Balance & Barrier

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In this Episode meet

  • Leah Bauer, Research Entomologist (retired)
  • Roy Von Driesche, Entomologist / Conservation Biologist, Amherst, MA (Professor of Entomology, Department of Environmental Conservation at the University of Massachusetts-Amherst - retired)
  • Jian Duan, Research Entomologist, USDA Agricultural Research Service

Episode 2: Wasps vs Emerald Ash Borer Beetles

Produced and hosted by Jonathan Yales - 25 min.

Research Entomologist Leah BauerJon Yales, Roy Von Driesche, and Jian Duan preparing to find eab infested ash trees.

With global trade and travel, organisms are moved around easily and abruptly, causing biological invasions. What’s our best hope to combat these rapidly spreading pests? Sometimes, it’s to do the exact same thing, to start moving around organisms — on purpose — to attack unwanted pests. This is called ‘biological control.’ It is one of the most cost-efficient and environmentally acceptable long-term approaches for managing invasive species. And, it’s been a crucial component to managing the damage caused by the most destructive forest insect in U.S. history — the emerald ash borer.

Jonathan Yales
This is “Balance & Barrier,” Part 2. I’m Jon Yales. On today’s episode: the emerald ash borer.
With global trade and travel organisms are moved around easily and abruptly causing biological invasions, and sometimes our best hope to combat these rapidly spreading pests is to do the exact same thing: to start moving around organisms on purpose — organisms that will attack unwanted pests. This is called ‘biological control.’ It’s one of the most cost-efficient and environmentally acceptable long-term approaches for managing invasive species.
Leah Bauer
And, we happened to be in China when we got an email about it.
Jonathan Yales
This is Leah. She’s part of our small group of entomologists based in Michigan.
Leah Bauer
And it was really a shock, but we didn't actually know the extent of the problem. I don't think anybody could have known how serious the problem was, how many dead trees that were already in southeast Michigan at the time that we were notified, which would have been, maybe, September-October, 2002.
Jonathan Yales
What Leah and the group were notified of was the first U.S. detection of emerald ash borer — or ‘E.A.B.’ It’s a beetle, native to China, which is maybe the most famous U.S. forest insect of all time.
Famous, because over the decades since its arrival, it’s become the most destructive forest insect to invade North America. But, back then, no one knew any of that would happen. They knew nearly nothing about the beetle.
Leah Bauer
Certainly, it was a shock. The literature was very limited. And we happened to be in China, so we talked to our cooperators there, but a lot of the early scientific literature had been destroyed during the Cultural Revolution. So, we were dependent on these little snippets in these books about Buprestidae, which is the family of beetle that Agrilus is in: the ‘Buprestidae of Asia,’ or the ‘Agrilus of China.’ Some of those books are around, they just had a short paragraph about it, that Agrilus planipennis was known to attack ash trees…
Jonathan Yales
Since the early 2000s, E.A.B. has killed hundreds of millions of ash trees from Maine all the way to Colorado — and it’s still spreading. Today, it’s in 35 states, and five Canadian provinces.
So, what did Leah and the team do when they got word of its arrival? They dropped everything they were working on.
Leah Bauer
Yeah, we finished off some things we were starting. There were various projects ongoing that got wrapped up. And it was right here, it fell in our lap, there really wasn't anybody else who could do it — this work. So, in our little unit here in the East Lansing [Michigan] area, there were three scientists that were involved in emerald ash borer research. And the decision was to put some of that money into looking for natural enemies, either here, or in China.
Jonathan Yales
The initial response to the E.A.B. invasion by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service — or A.P.H.I.S. — was to attempt eradication of the beetle, meaning, to find, cut down, and destroy all infested ash trees. But, the size and complexity of the outbreak demanded something more. It demanded biological control. So, Leah and the team started looking, looking for natural enemies — any organism that naturally attacked the beetle. And what they found were tiny stingless wasps that attack E.A.B.
Leah Bauer
These wasps are highly co-evolved to develop within a host and kill it. That's why we don't call them parasites, they’re parasitoids. It's sort of a specific term to an insect that specializes on eating other insects.
Jonathan Yales
And what these wasps eat are the beetles’ larvae, and eggs. And it’s those larvae that were causing all the damage to the trees in the first place.
Leah Bauer
Emerald ash borer finds an ash tree by flying around, and when they find one, they prefer to lay their eggs on the bark between little layers of bark or in bark crevices — they like really tight places, the females. And so, when the larva hatches out of that egg after another week-to-10 days depending on the temperature, it tunnels directly through the bark until it hits the phloem of the tree. Now, the phloem of a tree is the area that these larvae start feeding in, and so instead of drilling in through the wood they kind of stop and they begin to go feeding along the outer wall of a tree, it's called the outer sapwood. So, the phloem is right between the bark and the outer sapwood, and the phloem is a super important part of the tree. It is where the food flows from the leaves that's produced by sunlight and nourishes the entire tree just under the bark of a tree. Which is why your mom probably always said, ‘Never peel the bark off a birch tree because you're going to kill it!’ If you take the bark off a tree all the way around it, that fluid that runs down just under the bark, in the phloem, is intercepted, it can't feed the roots, the tree dies. And so, emerald ash borer feeds in that zone and there's so many of them that it actually can intercept the ability of the tree to develop, or continue being alive.
Jonathan Yales
In 2003, Leah and the group, along with various collaborators, began searching the beetle’s native range in China for the wasps that could attack these beetles. And they found three.
Leah Bauer
What we found in China was an egg parasitoid, one that can actually — tiny little thing, about as big as a poppy seed — that crawls around through the layers of bark and up and down the bark crevices. And, it will lay an egg in one emerald ash borer egg. So, that means no larvae can feed where that egg was — it won't ever move in and cause a gallery and damage to the phloem. The more common parasitoids we found in Asia are larval parasitoids — those are ones that only attack larvae. They come in various sizes, there’s three species now that are being released for biocontrol. So…
Jonathan Yales
Well, what was the first one? What was its name? The egg parasitoid?
Leah Bauer
Ok, so the egg parasitoid — after it was described — was given the name Oobius agrili. The larval parasitoids, there are two that we found in China. One of them was named Tetrastichus — it’s in the genus Tetrastichus — it was eventually named Tetrastichus planipennisi. And it is a larger parasite, and it will walk around on the bark of ash trees and look for the presence of larvae under the bark. There's a third parasitoid that's called Spathius agrili, and it is much larger. And the biology of the two larval parasitoids are quite different, but one is capable of drilling through thicker bark because it has a longer, what’s called an ovipositor. It’s the part of the wasp that you might think of as a stinger, but it's not a stinger. It's what they literally use to drill through the trunk of a tree, or through the bark of a tree, when they sense the presence of a larva under the bark. Tetrastichus planipennisi is an internal parasite, it lays its eggs inside emerald ash borer larva. And those larvae — literally — develop their tiny, tiny little first eggs, they hatch inside the blood of the insect, and they grow in there. And eventually when the larva is completely eaten, from the inside out, they will pupate under the bark of the tree in the gallery of emerald ash borer.
Jonathan Yales
So, Leah had her three wasps. But, before releasing them into the wild in the U.S., they first had to determine the potential risk of letting them go in North American ecosystems.
Leah Bauer
Okay, before you can introduce something to the U.S. — because it's going to spread to Canada and Mexico — you have to submit a request or petition to an organization called the North American Plant Protection Organization, or N.A.P.P.O. And they have very specific guidelines as to how you prepare a N.A.P.P.O. petition for release of a non-native, insect-feeding, insect. And in this case, they are these parasitoids, that we had three of them in 2006, that we felt very comfortable with the risk-benefit analysis. We’d had extensive conference calls with people from all over the U.S. — professors at universities that were experts in biocontrol, and some in Canada weighed in — and they were like, ‘Just get them all, introduce all of them. The risk to ash is so extensive.’ By this time, we were in 2006, where emerald ash borer had already spread to Maryland, pretty much the Beltway [Maryland, Virginia, Washington, D.C.] was infested, most of the states around Michigan it had been found in, Illinois, throughout Michigan, Ohio, Indiana, moving eastward into — I don’t know if it had been found in New York or Pennsylvania yet — but it was pretty shocking how quickly emerald borer was spreading. And so, the general consensus was like, ‘You have five years of research on these three species, the risk looks minimal.’ Certainly, Tetrastichus planipennisi, was specific to emerald ash borer — it never attacked any other native Agrilus. Spathius agrili crossed over, occasionally, to some other Agrilus only. Oobius [agrili], the same thing, in the lab, in a cup, with an egg and a little tiny wasp on, either, the ash or, let’s say, the Agrilus egg on oak, or birch, or raspberry — we have a lot of native Agilus that attack woody plants, and many of them are pests. But, I think anytime you have a species like Spathius agrili or Oobius agrili attacking native, you have to pause and think, ‘Is the benefit worth the risk?’ We didn’t really know how well these would work, but we knew how well they were controlling E.A.B. in China. So, the presumption was, we know emerald ash borer is killing all these trees, we know what the risk of doing nothing is, right? And so, the general consensus among the experts in the country was that it was worth the risk.
Jonathan Yales
And N.A.P.P.O., with representatives from the U.S., Canada and Mexico, also agreed that it was worth the risk. Then in the U.S., A.P.H.I.S. prepared an environmental assessment that was posted for 60 days on the federal register for public comment, and after evaluating the potential risks and benefits, A.P.H.I.S. and the state of Michigan, approved releases of these three wasps. And, in the summer of 2007, Leah and her team received permits from A.P.H.I.S. and began releasing small numbers of each parasitoid species at test sites in mid-Michigan.
Leah Bauer
For those of us that work here in Michigan, and other people who come here to work with me, like, Roy Van Driesche and Jian Duan, they've been working with me in Michigan [since] 2008 at a cluster of release sites, to monitor, not only parasitoid establishment, but abundance and impact on the emerald ash borer population, as well as, impact on the trees themselves.
Jonathan Yales
For over a decade, every year, Leah and the team has been visiting those same release sites.
Leah Bauer
We've been cutting down and dissecting trees and sampling bark, to look for the egg parasitoid. To find the larval parasitoid, we have to cut the trees down and dissect the larvae out from under the bark, and then dissect the larvae — if we don't see, obviously, that they're parasitized, we actually have to dissect each larva to look to see if there are internal parasites in it.
Jonathan Yales
This past spring, on a wet and grey afternoon, Leah and I checked in on how the wasps were doing at one of those sites. It was more of a swamp than a forest.
Leah Bauer
Well, we could go… I’m trying to avoid going into the swamp if we don’t have to, just because of the rubber boot problem. It’s literally, you know, deep water…
Jonathan Yales
Up to your calf or something?
Leah Bauer
I don’t know. Sometimes we’re up to the top or... Yeah, the boots get pretty helpful when you’re dealing with ash trees. So, we’re going to go up this little road here, or path…
Jonathan Yales
We were looking for Leah’s collaborators, Roy and Jian, who were looking for just the right tree.
Roy Van Driesche
Is this one too big, [Jian] Duan? He’s got one?
Jonathan Yales
He’s spotted one.
Roy Van Driesche
Duan is very good at spotting suitable trees…
Jonathan Yales
The perfect tree!
Roy Van Driesche
Like a bird dog nosing through the brush.
Jonathan Yales
He can smell them.
Leah Bauer
That’s pretty big Jian.
Roy Van Driesche
What? That crooked one?
Jonathan Yales
Jian, a research entomologist at the U.S. Agricultural Research Service, was looking for live ash trees that had signs of E.A.B. infestations.
Roy Van Driesche
See, you can see the… see the little divots out of the bark, those are woodpecker attacks…
Jonathan Yales
Roy Van Driesche is a professor of entomology in the Department of Environmental Conservation at the University of Massachusetts-Amherst, and he’s been working with Leah and Jian at these sites, since day one.
Jonathan Yales
So, in a sense this whole larger area that we're here at today is kind of like the original test ground for biocontrol for E.A.B.?
Roy Van Driesche
Well, it's one of 12 in central and eastern Michigan. So, when we set up plots starting in [200]7 and adding some in [20]10, we set up a series of six locations, each one of which we had two sub-locations: one for release and for not-release. But, most of the parasitoids spread into the not-release plots within a few years, so effectively we've got 12 plots.
Jonathan Yales
And why do you have 12 plots, not six, or two, or one?
Roy Van Driesche
Oh, because there's a tremendous amount of variation between sites, and even between trees. You can go to two different trees in the same plot and have quite a lot of difference in what you find.
Jonathan Yales
Over time, as the wasps proved effective in combating E.A.B., and as the beetles spread across the country, Leah handed off the rearing of her wasps to A.P.H.I.S. And in 2009, just down the highway from where we were, the U.S.D.A. built a breeding facility in Brighton, Michigan. Over the years, it has produced anywhere from a quarter to three-quarters of a million wasps each year, and they’ve been sending the parasitoids to researchers, land managers, and foresters working on the management of E.A.B. ever since.
Roy Van Driesche
It's typically done over three years. They ship you tubes with emerged adult parasitoids in them and you just take them to what you think's a suitable spot and you open the vial and kind of tap them and let them walk up the tree and then they're on their own.
Jonathan Yales
Besides shipping adult parasitoids, A.P.H.I.S. also ships some of the larval parasitoids when they are still inside the small ash logs that are used to rear them in the lab. After the logs are placed on a tree in the field, the adult parasitoids will chew their way out and fly away, seeking more E.A.B. larvae to attack.
With E.A.B. in 35 states and half the Canadian provinces, you can see the reason for needing to rear millions of wasps. But, no wasp is a silver bullet, biocontrol programs and forest ecosystems are complex systems, so it takes awhile to truly see the effect the wasps are having on a particular forest.
Roy Van Driesche
Well, every biocontrol project is different. This is a very complex system because you've got, of course, 16 species of ash, you've got a tremendous ecological range from maybe, eventually, Florida to Maine and out to the West, and so it'd be reasonable to think that there's going to be variation in how well it works. In most places where E.A.B. gets in first, what we expect to see is that there's a big die-off of the existing larger ash. But if you look even at this site, you can see that. You can certainly look around and see lots of larger 10-15-inch ash that probably died here maybe 10 years ago. But, we're sampling the sapling-size ash right now and we're having trouble finding infested saplings. There's a lot of saplings here that are in the 1-3-inch size that are maybe 20 feet tall that are too healthy for us to sample. And, of course, trees and their pests are a difficult system simply because trees grow slowly and they're pests grow slowly. So, you know, somebody famous once said, 'It ain't over till it's over,' and it ain't over, even by a long shot, yet. So…
Jonathan Yales
So, there’s still a lot of ‘we-don’t-knows’ and ‘we’ll-sees’ out there. And, somehow, there’s still a lot of uninfested places, too.
Roy Van Driesche
Because E.A.B. is just getting into Act II, and there's probably Acts III and IV to go…
Jonathan Yales
Jumping to Colorado…
Roy Van Driesche
Well, Colorado and Louisiana are sort of the west and southern limits, that at least I'm aware of. But, there's no reason why, given time, it won't reach California, Oregon, Utah, some of the mountain areas maybe of New Mexico, of northern Mexico as well. It'll keep spreading until it finds its ecological limits…
Jonathan Yales
Which could be … the oceans?
Roy Van Driesche
Well, I mean, ash doesn't grow everywhere, but there's ash all the way to California and all the way down into northern Mexico. So, the whole Southwest, California and the Pacific Northwest is still pest-free, but it won't stay that way. So, right now in Michigan, [Jian] Duan and Leah and the other people involved in the project, collected parasitoids that were from part of the E.A.B. distribution in China and Russia that climatically match sort of the [Great] Lake states and the New England/Northeastern region. We don't have any specific collections yet of anything that would, climatically, be a good match to Louisiana or any of these uninfested western and more southern areas. So, one of the projects [Jian] Duan's working on in China is to put out sentinel logs — or infested trees — in ash stands, like in Sichuan province, and other areas that are more down along, like, 23 degrees north latitude or thereabouts. So that if they can find... even if it's the same species of parasitoid their life cycles would be more successful in establishing if the climate of where you to take them from and where you put them are similar…
Jonathan Yales
And a climate that’s similar to the northern United States’ climate is the Russian Far East’s, where a fourth wasp — spathius galinae — was found. And in 2015 — nearly a decade after Leah and the team began releasing their three wasps — A.P.H.I.S., along with Connecticut, Massachusetts, New York, and Michigan, all approved its release. This fourth wasp was specific to E.A.B. larvae, highly tolerant of cold weather, and was well synchronized to the E.A.B. biology in the Great Lakes states and the Northeast.
But back in the field, after a day talking and working, and with a few trays of E.A.B. and parasitoid larvae, we headed out of the swamp and back to the lab with Leah to do some dissecting.
Leah Bauer
Let's see if we can find a whole one. Here's a whole one. So, this is one that has not been damaged during the tree dissection process. So, first we dissect these insects in the field out of the trees and then we put them in these little dishes in the tray. This is a typical prepupa of an emerald ash borer — it's kind of short and stout and cylindrical…
Jonathan Yales
It’s almost like, a toothpick-width? Maybe, a little smaller than that?
Leah Bauer
Oh, it looks wider than a toothpick to me — wider than a wooden toothpick.
Jonathan Yales
Yeah, I guess.
Leah Bauer
I'd say it's maybe as long as a grain of wild rice, but certainly more stout. So, what we're looking for when we dissect them is for the internal parasites. The external parasites, or parasitoids, we can see on the outside of the body, but the internal parasites we need to dissect the insect’s insides, and take them out of them, and look to see what's living in its body…
Jonathan Yales
Based on all this work by Leah, Roy and Jian in Michigan over the past 12 years, small ash throughout the northeast and central U.S. — now, and in the near future — will be protected by two of the wasps.
Leah Bauer
So, the first thing I'll do is look at the outside to see if I see any eggs or larvae of an ectoparasite, and then if I don't see any — which I don't in this case — I'm going to cut the back end off…
Jonathan Yales
With the wasps protecting the small ash, this should allow ash seedlings enough protection and time to grow into full forest trees.
Leah Bauer
So, I'm going to cut this open. Now, the prepupae don't come apart quite as easily. It's mostly — what we see is — what's called the fat body of the insect...
Jonathan Yales
So, it's mostly mush?
Leah Bauer
Yeah, it looks like mush, exactly, just to get polite about it.
Jonathan Yales
Within 5–10 years, as current young ash reach mid-size, that fourth parasitoid — spathius galinae — will be abundant and able to protect those trees as they enter the mid-size cohort, allowing ash regeneration and regrowth to continue.
In a study with the University of Michigan, Leah’s also finding that where these wasps were released, there are more ash saplings growing and fewer invasive and weedy saplings. Meaning, it may be, without these wasps invasive plants fill the gaps left by the dead ash, but with the biocontrol program — with the wasps — ash saplings outcompete the invasives.
As of today, nationwide, at least one type of these wasps has been released in 25 states. And their offspring have been recovered in 17. Which means, the wasps are establishing, reproducing, and more importantly, attacking and killing E.A.B. So, the plan is to significantly expand the distribution and number of wasps released to help stabilize and eventually reduce E.A.B. populations.
But, what happens when wasps like these aren’t available for your pest insect? Well, there are other options. Insect biocontrol can involve the three ‘p’s: parasitoids, predators, or pathogens. Next up, on Part 3 of “Balance & Barrier,” we’ll meet our next pest: the gyspy moth, and hear the Cinderella story of how a pathogen — a fungus — mysteriously slowed their spread.
Dave Smitley
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
Thanks for listening! See ya soon!
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 Leah Bauer of the Northern Research Station, and Roy Van Driesche of the University of Massachusetts-Amherst, and Jian Duan of the USDA Agricultural Research Service. And, as always, thanks to the Michigan State University Department of Entomology.

And, if you liked this podcast, please leave us a review on Apple Podcasts, it helps grow the show. And, 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.

Adult emerald ash borer. Photo by David Cappert.     Tetrastichus planipennisi – Gregarious endoparasitoid of EAB larvae introduced from China. Photo by David Cappert.

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