COS 64-3 - Can white fringetree serve as a refuge for emerald ash borer?

Abstract: Emerald ash borer, Agrilus planipennis, an invasive woodboring insect, has had devastating effects on ash tree species throughout eastern North America and has now been detected in the Pacific Northwest. While most North American ash species are highly susceptible to this insect, one non-ash host has been confirmed in the wild for this species, white fringetree, Chionanthus virginicus. While this species is known to be susceptible, it does not appear to be as preferred or as heavily impacted by emerald ash borer as most susceptible ash species. As a consequence, it may serve as a low-yielding, but sustainable refuge for emerald ash borer after the majority of its more preferred ash tree hosts have been killed. I examined wild populations of white fringetree to examine their status as a long-term host to this insect in three locations in southern Ohio. Monitoring of these populations was begun in 2015, prior to the arrival of emerald ash borer at these locations and when trees selected for monitoring were generally in good health. Through the main invasion wave of the beetle from 2016-2019, approximately 25% of the trees in these populations were attacked by emerald ash borer, and canopy health of attacked trees as measured by changes in canopy cover declined by about 30%. At this point, most of the previously healthy ash trees in the vicinity of each populations were attacked to the point of being killed. Reexaminations of these populations in 2022 and 2023 revealed that up to 80% of the white fringetrees had been attacked at some point in the last seven years, there was a great deal of individual stem mortality mostly without whole plant mortality, and that 50% of the trees had evidence of current attack. Populations with larger white fringetrees showed more evidence of long-term attack by emerald ash borer. These results indicate that as susceptible ash trees decline across the invaded range of emerald ash borer, wild populations of white fringetree may be sufficient to sustain small populations of emerald ash borer if the fringetree populations are large enough and contain enough larger trees.