Ash survival and growth in response to emerald ash borer invasion in Massachusetts riparian forests: Impacts of biological control

The emerald ash borer (Agrilus planipennis, EAB) is an invasive buprestid that currently threatens North America’s native ash (Fraxinus spp.) resource, particularly green ash (F. pennsylvanica), a species that is common in riparian ecosystems and floodplain forests. Loss of ash has widespread implications for the ecosystem services these habitats provide. Biological control, through the introduction of hymenopteran parasitoids from EAB’s native range in Northeast Asia, is a potential long-term management solution; however, it is unknown if the program can protect mature ash trees that often succumb to the initial wave of EAB invasion. We here assess the ash growth and mortality responses to EAB invasion at six floodplain forests in Massachusetts and the impacts of early releases of biological control agents. We released the parasitoids Oobius agrili, Spathius galinae, and Tetrastichus planipennisi at three sites from 2017 to 2022. From 2018–2024, we measured the diameter at breast height and assessed the canopy condition of approximately 100 tagged ash trees at each site plus three non-release sites. We found significant effects of site and initial crown dieback on ash tree growth. We also found that the percent ash mortality was negatively correlated to the level of parasitism of EAB larvae in sentinel ash bolts deployed across the six study sites in 2024. This is the first finding of a relationship between EAB larval parasitism and ash health outcomes in the literature. Future work could examine if these results extend to different habitats and environments.