Balance & Barrier

Episode 4: When A Single Clone Caused An Invasion

Produced and hosted by Jonathan Yales

Research Entomologist Nathan Havill looking at southern pine beetle specimens from a population genetics study. Photo by Andrea Gloria-Soria. Used with permission. Research Ecologist Talbot Trotter

With no parasitic wasps — like we have for emerald ash borer — and no miracle fungal pathogen — as with gypsy moth — our control options for the nonnative insect threatening Eastern hemlock forests, the hemlock woolly adelgid, have been tricky. Hemlock forests, and the hemlocks in your yard, are paying the price.

Jonathan Yales
You’re listening to “Balance & Barrier,” Part 4. I’m Jon Yales. On today’s episode: the hemlock woolly adelgid.
Nathan Havill
When you walk into a hemlock forest — a pure hemlock forest — it's really walking into another world. It's really, very quiet, kind of shaded, cool place. It's really kind of a magical forest. But they can get really huge. I mean, there's some out West that not quite rival the redwoods, but they grow right alongside the redwoods, and some of them are really giant trees.
It creates this unique habitat of cool, moist areas that certain species really kind of rely on. There's certain stream invertebrates that require that kind of cool habitat; certain trout require that cool habitat; there are certain birds that nest almost exclusively in hemlock trees. And, it's also very important for providing clean water, so hemlocks tend to grow places where the water table is high, and around streams. So, it's very important for filtering water as it goes into our water sources.
Jonathan Yales
In these hemlock forests, a pest called the hemlock woolly adelgid has been — literally — sucking the life out of hemlocks from northern Georgia all the way to Nova Scotia. With no parasitic wasps, like we had with emerald ash borer, and no miracle fungal pathogen, like with gypsy moth, our control options for these adelgids have been tricky. And, the hemlock forests — and the hemlocks in your yard — are paying the price.
Back in Part 1, we classified our insects into three categories. One, wood borers — like the emerald ash borer. Two, foliage feeders — like gypsy moth. And, three, sap feeders, which the hemlock woolly adelgid is… or isn’t. It’s classified as a sap feeder, but really, it’s not one.
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. And so, that causes discoloration, causes needle loss, yellowing of the needles, you get die back of the ends of the branches, and it has a lot of the symptoms of drought. So...
Jonathan Yales
This is Nathan. Yet another research entomologist here at the Station. He works from Hamden, Connecticut.
But, what the heck is an ‘adelgid?’ Let’s start there.
Jonathan Yales
What is that word adelgid? Like, what is an adelgid?
Nathan Havill
So, it's a group of insects, the adelgids, or Adelgidae, which is the name of the family, and they're closely related to aphids which are pests of roses and lettuce and things like that.
Jonathan Yales
They’re by far the smallest pest in this series. And it’s their size that’s one reason they’re a tough bug to control.
Nathan Havill
The largest ones only get to be about 2 millimeters — the largest adults, and the small ones, the crawlers — the first instars — are kind of like a speck of pepper. They’re very small.
Jonathan Yales
And, what's — obviously, every insect has its own life stages — what type of life stage does hemlock woolly adelgid have? Is it simple…?
Nathan Havill
Adelgids have very complex life cycles, so... They.... This is going to be complicated. I’m going to try to do the simplest version first, right? [laughs] ...
Jonathan Yales
You know, we haven’t gone deep — really deep — on a pest’s life cycle yet. Why not make it these adelgid?
Nathan Havill
So, after the adelgids hatch out of their eggs, they crawl around and find a good spot or they blow in the wind to another tree and then crawl around to find a good spot to settle. So, they settle at the base of needles on hemlock, usually on the underside of the branch — which makes them tricky to find. And they insert what's called their stylets — or their mouth parts — which are kind of like a long straw that they drive into the branch, and they begin sucking nutrients out of the tree. And then they have several, what we call, instars or various stages. So, they have four different nymphal instars, where they molt — they get a new, they cast off their old skin, they get a new skin — and they do that four times before they become an adult, and at that point they begin laying eggs, and they hatch, and they become crawlers and it starts all over again. So, that's the simple version.
And we have two generations here in eastern North America. So, that same process happens twice one generation that starts in early spring, and then lays its eggs, and then there's another generation that lays its eggs and then goes through what's called an estivation period in the summer where they're dormant as first instars. And they kind of wait until the fall when they start developing again. In its native range in Japan, it's a little more complex because a certain proportion of that second generation can develop wings, and they fly off looking for a different host. So, those winged adelgids, they can't survive on hemlock, they can only survive on, as far as we know, a couple of Asian spruce species called picea torano — or the tigertail spruce — where they land, those winged adelgids land, they lay eggs, and those eggs become males and females.
So what I didn't mention before is that all the other stages are purely asexual, they're all females. They don't need to mate. They can just develop to the adult stage and lay eggs. But, in Japan where they go to spruce, they have this one generation of males and females where they can mate and that female, the sexual female, lays a single egg. Which is a very strange strategy to put all your eggs in one basket, literally, all your egg in one basket. And so, that female becomes what we call the fundatrix, that settles at the base of a bud on this tigertail spruce tree and her feeding in the spring causes that bud to enlarge and become what we call a gall, which is an abnormal growth on a plant, that actually becomes a house for the adelgid. So, all the offspring of that fundatrix — and she has a lot of them, several hundred — they all crawl up inside this gall, as its developing, which look kind of like almost the size of a golf ball sometimes and it has little chambers inside of it and inside those chambers sealed off from predators and other dangers they’re inside feeding on this really highly nutritious tissue that's being formed by their mother in the previous generation. And so, at a certain time that gall dries up and opens up and more winged adelgids fly out of the gall and they look for hemlock, they come back to hemlock.
So, because there aren't any suitable spruce trees that they can survive on here in North America, we don't see that part of the life cycle, we only see the two asexual generations per year, and it's only the wingless ones that keep surviving on hemlock. We still get winged adelgid in the East and they fly off and they die. That's actually a very important source of mortality in the life cycle. If we didn't have those individuals flying off and dying there would be even more adelgids out there.
Jonathan Yales
I told you it was a lot. If you spaced out, that’s okay. Glad to have you back.
Now, let’s get to where these insects first arrived in the U.S., and where they originally came from.
Nathan Havill
The origin story of H.W.A. is not quite as precise as with some other invasive insects. A couple pieces of evidence that we do have is we know that the first record of H.W.A. in eastern North America — which is where it was introduced — is in Richmond, Virginia, and that was 1951. The other pieces of evidence that we have, and a lot of the work that I've done with hemlock woolly adelgid, is to look at its genetics. So, we can do kind of a forensic experiment to figure out what it's related to and how much genetic diversity there is within this invasive species. So, we were able to get DNA out of adelgids all throughout the introduced range in eastern North America, and also from western North America, and in China, and Taiwan, and Japan, and we can compare their DNA using various methods. And using those methods, we’re able to match the adelgid that's in eastern North America with adelgids in southern Japan.
Jonathan Yales
But, that’s not all they found.
Nathan Havill
And, we were also able to find out that there's a single clone of adelgids throughout the whole eastern North America — which is really remarkable, to have this devastating pest is actually a single genetic clone.
Jonathan Yales
Meaning what? That means it's just one… one insect came here, and because it reproduces asexually, it could be in a sense…
Nathan Havill
Yes, actually, one insect could have started this whole invasion in eastern North America. It's likely that more than one insect was introduced because many of the insects that are introduced probably die, but it seems like all the insects that are here are a single lineage from one female. So, they all are, genetically, mostly identical.
Jonathan Yales
And do we have an idea of how exactly those first insects did arrive?
Nathan Havill
It's likely to come on nursery stock. Around the turn of the century, and later, there was a trend of having Asian and Japanese gardens in the eastern United States, and so it's possible that somebody ordered a Japanese hemlock — which you could do back then, you could just order a Japanese hemlock and have it show up on a boat. And it could have been infested with hemlock woolly adelgid, and like we said, because they reproduce asexually, if just one of those adelgids had survived to reproduce — and a single adelgid female can lay up to 300 eggs — then that could have started the whole infestation.
Jonathan Yales
And where did things go from there? So, we have some set of woolly adelgids that have arrived from Japan on the East Coast. How did they spread?
Nathan Havill
Yeah, it was wasn't until about 1970 in Pennsylvania when people started noticing a problem on hemlocks. So, sometime between 1951, or before, until 1969/70, we didn't even know the hemlock woolly adelgid was here. So, then around 1970 people started noticing, around the 1980s it really started to become a problem in the mid-Atlantic states, and it got up to New England around that time too, and it really started killing a lot of trees. And after that, then it also started traveling south, where it's had even more devastation through the Appalachians and in the southern U.S.
Jonathan Yales
But, as Nathan mentioned earlier, these adelgid — the adults — are around the size of a sesame seed, and the young ones, they look like a speck of pepper. So, how the heck do you even find them on a tree? How did these people notice them? Fortunately, they do one thing that helps us spot them.
Nathan Havill
A lot of the monitoring for hemlock woolly adelgid involves just folks on the ground looking for it. So, there isn't a trap right now that will trap adelgids. So, it's really going out, physically looking for the insect, right now is the best way to do that. And so we just got to try to get people out there flipping over branches looking on the undersides, looking for the white woolly egg masses.
Jonathan Yales
When you flip over that branch, what does that look like to you?
Nathan Havill
It's a white kind of cottony balls that you would find on the underside of the branch. So, there are certain times of the year when we don't see that — when the adelgid is estivating and it looks just like little pieces of pepper on the branch — but when it is developing its producing these masses of wooly wax that look like little cotton balls that are pretty visible on the underside of branches. But, a tree...
Jonathan Yales
We’ll come back to Nathan. Someone who knows a lot about the underside of hemlock branches is, Talbot.
Talbot Trotter
See, that's actually... You're really taxing me asking me to organize a thought...”
Jonathan Yales
This is Talbot Trotter, our group’s research ecologist. He’s based in Connecticut, along with Nathan. The two of them are the best of friends.
Talbot Trotter
Can you do me one quick favor? When you send this out for people to review, when you send a copy to Nathan, can you splice in me saying, ‘That Nathan has no idea what he is doing, whatsoever. You know, he is just making things up as he goes along,’ just to see if he notices.
Jonathan Yales
And, yes, I did.
But, all series long, we’ve heard about what damage the pests do to the trees, but we haven't dug into what other organisms we lose when a host tree dies.
Talbot Trotter
If we lose the hemlocks, yeah, we lose lots and lots of other things that go along with the hemlocks. There's a huge community of arthropods — spiders, mites, bugs — that live on these trees. There are communities of fungi and mycorrhizae that live in association with these trees down in the soils. There are soil invertebrates, huge communities of soil invertebrates, living in the duff that's underneath these trees. And, we know that they're there. We know surprisingly little about them.
Jonathan Yales
There have been a number of studies done over the years to try and quantify what spends time on a hemlock, and which of those species might be at risk as we lose hemlocks. How many species do you think fall off a hemlock branch if you shake it? Twenty? Thirty? A hundred? Three hundred?
Talbot Trotter
We've been doing surveys of just the arthropods that are just in the canopy. So, these are if you go out and you shake a branch on a tree — this is who falls off — and what we have found is that we have somewhere on the order of around 850 species of critters that come off of these trees, which is a huge community. Now, that doesn't mean that all of these species are associated with the tree. Some of them are what we would call ‘tourists,’ they just happen to have landed there, or have been walking across that branch when we shook it. But, some of them are very closely tied to the hemlock.
Jonathan Yales
Now, how many of those 850 do you think are hemlock-specific?
Talbot Trotter
And, when we go through that exercise, we end up with about 400 species that show up just on the hemlock — we haven't yet detected them anywhere else. Now, whether or not these are all absolutely dependent on the hemlock, we don't know that yet, but it's reasonable to assume that if you remove the hemlock from the system, that a huge chunk of that community is going to disappear. So, you know, we think about losing the hemlock and losing this one species, when in reality, we're talking about losing hundreds of species that are associated with it. Many of those species, we don't know that they're there, we might never know that they ever were there, and so, we're trying to document as much of that as we can.
Jonathan Yales
Now, back to the trees. Currently, H.W.A. occupies over half of the eastern hemlock’s natural range — which is most of the Appalachian region — and it’s expanded northward at a pace that exceeds predictions. There’s two ways to look at how these hemlock forests are doing: one, how they’re doing right now?
Talbot Trotter
For right now, what we know is that the further south you go the more significant the impact by the adelgid becomes. It's not uncommon to find large, mature growth stands of hemlock where virtually every hemlock is dead, and that down South can happen in just a matter of say three or four years, it's a very fast process. The further north you go, the slower the adelgid grows, and we know that the adelgid is sensitive to cold winters, and as that becomes more and more of a factor, the trees get a little bit of a leg up, they kind of get a little bit of a reprieve. So, the further north you go the more survival you see in the trees in the forest. And so by the time you get up into, say, Vermont and Massachusetts and New York, we’ll see stands that have been infested for 15-20 years where we still see a substantial number of the trees surviving.
Now, of course, there's a difference between surviving and thriving and a lot of these stands where the adelgid is present and has been there for a while, the trees are alive but they don't look very good. They're very thin. They have very few needles. They are not that dense, lush, dark green that we think of with hemlocks. So, we know that we have probably some time, particularly up north, and we have very little time down South.
Jonathan Yales
And the other way to look at it: how we think they're going to do in the future?
Talbot Trotter
In the future, things are a little different because we have several processes that are taking place. One is that we can expect that the adelgid is likely to evolve. When you get up to its northern limits of its range, large portions of the population are wiped out — of the adelgid — every winter by really low temperatures, but there's a small chunk of the population that survives, and it's reasonable to assume that some of those survivors are more tolerant of cold temperatures. So, every year the adelgid at the north end of the range — the tough ones — are being selected for, and so we might expect to see those northern populations basically become more and more robust to winter temperatures, and that may facilitate them both moving further north, but also may allow them to get higher populations like they do down south and ultimately have those sorts of substantial impacts where we start to see even more mortality amongst hemlocks.
And then adding to that, of course, is our shifts in the climate. So, as we see warmer temperatures and we see earlier springs and later falls, these are all things that are likely to benefit the adelgid and again allow it to get to those higher populations and have a higher impact on the tree. So, right now the north has a bit of a reprieve, but it's hard to say how long that reprieve is going to last. I mean, I would certainly personally see that reprieve as temporary, and so there is an impetus to continue trying to find ways to manage it even as it looks like it's getting up against its northern boundaries, that boundary should be expected to shift and so we need to be prepared for that.
Jonathan Yales
Now, let’s get back with Nathan and see what ways we have managed H.W.A. — what’s worked, what hasn’t, and what types of control are in the works.
Nathan Havill
Yeah. So, there are several different lines of attack that folks have been using to try to manage this insect. One is chemical control. So, there are certain pesticides — there are contact pesticides like horticultural oils and soaps that work well, if you can really drench a tree. Those are difficult to use in the forest...
Jonathan Yales
Why are they difficult for forest applications?
Nathan Havill
Well, you’d really have to drench the whole forest [laughs] in these oils and soaps, and it’s just not possible with millions of trees in the forest. Another option is using systemic pesticides. So, there’s certain classes of insecticides that you can inject into a tree, or do a soil drench around a tree, and it gets pulled up into the tree and that will protect them from hemlock woolly adelgid for several years.
And then, another way is silviculture, which is ways in which you can look at how the trees are planted, and how they’re growing, to figure out a way that might lessen the impact of the hemlock wooly adelgid.
Another way, is trying to find resistant hemlocks. So, we know that the Asian species are resistant to hemlock woolly adelgid — either they kind of repel the adelgid like Chinese hemlocks — so, that’s an option for ornamental trees, if somebody wants to plant Chinese hemlocks.
They’ve also looked into whether we can hybridize those Asian hemlocks with our native hemlocks and it turns out that Eastern hemlock — Tsuga canadensis — won’t hybridize with the Asian species because... and when we look at the history of that species, the history of hemlock, we see that Eastern hemlock diverged from the Asian hemlock a long time ago, maybe something like 30 million years ago, so they’ve diverged far enough that so that they’re not going to hybridize with each other.
And finally, another management technique that we’ve been working on is biological control. One thing that’s really interesting about adelgids — the whole family Adelgidae, all 70-plus species — is that there aren’t any parasitoids of adelgids, and that’s very strange. Every group of insects, that we know of, has parasitoids. So, with hemlock woolly adelgid — and any adelgid — there are only predators, or possibly pathogens, to work with...
Jonathan Yales
Do we have any concept of why they don’t have any parasitoids?
Nathan Havill
No. No, I’d love to know. We really don’t know why they don’t. I mean, it could just be an accident, which doesn't really seem likely to me because they’ve been a group of insects that have been around a long time. But, they must have some kind of defense or some other reason why parasitoids just won’t find them, or won’t develop in them, and we really don’t know why that is.
Jonathan Yales
Because you said, nearly every other insect on Earth has some version of a parasitoid?
Nathan Havill
Yeah. Yes. Yeah, except adelgids. So, it’s very strange.
Jonathan Yales
But, there still are other options.
Nathan Havill
I still have some hope for biological control, that the predators that have been released so far... there's a beetle called, Laricobius nigrinus, from western North America. And as we mentioned before, the hemlock woolly adelgid seems to be native to western North America, so there are predators there that are easy to get to and easy to find, that we can consider releasing here in the east. So, there's that species that's been released. There's another Laricobius from Japan called, Laricobius osakensis, that one's just started to be released. Those are both established. And the western species, Laricobius nigrinus, is spreading, it is reproducing. And then there’s another ladybeetle from Japan, that was introduced called, Sasajiscymnus tsugae. And all three of those beetles species — those are the prominent ones that are out there right now — they feed almost exclusively on the first generation of the adelgid.
And, it seems like if you knock down that first generation, even if you knock it way down, that second generation, if nothing is feeding on it, can come back again. So, we've been trying to find predators that will feed on that second generation — on both generations, or on that second generation — and there are some flies called, silver flies, that are native to western North America, that do feed on both generations. And, they can be really high numbers — they can be found in really high numbers on Western hemlock woolly adelgid.
And so, now we're working on evaluating, and starting to release, those western silver flies into the east. So, I have some hope that the combination of those predators that feed on the first generation to knock it down — possibly in combination — with these silver flies, that will feed on the second generation. Hopefully, they will have an impact on hemlock wooly adelgid.
We don't have evidence yet [laughs] that that's going to be successful, but I do have some hope that that might be — in combination — that might do something.
Jonathan Yales
Coming up next, in Part 5 of “Balance & Barrier,” we’ll meet our last pest — the Asian longhorned beetle. Melody, will introduce us.
Melody Keena
The street trees alone, you're looking at 12-61% of all the cities’ trees being killed by this beetle if they are allowed to run unfettered through cities. And, that doesn’t count all the trees we have out in our forests, that were using for maple production of syrups, and bats, and all kinds of wood products, etc.
Jonathan Yales
And we’ll chat with Talbot again about how he forecasts the directions the beetle infestations could spread.
Talbot Trotter
It's a beautiful beetle, that's, that's definitely a real — that's also a nasty one [laughs]. It's a very cool beetle, but it's definitely a problem.
Jonathan Yales
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 Nathan Havill and Talbot Trotter of the Northern Research Station; as well as the Department of Entomology at Michigan State University; and the MSU student radio station Impact 89FM.

If you liked this podcast please leave 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.