Human Footprint

(soft, tense music) (Shane) They say if you want it done right, you should do it yourself.

And humans, we certainly aren't shy about that.

("Da Rockwilder" by Method Man and Redman) But we share this planet with millions of other species... -(dogs panting) -(snake hissing) ...some that stand in our way... -(mosquito buzzing) -(mouse squeaks) ...and others that stand in theirs.

So, we enlist our enemies' enemies and send them into battle.

(Redman) Come on (Shane) Action.

Reaction.

Cause... (chess pieces tumble) ...and effect.

-(wing flapping) -(bees buzzing) But our most powerful weapons can have agendas of their own.

(whooshing) In the game of life, humanity's playing to win.

But we aren't the only ones.

Can we forge an alliance with nature to create the world we want?

(birds chirping) Or will we start something that we can't finish?

(intense music) (chess piece thuds) Welcome to the age of humans... (energetic music) ...where one species can change everything.

And what we do reveals who we truly are.

This is Human Footprint.

(indistinct chatter) (rooster crowing) People have been transforming the Earth for thousands of years.

And we've made enemies along the way... ...species that destroy crops... (audible chewing) ...spread disease... -(mosquitoes buzzing) -(mosquito squelching) ...and disrupt ecosystems.

(dog ears flapping) But we've also got allies... (dog growling) ...species we've recruited to fight on our side.

(device beeps) And they've reshaped the world too.

But is the enemy of an enemy always a friend?

(soft, tense music) (creatures chirring) Thailand is one of the world's top rice producers.

Farmers here often harvest three crops a year.

(rice grains clatter) And after each harvest, there's work to be done.

(vehicle engine rumbling) (Shane) The rice farmers are at war with an invasive pest.

(whooshing) The golden apple snail.

(ducks quacking) And ducks love snails.

How many ducks will get released here?

(grim music) (Shane) It's about to go down.

(intense music) (ducks quacking) Oh, it's the most adorable little army.

Almost a quarter of Thailand's 30 million domestic ducks grow up like this, trucked around the countryside to devour pests.

(distorted quacking) After a few months, they can be sold for meat or returned to the breeder to produce eggs.

The ducks are one form of biological control, or "biocontrol" for short.

It's the way we harness one species to control another.

(soft, tense music) Biologists like me have been using the word for about a hundred years.

But biocontrol is much more ancient.

In fact, it didn't even start with humans.

(mystical music) In the Serengeti, whistling acacias grow hollow thorns to attract ants, which build nests inside the thorns and defend the tree against herbivores.

(elephant whines) This is nature's biocontrol.

The trees recruit the ants to help solve a problem.

Humans have been practicing biocontrol since at least the 4th Century, when farmers in southern China controlled pests by placing weaver ant nests into their citrus trees.

Villagers would even collect wild ant nests and sell them to farmers, making this the first known commercial biocontrol.

(music distorts) Today, biocontrol is everywhere.

(soft music) But you might never notice, unless you know what you're looking for.

(Aaron) So, they're super small.

They're hard to see.

The trick is not to breathe and blow 'em off the plate.

(Shane) Or inhale them, I imagine.

(record scratch) (chuckles) This is Aaron Avila.

He's got six dogs... (Lil' Bow Wow) Bow wow wow, yippie yo, yippie yay (Shane) ...and almost as many refrigerators to showcase his prized magnet collection.

("Bow Wow (That's My Name)" by Lil' Bow Wow) Aaron works with farmers in Washington to help solve their pest problems.

(insects chirring) -You seeing some?

-Yeah, they are much smaller than I anticipated.

(Aaron) Yep.

(fast-forward sound effect) (Shane) These tiny soldiers are western predatory mites, and they've got a job to do in this hop yard.

(mellow music) (Aaron) Over 70% of the nation's hops are produced right here.

This is a national treasure -is what you're saying... -This is, yep, yep.

(Shane) The backbone of America.

The backbone of beer drinkin' America.

Yes, that's right.

(Shane) Which is pretty much all of America.

(Aaron) That's right.

(foreboding music) (Shane) One of the toughest pests in a hop yard is another mite, the two-spotted spider mite.

And the usual method of controlling them isn't biological, it's chemical.

(energetic music) (Aaron) Historically and still widely used today -are miticide pesticide sprays.

-(spray hissing) (Shane) But miticides aren't working as well as they used to, because some spider mites are evolving resistance.

Enter the western predatory mite, a spider mite's worst nightmare.

The trick is delivering them where they're needed.

(Aaron) We wanted to explore the possibility of being able to-- to do that via drone.

Okay.

And that is solely for application purposes and not because it's really fun to fly a drone?

(laughs) (soft music) The drone pilots load the mites into a custom dispenser, set the drone to fly a pre-programmed route, and, well, bombs away.

(energetic music) If everything goes as planned, the predatory mites will get the spider mites under control... (whooshing) ...and we can all raise a glass to our little eight-legged sidekicks.

(record scratching) -Cheers, friend.

-Cheers.

(mellow music) Oh, yeah, there's the hops.

(Aaron) Mm-hm.

Pests and disease are something that we do battle with every day, every single day.

-Okay.

-If we're looking at food production as a whole, the number is 20-40% losses globally due to pests and diseases.

(soft, tense music) (Shane) When farmers come to Aaron's company for help, technicians diagnose their crop problems and propose solutions.

The prescription could be a pesticide or some form of biocontrol.

(Aaron) Our number one concern is to help our growers remain successful.

And whatever that answer might be, that's what we're seeking out.

(Shane) And do you think, moving forward, that biocontrol will continue to play a larger role-- -Yes.

-...in--in that?

Mm-hm.

Yeah, absolutely.

It's definitely growing and it'll continue to grow.

-Okay.

-No doubt about it.

(Shane) Humanity has some big problems to solve, and biocontrol can help us do it... ...as long as we're sure that we understand nature well enough to use it as a tool.

(ducks quacking) On the other side of the planet... ...that idea is under assault from a fast-moving army of toxic amphibians.

(energetic music) (Rick) There's very few things that all Australians will agree on.

(wings flapping) (frog thuds) But one of the things that will get general agreement is that cane toads are horrible and ugly... (projector slide changes) ...but, you know, you've really got to admire what the hell they've achieved.

(Shane) This is Rick Shine, a legendary biologist and one of my science heroes.

When he's not waxing poetic about one of Australia's most un-loved animals, you might find him out fishing.

(fishing line whirs) And keeping one eye on the water... (intense music) ...for these guys.

(mellow music) (tour guide) In the true Australian scientific description, he's referred to as a bloody big crocodile.

And if I tried to ride him like a horse, my tackle would tickle his back even on a cold day.

(Shane) Here in the Northern Territory, giant crocs are a source of pride.

Cane toads?

Not so much.

(Rick) You know, the conversation around the barbecue at night is, you know, "Bloody cane toads," and, "They're so ugly."

And it took me, you know, years before I actually looked at a cane toad and I thought, "Actually, that's a pretty damned attractive animal."

-Yeah.

-Uh, the eyes of a big female toad, you know, you can lose yourself in them.

They're gorgeous.

(mellow music) (Shane) Beautiful or not, cane toads are an invasive species in Australia.

They're native to wet tropical forests in the Americas.

And their Australian invasion began... ...with biocontrol.

(Rick) There was a World Congress of sugar cane growers in Puerto Rico in 1932.

Everyone got told about how fantastic these giant toads were at controlling insects in your sugar cane.

Mm.

At the time, Australia's farmers were wrestling with a native pest called the greyback cane beetle.

(Rick) And it was only a year or two before an Australian had the brilliant idea that, "Hey, these giant toads can solve our problems as well."

(Shane) So, a young Aussie named Reg Mungomery went to Hawaii to get some toads.

Caught 101 toads, brought them back, released the babies, and the toads began their conquest of Australia.

(foreboding music) (Shane) The toads didn't stop the cane beetle, but they did make a lot more cane toads.

(water lapping) Because a single female can lay 20,000 eggs at a time.

They spread across Australia, eating whatever they could fit in their mouths... (tongue lapping) ...and poisoning native predators that tried to eat them.

(Rick) And it became clear that we had no way to stop them.

(energetic music) (Shane) Rick began studying cane toads in the early 2000s.

Until then, he'd spent most of his career working on snakes.

(Rick) And I'd been doing that very happily, uh, for about 20 years when the cane toad invasion appeared over the horizon.

And it would have been crazy not to move sideways a little bit and become a bit of a toad biologist.

(soft music) (Shane) Rick's work reveals how cane toads are reshaping Australian ecosystems.

(Rick grunts) (Shane chuckles) (Rick) You can tell by the thousands of bugs around your face that there's plenty here to eat.

-Oh yeah.

-Yep.

And this is the dry season.

In the wet season, the bugs are about a hundred times worse.

(Shane) Mm.

(Rick) Do you wanna try your toad-capturing technique, Shane?

(Shane) Yeah, let's see what we got here.

(intense music) (Rick) Nice grab.

(Shane) Took all the speed and strength I could muster to get her.

(Rick laughs) (quirky music) (soft music) Cane toads arrived at Rick's study site in 2005.

But these were not the same toads that Reg Mungomery released 70 years earlier.

In the early years, the cane toads spread at 10 to 15 kilometers a year.

(Rick) But then as they hit the drier country, they begin to go faster and faster and faster.

By the time they get to here, they're moving at 50 to 60 kilometers a year.

(Shane) The toads that arrived in 2005 were built for speed.

(whooshing) (Rick) It is a genuine case of evolution within a human lifetime.

Are we talking about longer limbs?

Are we talking about metabolism?

Is it behavioral?

Very easy question to answer: everything.

Every characteristic we measure about cane toads, except maybe the number of legs, seems to differ.

That would be truly terrifying.

(energetic music) And the toads didn't just evolve fast; they revealed a whole new kind of evolution.

The toad showed us that evolution can occur through space as well as through time.

(Shane) It's a process Rick calls "spatial sorting."

As toads spread across Australia, the fastest individuals led the way.

And they bred with other fast-moving toads, because they were the only ones there at the edge of the invasion.

(Rick) We call that the Olympic village effect on the suspicion there's a bit of interbreeding in Olympic villages among athletes.

Some of the babies of those toads will inherit genes for stamina from mum and long legs from dad.

And so the next generation goes even further.

And that's a cumulative process.

(Shane) It's different from natural selection, because the fast-moving toads don't necessarily pass on more of their genes.

(Rick) It's simply that you end up with that kind of genes over there and those kind of genes over here.

Yeah, so, you basically-- you get the athletes on one end and the couch potatoes at the other end.

Absolutely.

(Shane) But as the toads swept across the continent, they weren't the only ones evolving.

Rick and his team discovered an extraordinary adaptation in some of Australia's snakes.

(Rick) The most remarkable change is that they've evolved smaller heads.

And so if you've got a small head, you can't eat a really big cane toad.

(Shane) That's a good thing, because if you can't eat a big cane toad, you can't get poisoned by one.

(whooshing) (mellow music) But other species haven't proven so adaptable... (whooshing) ...and cane toads have had a profound impact on Australian ecosystems.

(Rick) You know, it's not the toad's fault.

But yet people are very inclined to say, "Here's the foreigner, here's the invader."

And if we go out the backyard and hit 'em all on the back of the head with a golf club tonight, we'll feel like we're doing something to save Australian biodiversity.

(intense classical music) (ball whacks) (Shane) Hitting cane toads with golf clubs feels oddly specific.

(Simon) In terms of where toads love to live, the golf course is just a heaven for them.

(majestic music) (Shane) Meet Simon Middap.

("Jump Around" by House of Pain) He lives on a golf course but doesn't play golf.

His game... is cane toad busting.

Well, what are your personal feelings about cane toads?

-I dislike them intensely.

-Okay.

(Simon) So, I don't really like, you know, killing nature, but I have no problem with picking up a cane toad and disposing of it...humanely.

(Shane) Okay.

Humanely.

In other words, without the use of a nine-iron.

(Simon) So, we've come up with this thing, TTTT.

That stands for: Terrorize Toads on the Third Thursday.

Okay.

(Simon) We do it on the third Thursday because that made the name work.

(Shane) Okay.

The volunteer toad busters start their work when the golfers go home.

-(Western-style music) -(hawk cries) (Simon) And we'll see you back here at half past eight -with a bucket full.

-Okay.

-So, let's go and get 'em, eh?

-Good luck, everybody.

(Simon) Have fun.

(energetic music) (Shane) For a few hours each month, this is a dangerous place to be a toad.

(Simon) There's another little one right beside him.

Look at that, two for one.

Don't let it get away.

(Shane) The most exciting part of the hunt?

The tally at the end of the night.

(Simon) 24, 25, 26, 27, 28, 29, 30, 31... You know, you get really excited when you get a thousand toads a night.

But you're never gonna do anything with the population just by catching the mature toads.

The process of catching toads is really so they can go back to the lab, they can get the toxin gland removed, and we can make these tablets.

They're called Bufo Tabs, which are things we put in the trap to catch tadpoles.

(Shane) But why are cane toad tadpoles drawn to bait made from cane toad poison?

Simple.

Because the tadpoles... (dramatic music) ...are cannibals.

(Rick) In Australia, a cane toad's worst problem is another cane toad.

(Shane) Female toads infuse their jelly-coated eggs with toxins.

(Rick) And as the jelly coat breaks down, the poisons begin to diffuse out into the water.

And that's a signal for any cane toad tadpole in that pond to come roaring across and try to eat them 'cause you get rid of the competition.

(Shane) So, toad poison is the perfect bait for a tadpole trap.

(Rick) And as we speak, millions of cane toad tadpoles are being pulled out of natural water bodies across Australia.

(Simon) The purpose of really chasing the tadpoles is that's where you actually really do some numbers.

Last season, we caught over 250,000 tadpoles.

Whoa.

-A quarter million tadpoles!

-Yep.

(Shane) Removing tadpoles is a good way to control toads.

(hip-hop music) But Rick and his team realized they could also put the cannibals to work.

(Rick) How about if we stop tadpoles from transforming into baby toads, if we kept them as perpetual tadpoles.

(Shane) And here's where things start to sound like sci-fi.

(Rick) You can actually manipulate the genes inside the egg of a toad and knock out the gene that it needs to transform from the tadpole stage into the toad stage.

Oh.

Rick and his team did just that, creating tadpoles that never grow up.

Rick calls them "Peter Pans."

(Rick) They eat a lot more toad eggs than a normal taddy does -because they're bigger.

-Mm-hm.

So, a pond full of Peter Pans is a deathtrap for new eggs.

(Rick) Toads can never breed there again until those tadpoles die of old age.

(mellow music) (Shane) It's a clever twist on biocontrol, turning a species against itself.

Peter Pan tadpoles haven't been released in the wild yet.

But when they are, biocontrol might help solve a problem that biocontrol created.

The story of cane toads, you know, I think a lot of times it's presented as a cautionary tale.

Is that still, like, a fair example, or is it out of date now?

Yeah, look, I think it was probably out of date even in the 1930s.

But there was too much political weight behind the push to bring in the toad.

(Shane) It turns out, even Reg Mungomery had his doubts.

(Rick) So, this is Reg Mungomery writing in the Cane Growers' Quarterly Bulletin in 1934, "Such a project is not to be embarked upon lightheartedly, since a false step may have disastrous economic consequences through the upsetting of the whole biological balance."

And then a year later, they gave him a first class seat to Hawaii in a very nice ship.

And off he went and got the cane toads.

And did exactly the thing that he warned against.

-Yeah, I'm afraid so.

-Okay.

Why?

I guess, is the question.

Oh, why do young men do anything, Shane?

Come on.

(laughing) (soft music) (Shane) Was it the desperation of an industry that brought cane toads here?

Or the impetuousness of youth?

Maybe a bit of both.

Either way, Rick doesn't want to follow in Reg Mungomery's infamous footsteps.

(Rick) It's taken decades to get to the point that I'm comfortable we know enough about the system that we can carefully move forward.

I think maintaining that humility is a key to doing decent science and making responsible decisions.

(Shane) Australians could have turned their backs on biocontrol.

But they didn't.

Because fate dealt them another chance to get it right, this time with even higher stakes.

(Martha) The rabbit has arguably been the most destructive nonhuman animal to ever arrive on the Australian continent.

Woah.

(hip-hop music) This is Martha Sear.

And this is her amazing collection of vintage animal-print shirts.

Martha's a historian at the National Museum of Australia... ...and she knows a lot about rabbits.

(Martha) They have sent animals and plants to the brink of extinction and beyond.

(Shane) Like cane toads, rabbits aren't native here.

(Martha) In 1859, Thomas Austin decided that he would like to do a bit of hunting on his country property.

Mm.

(Martha) On Christmas Day in 1859, Thomas let 13 rabbits out.

-You know the exact day.

-Yep.

We know the day, we know the number.

From there, they ran wild.

(Shane) Australia was like an all-you-can-eat bunny buffet... (male voice) Perfect (Shane) And they reproduced... well, like rabbits.

(energetic music) (Martha) Within 50 years they had covered two thirds -of the Australian continent.

-Wow.

(Martha) It's the biggest mammal invasion that's ever happened on planet Earth as far as we know.

People talked about the land being covered by a moving gray blanket.

(Shane) Whoa.

The environmental and economic costs of this bunny bonanza were staggering.

Rabbits eat plants from top to bottom, roots included.

So, before long, there was nothing holding the soil down.

(Martha) Cities and towns were blanketed with huge dust storms, liberated by the little incisors of the bunnies.

(Shane) Of course, people also figured out a way to make a profit off them.

(cash register chimes) (soft, tense music) (Martha) They went in the freezer, and they got shipped off to feed people in Europe.

(Shane) And rabbit meat was just the beginning.

(Martha) If you had walked through the streets of London in the 1920s, most of the gentlemen who lifted their hats to the ladies were wearing Australian rabbit fur on their head.

(Shane) But all that trapping barely made a difference, and rabbits reached plague proportions.

(Martha) So, Australia turned to its scientists to find methods of biological control, and that's where you get the story of myxomatosis.

(grim music) (Shane) Myxomatosis is a rabbit disease caused by the South American Myxoma virus, a relative of smallpox.

Researchers wondered if Myxoma could solve Australia's rabbit problem.

They were cautious, first making sure Myxoma couldn't infect any native species, and then testing the virus in small, isolated rabbit populations.

Myxoma was finally unleashed in early 1950.

And by the end of the year, people began to report hundreds, then thousands, and ultimately millions of dying rabbits.

(Martha) The piles of dead rabbits were stinking out pastoral properties.

And the spread traveled hundreds of kilometers in just a few short months.

-Months!

Woah, okay.

-Months.

It was a big success.

(soft, tense music) (Shane) More than 99% of Myxoma-infected rabbits died.

Agriculture and ecosystems rebounded as the rabbit population crashed.

(whooshing) (vehicle engine rumbling) But 70 years later, rabbits are still an inescapable part of Australian life.

(Ian) We have got quite a healthy population of rabbits, in our urban reserves within Canberra.

(engine rumbling) (Shane) Ian Lenon is a wildlife manager.

He's using heat-sensitive cameras to help locate and dispatch rabbits.

(Ian) Everyone behind the shooter.

(soft, tense music) (insects chirring) (gun clicks) (gunshot) (Shane) Ooh!

Yep, he got 'em.

(Ian) That's a female rabbit by the look of it.

(Shane) Okay.

It's hard to watch.

But the rabbits killed in these control programs provide important data.

What are you testing for here?

(Tanja) So, we take blood, for example, we can analyze that later for antibodies against the various viruses that are circulating.

-Okay.

-So we can see what was active in the area.

(mellow music) (Shane) And it turns out, Myxoma is still here.

So, why hasn't the virus finished the job?

(Tanja) Biological control is not an eradication tool.

It's a management tool.

(Shane) Meet Tanja Strive.

When she's not spending time with her very pampered chickens, she's working at CSIRO, Australia's national science agency, on the next generation of rabbit biocontrol.

("Fight The Power" by Public Enemy) Because Myxoma isn't as effective as it used to be.

-(mosquitos buzzing) -(mosquito sucking) (Tanja) There is a window of time where the biting insects can pick up the disease and transmit it to the next animal.

So, as soon as the rabbit's dead, it's of no interest to a mosquito anymore.

(Shane) Okay.

The first strain of Myxoma was actually too lethal for its own good.

It killed so quickly that it limited its own transmission.

But new strains evolved that killed their hosts more slowly.

Since the less deadly strains spread more easily, they gradually replaced the older strains, just like the more infectious Delta and Omicron variants of COVID-19 replaced earlier strains of that virus.

(Tanja) Today, Myxoma virus probably still kills a very large amount of rabbits.

But on its own, it would not be sufficient.

So, we need to continuously look for alternative tools and strategies.

(soft, tense music) (Shane) One of these alternatives emerged in the 1990s.

(Tanja) So the Calicivirus causes an infectious hepatitis.

It's one of the most virulent viruses that we know to affect mammals.

It was on track to be released, but it actually got away from an offshore island in Australia where they were doing field trials.

(Shane) Luckily, the escaped Calicivirus had already gone through rigorous testing to avoid unexpected impacts.

(Tanja) And again, it was spectacularly successful.

It knocked down populations by over 90% in some areas.

(Shane) Calicivirus is spread by flies feeding on dead rabbits.

Unlike Myxoma, it doesn't need to keep its host alive, so it probably won't become less virulent over time.

But rabbits could still evolve resistance.

(Tanja) And rabbits being rabbits, you only need to miss a few and they'll build up again.

(tractor rattling) (Shane) So, Australia's using every tool they've got.

(smoking device rumbling) (Tanja) Controlling invasive animals is not a solution that gives everyone the warm and fuzzy feelings.

But if you don't do it, the problem isn't going to go away.

(soft music) (Shane) Which leaves Australians in a tough spot.

(Martha) You don't forget the sound of that crack, when the rabbit's neck's broken.

And even as a seven-year-old, I knew that the rabbits were decimating the populations of native animals and plants that I loved.

But I could also see that those rabbits were themselves living things.

It's a very conflicted story for Australians.

(Shane) Biocontrol has completely reshaped this island nation.

And it's probably here to stay.

(mellow music) (audible chewing) (Raghu) On average across all of the weed biocontrol projects we've done in Australia, the return on investment is over 20 to 1.

(Shane) Okay.

Raghu Sathyamurthy is a biologist who studies invasive weeds and leads the CSIRO Biosecurity Research Program in Brisbane.

(Raghu) There's over 10,000 plant species that have been introduced into the country, which is why when you would have come through an Australian airport, the emphasis on biosecurity is very strong.

(Shane) "Biosecurity" means keeping unwanted organisms out.

-They don't play around.

-No, no, they do not.

-They do not play around.

-They do not play around.

(Shane) Biocontrol often entails bringing in non-native species, but Raghu thinks even on this island continent, biocontrol done right has a role to play.

(Raghu) We have a social license for doing this kind of work in Australia because of a hundred years of having done this and people having seen the benefits of it.

(Shane) Before the cane toad, there was the prickly pear, an invasive cactus that blanketed 60 million acres of Australia in the 1920s.

Researchers identified a South American caterpillar that only ate prickly pears.

Legions of workers deployed 3 billion caterpillar eggs.

And the prickly pear was vanquished.

(Raghu) The cane toad example came right hot on the heels -of the prickly pear example.

-Okay.

(Raghu) So, there was this sort of sense that biological control was easy, it's not!

They got a little too cocky is what you're saying.

(Raghu) Well, they got a little bit more confident than they should have been, yeah.

-Okay.

- Just a little bit of history repeating ("History Repeating" by Propellerheads/Shirley Bassey) (Shane) Today, Raghu's team studies new biocontrol agents in one of the most biosecure facilities in the country.

But like the Calicivirus escaping containment before its release, the risk will never be zero.

And as a great movie scientist once said... (soft music) ..."Life finds a way."

(Raghu) There is always inherent risk.

The question is, how does that risk compare to the benefits?

-Mm-hm.

-Doing nothing is also a decision that has its own risks and benefits.

Doing nothing means you're willing to live with the problem and its consequences.

(Shane) The cost of doing nothing is often what spurs us into action.

Especially when it's not just our crops or ecosystems at stake, but our very lives.

(whooshing) (mellow music) (motorbike engine humming) (laughing) (Shane) This is Nelson Grisales.

He never misses a chance to take his wife salsa dancing in the nightclubs of Medellin.

By day, Nelson's a medical entomologist focused on preventing mosquito-borne diseases.

Dengue fever is transmitted by a mosquito called Aedes aegypti, which can also carry Yellow Fever, Zika, and Chikungunya.

Exactly what is a vector?

And the way mosquitoes share pathogens with us... ...you're not gonna like it.

(laughs) It was gross before, and I didn't appreciate it.

And now it--now it's grosser.

But what if we could stop Dengue transmission from mosquitoes to humans?

(lively music) That's what Nelson and his colleagues at the World Mosquito Program are trying to do.

(soft music) Wolbachia are bacteria that naturally inhabit insects, living inside their cells.

(mosquito sucking) Aedes aegypti doesn't have its own strain of Wolbachia in nature, but researchers infected them in the lab with Wolbachia from fruit flies.

And they discovered something pretty amazing.

A mosquito carrying Wolbachia can bite a Dengue-infected human without transmitting the virus to its next victim.

The scientists realized they could be looking at a way to stop Dengue in its tracks.

(hip-hop music) But only if the bacteria would spread in a wild mosquito population.

Wolbachia isn't contagious like a cold.

Instead, mothers pass it to their offspring through the egg.

So, how can Wolbachia spread?

By manipulating mosquito reproduction.

Infected females produce infected young, whether their mate was infected or not.

Un-infected females mating with infected males don't produce any young at all.

So, with every generation, a greater proportion of the mosquitoes carry Wolbachia, until the whole population is infected.

The program's goal is to trigger this bacterial chain reaction by releasing Wolbachia-infected mosquitoes.

-Per week?

-Per week.

(Shane) The workers in this "biofactory" mass-produce Wolbachia-infected mosquitoes from eggs, to aquatic larvae, to flying adults.

Ugh, that's so many mosquitoes!

Aw, I'm just getting itchy looking at that many mosquitoes.

Male mosquitoes are happy to eat sugar water.

But the females, they're out for blood.

Theoretically, if I were in there, how long would it take before I'd look like a mummy?

(laughs) (blood trickling) No one's gonna volunteer for that job.

So, Nelson and the team feed the females with blood donated by local blood banks.

(hip-hop music) They are furious.

They are really going at it.

After a hearty meal, the females lay eggs and the cycle repeats.

This one facility can produce 30 to 40 million eggs per week.

Today, the World Mosquito Program is working in more than a dozen countries.

And that means convincing local communities that a solution to their mosquito problem is releasing more mosquitoes.

(motorbike engine humming) (energetic music) On the ground, that outreach is led by local community workers, like Maryory Betancur.

(bright music) Gaining the community's trust meant full transparency about the project's goals and its methods.

Today, the communities involved are seeing results.

Colombian cities where Wolbachia-infected mosquitoes were released have far fewer lab-confirmed Dengue cases.

And in 2021, a larger, randomized study in Indonesia found that releasing the mosquitoes reduced the chance of a person contracting Dengue by 77%.

So, the World Mosquito Program looks like a biocontrol success story.

But some experts have doubts.

(phone ringing) (Jason) I think if there's one thing that defines humans, it's our ability to twist the environment to suit ourselves.

("Gimme the Loot (Instrumental)" by The Notorious BIG) (Shane) This is Jason Rasgon.

In his home workshop, he's got a knack for repairing vintage pocket watches.

(Notorious BIG) Rolex watches and colorful swatches I'm diggin' in pockets, ... can't stop it (Shane) In the lab, Jason studies mosquitoes and the diseases they carry.

When the World Mosquito Program first showed that Wolbachia could block Dengue transmission, he was intrigued.

(Jason) And since we'd been working on Wolbachia for a long time, we'd been working on West Nile for a long time, it seemed sort of a natural thing to kind of put those two questions together.

We were basically honestly trying to jump on the bandwagon.

(soft, tense music) (Shane) Jason was working with Culex tarsalis, a mosquito known for carrying West Nile Virus.

(Jason) We developed techniques to artificially infect that particular mosquito with Wolbachia.

When we did that we found something a little unexpected.

The mosquitoes that were infected with Wolbachia actually had a greater probability of becoming infected with West Nile.

So, it actually increased their susceptibility to virus.

(mosquitoes buzzing) (Shane) Jason and other researchers dug deeper and found that Wolbachia could affect mosquito-borne viruses in a variety of ways.

(Jason) You can have highly variable results depending on what mosquito or what arthropod you're talking about, what Wolbachia strain you're talking about, what pathogen you're talking about.

(whooshing) Variation in environmental factors... -(rain pattering) -(water lapping) ...such as temperature or relative humidity.

(rain pattering) (Shane) And all these factors can interact in ways you wouldn't expect.

(Jason) You can have a particular Wolbachia strain in a particular mosquito that actually blocks one pathogen but might have no effect on a different pathogen or even the opposite effect on a different pathogen.

And so those are the kind of things that concern me a little bit when these things are being released into natural populations.

(Shane) And as someone who likes to understand how things work, Jason has another concern.

(watch gears clicking) (Jason) We still don't really have a good handle on what the actual mechanism of how Wolbachia blocks pathogens in mosquitoes.

(motorbike engine rumbling) (Shane) He's worried that we might not know enough to intervene or even recognize if something's gone wrong.

Not everyone shares Jason's reservations, but they illustrate why communities should approach these solutions with eyes wide open.

That's something, Jason says, that the World Mosquito Program does really well.

(Jason) The public engagement and the public participation was never done as an afterthought.

It was baked into the very foundation of the project from the very, very beginning.

I think that's a model for how these things need to be done.

(soft music) So, while I might not agree with every sort of technical decision that they make, I really want these things to work.

(mosquitoes buzzing) These are devastating diseases.

They affect, you know, billions of people.

(Shane) The question is, when are these approaches ready for widespread use?

And the answer is... (mosquito buzzing) ...that it kind of depends on who you ask.

I think whenever we're faced with a huge problem, we always have to balance two things: knowledge and action.

And sometimes it's very difficult to know, you know, when it's time for knowledge and when it's time for action.

(mellow music) Nelson and his colleagues take these questions seriously.

But communities around the world have decided that the benefits of controlling Dengue are worth a degree of calculated risk.

(soft music) Time and again, we turn to biocontrol when we have big problems to solve.

But some of those problems seem self-inflicted.

(whooshing) Crop pests thriving in our industrialized food system.

(energetic music) Ecosystems ravaged by species we've introduced.

And diseases perpetuated by our modern ways of living.

We act like these problems must have technical solutions.

But maybe there's another way.

(whooshing) (Dru) People often ask us, "Why do your carrots taste so good?"

And I'm a big believer that our carrots taste like the love that is in the soil here.

(Paul) Phew.

I'm gonna be thinking about that for a very long time.

("California Love" by 2Pac and Dr. Dre) (bike bell rings) (Dr. Dre) Now let me welcome everybody to the Wild Wild West (Shane) This is Dru Rivers.

She and her husband Paul are co-founders of Full Belly Farm here in California's Central Valley.

(liquid pouring) They might look sweet and harmless, but on cribbage night, it's a whole other story.

(Dr. Dre) And pimps be on a mission for them greens Lean mean money-making machines servin' fiends (Shane) Dru and Paul started farming here in 1984.

And right from the start, they wanted to do something different.

(Roger Troutman) California (Paul) We watched a lot of farms run by good farmers, that were family farms, go out of business because they depended on a model of having one or two crops, and weren't diversified.

So, we grow how many crops?

(Dru) Um, probably a little over a hundred different crops.

-Wow!

-Yeah.

(mellow music) (Shane) All this variety doesn't just protect Full Belly Farm from volatile markets.

It also keeps the farm productive all year long.

(Paul) We grow plants that feed soil, and we grow plants that feed us.

We want to make a healthy ecosystem here that we're part of, and we enjoy being part of.

(Shane) But even on this super-diverse farm, not every species is welcome.

(Paul) Pests can be devastating, right?

But what those pests are indicators of, oftentimes, are of unhealthy ecosystems, right?

(Dru) Yeah.

(Shane) For example, monocultures, vast expanses of a single crop, are particularly vulnerable to pests and diseases.

(soft, tense music) But the diversity on this farm fosters a kind of preventative pest control.

(mellow music) Consider Dru's gorgeous fields of flowers.

They're more than just another crop to sell.

(Dru) The other thing that the flowers do for us is provide this habitat, amazing habitat for all these beneficial insects.

(bee buzzing) Here's a lacewing right there.

This is a great example of a really beneficial insect coming in and loving-- loving the marigolds.

Um, and that lacewing could then go over to our broccoli field and eat the aphids that we don't want.

(Shane) And the spaces in between the crops have a purpose, too.

(Dru) One of the things we've done starting from the first year we were here is planting hedgerows around the farm.

Having that un-tilled ground around the farm also encourages places where snakes can live, foxes can live, all of which are beneficial to us.

(cow moos) (Shane) Biologists have a name for this holistic approach to pest management: "conservation biocontrol."

(bee buzzing) It's about making space for all of our allies that already live here.

Choosing nature's rich web of ecological checks and balances over a narrow technical fix.

(birds chirping) (wings flapping) It's almost as though, like, you're not the only farmers here.

All of these other species are also doing their part.

(Paul) Absolutely, the farm has been the teacher.

(Shane) Mm.

Do the natural enemies that you invite in to control sort of what we call pest species, are they taking care of it?

(Paul) You're never gonna have a 100% clean field, but we probably have a 98% clean field.

-Whoa.

-You know, our stuff is really clean.

I mean, we can't send produce to market that's damaged by insects.

(mellow music) (Shane) Conservation biocontrol isn't a panacea for pests.

There's still a role for targeted releases of traditional biocontrols.

But by embracing the idea of the farm as an ecosystem, Paul and Dru are getting a helping hand from nature.

And their approach works for them, in more ways than one.

(Dru) Of the whole 40 years that I've been here, I've never had a day where I didn't feel like I was doing the right thing in my life.

And I--I feel like as a species, more people need to have that in their life, that conviction of rightness in what they do.

(Shane) That's so profound.

Conservation biocontrol means letting a little more nature into our highly curated spaces.

It means placing trust not just in our own ingenuity but in ecological machinery eons in the making.

And it's a lesson that has implications beyond the farm.

("What They Do" by The Roots) Seeing this version of biocontrol, I think it's really beautiful in its simplicity.

Being wise enough to know when the solution was the thing that existed before you got there.

And being wise enough to at least try to put that system back together.

(Black Thought) Lost generation, fast-paced nation World population confront they frustration (Shane) One little farm in California isn't a solution to global problems, but it's a reminder that technical fixes aren't the only tools we've got.

Humans are amazing problem solvers, but part of our genius is also knowing when to stand down and let nature help.

(soft music) (Martha) There is no deeper thing to know and understand in human history than to understand our relationship with nature.

It's not the backdrop to the human story.

It's a main character in the human story.

And it is only by understanding that relationship that we will be able to thrive as a species into the future.

(mellow music) -(birds chirping) -(insects chirring) (Shane) We didn't conquer this planet alone.

And as we reckon with an ever-changing future, our alliances will be more important than ever.

So, we should remember the triumphs and tragedies of the past... (mellow music) ...weigh the risk of action and the cost of inaction... ...and resist the all-too-human urge to innovate our way out of every problem.

Because if there's one thing biocontrol can teach us, it's that we have extraordinary allies in the Tree of Life... (whooshing) ...and we rise or fall together.

(female announcer) This program is available with PBS Passport and on Amazon Prime Video.