Balance & Barrier

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In this Episode meet

Episode 3: Slowing the Gypsy Moth's Spread

Produced and hosted by Jonathan Yales - 18 min.

Research Entomologist Sandy Liebhold Professor David Smitley

Insect biological control comes in all shapes and sizes — parasitoids, predators, or pathogens. So, what happens when neither a parasitoid nor a predator are feasible? Well, sometimes we have to turn away from using insects to attack insect pests, and turn to using an even smaller organism, something microscopic: a pathogen — in this case a fungus.

Jonathan Yales
A biological invasion is an enormous increase in population of some kind of living organism. It happens when an organism — like an insect — arrives somewhere beyond its previous range, when it breaks out past its natural barrier, unbalancing the biological order. More than 450 nonnative insects have invaded our forests and urban trees since European settlement. In this series, we'll explore four of these insects, and the scientists studying and combating these pests. In 1957, a British ecologist, Charles S. Elton, gave three radio presentations entitled “Balance and Barrier.” Within a year, he had expanded these ideas into what was to become a bible for practitioners of a burgeoning new science: invasion biology. In a tribute to those broadcasts, this six-part series will explore biological invasions — and their repercussions — in the Midwest and the Northeast.
Jonathan Yales
This is “Balance & Barrier,” Part 1, I’m Jon Yales. On today’s episode: biological invasions. Think of an explosion: a volcano erupting, a popcorn kernel popping, a nuclear bomb’s mushroom cloud rising. Now, think of those explosions, but if I took away their sound. And then, what if I slowed the speed of their explosion way down, to where it’s barely moving: an explosion in silent, slow motion? That wouldn’t be much of an explosion, would it? In our world, slow explosions like these do exist. They’re just a lot harder to see or hear than the glowing volcano, and they take a lot longer to explode than the fraction of a second a nuclear bomb or a popcorn kernel takes. And they’ve been happening all around you for hundreds of years, taking decades, or even centuries, to play out. What are these decade-long, slow and silent explosions? And what do they have to do with forest insects? Welcome to Part 1, where we’ll find out. But first, let’s meet the boss — my boss — Therese. She’s the leader of a special group of scientists — a crew of invasive species experts — who you’ll meet throughout this special six-part podcast.
Therese Poland
The Research Station is divided into research work units, which are more or less discipline oriented research teams that work on specific problems. And, our research work unit is the Ecology and Management of Invasive Species and Forest Ecosystems unit, and we have scientists located in, essentially, four locations: here in East Lansing, Michigan; in Morgantown, West Virginia; and then in Hamden, Connecticut and Ansonia, Connecticut. We have nine research scientists: research entomologists here in East Lansing; and then in Hamden and Ansonia, we have two research entomologists and a research ecologist; and down in Morgantown, West Virginia, we have a research entomologist, a soil scientist, a botanist, and silviculturist. So, we’re kind of a multidisciplinary unit and we have multiple locations.
Jonathan Yales
The Northern Research Station is one of five research stations. It’s a grouping of dozens of different research laboratories spread across 20 different states in the Midwest and Northeast. Along with the other four research stations, this group makes up what is the largest forest research organization in the world.
Therese Poland
The Forest Service has three different main branches: the National Forest Systems, State and Private Forestry, and Research and Development. So, Research and Development is divided into five research stations that are regionally based: there’s the Northern Research Station, Southern Research Station, Rocky Mountain, and then, Pacific Northwest and Pacific Southwest. And, the Research Stations kind of coordinate research that’s of regional importance, but then also work across regions to address national priorities and even international priorities. So, the Northern Research Station has some of the most populated states and largest cities in the country — the most people. We also have the most fresh water and water resources with the Great Lakes. And, we also have a lot of ports that bring in imports from other countries. So, we have the most introductions and establishment of invasive species in the country, in our region.
Jonathan Yales
So, I’m going to be doing this podcast on these non-native forest pest insects, and I kind of need a general concept of what are biological invasions, in general. Who should I talk to in our unit?
Therese Poland
Andrew Liebhold — who goes by, ‘Sandy’ — is one of our senior research scientists in our unit. He’s a research entomologist, and he specializes on invasion ecology, really looking at the whole process of invasion: the stages from early arrival, to establishment, and spread. He’s one of the world leaders in this area. So, he’d be an excellent person to talk to to find out more about that.
Jonathan Yales
Before we talk to Sandy, let’s get back to those slow explosions. What we’re talking about today — and all series long — are ecological explosions, which are the enormous increase in some kind of living organism. It can be a virus, a plant, a fish, or an insect. They’re slow motion wildfires — not of flames, but of organisms — slowly spreading across the landscape. Scientists call them: ‘biological invasions.’ And they happen when an organism arrives somewhere beyond its previous range, when it breaks out past its natural barrier, unbalancing the biological order.
Sandy Liebhold
I think [in] most of the world people are increasingly recognizing biological invasions as a major environmental problem, but the timeline kind of has differed in different parts of the world.
Jonathan Yales
This is Sandy.
Sandy Liebhold
But, really if you go back far enough, certainly, prior to 1900, there was virtually no recognition that there is potentially any problem with invasions — except for a very few farsighted people, like, Charles Darwin. He was perhaps maybe one of the first people recognize the whole phenomenon of biological invasions, and to recognize that non-native species sometimes behave very different — ecologically — than native species.
Jonathan Yales
Because no one really knew much about the impacts of biological invasions back then, we did things that we thought — and at the time — were quite good for nature, yet, looking back they were a bit absurd.
Sandy Liebhold
Prior to the 1900[s], there were some interesting things going on. There was actually a sort of phase in history where there were these organizations called ‘acclimatization societies,’ and these were people that actually loved nature, but they actually thought that one of the best things you could do to help nature was to add more species. And they would actually go around the world moving species from one part of the world to another thinking that that would make it a better place. And, you may not realize, when you move a plant from one part of the world to another you may be inadvertently moving insects and diseases. And so, it was really a few... in the early 1900's there were a few foresighted entomologists and plant pathologists in the USDA that really promoted the idea that we should have some regulation of plant imports because this is a dangerous thing that was going on and it was having an adverse impact on U.S. agriculture and forestry. But finally, in 1912, the U.S. Congress passed the Plant Quarantine Act, and this is the legislation that really is still in place today.
Jonathan Yales
But, in the last 100 years, it’s not only by moving plants that we’ve inadvertently moved insects.
Sandy Liebhold
The pathway that is probably the most important for newly introduced forest insects and diseases is wood. And that wood can be either logs that are moved around the world or, more importantly, wood packaging material and this is one of the things…
Jonathan Yales
And, what is wood packaging material? What’s an example?
Sandy Liebhold
Most shipping containers, you know, even if we're talking about a container that's filled with television sets, those boxes of television sets are sitting on a wooden pallet. And pallets are made from low quality wood, because you don't really need to have either a very beautiful wood or extremely strong wood. So, unfortunately this low quality wood, very often, is from the outer part of a tree and, very often, contains insects and diseases. And then there's finally a category that we refer to as ‘hitchhiking,’ and this is simply the phenomenon by which insects can become associated with things other than what they eat. So, like, for example, the outside of a shipping container, we know that insects like the gypsy moth will sometimes lay eggs on the outside of a shipping container.
Jonathan Yales
In the United States since European settlement, more than 450 non-native insects have colonized the trees in our forests and our cities — 2-3 new ones every single year. But, scientists like Sandy, didn’t start studying these invasions until the 1960s.
Sandy Liebhold
The scientific study of invasion biology is something that people didn't really become that interested in until… actually, the first person that we credit with sort of starting the field of invasion biology is Charles Elton, who was a British ecologist, and he published a book [“The Ecology of Invasions by Animals and Plants”] in 1958, that really started it. Since then the scientific research on biological invasions has just exploded, and so now, invasion ecology is really one of the most active areas of applied ecology. And in my field, when I went to graduate student school 30-35 years ago, very few people were actually studying biological invasions, but now, the field of forest entomology is dominated by people studying biological invasions. And a lot of that…
Jonathan Yales
What were they studying thirty years ago? Like, what was the majority of the focus?
Sandy Liebhold
We were mostly… until relatively recently, most of the problematic forest insects and diseases in the U.S., and elsewhere in the world, were native species. So, to be honest, most of those problems have sort of been eclipsed by the impacts of non-native species. And it's an interesting thing, even though, you know, these invasions have been going on for a long time, very often, the impacts take a long time to really be felt. It may take very long for that species either to grow to a level such that causes damage, or in other cases, there are situations where it takes them a long time to spread into part of the country where their hosts are very common.
Jonathan Yales
But, that doesn’t mean that every single insect that’s brought here turns into a problem.
Sandy Liebhold
We know that there are many, many more insects and plant pathogens that are moved around the world then actually establish. In fact, we know it's actually a tiny fraction of the species that are introduced to the United States that establish. And this is because, very often, these species may be introduced to a region where there is no plant for them to feed on. Certain species of insects may specialize on certain plant species that just simply aren't present in some parts of the world. But, another really important factor that comes into play is climate, that some species, if they're moved from say a tropical region and introduced to a temperate region, the climate just may be unsuitable for the successful establishment and reproduction of those insect species and they may fail to establish.
Jonathan Yales
But, once they’re established, what plays into them being able to spread like crazy?
Sandy Liebhold
To be honest, it's actually really difficult sometimes to predict why certain pests become problems and others do not. And, the one of the one reason is what we call ‘enemy release,’ that most organisms in their native range there are a host of other organisms that eat them, things like predators and parasites. And we know that for many organisms those species keep their prey populations in check, and so when you move a species from one part of the world to a new part of the world where all of a sudden they're not exposed to all these predators and other organisms that eat them, they're able to grow at phenomenally high rates and cause lots of damage. The other main mechanism or explanation for why, is the phenomenon of tree or host resistance. That, typically, throughout evolution there's many insects, and the plants they feed on, have gone through millions of years of evolution in something of what you could call sort of an arms race that, that maybe they're insects feeding on a certain plant, but that plant may evolve a mechanism of preventing that feeding either through the production of chemicals that are toxic to that insect or physical properties that prevent the insect from feeding on it. And so, instead of having, you know, thousands or millions of years of evolutionary history over which they build up resistance. They may have zero resistance to that insect, and consequently that insect may be able to, very easily, conquer the mechanisms that that tree normally used to prevent feeding, and consequently, the insect may be able to cause a lot of damage to kill a lot of trees and reproduce quite quickly.
Jonathan Yales
And, if you’re in the Midwest or Northeast like we are, you actually happen to be in the heart of the country’s hotspot for these types of invasions.
Sandy Liebhold
When we count up the number of these species that occur in different regions it sort of stands out that the Northeast has many more than other parts of the United States. And why that is, we don't know, to be honest completely. But, the factors that we believe are at least partially responsible for that are sort of twofold: one is, there's simply a much longer history of people — Europeans — living in the northeastern United States then, say for example, the western United States. And because there's a longer history of people there's a longer and greater opportunity for these accidental movement of life stages. But there's probably another important factor, which is maybe slightly less obvious, and that's that forests in the eastern United States are actually more diverse than forests in the western U.S. We actually have many more species of forest trees, many more genera of forest trees, and from a evolutionary or biogeographical standpoint, it basically creates a bigger target for establishment of non-native insects and plant pathogens.
Jonathan Yales
Now, what does all this damage Sandy’s talking about actually look like? Like, are these little insects just killing a couple branches on a tree? Or are they killing whole trees? Or whole forests? And does it cost anyone any money, or are these trees just happily dying and decomposing in some far away forest? We need an economist — a special type of economist — to wrap things up.
Bob Haight
So, I'm a forest economist, and I've applied my economics toolkit to a lot of different issues.
Jonathan Yales
This is Bob, a research forester in Minnesota.
Bob Haight
I became aware of invasive species about 15 years ago here in St. Paul because… actually because
Sandy Liebhold
introduced me to the problem and invited me to participate in a workshop sponsored by the National Center for Ecological Analysis and Synthesis in Santa Barbara, and at that workshop we were trying to estimate what the cost of the economic damage associated with non-native forest insects and diseases in the United States.
Jonathan Yales
I called Bob, to revisit that work, and talk about modeling the economic impacts of non-native forest insects.
Bob Haight
What I love most is to build what I call bio-economic models, and they’re models of biological systems, and incorporated into that, are models of what people like, what they're willing to pay for, and [what they] value. And so, I combine both the biology and economics into these modeling systems.
Jonathan Yales
What's an example of, you said, ‘What people like,’ like, what's an example of, you know, what some of those chunks of the model would be that you're trying to suss out?
Bob Haight
Yeah… Yeah, so, like, if you think of a city... well like here in St.Paul, one of the things that we’re known for is a lot of trees — we’re a leafy city. And the emerald ash borer and ash trees are an important component of the urban forest here. And so, when the emerald ash borer was discovered, the city foresters and people in general, recognized that it's a threat to kill most of the ash trees here, and the ash trees are, you know, 20-30% of the tree cover. So, what we tried to do was estimate how much people are willing to pay for their trees. I look at people's behavior and try to estimate, from that behavior — like home buying behavior — what they'd be willing to pay for a home say in a leafy neighborhood versus a non-leafy neighborhood.
Jonathan Yales
And they were also able to estimate how much this insect damage would cost us. How much it would cost our neighborhood, our city, our state, our country.
Bob Haight
One of our goals was to try to estimate the economic damage associated with three different guilds — feeding guilds — of non-native forest insects.
Jonathan Yales
And, a guild is just a group of insects?
Bob Haight
Yeah, it's a group of insects that all have the same sort of feeding strategy. So, wood borers are insects that bore into the tree and lay their eggs and the larvae tend to kill the trees. And so, we estimated that — nationwide — that there'd be over a billion dollars a year in local government expenditures to control wood borers over the next decade, and that there would be almost.. there would be slightly less, around $800 million in lost residential property values each year. In addition to that, there's also losses associated with homeowners absorb the loss of sort of replanting some other tree along their street or in their yard. So, it's local governments and homeowners are the ones that bear the costs of non-native forest insects at this point.
Jonathan Yales
So, it’s you who pays the most. It’s not us — the federal government — it’s mostly your state or city, or you with your own cash, paying to remove, replace, or treat these infested trees. But, let’s break these damaging insects down a bit more. Maybe there’s a certain type of feeder that causes more damage than the next?
Bob Haight
This is the story we have now, is that the boars have done more damage than the other two feeding guilds, which would be sap feeders, like the hemlock woolly adelgid, and then the defoliators, like gypsy moth. The sap feeders, obviously, they kind of bore into leaves and eat the sap, and the defoliators actually eat the leaves. And so those two groups, at least so far, haven't cause much damage.
Jonathan Yales
But, like Sandy mentioned before, these non-native damaging insects are just a small minority of the insects flying around out there.
Bob Haight
So, if you look at non-native forest insects, there's about 450 known to be established in the U.S. Most of them do not cause detectable damage. I think Sandy, and the group of scientists that worked together at N.C.E.A.S., we estimated that about 60 of those 450 have been reported to cause noticeable impacts to live forest trees, and then a very, very small number are extremely damaging. We call them ‘poster pests’ because they're really mean critters, [they] are like the emerald ash borer, the hemlock woolly adelgid, gypsy moth is another one.
Jonathan Yales
So, 60 of the 450 have caused noticeable damage — that’s only about 14%. Keep that in mind all series long. And for the rest of this series, that’s what we’re going to be focusing on — that 14%. We’re going to go deep on four “poster” pests, one from each feeding guild. One sap feeder: the hemlock wooly adelgid. One foliage feeder: the gypsy moth. And two wood borers: the Asian longhorned beetle and the emerald ash borer. Coming up in Part 2? A wood borer: the emerald ash borer.
Leah Bauer
...I don't think anybody could have known how serious the problem was, and how many dead trees that were already in southeast Michigan, at the time that we were notified, which would have been...
Jonathan Yales
Thanks for listening! And see ya soon! This episode was produced and edited by me, Jon Yales. My editors at the Northern Research Station were Jane Hodgins, Sharon Hobrla, and Gina Jorgensen. Special thanks to Therese Poland, Sandy Liebhold, and Bob Haight of the Northern Research Station. And thanks to the Michigan State University Department of Entomology. and the MSU student radio station, Impact 89FM. If you’re interested in learning more about biological invasions, start with the book Sandy mentioned, “The Ecology of Invasions by Animals and Plants” by Charles Elton. And if you liked this podcast, please leave a review on Apple Podcasts to help grow the show. If you have any questions, we’re on Twitter at @USFS_NRS. And, this podcast is produced by the U.S. Forest Service. The Forest Service is an agency of the U.S. Department of Agriculture, which is an equal opportunity provider, employer, and lender. Thanks for listening.

gypsy moth (USDA APHIS PPQ , USDA APHIS PPQ, Bugwood.org)

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