Increase the number of flies to decrease the number of screwworms??

I’ve been reading an unhealthy amount into the plague that is the New World Screwworm (NWS)¹ after I saw articles^{2 3} introducing the strategy to stop the spread of this live-flesh eating larva, which devastates livestock, is to air drop billions upon billions of these hell spawn onto infested areas.

I did a double take when I read that. However it’s something counter-intuitive that you’ve probably seen before such as the introduction of apex predators in the form of wolves into Yellow Stone actually improved its ecosystem⁴. I turned to Systems Dynamics (SD) to better understand the rationale of the against-the-grain strategy to parachute countless sterile flies and I think I have the gist of it.

In this model I am assuming an initial population of 1 million Eggs, 7 day delay between Larva and Pupa, another 7 day delay between Pupa and Fly, a Fly lifespan of 30 days, a 1:1 Male:Female ratio, and a single female fly can lay 100 eggs per day during its lifetime.

Given infinite breeding sites and a warming climate there is near to nothing stopping the NWS to exponentially grow in numbers.

However the introduction of sterile male flies means that the probability of a female fly mating with a sterile male fly increases therefore the laid eggs are less likely to hatch into larva (because the eggs are sterile). What I found is that there is a tipping point of the number of sterile male flies introduced, lower than the tipping point the population of larva will oscillate with positive feedback until it explodes exponentially, greater than the tipping point the population of larva will oscillate with a dampening effect until it tapers down to 0 eventually.

The SD model underwent two major iteration removing unnecessary complexities but I see opportunities to further analyze the model for reinforcing and balancing loops.

If you’re interested to have a look at the model then here’s the link.

1. Centers for Disease Control and Prevention (CDC). (2024, September 13). New World Screwworm Myiasis. Retrieved from CDC website. https://www.cdc.gov/myiasis/about-new-world-screwworm-myiasis/index.html ↩︎
2. U.S. Department of Agriculture, Animal and Plant Health Inspection Service. (2025, April). Eradicating New World Screwworm with Sterile Insect Technique. Retrieved from https://www.aphis.usda.gov/sites/default/files/factsheet-eradicating-nws-sit.pdf ↩︎
3. Cross, G. (2025, May 11). New World Screwworm: Parasite threat forces US to halt cattle imports from Mexico. USA TODAY. Retrieved from https://www.usatoday.com ↩︎
4. Farquhar, B. (2025, March 10). Wolf reintroduction changes ecosystem in Yellowstone. Outside Interactive, Inc. Retrieved from https://www.yellowstonepark.com/things-to-do/wildlife/wolf-reintroduction-changes-ecosystem ↩︎