How are the microbial communities of Minnesota’s trees altered by the arrival and presence of invasive Emerald Ash Borer beetles? This valuable inquiry into tree health was addressed by Andrew Mann (PhD ‘25) et al., with funding from the National Science Foundation, in a new publication documenting a four-year study of how ash trees at seven Minnesota sites are impacted on a microbial level during and following an Emerald Ash Borer (EAB) attack.
According to the US Forest Service, Minnesota has a larger ash tree population than any other state, making EAB a major source of potential change to our forest landscapes. First detected in Minnesota in 2009, EAB has become quite prevalent, spreading to 59 of Minnesota’s 87 counties by the end of 2025. Previous research shows EAB beetles having a 1 or 2 year life cycle, with the host ash trees typically experiencing dieback and death between 2 and 4 years after larvae arrive at the base of the tree. Existing research on EAB-associated microbes has not compared the microbes in tree phloem over time, though, leaving a gap in which important questions about how EAB may be altering an ash tree’s microbial community in ways that could impact its health.
To conduct this study, Mann and others sampled ash trees during the larval feeding stage at seven natural forest stands each fall during September and October from 2019 to 2022. This sampling included 30 black ash and 46 green ash, the majority of which were sampled across multiple years to assess changes over time. Types of samples collected included EAB-infested tree phloem (so, phloem with visible galleries), uninfested phloem in trees with EAB presence, uninfested phloem in trees without EAB symptoms, as well as frass and larvae from EAB galleries. After lab processing and DNA extraction, samples underwent DNA sequencing and identification. From there, Mann and colleagues could use the genetic information about fungi and bacteria present at each tree and site to analyze variation in microbial presences based on factors including geographic location, ash type, sample type, infestation category, and whether or not the EAB infestation had yet moved to base of the ash tree. For details about methodology in sampling, extraction, and data analysis, please read the original publication in Phytobiomes.
This study demonstrates that EAB infestations in the beetles’ invasive North American range alter the microbial communities present in the phloem of green and black ash trees, intensifying over time such that over time, more than 80% of taxa detected were absent from the control trees.
Many of the species that gained prevalence over the course of the EAB infestations were fungi that are known to induce cankers or decay in trees, an important consideration when thinking about how increased prevalence of EAB is degrading tree health. To name a few examples, canker causing fungi made up over 10% of present fungal taxa across both black and green ash trees in the study, soft rot fungi were four or more percent of fungal taxa, and white rot fungi were greater than 12% of the fungal taxa in uninfested phloem after EAB reached the base of the tree, as well as in infested phloem of green ash trees. The authors did not detect any bacterial pathogens of ash trees in EAB galleries, but they did observe a decrease of some endophytic bacteria, such as the Pseudonocardiales, as trees were attacked by EAB.
With the increased understanding this study provides about the bacteria and fungi present as EAB expands in its invasive North American range, researchers can move forward asking other important questions that will enhance our knowledge of both ecology and management of EAB. Key next questions include whether the few remaining living ash trees after a large die-off event host different microbial communities than the ones that had died, whether the microbial communities present when ash trees decline and die are also present when trees die due to non-EAB causes, and whether the fungi and bacteria found in the North American EAB galleries were also introduced from abroad.
Mann, A. J., J. T. Mogouong, D. N. Showalter, B. W. Held, K. E. Bushley and R. A. Blanchette. 2026. Does the emerald ash borer infestation alter ash phloem microbial communities over time? Phytobiomes 10: https://doi.org/10.1094/PBIOMES-06-25-0046-R