Emerald Ash Borers Found Attacking White Fringe Tree

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Originally posted on ecoipm.org

See an update to this story posted here.

As reported by Entomology Today, Emerald ash borer, Agrilus planipennis, has recently been found to attack white fringetree, Chionanthus virginicus, in Ohio. This is the first non-ash host recorded for emerald ash borers since they were discovered in Michigan in 2002. In a recent paper published in Environmental Entomology, researchers Don Cipollini and Chad Rigsby at Wright State University inspected white fringetrees and Chinese fringetrees, Chionanthus retusus, at Spring Grove Cemetery and Arboretum in Ohio and The Morton Arboretum outside of Chicago. They found 32% of white fringetrees at one site and 43% at the other were infested with emerald ash borers. In some cases it was clear the trees had been infested for multiple years.

No Chinese fringetrees were infested with emerald ash borers. This make sense because emerald ash borers are indigenous to Northeast China so trees there have likely evolved resistance mechanisms. Chinese ash trees are also more resistant to emerald ash borers than North American ash species. The researchers confirmed this resistance by trying to rear emerald ash borer larvae on both fringetree species and a closely related tree, Devilwood (Osmanthus americanus). No larvae developed on the Chinese fringe trees. On white fringetrees larvae survived the 40 day experiment developing to 4th instar but were smaller than larvae reared on highly susceptible green ash. Few larvae survived on devilwood and those that did were small 2nd instars rather than 4th so this does not appear to be a good host.

White fringetree flowers. William M. Ciesla, Forest Health Management International, Bugwood.org

White fringetree and Chinese fringetree are frequently planted along streets and in residential and commercial landscapes. Its popularity is increasing as an alternative to other white-flowering trees such as Callery pear. Along Raleigh streets there are 580 white fringetrees 60% of which have been planted in the last 5 years.

Host switching by emerald ash borer from ash to fringe tree is obviously bad news. We need more research to determine what is driving this phenomenon.

Does emerald ash borer only switch to fringetree after all the ash trees are gone? Do beetle populations have to be at a certain level so competition drives them to try new hosts? Was there a genetic anomaly in these populations that allowed these beetles to succeed in fringetrees when other may have failed? These are all things we need to know before treating or removing white fringetrees from our yards and streets.

So far emerald ash borer is patchily distributed in North Carolina and we do not have as many ash trees on streets or in natural areas as the Midwest and northeast. If this host-switch is a function high emerald ash borer density then maybe we will never get there. Much more work needs to be done.

See an update to this story posted here.