🪱 What is the Great Worm Wall?

Today’s newsletter covers a billion-dollar effort to keep the continent of North America free from a gruesome flesh-eating parasite:

The New World screwworm.

It’s known as the screwworm, but it’s really a type of parasitic fly whose larva can literally eat animals alive. (nobody wants to see that, so I’ve included this cute worm cartoon instead)

Screwworms were a major problem for U.S. farmers in the 1930s and 1940s, infesting farms and killing livestock across the southern U.S.

They haven't been a real problem in the U.S. for decades, so many people today are unaware of the threat posed by the screwworm — and the innovative solution that allowed the U.S. to eradicate the pest by the early 1980s.

Texas A&M livestock and veterinary entomologist Sonja Swiger explained to PBS NewsHour:

What is the screwworm?

The screwworm is a type of parasitic fly that is native to tropical regions of the Americas.

While many fly maggots eat dead flesh, the screwworm larva is unique in that the mother lays eggs inside living animals.

The eggs are typically laid in open wounds. When they hatch, the larva eat the surrounding flesh.

They became a major problem for U.S. farmers in the 1930s, causing $10M in annual livestock losses in Texas in 1935.

That’s over $200M in today’s money.

Severe cases would see ranchers in Texas lose up to 20% of their livestock and up to 80% of new-born calves.

It was like a biological apocalypse playing out across American farmlands.

Building the worm wall

In the 1950s, American ranchers petitioned the U.S. Department of Agriculture to find a solution to the screwworm problem.

This was the heart of the nuclear age, and U.S. scientists were learning a lot about the effects of radiation on living organisms.

A Texas-based research team discovered that bursts of gamma rays could sterilize male screwworm flies without preventing them from mating.

This was the breakthrough everyone had been looking for.

The researchers quickly developed a plan using the technology, which came to be called the “sterile insect technique” (SIT). (The graphic below refers to the use of SIT on mosquitoes, but the same principle applies for screwworms)

The plan involved breeding massive numbers of screwworm flies and sterilizing the males with radiation.

Specialized aircraft would then release the infertile males into infested areas to mate with the females.

Since the screwworm females would lay infertile eggs, later generations would not be born, killing the targeted population. (see a sterilized fly that researchers marked in order to track it below)

Building the Wormwall

In 1953, the USDA conducted their first field test on Sanibel Island, Florida.

By the late 1950s, the screwworm had been eradicated in Florida.

The extermination campaign was then taken west through Alabama, Mississippi, and Louisiana.

This campaign involved unprecedented cooperation between:

• federal agencies

• state governments

• individual ranchers

Veterinarians and ranch hands were trained to identify and report cases of screwworm infestation.

The USDA would then use the information to strategically distribute sterile flies from aircraft.

International Collaboration

The campaign worked well… until they got to Texas.

The problem: the screwworm population spanned the U.S. and Mexico.

Because the insects usually didn’t file immigration paperwork, killing them within U.S. territory wouldn’t be good enough.

The flies could just fly across the border. 🪰

In 1972, the U.S. and Mexico signed the U.S. and Mexico formed the Mexican-American Commission for the Eradication of Screwworms.

The agreement oversaw the eradication program’s efforts south of the U.S.-Mexico border and established:

• A sterile fly production facility in the Mexican state of Chiapas

• Screwworm surveillance networks across Mexico

• Training programs for Mexican veterinarians and agricultural workers

• The creation of quarantine zones and inspection points

Where we are now

The screwworm extermination effort moved further south.

Eventually it reached Panama, where a permanent barrier zone was established.

A sterile fly production facility was established in Panama to maintain this barrier and prevent reinfestation from South America.

The Panama facility has became a global model for pest control, with the technique later applied to other invasive insect species worldwide.

Thanks to widespread international collaboration on the sterile insect technique (SIT), the screwworm was eradicated in:

• the U.S. in 1982

• Guatemala & Belize in 1994

• El Salvador in 1995

• Honduras in 1996

The fight’s not over: the USDA still drops 14.7 million sterilized screwworms over the rainforest that separates Panama and Colombia every week.

In total, more than 1 trillion sterilized insects have been transported from the 50+ sterile insect factories built around the world since SIT first started in the 1950s.

ART OF THE DAY