Science Friday - How Millions Of Flies Can Help Stop The New World Screwworm

Episode Date: July 22, 2025

The New World screwworm has cattle ranchers, entomologists, and the federal government on edge. The pest was successfully eradicated from the US decades ago, but has recently been moving north from South America into Central America and Mexico, with concerns that it may cross the border into Texas. It’s notorious for laying eggs in the wounds of animals and slowly eating them from the inside out. Host Flora Lichtman speaks with entomologist Sonja Swiger about past efforts to get rid of the New World screwworm, and why that process involves dropping millions of bugs out of airplanes.Guest: Dr. Sonja Swiger is an entomologist and professor at Texas A&M in Stephenville, Texas.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

I'm Flora Lichtenen, and you are listening to Science Friday. Today in the podcast, an unwelcome pest has cattle ranchers and tomologists and the federal government on edge, the new world screw worm fly. It lays its eggs when you're still alive on the body. And that's kind of freaky, right? I mean, it's never good to actually be eaten by an insect. And that's one of those things that creeps people out. This fly was technically eradicated from the U.S. back in the 70s, but now, an outbreak has been creeping further and further north into Central America and Mexico, inching

towards the U.S. border. This fly is notorious for laying eggs in the wounds of animals and slowly eating its victims from the inside out. But scientists have a strategy to stop the fly, and it involves dropping millions of bugs out of airplanes. Here to tell us more is Dr. Sonia Swiger, entomologist and professor at Texas A&M. Sonia, welcome to Science Friday. Thank you. Tell me more about these screwworms. Are they as horrible as they sound? Yeah, they actually are. So, you know, as anemologist, we do try to control pests when we can, but it's not usual that we try to eradicate these pests completely. And this is one of those that we felt just had to be eradicated to basically better the world, so to speak. So, yeah, it is definitely a very bad insect to have around. Why do they need to be eradicated?

The main reason is their impact to the industries. Of course, cattle being a huge industry for the United States and other livestock as well, but it's not just them. These flies will lay their eggs in all warm-blooded animals. So that's basically everything, right? And that includes dogs, that includes cats. When we had an outbreak in the Florida Keys back in 2016, they had a raccoon, they had a cat, they had a dog, they had a pig, and all these other animals, plus the deer that were infested with these flies. but they will attack a number of animals, including humans, which isn't common, but it can happen. Okay, so they're moving north towards the U.S. What is your level of alarm? It's definitely a concern, right? We have been able to maintain them at the Dary and Gap, which is a small area, geographically speaking, of the eastern side of Panama. And they've stayed over on that side for over 30 years. So keeping them down there made it easy. to contain them, lowered the impact to areas, of course, that we do not want them in. So the fact that

they have moved out of that area and are now significantly moved further north than what we would really like them to be, it does put us on edge just a bit and does make it a very big concern. So where is the management program happening now? Is it in the U.S. yet? It's in Mexico. So the United States funds that program, and obviously the flies were being managed. maintained in Panama for, again, 33 years with no concerns. But since this issues happened, they are now putting, they're dropping and releasing those flies at the most northern point. So we just recently had an announcement that they found another case further north. So I'm sure that all efforts are now being put into that area. Prior to that, they were all being

dropped around the O'Haka-Varacruz area of Mexico. The way they do that is they literally drop the actual adult flies kind of out of the plane. They release them as they fly over. And then they also have these stations on the ground where they will place several thousands of pupa and then they will hatch in that area. And they usually put those in places where they know they have a confirmed infestation or an animal, you know, that has larvae present on them in that area. Tell me the history of these flies, our relationship with them anyway. Well, they've been around a long time, right? So we call them New World Screw Worms. So everybody kind of gets saying like, oh, this is a new pest, but that's not how it really works. There's two species in the entire world that are known to be of this nature. And one lives on our side of the world, and the other side of the world over in Africa and Europe and other countries. And it's called the Old World Screw Worm. So that's why ours is called the New World Screw Worm, based on old histories and coming to the New World. So they've been here a long time back into like the mid-1800s. And as our industry started to grow in the

livestock side, and we saw the impact of this fly, it was felt that they needed to be managed. And the way to manage them, of course, was with that eradication program. What's the program like now? The program is very similar to what it was when they started it. So the program was developed back in the early 1900s, so around the 1930s into the 50s, there were scientists that worked with USDA. that developed a system on how to manage these flies. And part of fly management is understanding flies. And in order to do that, they had to learn how to grow the flies, which sounds a little funny. So we basically grow them inside a laboratory in order to understand them better,

and then also to kill them, which is a unique concept. But that's how anemologist kind of have to work so that we have populations we can basically handle. Okay. So they're mass rearing them. But then how does that help manage them? Once they figured out proper ways to grow them in large numbers, they then had to figure out a way to, of course, control them in large numbers. And the way that they came up with this fly was with a radiation program.

So when they hit the pupa stage, they radiate them with the processes. And they've changed the procedure of the radiation since the 1930s till now. And when they do that, it basically destroyed. them in a genetic fashion that prevents them from producing future offspring. So that makes them sterile. The reason it works for this fly and not other flies, which has always been a question, well, why do we do it for this one and not these blowflies or these house flies or all these other flies is because this species is known to only mate one time in their lifetime. And that's unique to this fly, as opposed to house flies, which will mate with multiple males and then

produce multiple offspring. Okay, so you radiate the flies, you make them unable to have offspring, and then you dump all of these radiated screwworm flies in the areas where they're living, and you're hoping that they mate with the wild ones, basically. Correct. And this species of fly in nature doesn't have large numbers. Now, when they're around, they're a significant impact, but they don't have large numbers. They will make millions of flies in the lab, and then they will dump all those flies in one area,

and what you're doing is oversaturating the wild population with these now sterile males in hopes that these males will be found by those wild females. And then as they keep mating with the sterile males, they think that they are bred in order to lay eggs. And they do lay their eggs, but they lay eggs that they lay eggs that. don't hatch. Right. Are there unintended consequences of dropping millions of flies? No, not of this species. Again, we've been doing this program basically since the 1950s. Since they've started it, they've never really had any hiccups with it. They've never had any backfiring of it because these flies do not mate with other species. These insects particularly are not playing a viable

role in any ecosystems. So taking them out doesn't impact anything else. And it actually saves the lives of animals and saves the lives of humans. And if you find some of our farmers who were around back in the day when they did it here in Texas, because Texas was the last state that actually got rid of it. So there were folks that are still farming today that remember doing this back in the 70s. And then I have a lot of guys I talked to that remember going out with like grandpa, you know, to find these flies. And it's fun to hear the stories. Okay, so we pushed this fly out of the U.S. decades ago. Why are they creeping back up north? So, you know, that's really tough to pinpoint. One of the issues is mild winters, because this is a species that is tropically based. It doesn't do

well in cold temperatures, but as you have consistent mild winters in an area that can expand its range, movement through the Dary and Gap has also been noted as a concern, and this is both movement of people and people and animals, and probably lack of knowledge, in addition to probably some lack of surveillance. But as you bring people through an area that may not be aware of an impact of a pest, and they're not teaching them about that pest, obviously you have a lack of education and then things get moved around that we may not want to have move around. It's like these diseases that we've, you know, all but wiped out but are recurring now, you know, and doctors aren't familiar with them. It sounds similar. Yeah, and that's what happens, right? We do so good and we clear things up, but then we forget to talk about them in the future. And then people forget what they are. And I know that right now education is definitely a very important tool that we can use to our effects because we need to tell people what this thing does and have them prepared. Is this the kind of thing that you read about in textbooks when you were studying? It was. When I was in grad school, it's like one of our shining lights. Look what we did is, you know, animologists and veterinarians. We eradicated this new world screw worm and how terrible it was,

like as well as a tick species that we got rid of. But they have actually come out of the textbooks. And I'm a little sad about that because I preferred them staying in black and white. There's been coverage of this in the news. And the headline is like millions of bugs might be dropped over Texas. this. How's the public responding? It's interesting. As usual, you want to avoid Facebook comments, but it's always. As always, but it's hard not to. I'll look at a few and then I have to just step away. But what is the concern? Are there conspiracy theories about it? What's the concern? There is. There's a lot of conspiracy theories. And of course, it's made up by the government. It's another scare tactic. And it's, again, this is just lack of education.

And most folks don't know anything about it. I mean, I didn't learn about it until I was in grad school to be an entomologist. So it is kind of like out of a sci-fi movie because it's such a freaky insect that you think, this can't be real. But it is real. I mean, you say it's a freaky fly. It is. For an entomologist to say that is, you know, that's real. Things are getting real.

What elevates it to freakdom for you? Well, I mean, the way that it prefers to grow its larva, you know, blowflies in general are a cool group of insects. I actually kind of like blowflies. I studied them in grad school, but their role is as decomposers. They come in, they clean up a carcass, right? Which is a disgusting thing to think about, but somebody's got to do it. So we're happy they're there and want to do that for us. But this one decided to be a little different and decided to make the carcass, which means it lays its eggs when you're still alive on the body. And that's, that's, that. That's kind of freaky, right? I mean, it's never good to actually be eaten by an insect, and that's one of those things that creeps people out anyways. I agree. Even though it doesn't normally happen. Completely agree. Okay, what's your message? Is there anything you want to leave the public with? Yeah, I mean, I think it's important that people do some research, look it up and realize that this is a real insect. We do kind of target talking to livestock folks, right, because we know people who own cattle and other livestock are going to be impacted. So we just want people to be aware, to understand its importance and the significance and the reason why we need to keep it out of the United States.

Thank you, Sonia. That's about all the time we have. Appreciate you coming on to talk. I appreciate it. Thank you. Dr. Sonia Swiger, entomologist and professor at Texas A&M. Thanks for listening. Don't forget to rate and review us if you like the show. And you can always leave us a comment on this segment on Spotify. We'd love to hear from you. Today's episode was produced by Kathleen Davis. I'm Flora Lichten. Thanks for listening.