Emerald Ash Borer resistant ash trees

Asked September 29, 2019, 9:00 PM EDT

I have a 12 1/2 acre stand of mixed hardwoods of what comprised of a large amount of Ash trees until the Emerald Ash Tree invasion. Just when I thought I was down to "0" on my Ash trees, I started noticing what I thought to be surviving Ash trees. I ID'd the leaves and they appeared to be Ash Tree leaves. I did some further research and found out that some Green Ash, through studies done at University of Pennsylvania,show a resistance to the Emerald Ash Borer. I've been cutting and burning dead Ash for heat for at least 10 years and yet those Green Ash still thrive. What have your researhers come up with?

Gratiot County Michigan

2 Responses

Thank you for your question. I have sent an email to our EAB researchers for their input and will forward their collective response once it's received. I'll be back in touch as soon as possible. Thank you for your patience.

Thank you again for you question and patience as I gathered information from prominent Emerald Ash Borer (EAB) researchers at MSU, Drs. Deborah McCullough and David Smitley. Here is a synopsis of their responses:

Re-Growth of Previously Attacked Trees:

The trees you are seeing may be re-growth from the base of ash trees killed from the base up, i.e. the roots are not directly killed by EAB. After 10 years the new growth from these trees could be 6” diameter trunks or larger. In that case, these trees will eventually be attacked again. However, the EAB population may be so low that it will take a few years to build-up again. This is what researchers typically have discovered.

Tree Species Variation in Susceptibility:

Emerald ash borer only attacks ash (Fraxinus spp.) trees. The different types of ash trees vary in their susceptibility to attack by EAB. Green ash is most susceptible to EAB attack. Here are a couple ash tree identification resources:

https://www.canr.msu.edu/uploads/files/e2942.pdf and https://www.canr.msu.edu/resources/distinguishing_ash_from_other_common_trees_e2892

Based on this knowledge,

1.) Your trees could be young green ash that were seedlings or sapling when EAB blew through your area. Many areas like this have lots of those young green ash trees now.

2.) Your trees could be white ash trees. We know that white ash is an intermediate host for EAB. We have a 2019 research paper with data from 28 forested areas in southeast and south central MI. More than 50% of the stems and basal area were alive in more than half of the sites. Most of the live trees had some old attacks but those were “healing” i.e. wood was laid over those areas of attack and they are closing up. A few sites had zero survival but a few other sites had less than 10% mortality of the white ash. This may have something to do with population level genetics. We were unable to identify any factor that consistently predicted white ash survival rates.

There is a research group in OH that has worked with “lingering” ash trees – e.g., a solitary ash surrounded by a sea of dead ash (same species). Dr. McCullough’s understanding is that the lingering ashes pretty much all succumbed; the “lingering” ash trees just stayed alive a little longer than others. For more information on this research, contact Jennifer Koch with the US Forest Service in Ohio.

3.) It’s also possible that the ash trees on your property could be blue ash. Healthy blue ash trees are very resistant to EAB.

4.) And, the trees could be something like pignut hickory. Researchers have been out to see an array of “resistant ash trees” over the years. Most often they are hickory but black walnut and black cherry have also been on the list.

I hope this information from Drs. McCullough and Smitley have been helpful. I also wanted to share a national EAB resource where EAB research and educational materials are housed: http://www.emeraldashborer.info/

Thanks for using Ask an Expert!