Laurel Wilt: How Tiny Beetles Can Kill Huge Trees

Introduction and Welcome

Bud: The beetle vector is incredibly tiny, incredibly mobile, and very efficient at getting that pathogen into the tree. You know, the tree doesn’t have to be wounded or injured. It’s, it’s just healthy doing its thing. And the beetle can still get the pathogen in there. Um, a single beetle is carrying enough spore load to kill, to kill a tree.

Shannon: Nature isn’t just “out there” in some pristine, far-off location. It’s all around us, including right outside our doors.

Hi, my name is Shannon Trimboli and I help busy homeowners in the eastern U.S. create thriving backyard ecosystems they can enjoy and be proud of.

Welcome to the Backyard Ecology podcast.

In today’s episode we’re talking with Dr. Albert, Bud, Mayfield. Bud is a Research Entomologist with the Southern Research Station of the USDA Forest Service.

Hi Bud. Welcome to the podcast.

Bud: Well, thank you for having me. I’ve been looking forward to it.

Shannon: You are welcome. Yes. I’ve been looking forward to it as well. And in some ways this is kind of a continuation of a conversation that I had with Alexandra Bevins from the University of Kentucky Extension Service several years ago about Laurel Wilt Disease.

Because at that time, Laurel Wilt Disease had been in Kentucky for two years or so, I think around that. And it had recently been found in my county a little bit further away than I am. And I wanted to know what was going on and to learn more about Laurel Wilt Disease. I mean, I knew the basics, but I wanted to go a little bit more in depth.

And some of the trees that it affects are some of my favorite trees, and pretty common on my farm. So, I was like, I really want to know. And a lot of other people I was talking to wanted to learn about it as well. So, we had that conversation.

And then, like I said, a few years have passed, but I haven’t heard as much
about it in in recent years as I was hearing about it before. And it hasn’t spread, that I’ve seen at least, um, like I thought it would. Which I’m not complaining about.

Bud: We will take all the help we can get.

Shannon: Exactly.

But yes, I was like starting to think to myself, “I want to learn more.” And at about the same time, several other people had asked me, “Hey, we really enjoy that conversation, but it’s been a it’s been a few years. What’s going on? How about an update?”

So that’s why I was like, “Yeah, let’s, let’s have a conversation on the podcast about it.”

That’s when I reached out to you, because I was like, “I want to know more. What’s going on? What’s changed or hasn’t changed?”

And so yeah, I’m really looking forward to this conversation.

Bud: Excellent. Well, I’m honored that you invited me. Laurel Wilt’s been one of those things that I’ve been exposed to since the mid-2000s when I used to work in Florida. And, I’ve been following it since then.

So, I’m glad to talk about it and share what I know. I don’t know it all, but I can share my experience and, and insights on it. And, again, thanks for having me.

Shannon: Oh, yes. You’re welcome.

What Is Laurel Wilt Disease?

Shannon: I know for some people that might be listening, they’re newer listeners or they’ve just never been exposed to Laurel Wilt Disease. They don’t know what we’re talking about. So, let’s start at the beginning. What’s Laurel Wilt Disease?

Bud: Yeah. So, what is this thing? Laurel Wilt is a type of plant disease that we tend to call a vascular wilt. And vascular diseases in plants are things that mess up the water conducting system. Okay? The water pipes are not functioning as they should.

Vascular diseases can be caused by a number of different kinds of organisms, viruses, and bacteria. But in the case of Laurel Wilt, the organism that is messing with the water pipes is a fungus. And that fungus is called Harringtonia lauricola.

You don’t need to remember that, but the scientific name actually contains a clue as to what kinds of plants it affects. That lauricola indicates that it affects plants in the Laurel family. So yeah, it’s a vascular wilt disease that kills trees in the Lauraceae, the laurel family of trees. That’s the short answer.

Plants Affected by Laurel Wilt Disease

Shannon: You said it affects trees in the Lauraceae family.

Bud: Yeah, yeah. What are those?

Shannon: Can you give us an idea of what some of those plants are?

Bud: Sure. Absolutely. So, in the Lauraceae family, it’s a family that occurs throughout the world. Lots of species worldwide. In North America, we have relatively few, maybe a dozen, a dozen plus.

Sassafras is one of the most well-known species in the Lauraceae. In the Coastal Plain, we have several species in the genus Persea, and those are often referred to as bays. So, redbay, swamp bay. These are species that are broad leaved evergreens. Okay. They tend to retain their foliage all year long, but they are a broad leaf tree. Avocado is also in that genus. So avocado trees in Florida are susceptible to the disease.

We also have shrubs. Some of which are common, like northern spicebush Lindera benzoin, which is found throughout eastern North America, but also a few that are more rare, like pondberry and pondspice. So, these are some of our North American Lauraceae.

On the West Coast, there is a species called California Bay Laurel that’s in the family. One thing to keep in mind is that even though we’re talking about laurels and we’re talking about bays, there are plants with those names in the common name that aren’t susceptible.

So, people might say, “Oh, mountain laurel. Is that susceptible?” No. Laurel Oak is not a host. And in terms of bays, we have plants like loblolly bay or sweet bay magnolia. Those are in different families. They’re not in the true Laurel family, which is the Lauraceae. Our sassafras, spicebush, Persea bays are in that family.

Shannon: Yeah, the spicebush is one of our primary understory plants where
I’m at. And then also we’ve got a lot of sassafras. I love sassafras. I just think it’s such a beautiful tree.

Bud: It is a beautiful tree. We have a lot of those species here in the Southern Appalachians. In North Carolina where I’m located, the spicebush is a very common understory species. Produces berries that the wildlife and the birds love.

And sassafras is just one of those trees with beautiful fall foliage and beautiful wood. And lots of culinary and cultural medicinal uses. So yeah, they’re valuable, valuable, ecologically and culturally.

Shannon: Right. And so, that’s why when I saw it getting close to where I was at, I was interested in it anyway because of the ecological value of the sassafras and the bays and everything in the Laurel family, but then, I mean, it takes on a personal note when it gets close to you and it has the potential to really affect and take out a lot of your favorite trees.

Bud: Right.

The Role of the Redbay Ambrosia Beetle

Shannon: And I think one thing that is important to bring up with Laurel Wilt Disease is… Okay, we’re talking about the disease and it kills the plants, but what causes the disease?

Bud: It’s a good question. One of the interesting things about this disease is its connection to an insect that’s known as the redbay ambrosia beetle. And maybe to give it a fuller explanation, I’ll talk a little bit about the lifecycle of the insect and ambrosia beetles in general, and then we can cover some of the specifics about redbay ambrosia beetle and Laurel Wilt.

So, ambrosia beetles in general. We have native species of ambrosia beetles in our forests. And, they’re thought of as fungus farmers. So, they are insects that typically bore into the trunks or stems, or sometimes even small branches of usually weakened or dying trees, sometimes the small branches of healthy trees.

But their objective is to bore into the wood of the tree, then use the walls of their tunnels kind of like a garden plot. Ambrosia beetles are carrying spores of fungi on their bodies or in specialized little structures on their bodies that we call mycangia. And they’re carrying those spores kind of like a farmer or a gardener would carry a pack of seeds.

And when they create those tunnels, they inoculate or deposit those spores on the walls of the gallery of the tunnel, and they grow that fungus as food for their offspring. So, after they lay the eggs in the tunnel, the larvae hatch and the larvae are eating the fungus that has grown in the tunnel and developed there.

All of that’s well and good. It’s part of the natural ecology whenever the tree is a native species and the ambrosia beetle is a native species and the fungus is a native species, and everything has been living in harmony for millennia.

In the case of the redbay ambrosia beetle, it was brought here accidentally from Asia, and its fungi came with it because it carries it in those specialized parts on the beetle. And so, how did it get here?

One of the ways in which we get introductions of beetles from other parts of the world is through solid wood packing material crates and pallets and things that we use to pack shipments in international trade. And so, that’s probably how this insect came to the southeastern United States. It emerged from wood packing material at a port and live beetles found their way to our native redbay trees, in this case.

Unlike a native ambrosia beetle with a native tree, the redbay ambrosia beetle initiates at least some of its attacks on healthy trees. And that initial attack, even though the beetle might not get very far into the tree because the tree’s still healthy, that little attack is enough to get the fungus into the tree.

And the tree, being kind of naive to this foreign pathogen, has a hypersensitive reaction to the fungus. It’s trying to close off the water conducting cells in the wood to wall off the pathogen and keep it from spreading in the tree. However, because it has not been exposed to this pathogen before, it’s a hypersensitive reaction, almost like an allergic reaction. And it shuts down most or all of the water conducting function of the sapwood.

You’re trying to wall the fungus off, but you’re actually shutting the water down for the whole tree. And so, you’re going to be in bad shape because you’re not getting water to the leaves.

So that’s a little bit about how the disease operates. It’s kind of a symbiosis between a fungus and a beetle that happen to be here in North America with trees that aren’t co-evolved to deal with those species. Does that make sense? Is that too much technical jargon?

Shannon: Oh, no! That was great.

Bud: Good.

First Detection of Laurel Wilt Disease

Shannon: When was it found here? Or first detected, I guess that’s the
best way to say it because we don’t know exactly when it arrived.

Bud: That’s a really good point. Often when we detect something, we have no idea how long it’s been here.

But the first detection of the redbay ambrosia beetle was made in 2002 in a trap near the port of Savannah that was put out exactly for the purpose of trying to detect the entry of non-native bark and ambrosia beetles.

The trap only caught a few beetles and at the time there was no knowledge of what the beetles were doing to any of our native trees. There was no known forest health problems around the port at the time. And so, it was very difficult even though the beetle was identified.

What did it mean? Did it mean that it just flew out of the port into the trap? Did it mean it was established in our native trees? Nobody knew. And the trap was being baited with lures that were not specific to the beetle. So subsequent surveys for the beetle didn’t show anything. Two years went by before any real progress was being made toward linking this insect to tree disease.

It was in 2004, near the Hilton Head Island area of South Carolina, which is just north of Savannah. A scientist who used to work in our research unit who has since retired, Steve Frederick, went down and investigated dying redbay trees. He discovered not only the redbay ambrosia beetle in those trees, but a pathogen, a fungus that was responsible for killing them.

And, the rest is history from there. The disease continued to spread, and the fungus was identified as the causal agent of the problem. So, it’s been 20 plus years ago that it first made its arrival on the southeastern coast of the U.S.

Native Range of the Redbay Ambrosia Beetle

Shannon: And I think it’s probably worth pointing out too, that yes, the common name for the beetle is the redbay ambrosia beetle, but it doesn’t mean that it’s native to here.

Bud: That’s right. The common name originated because redbay was the first species in North America in which it was found. But the beetle is native to Asia and it occurs in species in the Lauraceae in its native range in Asia. Of course, there are different species than we have here.

That’s a very good point. It’s not native to redbay, but its initial impact was detected in redbay. Redbay was the first species, but not the only.

Shannon: Right. And in it’s native land, what does it do? Does it kill the trees there?

Bud: No, in its native range, it operates, from what I’ve read and have talked with other colleagues, it operates very much like an ambrosia beetle typically does. You know, it’s found in dead and dying trees. It’s not killing a lot of trees.

It may be killing parts of a tree and doing its breeding in maybe a branch or a weakened part of a tree, but it’s not killing thousands or millions of trees in its native area because presumably those trees have defense against the pathogen and against the beetle.

Shannon: That’s usually the way it works. But I think it can often be forgotten sometimes, just that link and how crucial that is between something causing problems in one area because the plants, the animals, the you name it, didn’t evolve with it. They have no, like you said, immunity to it. Or way deal to with it. Whereas in someplace else, it’s no big deal at all. And that goes both directions.

Bud: Both directions. Yes.

Shannon: We’ve had so many of our native plants, animals, fungus cause problems in other parts of the world. It’s not good plant, bad plant, good fungus, bad fungus, anything like that.

Bud: It’s foolish people.

How Laurel Wilt Disease Spreads

Shannon: So, we’ve talked about how the beetles are what’s spreading the fungus. It’s the fungus that’s causing the problem, not the beetles per se.
Correct?

Bud: Right. And one of the ways that’s illustrated is that you can get disease in plants without the insect. For example, if you take the pathogen and artificially inoculate a tree, for example, drill a little hole and insert some spores of the fungus suspended in water, you can initiate disease in the tree.

Trees that have underground root connections, and this is particularly the case in sassafras, which grows clonally and has an interconnected underground root system. The pathogen can move from stem to stem underground, just through those root connections. There was a graduate student at Clemson University a few years ago who was tracking that movement of the pathogen underground and found that pathogen could move more than three meters a year through the roots of small sassafras trees. So yeah, that’s another way that the pathogen can move.

Of course when when a tree dies, it gets attacked by a number of insects. Not only the redbay ambrosia beetle, but other native and introduced bark beetles and ambrosia beetles that are in our environment already.

And it has been shown that when a tree diseased with Laurel Wilt is attacked by some of these other beetles, those insects can also pick up the pathogen and emerge from the tree with it. Now, how much those insects are causing infections on new healthy trees is uncertain because many of those insects are not attacking healthy trees as part of their normal behavior.

I think the managers and scientists in Florida have shown that some of the insects in those avocado orchards in Florida are actually responsible for moving the disease from one tree to another because interestingly their redbay ambrosia beetle populations in those avocado orchards are very low. The redbay ambrosia beetle brought the pathogen there, but in terms of how it’s getting around, some of those other insects are probably more important vectors than the redbay ambrosia beetle.

In our forests in Kentucky and North Carolina and parts of the range that are now infested, it’s really uncertain how much other insects are playing a role, but it’s something that as we continue to learn about the disease, I think will become clearer over time.

Shannon: Yeah, that was one of the things I was kind of wondering about too, because there are so many other insects that use the trees. Or even just thinking about woodpeckers drilling into the tree.

Could…. I mean, it’s a fungus, it’s spores. Spores go everywhere and can be passed so easily. I was wondering if that could be helping to spread it or if there was something special about the way the ambrosia beetle carried the fungus or they had some sort of “activation,” I guess, that was causing it.

But it sounds like no, they’re spores and they do what fungus spores do, which is go wherever. And attack when they get the chance.

Bud: You know, ambrosia beetles, as I said before, have these specialized structures on their bodies that carry spores and that house spores.

Many insects have lots of little “hairs,” I’ll call them for the common person, that when they’re tunneling through the sapwood, which is where this fungus grows, those hairs are picking up the spores. And so, if they are actually tunneling in the wood and chewing the wood, they may be ingesting those spores. So, the beetles are very good at picking up fungus.

Another area, again, this is speculation I don’t have data or studies to point to, but there have been cases of other vascular wilt diseases that can spread through sap feeding insects. So, insects that pierce into the leaves or stems of plants and draw fluid out. The Laurel Wilt pathogen moves in the water conducting cells.

And so, it’s conceivable that a sap feeding insect feeding on a diseased tree, flying to a healthy tree, sticking those same mouth parts in a healthy tree, that vascular pathogens can spread that way. Again, we don’t have any evidence that Laurel Wilt moves that way, but it’s not out of the realm of what’s possible with moving a wilt fungus.

Shannon: Right, and I mean, we were talking about it being here 20 years. We’ve learned a lot in those 20 years. But it’s only been 20 years. There’s
a lot more to learn. And those first few years are always the, “Oh my gosh, what’s going on?”

It’s triage. It’s just learning as quickly as you can the basics and what to do to try and stop it. And then, now we’re getting to the point where we can say, “Okay, let’s look at some of these more intricate and complex possibilities and questions.

Bud: That’s right. I like that word that you use, “triage.” You’re trying to stop the bleeding, reduce the impact, and at the same time try to answer some basic scientific and biological questions that are going to help you as building blocks for future management options if you can develop them.

People Accidentally Spreading the Disease

Shannon: We’ve talked about how it’s in the wood and can be so easily moved. That would also be the same thing as with pretty much most of our wood borne pathogens. People can also move it as well through moving firewood or other types of wood if it’s from these Lauraceae family trees that have potentially been affected.

Bud: Yes. And in fact, that is one of the ways in which this insect and disease issue has spread in the southeastern United States. Again, it’s very hard to pin it down exactly. We don’t have little GPS trackers on all these insects and spores to know just exactly how they traveled and when.

But, we’ve had a very good effort, and I say “we” meaning collectively forestry and forest health professionals throughout the southeastern United States who work for federal and state and even local or nonprofit groups, have really come together and tried to track and manage this problem.

And over the 20 plus years that the disease has been here, there has been an effort to map its spread on a county by county level over time. The map is updated regularly. And again, it could be that the disease is in certain places and we just haven’t found it yet. But anytime it has been confirmed in a county, that county is lit up on a map. So, we know, okay, it’s at least here.

And so, one of the things if you look at the progression of the disease spread in the southeastern United States over time, you see that yes, the disease does spread slowly from county to county in many places, but sometimes you’ll get an outlier. You’ll get a county that lights up that’s maybe a hundred plus miles away from the nearest known diseased trees. And you think, “Well, how does that happen?”

And one of the ways it can happen is exactly what you’re saying. Someone moves some firewood or some logs or maybe even a load of chips. Something where people are carrying the beetle a long distance and then the beetle emerges in a new place.

One of the interesting stories – I used to work in Florida, I work in North Carolina now, but I used to work in Florida – and one of the first detections of the disease in a new county in Florida was because there was a homeowner who was a woodturner. He was in the Jacksonville, Florida area and he was visiting there and he saw some redbay logs on the side of the road.

Redbay has a beautiful wood. And of course, when it’s diseased it’s got this beautiful stain in it. And he thought, “Oh, what a piece of wood to work with.” So, he gathered the wood, it was just on the side of the road, took it to his home county, you know, several counties away, and was utilizing that wood at his house. And the tree in his yard ended up being the first detection of Laurel Wilt in that county.

It’s very likely that he spread it, you know, totally unintentionally, non-maliciously, something you or I would do. It was like, “Oh, that’s some beautiful wood.” And it wasn’t until afterwards that he learned about Laurel Wilt and thought, “Oh my goodness, maybe I helped move this thing.”

And so, it’s one of the reasons why you might notice that different states have don’t move firewood programs or even a national effort to don’t move firewood. That’s the reason – because we can unintentionally move these insects to new areas through the movement of wood, not just redbay ambrosia beetle, but other wood boring, invasive pests like you’ve probably covered on your podcast with other people like emerald ash borer or something like that.

Shannon: Yes. Emerald ash borer was when I really first started hearing the BIG push for don’t move firewood. That was when the movement seemed to really get started. And then, I mean, Laurel Wilt Disease was just the same sort of thing. And it’s picked up momentum a lot I think in the past 20 something years.

Bud: And it’s something that people can relate to when you’re talking pathogens and minute insects. It’s sometimes hard for people to relate. But moving firewood to some place where you’re going camping, people can wrap their head around that and feel like they’re making a difference.
So, I think it’s a good movement.

Fire and Heat as Management Strategies

Shannon: Yes. And speaking of moving wood, I mean it could be lumber, it could be firewood, anything. Does kiln treating it or heating it up kill the beetle and the fungus, or do we know that?

Bud: That’s a great question. So, one of the phytosanitary treatments that is widely used and research based is part of what’s called ISPM 15. The ISPM stands for International Standards for Phytosanitary Measures. These are standards that can be used to treat wood so that you can safely move it to a distant location without great risk of moving insects and pathogens.

For example, one of the standard treatment protocols for most insects and pathogens in ISPM 15 is that the wood needs to be heated to at least 56 degrees Centigrade or Celsius for 30 minutes or more to the core of the wood, to the center of the wood. So yes, heating, heating up wood products to a certain temperature or fumigation with certain materials can make wood safe to move.

And that’s true, not just for international shipments, but also for firewood. So yes, kiln drying firewood will allow that wood to reach temperatures that kills the organisms in it.

We recently, with the Forest Service, conducted some experiments in Tennessee where we were trying to see if, okay, not everybody can take their wood to a kiln or even have a chipper or a place to burn it, but could you solarize the wood? Could you take clear plastic and put the wood under clear plastic in the bright sun? Would that bring the temperature up high enough to kill the beetles?

We weren’t able to completely eliminate beetles, but we were able to greatly reduce the number of beetles that emerged from wood by doing that. So, yeah, temperature, kiln drying, those kinds of things help the movement. And certainly, if you’re using kiln dried firewood, you should be in good shape to move it.

Shannon: And I’m assuming that fire, just burning it, is going to get rid of it.

Bud: If it’s consumed completely. These insects are incredibly tiny and, with the case of ambrosia beetles, they actually bore all the way into the sapwood. Some of the insects in this group of beetles we call bark beetles and they just bore underneath the bark in what’s called the phloem those are easier to kill with fire because it’s not as deep. Ambrosia beetles go into the wood further. So, when you’re burning wood, you don’t want to just char it on the outside. You want to burn it up.

Shannon: You were talking a few minutes ago about the sassafras being clonal and that the fungus can move in their roots as well. So, that would even mean that running a fire through a larger property like a prescribed burn and top killing the sassafras, especially if you’ve got little saplings, that’s not going to help either because it could be in the root system. So when it resprouts, you’ve still got it.

Bud: It’s not necessarily going to eliminate the pathogen from the site. I think that the area of prescribed fire is an area of Laurel Wilt management that deserves more attention from a research standpoint. I happen to know that Virginia Tech is doing some research on prescribed fire and Laurel Wilt.

I think that is an area that’s worthy of some more attention because the number of management options that we actually have for Laurel Wilt, especially in forests, is relatively few to none.

It’s much easier if it’s in a managed landscape like an avocado orchard where you have high value, at least economic value or public perception value, of the trees and there’s some money behind it. But, for our species like sassafras and redbay, that many people may not even know that these I species exist, there’s not a lot of attention given to it.

So yeah, I think prescribed fire, especially in natural forests, is one of those areas that don’t know a lot about the potential management use of it.

Shannon: That’s interesting and sounds like another podcast.

Bud: There you go.

Flight Range of the Redbay Ambrosia Beetle

Shannon: With the beetles, because we’ve talked about the beetles spreading the fungus. How far can they fly? Because I was thinking that they don’t fly very far. What’s, what’s their range?

Bud: Yes, that’s a great question. In terms of potential, like how far can one beetle fly? Um, I don’t know the exact answer to that question.

We had a research study in Georgia that was looking at an infestation that started in a wetland and monitored beetles with traps out into a pine stand where there was no host. So, there were no Laurel family species out there away from the creek. And you could catch beetles 300 meters away from the creek and that was as far as the traps went. Maybe you could catch them even farther.

One of the observations in the early stages of the Laurel Wilt epidemic on the coast of Georgia and South Carolina was that there were barrier islands or, you know, kind of like the, um, what do you call it in North Carolina there, the, um…

Shannon: The Outer Banks.

Bud: The Outer Banks! Thank you. Outer Banks and islands off the coast where there’s no conceivable way that people were driving the wood out there, but yet there were infected trees miles off the mainland, and so presumably beetles were flying or being windblown across those distances.

Now does that mean that the beetle is spreading at that rate, you know, miles per day? No. Or, that the disease is spreading miles per day? Not necessarily.

I think the spread of Laurel Wilt through the Southeast has been a mixture of, okay, maybe beetles spreading a few hundred meters a year on the landscape, but also people moving it dozens or hundreds of miles through a single event. And then those spots begin to grow, and then another human assisted movement happens.

Current Distribution of Laurel Wilt Disease and Potential Spread

Bud: There was a paper that showed in the early stages of the outbreak, if you look at how the county infestations showed up the first five years or so, the rate of spread was like 25 miles per year in terms of new counties. But then that has slowed since then.

It may be part of, okay, we’re filling up more and more of the range of sassafras and redbay and so there aren’t as many new detections to be made. It also could be that the new area for it to expand in the Eastern United States is northward. And so, it’s cooler.

Even though research has shown that temperatures in the range of sassafras probably would allow the beetle to survive even all the way up into Canada. It doesn’t mean that the environment is ideal for the development of the disease at every latitude where sassafras occurs. And so maybe as this disease now has to move northward if it’s going to spread, that may be more difficult as temperatures become cooler as you move north.

Shannon: Let’s give people an idea of where can we find it at now. So, this is 2025. What’s kind of the range where we know Laurel Wilt Disease exists?

Bud: If I had a map I’d show you. But, describing it…

So we know it came, we strongly suspect that it came, into the area around Savannah, Georgia on the southeastern coast. It spread all the way down through the peninsula of Florida and across the panhandle west to Texas, the eastern part of Texas.

And then you can find it from there up through Louisiana, Arkansas, Kentucky, up to around the Louisville, Kentucky area, and then back eastward again through very southern part of Western Virginia, and over to the coast of North Carolina.

We actually have not found it here in the higher elevation areas of Western North Carolina, yet it’s right on the border with Tennessee in terms of known infections. But we have not detected it in Western North Carolina per se. But when you get down into the Coastal Plain of North Carolina and the Piedmont of South Carolina and Georgia it occurs there.

So yeah, it’s still a southeastern distribution. You could find it in those areas, and there’s a website where you can go and look at the current distribution. But as we sometimes say, an uninfected area is just a truckload of firewood away from being infected. But we hope not.

Shannon: Yes, hope not. And, I’ll put a link too in the show notes for anybody who wants to see that map, because I think it is very important and helpful to see it.

And that’s really interesting too, at least I find it really interesting, that it hasn’t spread north of Kentucky. Because when I was talking to Alexandra, like I said several years ago, we knew that the beetles, from everything that they’d done in the research, should have been able to move on north. But like you said, the disease isn’t being seen there, which is reassuring in some ways. But it’s also interesting.

Bud: Yeah.

Northern Limitations?

Shannon: Is the fungus able to survive that far north?

Bud: I think it’s very likely that that the fungus can. It’s just that… for disease, if we think back to our basic ecology or biology classes, you know, a disease can be thought of as a triangle with three different corners. We have host, and we have pathogen, and we have environment. And you have to have all three of those to make disease.

Often the environment is the most important part of that triangle. You can have this organism that causes problems and you can have the plant that it causes problems in, but if the environment isn’t right you’re not going to have the disease. And so that’s one of the questions that we have to keep in mind about how far north will it go.

And again, it’s not as though there is a generously funded monitoring effort for Laurel Wilt annually across the United States. There definitely is not. And folks, as I’m sure you well know, folks involved in forestry and forest health, it’s like drinking from a fire hose. You’ve got so many different things you’re trying to keep track of and help people with.

So our Laurel Wilt monitoring effort is really, you report it as you see it, as you kind of run into it. And so again, there could be Laurel Wilt infections that we don’t know about yet. There’ll be some time before they show up, just like it was in good old coastal Georgia back in 2002.

The beetle was there. It probably was in our native trees because it showed up in a trap. But there was a lag time before realizing it, and that there were enough dead trees to generate some public attention.

How to Identify Laurel Wilt Disease

Shannon: So how do people know if they’ve found a tree that’s got Laurel Wilt Disease?

Bud: Oh, that’s a great question. One thing to keep in mind is that to confirm that you have a tree infected with Laurel Wilt you have to get an identification of the pathogen. That’s typically done by sampling a part of the tree and that going to a lab where they either isolate the pathogen from the wood into a Petri dish and they can identify the pathogen because of its physical characteristics or, and what’s often done, even if the physical characteristics are recognized, confirmation is also done genetically or a molecular confirmation with the DNA to say yes, this pathogen matches the genetics of what we know to the Laurel Wilt Fungus, Harringtonia lauricola.

There are even some really cool equipment now that you can do that molecular identification in the field with a little piece of equipment that allows you to do what’s called a LAMP assay. Not everybody has access to that, but there’s been a lot of progress. Things have come a long way for molecular identification of pathogens. So that’s, uh, that’s how you know for sure, “yes, this was caused by Laurel Wilt.”

But nonetheless, there are symptoms, ways in which the tree is expressing the disease, that although they aren’t a hundred percent diagnostic for Laurel Wilt, they help give you a clue that, okay, this may be Laurel Wilt and this would be a tree that is worth sampling to identify the pathogen.

So, when trees are in the early stages of Laurel Wilt they may express discolored foliage. For example, in sassafras when trees are infected and it’s the middle of the summer and that tree’s struggling to get water, you may notice that the leaves almost look like they should a few months later in the fall when they’re beginning to change color for autumn, yet it’s July. You’re just like, “oh, that’s, that’s too early to see those pretty pink and pink and orange colored leaves.” So, that can be an early symptom.

Another is that as those leaves fail to get enough water, they droop. Hence the name of the disease – Laurel Wilt. So, you got wilting foliage in the Coastal Plain with the broad leaved evergreen trees that we were talking about, like the Persea bays. Those trees tend to hold onto their wilted and discolored leaves for weeks or maybe even months.

In contrast more deciduous species, like sassafras and spicebush, when they become infected they may drop those wilted leaves relatively quickly compared to the bays. So, you may miss that wilting stage, and it’ll just look like, “oh, this tree just doesn’t have leaves.” Or a portion of this tree doesn’t have leaves.

Another thing you can see, especially in the spring when the tree’s starting to leaf out, is that the foliage may look yellowish or stunted. So, smaller leaves than you’re used to seeing. And sometimes when there’s a partial infection in the stem, like the disease has only affected part of the stem or crown so far, then that one branch will look kind of sick, yellow, small foliage.

But probably the most helpful symptom in the field that I find, as somebody who goes hunting and looking for diseased trees, is actually in the wood. So, you see a tree. It’s wilting or it’s missing its foliage. And it’s a sassafras or it’s a spicebush or a redbay. If you have a hatchet, you can go up and take some of the bark off of the tree and look at the surface of the sapwood, which is normally very light colored white or cream colored. When the fungus is in the tree, the tree has a reaction, that causes that sapwood to be discolored.

From the side angle, it’ll look like streaks of dark blue or black or brown discoloration against what normally would be a creamy, healthy color. Or if you have cut the tree, either the main trunk or a branch. Let’s say you have a wilted branch and you cut off the wilted branch and you look at the cross section of the cut portion, it’ll look like a darkened ring or maybe even multiple rings near the outside of the radius that are dark compared to the light color of the wood. That material actually is the best material to sample if you’re going to get a laboratory confirmation.

So discolored leaves, wilting leaves, dead branches, discolored sapwood. All of those are general symptoms that don’t tell you for sure, but if you’re looking at a Laurel Species tree in the range of Laurel Wilt, those symptoms will tell you, “Hey, this could be, this could be it.”

First Step If You Think You Might Have Found an Infected Tree

Shannon: So, let’s say a listener’s going out in their woods and they see something like this, or like you said, they’ve got a big sassafras in their front yard or their backyard. It looks like it’s not doing so good, so they chop it off, whatever, and they see some of this stuff and they go “uh-oh.” What do they do?

Bud: One thing I would do is I would call somebody local, either your county extension or your local forestry office, or maybe your local Department of Agriculture. There’s a plant industry division in your state. Folks like Alexandra Blevins and her office that you had on the podcast earlier. And have somebody come out and take a look and maybe even take a sample because, as we all know, trees die for multiple reasons.

That was one of the things that was so evident to me as a young scientist in Florida trying to figure out, “Where’s Laurel Wilt?” “How fast is it spreading?” is that you get calls all the time and you go out and look at it expecting it to be Laurel Wilt and then you realize, “oh no, this is something else.”

So, it helps to have somebody with some experience. Even if they don’t have experience, they can send in a sample to a lab that is set up to identify the pathogen and take closer look.

Challenges in Managing Laurel Wilt Disease

Bud: In terms of what you can do, it’s, I tell you, it’s really tough. This has been a tough disease to manage. And there’s some reasons for that.

One is the beetle vector is incredibly tiny, incredibly mobile, and very efficient at getting that pathogen into the tree. You know, the tree doesn’t have to be wounded or injured. It’s just healthy doing its thing. And the beetle can still get the pathogen in there. A single beetle is carrying enough spore load to kill a tree.

There does seem to be some resistance in both sassafras and redbay populations. You know, not every tree that gets the – I shouldn’t say necessarily resistance, but at least tolerance – not every tree that gets infected is doomed to die within a year. Many do, but some don’t. So, there may be some resistance or natural tolerance in the population out there, but even just a little bit of fungus carried by one beetle is enough to bring down, let’s say a big redbay tree or a big sassafras tree. It’s not like you need an overwhelming number of these insects making holes in the tree to kill it. So that makes it tough to manage.

For vascular wilt diseases in general, in concept you could use strategies that have been used against other wilt diseases like Dutch Elm disease. Dutch Elm disease has a very similar progression where you have bark beetles spreading a fungus onto healthy trees. The little beetles are feeding in the little branches. The tree wilts, the beetles breed in the tree.

American Elm was an extremely valuable street tree, shade tree in cities and parks and communities with a big beautiful umbrella, arching crowns. People really did not want to lose their elms. And so, you had public support for a community effort for things like sanitation. So, “Hey, we see an infected tree. Let’s go cut it down. Let’s chip it up, let’s burn it. Let’s process that wood and reduce the number of beetles on the landscape to help protect the trees we still have.”

It’s hard to do that when the tree is a sassafras or a redbay, just because they aren’t this big beautiful street tree that’s shading my house. So, in theory, could you use sanitation? You probably could. It would take a community wide effort that probably most people aren’t going to do. And the beetles are so tiny and so few are needed to infect a tree.

There’s been some work done as, as there has been done for Dutch elm disease, to look at infusing fungicides into trees in order to prevent spread of the fungus and infection of the fungus in the tree. It’s what you would call a preventive measure. So, you would need to treat a tree before it has symptoms. Get the fungicide in there.

The way it’s typically done is you drill holes in the root flare of the tree and infuse a volume of fungicide and water into the tree. So, the tree’s taking it up just like it would take up water from the roots and distribute the fungicide through the tree. It has been demonstrated that you can protect a tree for about 12 months, both for Redbay and sassafras by infusing a fungicide in that manner.

But the fungicide doesn’t move into new wood as the tree is growing. So, after you’ve treated for one year, you’re not guaranteed to have protection from that same treatment for multiple years. And do you really want go drilling holes in the base of your tree every year and paying for it? Most people don’t.

So, could it be an option for a really high value, big tree that you’re going to do this and you’re going to practice sanitation on all the other little trees around it? Maybe. But, again, it’s not an area wide type of solution and not for a forest level stand.

So it’s tough. It’s tough. That’s why I was saying, I think we need some more research into other treatments, like maybe prescribed fire in natural areas.

One of the interesting things about the beetle… I’m sorry, I’m rambling a lot here, but I keep thinking of things.

Shannon: Oh, you’re actually covering a lot of what I was going to ask.

Bud: Oh, good. Okay.

One of the things that we found out about the redbay ambrosia beetle is that it has a preference visually for larger diameter stems. And so, it doesn’t necessarily mean that smaller stems are more resistant to the pathogen, but smaller stems are not as attractive to the redbay ambrosia beetle in flight as a bigger stem.

So, reducing the prevalence of large diameter trees on the landscape could help you reduce the number of beetles in theory, and yet maintain a presence of the tree. So, maybe you’re not going to have 24 inch, 12 inch sassafras trees anymore. But could you still have small diameter sassafras and redbay and keep that as an ecological component on the landscape with stems that are not as big and visually attractive to the insect?

Ecological and Cultural Impacts of Laurel Wilt Disease

Shannon: Would it still serve the same purpose in the ecological landscape, though? If you take out all the bigger trees.

Bud: Not necessarily. It won’t be the same. It may be able to function for certain organisms, for example. As I think about ecological impacts, and this might be one of the areas you wanted to talk about, one of the concerns with Laurel Wilt is reductions in biodiversity.

The Laurel family has insects that are very specialized on Laurel family plants. For example, there’s a butterfly species called the Palamedes Swallowtail. That butterfly uses only Laurel family plants like sassafras and redbay as its larval host. The Caterpillars only eat Laurel family leaves.

In the Coastal Plain on redbay trees, there’s a little Hemipteran insect called a redbay psyllid. One of those kinds of sap feeding type of insects that creates a gall on the edge of redbay leaves. And in fact, that’s a great way to identify. redbay is, does it have these little gulls on the leaves because the redbay psyllid is so common. That insect is completely dependent on Persea, on that genus of bays.

A graduate student at the University of Tennessee recently, Matthew Longmire, was looking at what are the insect associates of northern spicebush. And he found several species, some of which were curiously insects that build leaf shelters with spicebush leaves, that are very specific to spicebush.

Even the natural enemies of those insects may be at risk because you have little wasps and things that are the enemies of the herbivores that eat the spicebush. Those are insects that are at risk as we lose more of our sassafras or spicebush populations.

Now are those Laurel species going to go extinct? I don’t know. It doesn’t feel like it. As you look at the landscape where the disease has been a long time, there’s still small diameter plants out there. Even of redbay in areas that have been hit hard on the coast. But, there’s still concern that as the populations of that host are reduced dramatically on the landscape, that we also are at risk of losing biodiversity of organisms that are dependent on those plants.

Shannon: Right, and that’s one of the things that I think is so important to bring up, and really kind of discouraging in in some ways and very concerning with this disease is that we don’t have any of these landscape level ecological management options that we know of, that we’ve found, yet that could potentially control this.

And then it’s like you said, all these ecological impacts of, okay, if we start losing these larger diameter trees, or if even we start to lose a majority of the smaller diameter, plants in these families, what’s that going to do? What’s the ripple effect?

Bud: Right.

Shannon: Because there are so many, like you said, so many specialized insects. I mean the spicebush swallowtail butterflies. I mean, that’s the one that’s really big here because we’ve got spicebush and sassafras.

So, yeah. What’s those ripple effects? And those long-term ecological impacts going to be? And it’s so hard. So hard to calculate them or to even predict.

Bud: Yeah. Hard to calculate either their amount or their value. It’s not something that’s easy to put a dollar value on if you’re trying to justify it for some funding to work on it or to manage it. It’s hard to do.

The same is true of the cultural impacts. The plants in the Lauraceae – one of the reasons that they’re so beloved is that they’re rich in essential oils. So, they’re very aromatic and they have culinary uses. You know, you may have had bay leaves in your gumbo or the cinnamon plant is in that same family, avocado as we talked about. So, if you like guacamole with your chips… So, there’s culinary uses.

There’s ceremonial uses. Certain Native American tribes, like the Seminole Indians in Florida or the Cherokee Indians, Laurel family plants are part of their sacred plants and ceremonial plants. And so, losses to those uses get easily overlooked, but they’re very important, even if you can’t put a dollar value on it.

Shannon: Yes. But I do find it encouraging that you said that even down in Florida and on the coast where it’s been for so long, I mean it’s been 20 something years at least that it’s been there, we’re not losing everything and there seems to be some tolerance.

That doesn’t minimize the importance of studying and learning more, trying to find hopefully, ways to manage this on a landscape, ecological scale. but it gives us a little bit of hope that yeah, there’s at least possibilities here.

Bud: Some possibilities. Yeah. When I was in Florida, there was a graduate student there, Marc Hughes, who did his PhD at the University of Florida. And he was one of the first people out looking for resistant genotypes of redbay.

So, he would go to areas where the disease had gone through and killed the vast majority of redbays in the area. And usually all that’s left are these small diameter trees. The question was are they left just because the beetle missed them or are they actually resistant to the pathogen? And so, Marc went and took cuttings of those trees and figured out a way to root them and grow them, and grew them up to large sized plants, and then challenged them with the pathogen.

And he was successful in finding some genotypes of redbay that were resistant to Laurel Wilt. And so, that was one of those little silver linings of, of hope.

A similar effort has been initiated for sassafras. It isn’t quite as far along, but I think those are part of the options of moving forward for managing the disease.

Possible Reservoir Hosts for the Fungus

Shannon: Has there been any research or anybody looking at whether there are, reservoir species for the fungus?

Because, like with chestnut blight, some of the oaks can be a reservoir for the fungus without the chestnuts themselves. So, if a chestnut started to come back from the roots, because with American chestnut they can still resprout from the roots sometimes, the fungus would still be around.

Is that something that we have to worry about? Or do we even know yet? I mean, that’s one of those kind of deeper level questions we may not know yet.

Bud: It’s a good question. I don’t know the answer. I’m not aware of any research on it. I also am not aware of any species that are not in the Laurel family that have harbored the pathogen or the disease. But to say that it doesn’t exist, I don’t know. But that’s a good point. I’m glad you brought that up about other diseases.

The fact that this pathogen has been so family specific, both in its native range and here in the U.S., I guess I would lean towards thinking maybe there aren’t other plant families that would harbor it. But, um, yeah, I don’t know. It’s a great question.

Recent Monitoring and Research Efforts

Shannon: So, we’ve kind of edged up on this a few times, but can you quickly tell us about the monitoring work that is going on now?

Bud: So as, as we said before, there’s an awareness of the disease in forestry and management agencies throughout the southeast. And now as the disease is kind of pushing up into the central hardwood region, people keeping an eye out for it and contributing to that regional map of where it is.

There’s been a lot of work on the chemical ecology, we call it, of the beetles. What is the beetle attracted to and can we develop lures for it or traps? Is it possible to trap it out? Could you trap so many beetles that you could reduce the population? Unfortunately, the data suggests that’s not been a good strategy or a potential strategy, at least not in forests.

There’s been a lot of work for the avocado industry looking at can we do things like sever root transmission between trees? Can we try to trap beetles over here. Maybe we can even use some chemicals that repel the beetles. There’s been some work looking at a compound called Verbenone that appears to have some repellency, not only to the redbay, ambrosia beetle, but just in general it tends to be a compound that repels beetles. And so kind of like this integrated pest management system within an avocado orchard.

I think you have many more limitations whenever it’s something on a forest landscape and the trees aren’t predictably arranged on the landscape and maybe even go unnoticed. But the lures that are available for redbay ambrosia beetle, and again there’s been a lot of good work by entomologists and chemical ecologists looking at those attractants, there isn’t what we call a pheromone for the beetle.

A pheromone is a compound that’s produced by the insect that communicates messages to members of its own species. So, an aggregation pheromone is a great chemical to have if you want to lure in large numbers of insects because the insects think, “oh, that’s someone from my own species producing that and saying, come on over.”

The redbay ambrosia beetle doesn’t have that. And so, the lures that are developed for it are primarily host based compounds – compounds from the plant. And while those can be attractive, they don’t seem to be attracting beetles from long distances. It may be on the order of just a few feet or meters away.

That helps that insect that are navigating through the landscape and as it gets close to a tree helps it decide, “all right, I’m going to land on this because it smells like a sassafras.” But in terms of pulling insects in from long distances, no. So, the traps are not particularly a good early detection tool whenever you don’t know that the problem is already in the area.

Can they be used to monitor populations of the insect once it’s in an area? Yes, you can. But, if it’s an area where you’re not even sure if you have symptoms, putting out a trap isn’t necessarily going to pull in the few beetles that might be there and the trees just haven’t shown symptoms yet. So, that’s why a lot of the monitoring is symptom based rather than beetle trapping based. If that makes sense.

Shannon: Yeah, it makes a lot of sense and also kind of tells me that it’s really important for any of us normal, everyday citizens who really care about things if we see symptoms, whether we’re hiking or on our own property or something like that, to let people know.

Bud: Let people know. That’s right.

Shannon: So that we can help monitor and determine what the spread really is. And if it’s not in a county, is it not in the county because it hasn’t been reported? Or is it really not in the county – doesn’t exist there yet?

Bud: Absolutely. Yeah. And make use of those resources in your county and in your state. It’s your tax dollars at work. I used to work for the state of Florida, and that’s part of our job is to go and investigate things that could be pest problems for our forest resources. So yeah, give your county extension or your county forester, your local Department of Agriculture a call.

Future Directions

Shannon: As you look at where we’re at with what we know about Laurel Wilt Disease now, the research that’s been going on and is going on, what are you most excited about or what do you think is most interesting?

Bud: Hmm. This is good. I don’t know a lot about it, but as I said before I’m excited about the potential for other silvicultural tools. Not just sanitation, but are there ways in which we could manipulate stand composition of trees, such as something through prescribed fire that might, change the forest environment or change the structure of the host and the forest in a way that that decreases the impact?

Because I think it was recognized pretty early on that this isn’t the type of pest problem that you’re going to eradicate. We’re not going to remove Laurel Wilt from North American forests. We’re going to have to learn how to live with it. And so, we need to find tools that reduce the impact, because we’re going to have some impact, but how can we reduce it? I think that shows promise.

I think the fact that there is evidence of trees that don’t die as quickly or have some tolerance means that there is some component of the population that may have genetic resistance to the tree that could be capitalized on. Those types of programs require a lot of funding and research focus to bring to fruition. I mean, if you think about the example of the American chestnut, we still are not out planting resistant American chestnuts in our forests. And, we’ve been working on that, we the people, have been working on that for a long time.

It’s a complicated process. Sometimes resistance in plants, and usually resistance plants, it’s not due to just one gene. It’s multiple genes that are playing into it. Sometimes trees are surviving on the landscape, not because they are resistant to the pathogen, but because they have somehow escaped it. So figuring that out, is tough.

Sometimes if you are breeding for resistance to one pathogen or problem, you may not be breeding for resistance to something else that ends up being a problem later. So, it’s a challenge. It’s an uphill, long-term battle. But I think it’s encouraging that there is some evidence of resistance to Laurel Wilt in some of our native species. And so, that’s good.

I think there’s been some really cool and creative management and monitoring things that were done in Florida in the avocado industry. One of them was using dogs to identify the pathogen before symptoms show up in the avocado trees.

So, we’re probably all familiar with the fact that law enforcement has used dogs to detect drugs or bombs or even missing persons. You know, you train a dog on an odor and then send them out to find that odor.

This was done to identify the Laurel Wilt pathogen in infected trees and avocado orchards before any symptoms were expressed. And the dogs were incredibly good at it. I mean like 95 plus percent of the time they would find the infected tree. So, that’s really neat. Are there ways in which we could utilize that type of technology, animal technology, in forests that could be explored?

Also, you know, there’s more and more being done with remote sensing. Can we remotely sense where the pathogen is either from satellite imagery or a low altitude aerial imagery with species that are small on the landscape, like a sassafras or a spicebush? That can become hard because of the canopy coverage, but there might be more that could be done in that in terms of detecting it.

So yeah, I think those are some of the encouraging things or maybe where there’s some hope for progress.

Shannon: Yeah, it sounds like there are a lot of really interesting things going on, especially thinking about some of the other things that we’ve talked about
today.

Final Thoughts

Shannon: This has been really helpful and really educational. Interesting for me at least. And I hope for a lot of our other listeners as well.

If there’s one thing and one thing only that our listeners remember from this whole conversation, what would you want that to be?

Bud: I think it’s that Laurel Wilt is but one example of many non-native invasive species problems that we’re dealing with that are causing ecological and economic and cultural damage, aesthetic damage in our forests. And there are things that we can do to minimize those impacts. I think that’s what I’d say.

And, you know, maybe the following is more my opinion. This isn’t “Bud the data scientist” with evidence for this, but non-native invasive species problems are a symptom of our global economy. And the way we do business and the way we get our goods. And I think as a society, we’re going to have to ask the question, “Am I okay with the status quo of the way I get my stuff? And the consequences that come with a global economy with non-native invasive species? Or, does something need to change?”

And, I think that’s something that as a society, we need to think about.

Shannon: And it’s not a simple question either. Or a simple answer.

Bud: It’s not a simple answer by any means.

Resources

Shannon: Thank you so much and I will have a link in the show notes, like I said, to the map and some other resources for learning more about Laurel Wilt Disease because this is an ongoing issue and question.

Bud: Thank you, Shannon. I really enjoyed it and all the best to you with this backyard ecology effort. I think it’s great and I’m glad you have a faithful following of listeners. I’m glad I could be a part of it.

Shannon: Yes. Well, thank you so much and talk to you later.

Bud: Thanks. Bye-bye.

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