Balance & Barrier

Subscribe to Forestcast

Subcribe to Apple Podcast Subcribe on Spotify

In this Episode meet

Episode 3: Slowing the Gypsy Moth's Spread

Produced and hosted by Jonathan Yales

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
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.

This is “Balance & Barrier,” Part 3. I’m Jon Yales. On today’s episode: the gypsy moth.There’s no denying when the gypsy moths have invaded. They aren’t as tricky to spot as the emerald ash borer was.
Sandy Liebhold
Very often, it's not just these live caterpillars crawling around that are unpleasant, but you have dead caterpillars. You have oftentimes have a lot of diseased caterpillars that are dropping from the forest canopy into your patio and it's just not a really pleasant thing to experience.
Jonathan Yales
We met Sandy in Part 1. He’s a research entomologist.
Sandy Liebhold
I love insects, and I've experienced a gypsy moth outbreak in my yard, and I have to admit, it wasn't fun.
Jonathan Yales
Gypsy moth was accidentally introduced to North America over 150 years ago, but has spread relatively slowly. Yet, when they’re in an outbreak, they seem to quickly eat every single leaf around.
Sandy Liebhold
Well, it's just millions of these caterpillars crawling everywhere, and it's not just forest trees they're defoliating, but they're defoliating, you know, your prized shade trees in your garden, fruit trees, for example, apples are one of the favored gypsy moth hosts. Even things like some conifers, so for example, Norway spruce or Colorado blue spruce, which are commonly planted conifers in landscapes, they’ll sometimes totally defoliate it. And in fact, most broadleaf trees can be defoliated at least in a single year and they'll put on a new flush of foliage and survive, whereas a conifer if it's totally defoliated, they almost always die. And so, it's not unusual in a landscape setting in someone’s yard, if they have intensive defoliation by the gyspy moth, it’s not unusual to have large numbers of the trees in their yards dying.
Jonathan Yales
And, why does that happen? How does the gypsy moth end up actually injuring and/or killing various species of trees?
Sandy Liebhold
Well, trees rely on leaves for photosynthesis and so when gypsy moths totally consumed the leaves, they obviously can't photosynthesize. Broadleaf trees, typically have energy reserves in their roots and so they're able to produce more leaves and in many cases recover from the defoliation. But, in severe gypsy moth outbreaks it's not unusual to get two or three or more years in a row. And when that happens, it simply depletes the resources that trees have.
Jonathan Yales
And, surprisingly, unlike most pest insects, we actually know exactly how this moth originally entered the country — down to the exact person.
Sandy Liebhold
In the case of the gyspy moth, we know there was a man, Etienne Leopold Trouvelot, who, was at the time accidentally introduced gypsy moth in 1868 or 1869. We actually don’t know which of those years it was.
Jonathan Yales
He lived in Medford, Massachusetts, which is a suburb of Boston.
Sandy Liebhold
He was the type of person that liked to rear any kind of insect that he could get his hands on, and so, apparently in his backyard he had hundreds of different species of insects that he was growing on the trees.
Jonathan Yales
And after a trip back to his native France, he returned home to Boston with some gypsy moth eggs. He was trying to cross gyspy moths with a silk-producing species to develop a strain resistant to a disease which had decimated the European silk industry. But, the moths had plans of their own.
Sandy Liebhold
Apparently, some of the gypsy moths got loose and he realized that they had escaped. More importantly, he realized that it potentially was a problem, because in France and elsewhere in Europe gypsy moth is a bit of a problem. And, he notified several entomologists in the area, but no one really cared about it. No one did anything about it. They just didn't think it would be a problem.
Jonathan Yales
But, give it some time.
Sandy Liebhold
It wasn't until about 10 or 11 years after that Trouvelot had this accident that the first outbreak started in his neighborhood and, as you can imagine, people were pretty upset, there were gypsy moths crawling around everywhere. And, Trouvelot, at that point, left the U.S. and went back to France.
Jonathan Yales
But, his gypsy moths, they stuck around and started to spread.
Sandy Liebhold
Well, up until the 1960s, the spread was really just something like... it spread in essentially concentric circles outside of Medford, outside of the Boston area. So, that meant it was, you know, expanding down into Pennsylvania, and up into Canada, into Quebec, and those areas. But, then in the late 1950s early 1960s, there was — a population was accidentally transported to the southern peninsula of Michigan and there was an effort to eradicate it, but that effort failed. And so, the second isolated population established in Michigan and so, somewhere in, well, maybe mid 1990s, the population in Michigan and the main population around Boston sort of merged and coalesced. So, now we have one big population that, in the U.S., it ranges from the northern Minnesota, through Wisconsin, down into Iowa, Illinois, Indiana, Ohio — I’d have to look at a map here, to make sure I don’t miss a state — but into West Virginia, Virginia and the very tip of... northeastern tip of North Carolina.
Jonathan Yales
In Michigan, in the 90s, Dave Smitley, an entomologist at Michigan State University, was dealing with that state’s recent outbreak.
Dave Smitley
So, we had about 800,000 acres defoliated in the early 1990s for a couple years. I mean, our forests the size of all the forests in the state of Massachusetts was defoliated by gypsy moth here.
Jonathan Yales
And, what was the plan back then for controlling it?
Dave Smitley
Well, there wasn't…
Jonathan Yales
There wasn’t?
Dave Smitley
Well, we didn't have a lot of choice, but the only thing we could do was an aerial spray program. So, we had a pretty active aerial spray program. It was all Bt in Michigan, which is the safest product you can spray, and — gosh — there were years there were 500,000 acres sprayed. It was pretty amazing.
Jonathan Yales
And what is Bt?
Dave Smitley
Bacillus thuringiensis, is what it stands for. It's a product made from a bacterium. It produces spores that are toxic to gypsy moth when they ingest them, but it only affects caterpillars, and nothing else. So, that's why it's safe to spray over forest blocks.
Jonathan Yales
A few years later, as Michigan was still spraying Bt, a mysterious fungal pathogen reappeared, out of the blue, in Massachusetts. It could be used as a biological control, along with the Bt.
Dave Smitley
It was a mystery at first. They had all this gypsy moth defoliation, and then, one spring — I think it was 1988 towards the end of June — they walked into a forest block and there was just thousands of dead caterpillars on the trunks of trees — practically coating the trunks of trees — and they were all lined up facing head down. And they didn't know what happened, so they took them into an insect pathologist and within a few days they determined it was a fungal pathogen of gypsy moth that had not been seen in the United States before.
Jonathan Yales
That fungus is Entomophaga maimaiga, and a strain of it had originally been introduced in the Boston area a few decades after Trouvelot’s accident. But, it never took hold, and then, it just disappeared for 80 years until mysteriously reappearing just in time for Dave and Leah [Bauer] — who you met in Part 2 — to introduce it into Michigan to slow their outbreaks.
Dave Smitley
When Leah and I heard about Entomophaga maimaiga being discovered, we were pretty interested and pretty excited about the possibility of it as a biocontrol. So, then the next year, when they followed up on it and they found all these dead caterpillars on the trunks of trees over there — where practically every caterpillar died — we immediately jumped into a biocontrol program so we could start introducing it into Michigan, as soon as possible.
Jonathan Yales
And, what did that on the ground work look like? How did you…
Dave Smitley
Yeah, well, it was kind of interesting to do. We set up research plots up near Grayling, Michigan in a big forest block with lots of oaks. And, we set up these plots — they were maybe 50 meters apart — and one plot we’d introduced the pathogen by... actually we actually injected caterpillars with the pathogen and then put them out, and then we had another treatment where we put some of the soil containing resting spores around the base of the trees, and then we had a third treatment as a control. So, we wanted to compare, o.k. what happens to the control plots to the plots that we put out the pathogen? And in the first year, the pathogen started to take off pretty fast and affected like 30% of the caterpillars. In the second year, it wiped out all the plots, including all the control plots and everything within about an eighth of a mile of our introduction plots. So, it spread so fast that we couldn’t get much data. For an experiment, when you wipe out the control plots, you don’t really have good data to show, because it just spread so fast. I mean, the most amazing thing was, we had a separate experiment where we just use one quart of soil from Massachusetts, put it around a single oak tree in a forest block — so, the block could be a mile long. One tree, little bit of soil around the base of one tree. Three years later, it had infected caterpillars within a quarter mile in each direction of the single tree. That's how fast it spread. So, we know it would have come to Michigan naturally, eventually, but I think we sped up the process by maybe five years by introducing it ourselves.
Jonathan Yales
And, how does it actually work? Why does it spread so fast?
Dave Smitley
Yeah, when the caterpillars become infected their entire body fills up with fungal tissue. The fungus just grows until you have a mushroom in the shape of a caterpillar. It's not really a caterpillar anymore. The younger caterpillars, when they die, the fungus sporelates — as long as there's enough moisture — and it produces about 500,000 spores. They rain down through the canopy of the tree and infect other caterpillars.
Jonathan Yales
And because this fungus spread so fast, the outbreaks in Michigan were able to be controlled within a few years.
Dave Smitley
By about 1994-1995, it pretty much spread throughout Michigan. So, if you look at the defoliation maps, we had heavy defoliation in 1990, '91, '92, into '93, by the time we got the 1995 and '96, it had dropped to very low levels — I mean, down to about 5% of what the defoliation level was before — and it stayed low ever since. I mean, we've had a couple little cycles where it pops up, we get defoliation of blocks, especially after a couple dry springs, but then it crashes again because the fungus spreads and kills all the caterpillars. So, I don't think we've even reached 10% or 20% of that level of defoliation we had in the early ‘90s again, because Entomophaga has been working all that time.
Jonathan Yales
So, it sounds like a pretty successful biocontrol?
Dave Smitley
Yeah. Even though the introduction was accidental, it has been one of the best biocontrol organisms of a defoliator that we have ever seen.
Jonathan Yales
So, the use of this fungal pathogen is one form of controlling the moths, but in general — nationwide — Sandy says there are three strategies for detecting and controlling gypsy moth across the U.S.
Sandy Liebhold
One is what’s called ‘suppression,’ and that refers to when you have a gypsy moth outbreak — which means essentially an epidemic — people can take measures to suppress populations, to reduce their populations below damaging levels. And, currently in the U.S., most of the suppression is done using aerial applications of bacillus thuringiensis. And so, the suppression really is targeting just reduction of populations. We know that in areas where gypsy moth is widely established, it’s virtually impossible to totally eliminate it. The other technique... well a second approach to managing gypsy moth is what we term ‘eradication.’ And eradication refers to completely eliminating a population, essentially forced extinction. And, the first gypsy moth eradication project — which actually we think was the first attempt ever to eradicate an insect anywhere in the world — was the attempt to eradicate gypsy moth in Medford, Massachusetts following Trovelot's accidental introduction. There's a third type of gypsy moth management which is… we term it ‘Slow the Spread,’ and this effort focuses along the expanding population front gypsy moth. And, the objective of this ‘Slow the Spread’ barrier project is not to eradicate, but it’s really just to slow the continuous invasion. And, it’s a program that’s been in place since 1999 and it's been quite successful at slowing gypsy moth spread. And basically the way that the program works is, essentially there’s a 100-kilometer band stretching all the way from the Canadian border down to the Atlantic coast and North Carolina, and along that that band there's a grid of pheromone traps that are placed about every two kilometers. Every year, these traps are deployed and the grid of traps is then used to detect isolated populations in this area along the expanding population front and when an isolated population is detected then it's in similar to eradication the next year, more traps are put out to limit the population.
Jonathan Yales
So, how many traps are we talking? Thousands? Hundreds of thousands?
Sandy Liebhold
In these uninfested states, it’s about a 100,000. So again, the big trapping program there’s about a 100,000 traps every year put out for eradication, and another hundred thousand put out every year for ‘Slow the Spread.’
Jonathan Yales
So, compared to a lot of other insects, gypsy moth is easy to eradicate because the traps are so effective at detecting populations and the powerful control methods that are available for eradicating the moths.
Sandy Liebhold
It's been tremendously successful. Before this ‘Slow the Spread’ population was in place, gypsy moths was expanding at about a little over 20 kilometers per year. And since the program has been in place, the rate of spread has been, on average, dropped well below 10 kilometers per year. So, we think that we've slowed the spread of the gypsy moth by somewhere between 50%-75%. And, sometimes people will say, ‘Well, why do you really want to slow the spread of the gypsy moth? Why not just let it spread and let populations take their course?’ And, the explanation here is, that there's actually great benefit in delaying the establishment of gypsy moths. And, the key thing here is, that once gypsy moth establishes in an area, it's not just a matter of having an initial outbreak and then it goes away, once it establishes in an area you have, essentially, recurrent outbreaks that come back over and over and over again and by postponing the starting of these rather unpleasant outbreaks, that brings tremendous benefits to residents and forest landowners in those areas.
Jonathan Yales
But, what do you do when a pest can’t be easily trapped like the gypsy moth, and no miracle pathogen appears out of thin air, and no parasitoids even exist to control your pest? On Part 4 of “Balance & Barrier,” you’ll find out. When we’ll meet: the hemlock woolly adelgid.
Nathan Havill
They have these long stylets that they insert down into the tree and they're feeding, not on the sap as you might think, but actually on the storage cells inside the tree, so they're sucking the nutrients out of those storage cells in the tree.
Jonathan Yales
Thanks for listening! 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 again to Sandy Liebhold of the Northern Research Station; and also to Dave Smitley of Michigan State University’s Department of Entomology; and to Jeremy and the MSU student radio station, Impact 89FM.

And, if you liked this podcast and want more, please leave us a review on Apple Podcasts. If you have any questions, we’re on Twitter at @USFS_NRS.

And, as always, 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.