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Sterile Flies vs. the Screwworm

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Name Date

Predict

The screwworm is a fly. Its babies, called larvae, eat the living flesh of animals like cows and deer. Scientists want to get rid of every screwworm fly in the United States. Could releasing millions more screwworm flies help get rid of screwworm flies?

Circle one. Yes / No

Why?

Watch

  1. Sterile flies beat a flesh-eating flyyoutube.com/shorts/1-vnaUjSylQ

Word of the day

Eradicate (ih-RAD-ih-kayt). To remove or destroy completely.

Word partComes fromMeaning
e-Latin ē-out
radic-Latin rādīxroot
-ateLatin -ātusmakes the word a verb
  • A radish is a root you can eat. Radish comes from the same Latin word rādīx.
  • In math, a radical is a root, such as √2, written with the radical sign (√).

Bonus word. Hominivorax (hom-ih-nih-VOR-ax) is the second half of the screwworm’s scientific name, Cochliomyia hominivorax. It joins Latin hominis, “man,” and vorax, “consuming.” Voracious shares the root, from Latin vorāre, “to devour.”

Questions

  1. Female screwworm flies mate only once in their lives. Males can mate many times. Why would releasing sterile males work less well against an insect whose females mate many times?

    Hint. Picture a female who mates first with a sterile male and then with a wild male.

  2. In 1954, scientists tested sterile flies on the island of Curaçao at two release rates. Three weeks in, 28% of egg masses were sterile where they released 200 sterile flies per square mile, and 53% were sterile where they released 800. Why do more sterile flies per square mile mean a larger share of sterile screwworm egg masses?

    Hint. Think about the share of males a wild female could meet that are sterile.

  3. The US Department of Agriculture (USDA) sterilizes screwworms by exposing the pupae, the stage between larva and adult, to gamma radiation. The sterile flies can still mate, but eggs from those matings never hatch. Why does the radiation dose have to sterilize screwworms without leaving the males too weak to fly and mate?

    Hint. A sterile male helps only if a wild female mates with him instead of a wild male.

  4. Since 2006, sterile flies released at a barrier in Panama kept screwworms from moving north out of South America. In 2026, the fly reached Texas after spreading north through Central America and Mexico. Why does a sterile fly barrier have to keep running every week, even in years when no screwworms live north of the barrier?

    Hint. Think about flies and infested animals that cross the barrier from the south.

Claim, evidence, reasoning

In a model valley, Plan A sprays the flies and Plan B releases 18,000 sterile males every generation. Both plans start with 2,000 wild females and drop to 1,000 in generation 2. By generation 5, Plan A still has 125 females and Plan B has 0. In Plan B, a female’s chance of finding a wild male falls from 1 in 10 to 1 in 901. Which plan works better once a screwworm population gets small, spraying or sterile flies? Make a claim, back it with evidence from the model and explain your reasoning.

Hint. Compare what happens to a female’s chance of finding a wild male in Plan B with what the spray does each generation in Plan A.

Claim

Evidence

Reasoning

Going further: paying for the barrier

  1. A sterile fly plant in Panama makes about 100 million sterile flies a week. USDA is building a plant in Texas planned to make 300 million a week. The early screwworm research cost about $250,000, and getting rid of the screwworm saved ranchers over $200 million a year in the 1950s. Should the United States keep paying to release sterile flies every week, even in years with no screwworm cases? Weigh the cost, the safety of the flies and the risk of the screwworm coming back.

    Hint. Think about what happened in 2026 when screwworms spread north. Then think about what a sterile fly can and can’t do to animals and people.

Exit ticket

The sterile insect technique depends on two things. Each female screwworm mates once, and sterile males far outnumber wild males. How do the female’s single mating and the ratio of sterile males to wild males work together to wipe out a screwworm population?

Hint. As the wild males drop and the sterile release stays the same, what happens to a female’s chance of finding a wild male?

Sources