This week we talk about flesh-eating screwworms, weeds, and the US cattle industry.

We also discuss genetic modification, procreation, and tsetse flies.

Recommended Book: 1177 BC by Eric H Cline

Transcript

The term ‘autocidal control‘ refers to a collection of techniques that are meant to control populations of some type of living thing, animal or plant, by disrupting their procreationary capacity.

So rather than attempting to control pest by spraying poisons all over the place, or controlling plants you consider to be invasive weeds by launching huge weed-pulling efforts in the afflicted areas, you might instead figure out how to keep this current generation of pests and weeds from having as many offspring as they might otherwise have, and then repeat the process with the next generation, and the next, and so on, until the unwanted species is either eradicated in the relevant region, or reduced to such a small number that its presence is no longer such a big deal.

There are all kinds of approaches one might take in trying to achieve this sort of outcome.

Experimental genetic modification measures, for instance, have been tried in, so far at least, limited ways, the idea being to either make the disliked species less competitive in some way (by making them slower, and thus more likely to be eaten by predators, maybe), or by making them less likely to have offspring, or less likely to have fit offspring—the next generation becomes super slow and clumsy, or they’re carriers of a gene that keeps them from procreating as much, or at all.

That approach seems like it could be effective, and there are quite a few efforts, globally, that’re working to refine and perfect it with mosquito species in particular, specifically the ones that are carriers of malaria-causing parasites and similar maladies that cause immense harm to local human (and other mammal) populations.

There have also been attempts to spray mating grounds with pheromones that disrupt mating behavior, or to use what’s called the Autodissemination Augmented by Males, or ADAM approach, which has been used to decent effect in some trials, and which involves basically just sprinkling a bunch of male mosquitos with pesticide, releasing them into mosquito mating grounds, and then having them deliver those pesticides to the females they mate with.

All of these efforts are meant to reduce populations via some procreationary mechanism, while also attempting to ameliorate some of the other issues associated with other, widely used pest- and weed-control approaches. Most of which rely on some kind of chemical being introduced into the right environment, that chemical helping to kill or disrupt these populations, but in many cases also leading to unwanted, and often initially unforeseen side effects, like those chemicals messing with other species, getting into the groundwater and possibly being associated with maladies in humans, and so on.

What I’d like to talk about today is another approach, the sterile insect technique, why it’s become so popular in recent decades, and how it’s being used, today, to address a burgeoning population of a pest that was previously eliminated in North America using this technique, but which has recently become a problem, once more.

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The New World screwworm fly is thus named because its larvae, its baby offspring, are planted in warm-blooded animals. These offspring eat not just dead tissues, like the maggots of other flies, but healthy tissues as well.

These maggots are often deposited near wounds, like cuts or scrapes, but also injuries caused by the castration or dehorning of cattle, or orifices and other sensitive areas with soft tissue, like the corner of a host’s eye.

They don’t typically infest humans, but it does happen, and they’re most likely to be found on wild and domesticated mammals, the females of the species depositing somewhere between 250 and 500 eggs in the flesh of their hosts, the maggots screwing their way deeper into their host’s flesh as they grow, burrowing and eating for the next three to seven days, at which point they fall off and enter the next stage of their lifecycle. By that point the host may already be dead, depending on the extent of the damage these things manage to cause in the interim.

These flies were originally found across the Americas and on some Caribbean islands, and they have long been a headache for cattle ranchers in particular, as they will sometimes infect one cow or goat, and then work their way through the entire herd in relatively short order, causing enough damage to seriously injure or kill a whole lot of the rancher’s stock.

As a result, humans have been trying to get rid of these things for ages, but nothing seemed to make much of a dent in their populations until the emergence of what’s called the sterile insect technique, which is exactly what it sounds like: a method of autocidal control that involves sterilizing members of the species, usually the males, and then releasing them back into the population.

Variations on this concept were developed by a few different researchers in a few different places around the world in the lead-up to WWII, but just after that conflict, scientists working at the US Department of Agriculture realized that they could use x-rays to reliably sterilize male screwworm flies, and that if they did this to a large number of them, then released those males into the local population of screwworm flies, to the point where there are more sterilized males than non-sterilized ones, that would serve to dramatically reduce the size of the next generation. If you then repeat this over and over again, you can eventually wipe out the species in a given region, as they successfully showed in the early 1950s by eradicating all the screwworms on Sanibel Island in Florida.

The same technique was then used to kill all the screwworms on the island of Curacao, off the coast of Venezuela—that kill-off achieved in just seven weeks. Over the next few decades, sterilized male flies were then released across other afflicted US states, and both Mexico and Belize were able to kill all their screwworms in the 1980s, followed by Central America in the 1990s.

This approach was also applied to other pests, almost always those that either spread disease to humans, or threatened local industries, like cattle or agricultural industries.

For instance, tsetse flies, carriers of a parasite that causes sleeping sickness, were entirely or almost entirely eradicated from Tanzania, Zanzibar, Senegal, Burkina Faso, Nigeria, and Uganda between the 1940s and late-1990s, Aedes aegypti mosquitoes, the carriers of dengue and yellow fever, were sterilized by a bacteria called Wolbachia in Queensland, Australia, in the late-20-teens, which reduced the populations of this disease-carrier in trial areas by 80%, and Japan eradicated the melon fly, an agricultural pest, in 1993.

This approach to pest-control has become so popular that dozens of facilities have been set up in countries around the world, exclusively to breed and sterilize different species, which can then be shipped to where they will be released. The first of these facilities was built in Mexico in the 1960s, where Mexican fruit flies were bred and then shipped for release in Texas.

It’s maybe fitting then that a new round of construction is happening, today, intending to combat the renewed presence of screwworms in Mexico, which have been making their way up into Texas via these two nations’ cattle industries.

The US Department of Agriculture recently announced that it will be building a sterile screwworm fly facility in Texas, which has suffered due to the US’s recent decisions to halt the import of cattle from across the border in Mexico due to issues with screwworms hitching a ride on that cattle stock, and thus infiltrating US herds. The government tried several times to drop this cessation of imports, as the US cattle industry is pretty reliant on those imports, but each time they tried, new screwworm infestations were found, and the import halt was put back into place.

US cattle populations are already at their lowest level in decades, and that’s impacting meat and dairy prices, while also putting other warm-blooded animals in the afflicted regions, especially Texas, at risk.

The folks behind the new facility have said they hope to be up and running in relatively short order, aiming to be releasing sterile male New World screwworms into the wild within a year. This deployment will operate in tandem with other, more direct efforts, like fly traps and parasite-sniffing dogs stationed at ports of entry.

The concerns here are not just theoretical: screwworms alone cause an estimated $1.5 billion in damage each year, and the cost of implementing a sterilization program of this kind usually adds up to something like a billion dollars, spread across decades; not a bad return on investment.

These programs are not universally effective, though, as in some rare cases non-irradiated males have accidentally been shipped to their intended mating location, temporarily inflating rather than deflating population numbers. And while these programs are relatively cheap to operate on scale, the cost of producing enough sterilized males to make such an effort effective can be prohibitive when aimed at smaller regions, or when attempted by governments or agencies without the budget to see what can sometimes be a long-term project through.

That said, this approach does seem to work very well when done correctly, and while its ecosystem impact is not zero, as, for instance, predators who eat these pests might suddenly find themselves without one of their staple food sources, which can lead to knock-on effects across the food web, it does seem to be one of the least foodweb ripple-producing approaches, as genetic modifications can theoretically lead to far more elaborate unforeseen consequences, and the widespread spraying of chemicals has semi-regularly led to die-offs and maladies in other local species, in addition to sometimes causing long-term, even fatal health problems for humans who rely on local food or water sources.

Show Notes

https://archive.is/20250815192422/https://www.reuters.com/business/environment/usda-build-texas-facility-fight-flesh-eating-screwworms-2025-08-15/

https://www.oregonlive.com/business/2025/08/how-to-stop-flesh-eating-parasite-from-devastating-us-cattle-government-will-breed-billions-of-flies.html

https://apnews.com/article/fly-factories-flesheating-parasite-cattle-texas-429ce91225bbab4a45c9040f1be356a5

https://en.wikipedia.org/wiki/Cochliomyia_hominivorax

https://archive.is/14Rdk

https://archive.is/afmt2

https://archive.is/QfTvG

https://archive.is/dxbcZ

https://www.oregonlive.com/business/2025/08/how-to-stop-flesh-eating-parasite-from-devastating-us-cattle-government-will-breed-billions-of-flies.html

https://en.wikipedia.org/wiki/Sterile_insect_technique

https://en.wikipedia.org/wiki/List_of_sterile_insect_technique_trials

https://web.archive.org/web/20210416164524/http://www-iswam.iaea.org/drd/refs_files/195_The-Area-wide-SIT-Screwworm.pdf

https://www.iaea.org/newscenter/news/sterile-insect-technique-used-to-suppress-mosquito-disease-vectors-in-florida

https://www.cdc.gov/mosquitoes/mosquito-control/genetically-modified-mosquitoes.html

https://www.nature.com/articles/s41598-023-30722-9

https://pmc.ncbi.nlm.nih.gov/articles/PMC4313646/

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MacVoices #23065: MacVoices Update - 2023-01