[soft music] - [Narrator] We're here with graduate student Courtney Smith to release a species of wasp that technically doesn't belong here.

This is Spathius agrili, a wasp that's native to China, and I promise that releasing it is actually a good thing.

Here's why.

This grove of ash trees is a very rare site in the Eastern US.

Most ash trees here tend to look like this.

Yep, dead.

And it's all the fault of an invasive beetle from Asia, the emerald ash borer, which showed up in the US about 20 years ago.

- And since that happened we have lost millions, tens of millions, probably hundreds of millions of ash trees to this one tiny insect.

- [Narrator] Yeah, it's kind of pretty, but pretty or not, these bugs, which scientists call EAB, have pushed ash trees to the brink of extinction.

They lay their eggs inside ash bark, and when the larvae hatches, it feeds on the same tissues that the tree relies on for nutrients and water.

So the larvae basically cut off the tree's life support.

You can see the tracks they leave behind, called galleries.

This patch in Wayne County, North Carolina, is only alive because foresters chopped it back before the beetle could fully establish here.

Now Courtney is trying to head off the Emerald ash borer again by releasing its old enemy, the wasp.

- [Courtney] This wasp is native to where the emerald ash borer is native, so it was brought here from the emerald ash borer's range to control it here as well.

- [Narrator] Okay, yes, I know what you're thinking.

Are we playing with fire by releasing more non-native insects?

But this is a time-tested strategy of the United States Department of Agriculture.

It's called biocontrol, and it relies on using the natural enemies of invasive pests to control them.

We've used this method many times before, lady beetles to eat citrus pests, wasps to eat alfalfa weevils, another wasp to kill mango-eating fruit flies.

The USDA provides wasps for free that have evolved to target a specific species, so they won't go after any other insects that we actually care about.

How do the wasps actually control emerald ash borer?

The answer is pretty gruesome.

- They have a very long ovipositor, or egg-laying device that they can insert into the bark, so it's long enough to actually go through the bark and attack the emerald ash borer larvae.

- [Narrator] Depending on the species, the wasps lay their babies inside the beetle's eggs or on top of its larvae.

Then the wasp babies feed on the larvae before it can grow up and eat the ash tree.

- It's a little bit like the movie alien where, you know, the parasitoid is inside the hosts and then it bursts out and kills its hosts on the way out.

That's exactly what it's like.

[laughs] - [Narrator] And this has worked really well in some Northern states, but down here in the South it's been tricky.

The types of wasps we release here actually prefer larvae when they're a little older.

- So it's really tricky to make sure that we know what stage the emerald ash borer is in at what time of year to be able to release them.

- [Narrator] Scientists are just starting to understand these life cycles, and for many ash trees it's too late.

Biological control is just one tool scientists are trying.

There's also a more subtle tactic, this one centering on the trees that against all odds are beating back the beetle.

Here's the dead trees again, but do you see those patches of green?

Those are what scientists call lingering ash.

Despite their neighbors being long gone, they've managed to survive year after year.

- It definitely gives you a feeling of hope.

I think each year you're crossing your fingers, you come back out here and they're still alive, and in a field that often could be quite depressing, it helps a lot.

- [Narrator] Jon and his team think there's probably some genetic resistance to emerald ash borer among these lone survivors, but identifying that genetic resistance is harder than you'd think.

- There's not really a magic gene or anything like that that we know of yet.

It's still being heavily researched.

- [Narrator] If they can identify the genes responsible for saving these trees, they can genetically modify a whole new crop of trees to be resistant to emerald ash borer.

We've used this tactic before too, this time on the American chestnut tree that was decimated by blight.

After almost 30 years of research, scientists developed a blight-resistant American chestnut tree, and ash trees are also a beloved tree in North America.

They're a crucial member of our forests.

They provide habitat to more than 100 insects and all the animals that eat those insects.

- [Jon] When you lose ash, you lose that genetic diversity, not just the tree itself, but everything else with it.

- We have about 258 million ash trees in North Carolina, so losing them as a species is a pretty big deal.

- [Narrator] Oh, and did I mention that baseball bats are made of ash?

Yep, it's an all-American tree that's quickly becoming just a memory.

- It's devastating.

Like seeing the mortality that one small beetle could do is really impactful.

But then as we're doing research, we also see that one surviving tree amongst the more than 32,000 trees that we have here, and then we see promise for the future.

We see hope, we see, hey, maybe there's something going on here and we have the opportunity to study it and then hopefully eventually establish ash again in our ecosystems.

[peaceful music]