Sterile Flies vs. the Screwworm

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 utterly.

• 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, from Latin hominis, “man,” and vorax, “consuming,” names the screwworm, Cochliomyia hominivorax. Voracious shares the root, from Latin vorāre, “to devour.”

Predict

1. An egg mass is the cluster of eggs a female screwworm lays on a wound. A sterile egg mass never hatches. Before you read the paper, take a guess. What’s the lowest percent of sterile egg masses that would make screwworm cases in a county start to fall? Give your reason in one sentence.

   Hint. Think about how many young flies one fertile egg mass can make.

Read the paper

Baumhover, “A personal account of developing the sterile insect technique to eradicate the screwworm from Curacao, Florida and the southeastern United States,” Florida Entomologist, 2002. The paper is free to read.

Alfred Baumhover, a USDA scientist, worked on the first screwworm eradication programs. His paper tells the story of the 1954 test on the island of Curaçao and the 1957 to 1959 program in Florida. Late in 1958, cases flared up in Broward County, Florida, even while sterile flies were being released there. Table 1 of the paper tracks that outbreak in four-week periods. Notes from the paper. Every period is 4 weeks except the last, which is 2 weeks. Cases are weekly averages and include confirmed and reported cases. Inspectors are the people collecting larvae from wounds. The egg masses for Jan 18 to Feb 14 come only from Jan 18 to 26, 1959. The one egg mass in the last period was fertile, the last sign of wild screwworms in the county.

1. To fill in the Percent sterile column, divide sterile egg masses by egg masses collected and multiply by 100. From Nov 23, 1958 to Feb 28, 1959, how did the percent of sterile egg masses and the cases per week change? And how many times larger was the last release rate than the first?

   Hint. Look at what cases did while sterility stayed near 75%. Then compare the first and last numbers in the release column.

2. From Nov 23, 1958 to Jan 17, 1959, about 3 of every 4 egg masses were sterile. The paper calls those eight weeks “time for almost three generations,” yet cases rose. How can a screwworm population grow even when about 3 of every 4 egg masses are sterile?

   Hint. If 1 egg mass in 4 is fertile, how many breeding females must each fertile egg mass produce for the next generation to be larger? Then look for conditions in the paper that helped young screwworms survive.

3. A note under Table 1 says the Jan 18 to Feb 14 egg masses include “collections only from 1/18-26/59 when released females began ovipositing sterile eggs due to anoxia.” Released females are sterile flies from the rearing plant. Why would counting egg masses laid after Jan 26 make sterile matings among the wild screwworms in Broward County look more common than they were?

   Hint. What does a sterile egg mass from a plant-reared female tell you about the wild females’ mates?

4. From Nov 17, 1958 to Mar 13, 1959, calf navels made up 415 of 491 screwworm collections (84.5%), and more than 36% of the wounds held mature larvae ready to pupate. Some ranchers stopped treating infested calves. Why does the sterile insect technique depend on ranchers inspecting and treating wounds?

   Hint. Every larva that leaves a wound and pupates becomes a wild fly. What does that do to the ratio of sterile males to wild males?

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