The emerald ash borer is in Denver. What does it mean for your trees?
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DENVER — The invasive emerald ash borer is in Denver, and now the city has a big ash problem.
This wood-boring beetle, native to eastern Asia, has devastated tens of millions of ash trees across 37 states. Boulder forestry staff detected the insect in Boulder 12 years ago, and since then, foresters have been preparing for its arrival in Denver.
Emerald ash borers move slowly on their own — traveling about one mile a year — but will hitch a ride on firewood, logs or trees transported by humans to cover more ground. EABs infect ash trees by laying their eggs on the bark. The larvae tunnel into the tree, feeding on its nutrients until the beetles emerge as adults.
The Denver metro area is home to 1.45 million ash trees. Denver’s Office of the City Forester launched its Be A Smart Ash campaign in 2016 to identify, treat and replace ash trees on city property.
Entomology experts confirmed the presence of the emerald ash borer in Denver June 17. One out of every six trees in Denver is an ash tree, so you might have one in your yard or neighborhood.
Rocky Mountain PBS spoke with certified arborist Amanda Westendorf from the Park People about what the EAB means for Denver’s ash population and how tree owners can respond.
This conversation has been edited for length and clarity.
Rocky Mountain PBS: So what is the emerald ash borer and why is it such a big deal that it's been detected in Denver?
Amanda Westerndorf: The emerald ash borer is a wood boring beetle, which means it bores into the wood of our ash trees. So what is particularly damaging about this insect is that it lays its eggs on the bark and then the larvae burrow into the tree and it feeds on the vascular tissue of the tree.
So we have these larvae that are eating at veins and arteries of our trees. That's going to disrupt the water and nutrient uptake for trees, which leads to a decline. So really the eating away at that vascular tissue is what's the most damaging.
RMPBS: In other places on the Front Range, these bugs have been detected before. Historically, what have we seen from previous infestations of these insects and their impact on ash trees?
AW: For EAB, it's an exotic or an invasive pest that doesn't have natural predators here, so there's no natural control of the pest. This insect was originally found, like, 20 years ago, in 2002. [We] had no idea what it was or what it could do, and so when it spread, it just devastated ash tree populations. Because it doesn't have a natural predator, it got to go haywire. Almost like a kid in a candy store.
That's why it's so significant that we've confirmed it in Denver because it doesn't have that natural predation. It can just kind of jump from ash tree to tree with very little biological control. Back in the Midwest, they didn't see it coming and so that's why it was able to devastate as much as it did. I mean we're talking hundreds of thousands, if not millions of trees.
So Denver saw that, and they were smart enough and progressive enough to say, “OK, it's only a matter of time before it gets here.” That's why they launched that BASA program — that Be A Smart Ash program — a few years ago because they saw how devastating it could be.
RMPBS: Twenty years ago when we first discovered this insect, was that in the Midwest area?
AW: Yeah. The theory is that it was probably brought in with packing material, maybe pallets. The hot spot was kind of around Detroit and Ontario and then it just kind of spread from there.
RMPBS: Why does Denver have so many ash trees? Are ash trees native to Colorado?
AW: No, not really. We have a lot of ash trees in Denver because, just like in fashion and hairstyles, trees and plants go through trends as well. So we have a tree with a desirable trait, people want to plant them, and sometimes it gets over-planted. When you take something like an ash tree, which has a great form, has lovely shade [and] it works well in our environment, people want to plant them.
When we get a good tree, we want to hang onto it. But now, with the presence of EAB, we're really starting to highlight the benefits, like biodiversity, of why we should plant different kinds of trees.
RMPBS: How can people identify — if they didn't plant it themselves — if the tree in their yard or in their neighborhood is an ash tree?
AW: I usually tell people when we're identifying any kind of tree, we want to look for three characteristics, just because you can't always just go by one characteristic.
A positive ID for an ash tree would be oppositely arranged twigs and branches. So you see twigs that will grow directly across from each other at a growth point that we call a node. That opposite arrangement is a good indicator.
The compound leaves — so we have one leaf stem that has multiple leaflets on it — that's pretty indicative of an ash tree. These particular leaflets, it [ranges] from like 5 to 11 leaflets per stem, and those leaves are kind of almond-shaped.
We also can look at the bark. So if the bark tends to make diamond shapes in the bark, that's a pretty good indicator of an ash tree.
But I think one [indicator] that people will really know is the seeds. Ashes produce a winged seed that looks kind of like oars, like you would have on a boat. Those oars tend to be grouped pretty closely together, almost like keys on a keychain.
If you can positively identify three out of those four characteristics, it's a pretty good chance that you have an ash tree.
RMPBS: If people are like, “OK, I have an ash tree in my yard,” what steps would you recommend that they take next, now that we have identified that this insect is in Denver?
AW: Definitely look for signs and symptoms of that emerald ash borer because early detection is highly, highly important.
Things to look for would be a thinning canopy, so the canopy — the leaves and stuff — don't look quite as dense. You can look for dieback, which is when the branches and twigs start to die back from the tips towards the trunk.
The really distinct indicator are the D-shaped holes. So after that larvae are feeding, the adults emerge out of these D-shaped holes, and they're very distinctly D-shaped. They're very tiny. They're only about an eighth of an inch wide, but that's kind of like the mark of an EAB. If you're able to see under the bark, that larvae will create these S-shaped tunnels under the bark. That's just their feeding pattern.
If you're looking at all of this and you still don't really see anything, an increased presence of woodpeckers at least tells you that something is under the bark because the woodpeckers are trying to get at it. Those are all like pretty good signs and symptoms that EAB might be a possibility.
But even if you don't see any of these signs of symptoms, it's still a good idea to talk to an ISA-certified arborist or a licensed tree care company or a licensed tree care expert to determine those next steps, either treatment or replacement, for your ash tree, especially since we have confirmation now.
I know that there are trunk injections that plant health care techs will do on ash trees, kind of like a vaccine almost to help prevent EAB from tunneling into that ash tree. So talk to a tree care specialist, talk to a licensed tree care expert and determine those next steps because preventative measurements can really help in the long run.
RMPBS: If someone does identify these symptoms, where’s a point that it's like, okay, we're past the point of treatment and we have to talk about removal now? Is it as soon as you see those symptoms or is there a severity that we're looking for?
AW: If your ash tree is showing kind of significant dieback or significant thinning, you may be past the point of no return. The best option would be to remove and replace. Sometimes if that dieback isn't as severe, you know, 20% or less of the crown, then there's a chance that the tree could be saved. But if you're starting to see significant damage, then it would probably be a removal. In that case I would absolutely call in an arborist.
RMPBS: People may think, “Shoot, I have an ash tree, and now I have to pay someone to come look at it, potentially treat it, potentially remove it.” What are resources that tree owners should be aware of that might help reduce the cost of this care or make it more accessible?
AW: If it's a tree in the right-of-way, they can call the city, and find out their options. The city has this Be A Smart Ash program specifically for trees in the public right-of-way. Depending on the size of the tree, they've been doing preventative injections and also removals and replacements.
For trees on private property, right now I don't know of too many resources for that other than, you know, you may be able to work out like a payment plan with the tree care company that you're working with.
RMPBS: Is it generally more expensive to treat a tree or remove it?
AW: There's a lot of factors that come into play with that. [Laughs.] If you have an ash tree right now that is mature and relatively healthy, then I would say treatment would probably be the most cost effective option. I mean it's a couple hundred dollars and that treatment can last 2 to 4 years. Whereas if you're just taking it down — this healthy, mature tree — then we're talking about thousands of dollars.
If it's a mature ash tree that is not in very good health, in the long run it would be cheaper to take it down just because a tree that is already stressed is going to be susceptible to more attacks. It would be almost like throwing your money away if you're trying to treat it when it's already on the decline. It really depends on the size of the tree, the health of the tree, sometimes where it's located.
RMPBS: Is there a world where we've treated all the ash trees that we can, we remove the ones that are too far gone and now this insect is like, “OK, I have nowhere to go,” and it will move on? Or is this something that we're going to be dealing with for the foreseeable future now that it's here?
AW: We're likely going to be dealing with it for a while because there's still plenty of places that haven't experienced EAB yet. So even if we treat things and we remove everything, that doesn't mean we go back to planting a bunch of ash trees and not treating everything because there's a chance that it could come back.
If we're trying to completely eradicate this beetle, it would have to be nationwide, you know, Canada’s involved, Mexico's involved. We're talking a very, very large amount of area that would have to eradicate this beetle. So, long story short, I think we're in it for the long haul for this.
RMPBS: There are a few different types of ash trees around Denver, like white ash and green ash. Is there a type of ash tree that the EAB is particularly prone to, or is no ash tree really safe?
AW: This is where things get a little tricky because we do have a tree that we call a mountain ash that's not technically an ash. So it doesn't really mess with the mountain ash. When we’re talking about ash trees, we are specifically talking about ash trees that are in the genus of Fraxinus. Any ash tree in the genus Fraxinus is going to be susceptible to EAB.
This really, really highlights the need for biodiversity in our urban forest. We are at risk of losing a lot of trees. So if we have a very varied palette of trees, if we start to lose one species, then we have multiple species that can kind of act as backup for those ecosystems services that we enjoy and that we need.
Type of story: Q&A
An interview to provide a single perspective, edited for clarity and obvious falsehoods.
An interview to provide a single perspective, edited for clarity and obvious falsehoods.
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