Emerald ash borer

As many of you know, the Emerald ash borer has been running rampant through North American Ash. It has recently been found in my province, Manitoba, and poses a threat to our urban tree population.
I have been thinking about this as I walk through bush that has old borer damage of some sort - why is the Elm ash borer so devastating? There are all sorts of wood boring beetles here and elsewhere, and the local trees don’t seem to suffer too much.
So, why is this beetle so destructive? Lack of disease/predators? I thought someone in this group could provide some sort of answer.


I think the EAB is worse than other insects because it’s introduced/invasive and has no natural predators and the trees haven’t evolved defenses. There are a few ash trees with more resistance but unfortunately they are few and far between.
The EAB just arrived here in Vermont too including right near our home. I made a collection project and am trying to map as many ash as possible to determine their range before EAB and find any resistant trees. https://www.inaturalist.org/projects/emerald-ash-borer-in-vermont


We do have some natural controls here (I’m in Ontario), like woodpeckers, but its nowhere near enough. I agree with @charlie that one problem is the trees not having evolved their own natural/chemical defences. Regular treatments of neem oil-based product on individual trees are the only fortification we seem to have to offer them here. The stuff is pricey and iffy. Its also not a landscape-level solution because, compared to external insect pests like defoliators, borer larvae are tucked away inside the tree and harder to target. Of the species I’ve seen in Ontario, Fraxinus pennsylvanica, F. americana, and F. nigra seem to be preferred hosts. (Or maybe it just looks that way because there’s more of those on the landscape?) F. excelsior will get hit too, but its only an occasionally occurring, generally urban, tree. F. quadrangulata, on the other hand, seems to not be to EAB’s liking.


There have been attempts to use introduced parasitoids to control it. (https://www.nrs.fs.fed.us/disturbance/invasive_species/eab/control_management/biological_control/)
From my understanding there is no serious prospect at this point that these will prevent the continuing spread and destruction from the borer - the parasitoids just can’t travel fast enough and/or reproduce enough. My hope though is that the parasitoids will allow some degree of ash recovery in areas that have already been hit.

I’m in an area where essentially every single ash tree is dead. Really heartbreaking, especially in moist floodplain/lowland habitats where the double whammy of Dutch Elm Disease and EAB has removed a huge portion of the trees and several of the most dominant species. And its really opened up opportunities for invasives. One of the most common habitats in my area now is European Buckthorn monocultures with dead Green Ash trunks sticking up out of them.


i’d really love a push to establish resistant elms where the ash die. At least we can start replacing the elms in the gaps where the ashes disappear, and hopefully do the same with ash later. We only barely got started with the EAB here and i really worry about what will happen with the buckthorn and other invasives also.


Thank you all for the help. As a non-plant person, I had not realised that plants/trees can develop their own defenses to a a hostile organism. A case of “plant blindness”?
We have already lost most of out elms to DED. The city spends a lot of effort to preserve them, so there are more in Winnipeg than elsewhere. They had planted Ash as a replacement, and now this. @charlie I will look at your project and pass it along to the city arborist for her consideration.


Of course, my project is just for Vermont, but you could make one too, or if there’s lots of interest I could broaden it. It’s just a collection project for tracking and such, though more could be done with it.
In terms of the elms, for New England there are resistant strains being planted by The Nature Conservancy at restoration sites, though there still aren’t that many available yet. I would love to get one or two for my land as native elms grow nearby and perhaps some cross pollination might occur and dump some resistance into the gene pool. In terms of the street trees we had similar issues with the elm-ash crossover here, i think the best take home is that it’s good to plant native species as street trees but we should be planting a diversity of species so that things don’t knock them all out at once.


Better urban forestry programs no longer plant single species on a given road or neighbourhood. Variety is indeed the better way to go. I’ve heard some make the argument that we should plant more Asian species since they’ll at least have some resistance to things that come along in the future. Of note: recent success with chestnut trees has apparently produced blight-resistant trees. Maybe someone can CRISPR some resistance into our ashes (and elms, and butternuts, and …). Until then, count on massive ash mortality. Ash die and become brittle quickly, and tend to fail at the base, and so require urgent removal/felling everywhere they pose risk. If the stumps are not ground out, expect copious regeneration via dormant buds (i.e., stump sprouts). Prompt replanting of other species and active ongoing control can go a long way to keeping buckthorn down. I hate to tell you, but next up are the oaks (oak wilt) and hemlocks (aphid). Such is the very very high cost of cheap Chinese stuff.


Man we are going to need some intense reforesting if all of these things go. Are there already efforts to find resistant forms of these?

As I said, they’ve been working on chestnut for a long time:
I thought there was some major chestnut news in the last year or so, but I’m not finding a link.
Efforts to find potentially canker-resistant butternut are ongoing:
Not sure where we’re at with elms.
No work of that sort on ash, oak, or hemlock yet that I know of. Some reading:
For a different look at hemlock:

Also worrisome, though a ways down the road for us more northerly (I hope), is a thing called the spotted lanternfly:


Ahh, that’s bad news. I thought oaks were basically indestructible. The Winnipeg climate also restricts our choices. We don’t have butternuts here, or Maple etc. We do have Manitoba Maple, but they grow fast and die fast. A fellow I knew called them weeds.
Replanting in Winnipeg is slow - the forestry department is busy trying to contain DED, so there is not a lot of cash or workers to do replanting or even pruning of trees. Ironically, we are ending up with a mixed forest. When elms are removed, and some replanting is done it mixes things up!
Your point about cheap Chinese goods is well taken. Most of our recent invasive pests have come from Asia
Thank you for all the links - I’ll take a look at them.

Reading this thread made me want to chime in with interesting fact about Hemlock resistance I recently came across: The HWA species currently invasive in the eastern US mainly coexists in the wild with Southern Japanese Hemlock (Tsuga sieboldii). Southern Japanese Hemlock has very little innate resistance to HWA, and Eastern Hemlock (Tsuga canadensis) grown horticulturally in the same range is able to thrive without chemical treatment in spite of coexisting with the same HWA that wipes it out in north america. The interaction of T. sieboldii and introduced T. canadensis with HWA in southern japan is entirely mediated by native predators of HWA, especially Sasajiscymnus tsugae.

Establishment of this HWA predator (and some others) has been slow but the overall arc of it has been increasingly successful. It will come too little, too late for much of the species’ range (especially the titanic specimens that grew in the southern appalachians), but there is greater reason for optimism about preserving populations that have yet to be invaded, and the prospect of future generations in areas that have been decimated, than I was led to believe when first learning about the issue.

Similarly, Fraxinus americana at least is starting to look a bit more optimistic with “survivor trees” in MI and OH being identified for study.

Where I live and in all the more temperate parts of Ulmus americana’s range that I’ve explored, while Elms don’t grow as large or old as they used to, they’re still a major ecosystem component. I look forward to the idea of resistant elms, but even today I’m often treated to 2-4’ DBH second-growth elms well over 100’ tall, even the occasional old-growth specimen.

One pathogen I usually don’t hear as much about is Beech Bark Disease. I’ve spent time recently documenting old-growth areas in Allegany State Park (NY), most of which were beech-dominant. Now the lumbering old sugar maples, black cherries, red oaks, hemlocks and others stand widely spaced with wind-battered crowns above a sea of endless beech sprouts where they used to be sheltered within a beech canopy. It feels like a haunted ecology. I see the effects at many different sites but nowhere has it been so dramatic as in old-growth areas of ASP. The newly emerging Beech Leaf Disease may be devastating to other areas in which the bark disease was impactful but not as severe, which is most other areas I’ve been in.


It’s not fully true that it has no natural predators. Woodpeckers eat huge volumes of this beetle, and if anything, infestations seem to be a temporary boon to woodpeckers.

It may be helped somewhat by the lack of other types of predators or pathogens that attack it, but I suspect that its massive spread is due to lack of resistance in the tree hosts.

But it could also still be lack of predators, even if there are some, like the woodpeckers, that there aren’t enough.

I have no full explanation of why this particular pest has become so problematic, but I do have a theory (at the end of this post) about how I think humans may be making the infestation worse or at least not doing as much as we could to prevent it or protect ashes for it.

In order to understand this theory, I think it is useful to focus on factors that could potentially lead to ashes surviving in the long-run. These reasons include:

  • White ash, one of the most widespread species here in North America, naturally seems to have some resistance to it. Blue ash, another relatively common species, seems to have even more.

  • Black ash, which is one of the most vulnerable species, has a range extending extremely far north, so far north that the EAB’s spread may be limited by cold. If the EAB cannot kill this species off from the north of its range, it will likely be able to recolonize from the north, by evolving resistance.

  • From what I’ve read, ashes can hybridize and there is some gene flow between different species, so there is hope that genetic resistance in one species could be transferred to others.

  • Green ash, the other most vulnerable species in the northeast, is an extremely weedy species, reaching reproductive maturity quickly, producing a high volume of seed, and thriving in disturbed habitats. I live in an area where EAB has killed most mature ashes and I see some green ashes of reproductive maturity already, that haven’t been killed by it. I also see a ton of green ash seedlings. It may survive just by virtue of its prolific seeding, rapid growth, and general adaptability. In these respects green ash reminds me of American elm, and American elm seems to be bouncing back already. I see some ones reaching into the canopy near me, with no signs of the disease.

We can also help along all of these processes.

But what has troubled me is that, in seeing the widespread human responses to the EAB, I see little signs that people are thinking about resistance and long-term sustainability of ash populations. All I see is people discussing cutting down every tree in sight, or chemically treating individual trees.

Chemically treating trees could harm ash populations, if it keeps alive trees that are more vulnerable and allows them to reproduce. I would want chemical treatment to be carried out very cautiously, selecting only the most naturally resistant trees and allowing the most vulnerable ones to die. I am finding, instead, the choice of chemically treating trees or not are usually made on other features, like aesthetic considerations, desires of the property owner, and the property owner’s feelings about chemical treatments. This is bad news.

Also I think cutting down all the trees pre-emptively may be a mistake. Basically, by doing this we’re wiping out the populations and not giving them ability to evolve resistance.

So I’m worried that the human response has actually worsened the EAB infestation, relative to what would have happened “naturally” (i.e. with a “hands off” approach once the EAB was introduced.)

Like, basically I want us to be helping along the natural processes of adaptation and I worry that we have been doing the opposite, and that is why this infestation has been so severe.


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