United States airdropped millions of live flies over the Panama-Colombia border.

And no, it wasn't one of our classic pranks, like when we dropped that big pie on Belize.

No, this is a part of a decades-long collaboration that keeps nearly the entire North American continent safe from—and bear with me here—the exact species of fly we're dropping.

But this massive, intricate, and deeply weird, quote, idiotic insect sex scheme is actually working, but how?

What on a day-to-day basis does it take to keep flying, flesh-eating freaks out of this much space?

The short answer?

A 24-7 bug factory, planes, boats, trucks, loads of inspectors, x-rays, cow blood, wound paint, and—this is true—donut holes.

Also grit and gumption and the spirit of a champion.

But as for the long answer, it starts with knowing our enemy—the New World Screwer.

And I know neither you nor I want to look at them, so here's the plan.

One, we're putting in a progress bar, two, I've encouraged my editors to make this as metaphorical as possible, three, dancing clown.

So here we go.

First, they're not nice guys.

If they were still bugging around the US today, they'd be costing farmers somewhere in the ballpark of a billion dollars in dead livestock a year.

And it's not just cattle at risk—these things can infect, even kill, any living, warm-blooded mammal.

Yes, including people.

Yes, including your dog.

Yes, including gritty.

These bad boys are about twice the size of a normal housefly, and a thousand times as rude.

Their 10-30 day lifespan begins when females lay their eggs hundreds at a time in a live animal's open wound.

Tick bites, scratches, fresh belly buttons.

They hatch as worms and hook into the animal's flesh, which they then eat, which is extremely painful and often deadly for the host animal.

Once the wormies are fattened up, they drop to the ground, pupate, emerge as flies, and the cycle repeats.

Okay, we're done.

This is Edward F. Knitley.

He grew up dealing with screwworms on his family's farm and went on to spend most of his career researching them, and here was his big finding—female screwworms only mate once in their lives, which means that if you can mess that up, they'd destroy themselves in a couple generations.

And after many years and many Thursday afternoon trips to the military hospital with his buddy

Raymond to blast worm larvae with x-rays, Knitley learned that you can sterilize a fly if you x-ray it 5.5 to 5.7 days into its pupal stage.

That discovery is the backbone of what I'm calling the Great American Worm Wall.

You can't see it from space—or from the ground, it's invisible—but it's nevertheless an extremely tight border.

Here's how it works.

Here in Pecora, Panama, this factory employs 115 people to cook up and churn out sterile screwworms—20 million of them a week.

The young flies are raised in a series of humidity- and temperature-controlled rooms that mimic the warm and inviting atmosphere of an open wound.

They eat a nasty brown goop made of reconstituted milk, egg, and powdered cow's blood thickened with some cellulose—which is actually good to know because I have all this powdered cow's blood in my cabinet, which I bought for like one recipe, and now I have all this leftover and nothing to use it for—so annoying.

Once the worms are old enough, they each get hit with radioactive cobalt-60, and it's off to the airport!

The sterilized pupae arrive in coolers, then get sorted into trays, and once they grow into adult flies, they go to a cold room where they get nice and sleepy before what is sure to be one of the craziest possible days any creature on this planet can have.

But now, the fun part—airplanes!

COPEG, the joint US-Panama commission that runs this whole thing, and also a heavy-lifting acronym, uses retired military turboprop planes that have been specially retrofitted to drop flies through the floor of the plane at an adjustable rate.

Once upon a time, technicians just threw cardboard boxes out, but those didn't always open on impact, which defeated the whole purpose.

Over the course of a four-hour flight, the plane will drop 2.1 million flies over this region of Panama—flies that will wake up as they hurtle down to Earth to eradicate their own species—or, as I like to put it, the screwworms screw worms to screw screwworms.

Now, the math-wizzes among you may have worked out that 2.1 million flies over six weekly flights doesn't add up to the 20 million the factory produces weekly, but there's plenty of use for the leftovers.

See, the New World screwworm hasn't actually been eradicated from all of North America—Cuba, for example, still has them, and they're still rampant in South America.

So, all those excess worms fight outbreaks when they happen—Aruba had ones in 2004 and 2011, and when they popped up in the Florida Keys in 2016, the USDA rocked up with 190 million flies, released three generations, and re-eradicated them in a matter of months.

Also part of that effort, they had 200 people trawling around the Keys feeding infected deer anti-parasite meds stuffed in donut holes, which implies the existence of a federal donut budget or FDB, which is exciting to me.

But this also brings up another part of the worm wall—the ground game.

Yes, the It's Raining Men strat does a lot, but COPEG also monitors animals across the

Darien, ensuring fertile screwworms that slip through the cracks don't make it far.

They work from 13 field posts, traveling on boat, horse, and motorcycle to monitor at-risk farms.

Every farm gets an inspection, either monthly, four-monthly, or annually, depending how COPEG has assessed their risk, and cattle leaving the Darien all get inspected, and any worms they have get coated in anti-parasite paint that would kill any screwworms that might be in there.

In all, the worm wall costs about $15 million a year—more money than the worm wall in my apartment costs, but an incredible deal for the money and lives it saves.

But the project didn't come out of the gates this well-oiled, or this cheap.

See, after scientists proved the sterile insect technique would work on screwworms in Curacao, the USDA took up the project here, east of the Mississippi, in 1957.

Two years later, screwworms were gone from the southeast, and the effort moved west.

We declared ourselves screwworm-free by 1966, and had a sterile fly factory up and running in Texas.

But maintaining the border in the west proved challenging and expensive—maybe because, while the eastern range of the fly was bordered by water, the western range was bordered by… well, more range.

With that in mind, in 1972, the US struck up a deal with Mexico—instead of having to maintain a 2,000-mile Great American Worm Wall at the US-Mexico border, how about we eradicate all the way down to here, the isthmus of Tejantepec, which is only 120 miles across?

The two countries agreed to split the bill based on the value of the livestock it would save in each place—Mexico paid 20%, while the US paid 80%, and by 1982, the Texas worm factory was closed and replaced by one in Tuxla, Mexico, and Mexico declared Sonora and Baja California Norte worm-free.

But why stop there?

If you're going to move the worm border here, why not move it here?

And so began the not-always-smooth process of trying to work an agreement with every individual country in Central America, having them work alongside the US, with whom they didn't all have the… awesome relationships at the time, to go screwworm-free.

And it worked.

From 1987 to 2006, they eradicated the screwworm from all these places, with the Central American country splitting costs with the US at a ratio of 15 to 85, and the US and Panama ending up with shared responsibility for staffing the border, and Colombia still having to give monthly permission to COPEC to drop a bunch of screwworms in the part of the buffer zone they control.

All to say, the Great American Worm Wall is a pretty amazing thing—it's effective, it saves tons of lives and money, and it does its job so well that most of us don't even know it's there.

Put another way, it's a great example of what's possible when you combine passionate, if slightly wacky scientists with government agencies that are actually willing to foot the bill for weird, expensive, but ultimately great solutions to big problems.

Too bad that lesson isn't applicable literally anywhere else.

Oh well.

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