Original subtitles

Animals can sometimes reach such numbers they defy our understanding.

They seem to rise up and invade our lives.

They become super swarms,

So immense they're impossible to ignore.

From the outside, they can be the stuff of nightmares.

From the inside, they're a thing of beauty.

As we enter the heart of the swarm,

we'll discover what happens when they meet our world.

It's 1995 at a football match between Costa Rica and Italy.

Killer bees stage a pitch invasion of a different kind.

The players' panic is understandable.

These bees have been known to Kill.

To understand what the swarm was thinking, we recreated the encounter

and slowed it down to match the bees' reaction times.

In Costa Rica, pitch invasions by killer bees are surprisingly common.

Swarms like to make a beeline across open areas,

which can bring them right into the field of play.

By showing the action in slow motion,

it becomes clear that the footballers

are making the situation worse.

Swiping at the bees is a complete waste of time.

Their reactions are so fast, they can easily swerve out of the way.

It's also a very bad idea.

Movement not only makes them angrier,

it gives them a target to aim for.

The bees' lives depend on teamwork.

Retaliation is swift and devastating.

Although each bee can sting only once,

the sting left behind pumps out a pheromone,

that whips nearby bees into a fighting frenzy.

They join the attack and each new sting brings in more angry bees.

Unlike honey bees, killer bees rarely give up a fight.

But this swarm has other things on its mind.

It's house-hunting.

Swarms arise when a hive gets too big.

They view the camera as a possible new home.

Bees are sensitive to electrical fields.

That seems to make the camera even more desirable.

A killer bee sting is no worse than that of a honey bee,

but hundreds will sting at any one time

and it's this behaviour that makes them dangerous.

Lying down is a surprisingly good idea.

The players merge with the ground

and the bees simply fly over them, unable to find a target.

SIRENS WAIL

Killer bees were accidentally created in Brazil in 1957

by crossing honey bees with aggressive African bees.

Since then, they've spread northwards

as far as the southern states of America.

But there is a swarm that has created far more devastation

than any killer bees.

Morocco, December 2004.

One of the oldest swarms on the planet is on the move.

One of the ten plagues of Egypt, they still invade one fifth

of the world's land surface

and affect one tenth of the human population.

Even today, meeting a swarm is unnerving

as this home video footage shows.

Eurgh!

Mommy! This is crazy.

Oh, my God, there are going to be millions of these and they're all flying off.

There's so many of them just swarming over here.

They're still just coming, across the road on both sides of the car.

Oh, my God! ..Complete infestation...

Argh! Mommy! Mommy!

Swarm locusts start life as hoppers,

the offspring of a solitary locust,

forced together by a shortage of food.

The hoppers crowd together to create an insect army.

It's an army that marches on its stomach, eating everything in its path.

When they run out of food, they march some more,

crossing open ground in vast battalions,

travelling up to 1.5 miles each day.

They stay as a ground army for four weeks,

before changing into an air force.

As they fly, they synchronise their wing beats.

This reduces turbulence, making their flying more efficient.

Like fighter pilots, they fly in perfect formation.

But here, one billion locusts

are covering an area the size of Heathrow.

It would be an air traffic controller's nightmare,

but the locusts maintain formation

by keeping a set distance from each other.

They even have anti-crash detectors, to avoid mid-air collisions.

They react six times faster than a human pilot,

so even in such crowded air space,

split-second manoeuvres can get them out of trouble.

Such state of the art flight systems make locusts a force to be reckoned with.

When they reach the human world,

they're almost impossible to control.

They eat their own weight in food in a day.

A large swarm can get through 200,000 tonnes, enough to feed half a billion people.

Locusts aren't the only ancient swarm that still plagues us today.

The house mouse originated in Asia and spread with human agriculture.

When conditions are right, they can quickly become plagues.

In South Australia,

mice outbreaks happen on average once every ten years.

They erupt after a period of wet weather creates bumper harvests.

By the time they're discovered, it's usually too late.

It's every farmer's living nightmare.

One unfortunate farmer filmed the actual moment she realised her pest problem was out of control.

Oh! They're in my boots! Eugh!

Concerned for the safety of her pigs,

she throws caution to the wind.

Yuk! Oh!

SHE SCREAMS

Among the pens, the mice found a limitless supply of food and safety from predators.

It didn't take long for their numbers to go through the roof.

The secret of their success is a rampant sex life

that turns breeding into a weapon.

A pair of mice can have babies every three weeks.

These, in turn, can breed just five weeks later and their offspring, five weeks after that.

Unchecked, the numbers just keep growing.

In theory, one mouse and her offspring

can produce 3,000 mice a year.

With few predators to control them, theory rapidly comes fact.

In one outbreak, 35 million mice were killed in just one month.

Even this hardly dented their numbers.

Eventually, stress and disease take their toll and the population crashes.

For every 1,000 mice, just two will remain.

But they're breeding machines,

ready to surprise us at any time in the future.

Some swarms are more predictable.

This eerie invasion happens like clockwork

once a year in the Midwest of America.

It can be so dense, it slows traffic to a crawl.

The swarms can smother an area of 4,500 square miles

and even show up on radar.

These are mayflies.

They have emerged from the Mississippi River,

where they lived as larvae for the past year.

They will exist as adults for only a day.

Their cue is a water temperature of exactly 17.5 degrees centigrade.

They hardly have a chance to try out their new body and wings

before it's all change again.

For some unknown reason, the body splits

and an almost identical mayfly emerges.

The previous form may have existed for less than an hour.

They soon spread their second set of wings, this time in a mating flight.

By emerging together, there's safety in numbers.

Nothing could eat this many.

They soon return to the Mississippi to lay their eggs.

Each female lays up to 8,000, making up for the billions of mayflies

that never make it back to the river.

It's easy to get lost. They navigate using the moon as a reference point.

But near towns,

this ancient navigational system is next to useless.

Artificial moons are everywhere and these beckoning lights cause

mayflies to become hopelessly confused.

The road also reflects light like the river,

attracting the mayflies down to lay their eggs.

As the mayflies are lured further into town, they spread their own haze of confusion.

The locals have seen it all before.

It's mayflies' breeding day...

in South Ballston Virginia.

I don't know if you can see this, but this is mayflies, and they come out in the gajillions!

And these things, they come out like this

on certain days, all at once

and they cover the streets of downtown South Ballston,

and probably just about anywhere where the damn river runs through.

It sounds like the wind is blowing out here, because there's so many of them.

It's just amazing, you just wouldn't believe it.

There are certainly huge numbers,

an estimated 18 trillion emerge for one night only.

3,000 times the world's human population.

Many fly straight to the bright lights of the city,

10,000 may cover a single neon sign.

By extraordinary coincidence, the invasion often coincides

with one of America's most important celebrations -

Independence Day.

As crowds fill the streets of La Crosse, Wisconsin,

they are outnumbered by mayflies 25 million to one.

Stand too close to a light and you certainly know it's mayfly day.

Fortunately, the mayflies are totally harmless.

They have no mouth parts, so they can't even feed, let alone bite.

To us, they seem out of control,

but inside the swarm, it's a different picture.

These angelic forms easily avoid each other,

even in the thickest gathering.

As America celebrates its independence,

the mayflies enjoy the last day of their life.

Bird swarms invade our cities too, a fact which many find unnerving.

There's some of them. Oh, my God!

It's like The Birds, it's like the invasion of The Birds over here.

Look at this!

We're driving through them.

I have never in my life experienced this.

Ooh, my gosh!

I'm very shocked at this.

In recent years, huge flocks of roosting birds have become a common

sight in many cities, but nowhere do they reach the numbers seen in Rome.

Here, they have become a super swarm that gets bigger year on year.

Ten million starlings flock together

to create the most astonishing aerial display in nature.

These spectacular aerial displays are seen

between October and February,

when birds pour into Rome from all over northern Europe.

To create such perfect synchrony,

it was once thought that the birds might use telepathic communication.

But in reality, the birds react 13 times faster than any person.

They turn in exact synchrony with their nearest neighbours,

but the tiniest delay in their reflexes help create these mesmerising waves in the sky.

Until recently, the spectacle could only be seen from the ground.

Now it's possible to take cameras into the swarms and fly among them.

This aerial ballet acts as a beacon to others still entering the city.

Smaller parties join the bigger flocks, increasing their numbers.

As they swirl in the sky, they collectively make decisions on where they should spend the night.

After an hour of pirouetting, the birds dive downwards,

having decided together where they will roost.

Birds are drawn to cities because they're warmer than the surrounding countryside.

Rome, with its balmy Mediterranean climate,

is an especially popular destination.

But the city pays a heavy price for hosting this incredible air show.

Each night, ten million birds produce

seven tons of excrement, playing havoc with the paintwork of cars.

As the birds settle down for the night,

there is a constant rain falling below.

Slippery roads and pavements cause accidents too,

it's a health and safety nightmare.

But the starlings don't have it all their own way.

They may look like extras from a sci-fi B-movie,

but these people really are wearing their work clothes.

They are starling busters, hired to clear the streets of this urban menace.

Their weapon of choice is a loudspeaker.

BIRDS SCREECHING

It plays back a starling's alarm call, amplified 100 times.

For these birds, there is simply nothing scarier.

This roost of 100,000 starlings will have to up sticks and find somewhere else to roost.

This bizarre experiment has been running for the last 12 years.

It certainly moves the starlings,

but Rome offers plenty of alternative accommodation

and the number of winter visitors keeps rising.

Some swarms only invade our cities once in a generation.

It's July 2008, and in the suburbs of Cincinnati,

an alien invasion is about to begin.

These periodic cicadas have lived deep underground

for the last 17 years, but now, under the cover of darkness,

they start to emerge.

Across vast areas of the state of Ohio,

ten billion cicadas join the alien invasion.

They have spent their subterranean lives sucking the sap

from the roots of trees.

Now, in the 17th year, when the ground warms

to 18 degrees centigrade, they all come to the surface.

They make their way towards the nearest tree and immediately start to climb.

Their marching orders are genetically programmed

and so is their sense of timing.

But genetics couldn't predict the changes made by those living in the world above.

These cicadas are the lucky ones.

Each year, more of the surface becomes concreted over, leaving millions trapped beneath.

It's Memorial Day.

In the 17 years the cicadas have spent underground,

America has fought three wars and had three Presidents.

And the human world has been transformed.

With people absorbed in the commemorations,

the cicadas go mostly unnoticed.

But all this is about to change.

The cicada nymphs break out of their old body,

a miracle that can take as little as ten minutes.

Soon, all the cicadas undergo the same extraordinary transformation.

But before these crumpled wings can fly,

they must be pumped full of blood, a marvel of insect hydraulics.

Their gossamer wings need time to harden.

While they wait, they're often noticed by their human neighbours for the first time.

The combined racket reaches 100 decibels, far louder than a jet flying overhead.

The male makes the sound by vibrating a drum-like organ hidden in his body.

It's not a good time for those with sensitive hearing.

Adults have seen and heard it all before, but for kids it's a totally new experience.

One they won't see again for 17 years.

By appearing all together, cicadas swamp predators with too much food to eat,

but their trick of using such an odd intervals of years is a masterstroke.

It stops any predator breeding to exploit the feast.

They count their 17 years below ground by measuring seasonal changes in the pressure of tree sap.

Although millions are eaten, the majority manage to find each other and mate.

After mating, they use their egg-laying tube as a saw

to cut a slit under the bark, laying 30 eggs in each incision.

Job done, they drop dead in their millions.

Having spent the span of a human childhood underground,

their adult life is over in just three weeks.

17 years is the longest cycle of any breeding swarm.

Most occur once every year.

These Cuban land crabs have spent the last 11 months

hidden in the rainforest.

Today, they begin a six-mile march back to the sea.

The crustacean army is 100 million strong

and their cue to migrate is triggered by the spring rains.

The hordes have to cross a coast road

that forms a barrier between land and sea.

It's hardly a motorway, but for the crabs it's an accident black spot.

For unsuspecting drivers, coming across the invasion

is 2 memorable experience, making some reach for the camera.

Those that ignore the crabs usually pay the price.

One garage mends 100 punctures a day.

Despite the carnage, the crabs are distinctly unsentimental.

Seizing the chance for a roadside snack.

Fortunately for the crabs' ultimate survival,

most of the millions make it to the other side,

then it's an easy sprint to the sea.

Land crabs came from the ocean.

They need to return here to lay the eggs

that will secure the next generation.

Crabs only swarm in the breeding season, but some animals live their entire life as a swarm.

It's May 2008 and the rains have arrived

in the mountains of East Africa.

One of the most organised swarms on Earth has appeared above ground.

They're driver ants, an insect with a fearsome reputation.

In the rainy season they form these extraordinary hunting trails.

Large soldier ants line the trail,

protecting the smaller workers inside.

Their massive jaws create an impregnable barricade.

The soldiers create these protective tunnels whenever they cross open ground.

The trails radiate from the nest in every direction,

some stretch 100 metres.

A worker running at full pelt takes 45 minutes to run its length.

The soldiers protect the trail at any cost.

Although blind, they're highly sensitive

to vibrations and air currents

and become instantly defensive when under attack.

Stress pheromones put the whole task force on alert.

They're ready to take on anything, including people.

They're programmed to keep the trail moving or die trying.

Driver ants are the stuff of legend.

It is said that they Kill everything that crosses their path.

It is said that no animal is safe when they're on the warpath.

It is even said that they'll enter huts to attack people

or kill babies left unattended in their cots.

BABY CRIES

The truth is somewhat different.

Although the bites are painful, against people they're purely defensive.

The jaws may slice through human flesh like butter, but it's simply a warning.

Despite the myths, driver ants are still ruthless killers,

but in a way that often benefits the villagers.

Dangerous pests like scorpions are quickly set upon by the ants.

Even the scorpion's deadly sting is powerless

against this invincible army.

Both workers and soldiers join the attack, dividing up their roles according to their size.

As some look for a chink in its leg armour, others prise open its body plates like a tin opener.

With its sting immobilised and faced by such overwhelming odds,

the scorpion eventually gives up the fight.

The dismembered body is hauled back down the trail to feed the nest.

The farmers' fields provide even more opportunities for the attack force.

Pests disturbed by digging are soon dispatched by hundreds of razor-sharp jaws.

The ants make a clean sweep,

capturing up to 100,000 insects in a single raid.

Despite the ants' formidable reputation,

most farmers value their role as pest controllers.

Their feelings about another African swarm are equally surprising.

Once a month throughout the rainy season,

mysterious clouds rise over Lake Victoria, Africa's largest lake.

This smoke on the water is not quite what it seems.

It's actually a living entity

comprised of trillions of individuals.

They're lake flies, a kind of midge that emerges from the water

in astronomical numbers at the time of the new moon.

There have always been flies here,

but increased pollution from fertilisers and sewage,

while damaging the lake,

is encouraging these huge columns of flies.

In terms of sheer numbers, these are now the biggest swarms on earth.

Eventually they make landfall

and people encounter the flies for the first time.

Although the air is choked with billions of flies,

daily life continues.

It's rumoured that the flies can suffocate people by sheer number.

In reality the lake villagers are surprisingly blase

about this living smog.

Like many swarms, the midges emerge together to swamp predators

with too much food to eat at any one time.

Birds even migrate to the lake

to make the most of this monthly free lunch.

But it's not only birds

that can't bear to see so much protein go to waste.

Such incredible quantity of biomass is equivalent to huge herds of game,

but far easier to catch.

A frying pan makes the perfect weapon.

The flies are so valuable the whole village joins in the hunt.

The saucepans have been dampened first to make the flies stick and then clump together.

These midges are so nutritious,

they're valued as a supplement to the local diet.

Each patty contains half a million flies

and has seven times more protein than the average beefburger.

For people who have little protein in their diet,

this burger bar is a health food centre.

Each village has its own recipes,

but fly burgers always go like hot cakes.

In times of famine, this traditional method of dealing with the monthly invasion becomes ever more relevant.

Both ants and flies are swarms with a positive side,

but one African plague has no such redeeming features.

These are the most numerous birds on Earth with a growing population

that may now be more than ten billion.

The largest flock on record consisted of over 40 million birds

and took five hours to pass.

Sometimes known as the locust bird,

these are quelea, a type of weaver.

They follow the seasonal rains in an endless search for food.

They eat grass seeds but, unfortunately for African farmers,

they prefer the more succulent domesticated varieties,

getting through 10,000 tonnes a day.

Smaller parties break off from the main flock to make sneaky raids on the farmers' fields.

Even a relatively small flock will demolish

an acre of rice in less than an hour.

SCREAMING

Subsistence farmers suffer the most.

It's a full-time occupation keeping the crop raiders at bay.

HE SHOUTS

It's like trying to dam an ever-flowing river.

As the flocks separate and reform,

predicting where they might land is anyone's guess.

With their livelihood at stake,

the whole village creates a taskforce

to try to stop them settling.

HE BANGS DRUM AND SHOUTS

A flock of one million quelea consume 60 tonnes of food a day

but, as they're always on the move,

they're almost impossible to control.

Only when the birds roost do the farmers get some respite.

Eventually the birds have to stop their endless travelling.

Nesting is their most vulnerable time.

The red-billed males do the main work,

weaving up to 20 of these intricate,

bowl-shaped nests to attract different females.

He takes the greatest of care, as his quality of work is being judged.

Only the finest weaving passes the test.

If she approves, she moves inside.

But stopping to breed has made them vulnerable.

A colony may cover four square miles and hold 300,000 birds,

all breeding a new generation of crop raiders.

As sunset draws the birds back to the nest,

they pay the ultimate price for feeding off our success.

COUNTDOWN IS SHOUTED

In the birds' world, this is Armageddon.

Over 100,000 annihilated in a single blast.

The inferno brings temporary relief to nearby farmers but does nothing

to stop the ever-expanding growth of Africa's quelea population.

We may unintentionally encourage swarms

but we cannot control them.

It's August 2008.

In the Mississippi and its many tributaries,

a purely man-made swarm is on the rise.

In the 1970s, silver carp were accidentally

introduced into these waters after escaping from a fish farm.

They now outnumber local fish by 10-1.

They have some truly bizarre behaviour.

Some can clear three metres in a single leap.

They say when fish are leaping, it's good for anglers.

It's certainly the case in this part of the river.

Here, fishermen don't even need a rod and line.

Fish are only too willing to do the job for them.

It's a phenomenon that takes some getting used to.

Whoal!

I got him too! There's a guy.

Look out! SQUEALING

Hey! LAUGHTER

That could be cut bait.

That could be cut bait.

Hey! We don't even have to get the fishing pole wet! Look out!

Damn it, let's eat that thing, see what it tastes like!

It's not just anglers who are affected.

Pleasure craft have to run the gauntlet too.

The secret of this extraordinary behaviour lies in the boats' motors.

The fish mistake pressure waves from the propeller

for the movement of predators

and literally jump with fright.

They react in alarm to every passing boat.

With numbers close to bursting point,

each leaping fish scares its neighbour,

creating a dangerous chain reaction.

As some weigh over 40 pounds, collisions can be deadly.

In some parts of the river, 200 fish missiles can launch at any one time.

They've already caused serious injuries and,

as they spread to other rivers, the danger increases by the day.

Silver carp are a swarm that we accidentally created,

living proof that, as we interfere with the balance of nature,

we create more swarms.

The honey bee is the only swarm we have ever domesticated,

but we're still unravelling the mysteries

of much of their behaviour.

For those who understand the swarm's inner mind,

it's possible to control them in a way that is quite extraordinary.

The bees are being summoned by a secret signal,

one that causes them to act with a single mind.

Swarms are made from thousands of organisms

but they work by acting as one.

By understanding the swarm's secret language,

it's possible to make them do the most extraordinary things.

100,000 bees cover his body, creating a living suit.

He has such control over the bees he rarely suffers a single sting.

He is a professor of entomology,

and has been studying bees and their communication all his life.

By covering his body with the pheromone that the queen bee

uses to control her hive, he has them at his command.

They react by protecting him with their bodies.

To them, he is the queen bee.

Using science, he has learnt to control what most of us dread,

and the result is quite disturbing.

As he brings a swarm into our world,

people are forced to confront their fears.

They also see that, with knowledge,

such an incredible force of nature can be controlled.

When swarms invade, we are inevitably separated

by an invisible barrier created

by our different senses and perceptions.

Swarms may share our world but the gulf between us couldn't be greater.

To control the swarm, we need to understand how they see the world.

But swarms not only experience things differently from us,

they think in a different way too.

The next programme will explore the extraordinary alien intelligence

that controls the swarm.