. The first study (Chapter 2) examines differences in host selection and colonization success among native and exotic species of ambrosia beetles across various tree species subjected to simulated flooding. The aim was to determine which tree species are more susceptible to beetle attack and which beetles may be more aggressive in a future climate scenario of increasing floodings. The second study (Chapter 3) builds on the same field experiment, comparing beetle behaviour between flooded trees and ethanol-injected trees. This study aims to evaluate the potential use of ethanol-injected trees as a management strategy for ambrosia beetle control. The second section of the thesis explores the interaction between ambrosia beetles and microbes, with a particular focus on bacteria, a group less studied than the beetles' fungal symbionts. The first study in this section is a mini-review (Chapter 4) summarizing current knowledge of the bacteriome associated with ambrosia beetles, outlining methodologies, and identifying existing research gaps. The final study (Chapter 5) investigates the functional role of a specific bacterial phylotype associated with multiple scolytid species, whose importance was highlighted in the review. Overall, the four studies presented in this thesis provide new insights into the interactions between ambrosia beetles and other organisms, laying the foundation for future research aimed at developing monitoring and control strategies for this important group of pest insects.