Associations between phloem microbiota and metabolomes in three North American ash species (Fraxinus spp.) susceptible to emerald ash borer (Agrilus planipennis)

Authors: MOGOUONG, JUDITH - Cornell University; YAGER, CLAIRE - Cornell University; Bushley, Kathryn

Submitted to: Environmental Microbiome
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2026
Publication Date: 4/1/2026
Citation: Mogouong Tambue, J., Yager, C., Bushley, K.E. 2026. Associations between phloem microbiota and metabolomes in three North American ash species (Fraxinus spp.) susceptible to emerald ash borer (Agrilus planipennis). Environmental Microbiome. 21. Article 66. https://doi.org/10.1186/s40793-026-00884-w.
DOI: https://doi.org/10.1186/s40793-026-00884-w

Interpretive Summary: The emerald ash borer (EAB) represents one of the greatest threats to forest ecosystems worldwide and has become the most economically costly pest in U.S. history. Very little is known about the microbiome of ash phloem or the emerald ash borer larvae that feed on it and their relationship to plant defense chemistry. We characterized the fungal and bacterial communities in phloem, gallery, and larval samples using a metabarcoding approach and conducted widely targeted metabolomics to identify all compounds present in phloem of the different ash species and those differentially regulated in response to EAB. We analyzed relationships between them and microbial communities in both phloem and larval guts to identify some compounds positively associated with either Phloem or larval gut communities. We found that green ash had microbial communities and metabolite profiles distinct from those of other countries and also showed a distinct response to EAB infestation. Terpenoids were among the class of compounds most upregulated during EAB infestation in green, and to a lesser extent white ash. This research provides knowledge about the microbiome of susceptible ash trees and its relationship with the metabolome.

Technical Abstract: Background: Microorganisms play crucial roles in the survival and wellbeing of their plant and insects hosts, including invasive species. The emerald ash borer (Agrilus planipennis, EAB), an invasive insect from Asia, represents a significant threat to North American forest ecosystems. It has caused widespread mortality in susceptible native ash (Fraxinus spp.) species across eastern North America and now threatens ash trees in western North America and Europe. While previous studies have shown differences in specific plant defense metabolites between susceptible North American ash species and their more resistant Asian counterparts that may help explain their susceptibility to EAB, global metabolite profiles and their interactions with phloem microbiota in response to EAB infestation remain underexplored. This study presents a comparative analysis of fungal and bacterial communities associated with ash phloem and EAB larval guts and their relationship to ash phloem metabolites in three native susceptible North American ash species: Fraxinus pennsylvanica (green ash), F. nigra (black ash) and F. americana (white ash). We employed a metabarcoding approach to characterize the microbial communities associated with the larval gut and host tree phloem, established the first global metabolomic profile of phloem in these ash species, and examined interactions between microbiota and metabolites in response to EAB infestation. Results: Multivariate analysis revealed that F. pennsylvanica (green ash) harbored microbial communities distinct from black and white ash and showed a significant response to EAB attack. Green ash also showed the strongest induced defense response, with the highest number of differentially regulated metabolites, predominantly terpenoids. A subset of these metabolites exhibited correlations with the green ash phloem or the EAB larval gut bacterial community. Conclusions: Our results provide the first comparative analysis of phloem-associated microbial communities and metabolomes across three native susceptible North American ash species and their response to EAB. We found that microbial communities and metabolites in green ash showed a response to EAB infestation distinct from the other two ash species. These findings contribute novel insights into interspecies variability in host-associated microbial communities and metabolomes and their response to an invasive insect.