America Beat a Flesh-Eating Fly by Farming Hundreds of Millions of Them

The way to wipe out the screwworm fly was to make vastly more of it.

After a successful test on the island of Curacao, the U.S. Department of Agriculture built a factory outside Orlando in 1955 that could turn out two million sterile flies weekly. It bred a special strain selected for frequent mating, nicknamed “the sexual athletes.” In April 1958, airplanes began dropping those sterile flies in a 100-mile-wide band across central Florida, forming a living barrier. No cases were reported north of the line by July. With help from a second, larger factory in Sebring that could make 50 million flies a week, infections in the Southeast fell to virtually nothing by September and to zero by February 1959.

Impressed by the Florida success, cattle owners in Texas and nearby states clamored for a similar effort. A small insectary near Kerrville made 20 million flies a week, but a real barrier needed more than 10 times that number. In 1962 the USDA opened a factory at a former Air Force base outside Mission, Texas, a 76,000-square-foot building that operated nonstop every day of the year. In effect, it was an artificial wound.

Trays of meat, blood, and water, heated to the precise temperature that triggers growth, moved through the plant on a monorail timed to the fly’s life cycle. Eggs hatched and larvae fed together on the bloody sludge in what one account called a “seething mass.” Mature larvae wriggled out and dropped into a water trough, which floated them to sawdust trays where they formed pupae. Workers packed the pupae into canisters and placed them in casks holding highly radioactive cobalt-60, which sterilized them. The sterile pupae went by refrigerated truck to distribution centers, hatched, and were loaded onto planes to be dropped by the millions.

Production kept rising because the fly was found to travel farther than believed. Scientists had thought it flew about 35 miles a week, so a 100-mile barrier would hold. Then research found that in some cases it could cover as far as 180 miles over seven days, forcing the USDA to widen the barrier to 400 miles and push flights deep into Mexico. It worked. By 1963, U.S. cases had fallen by 90 percent, and in 1966 the United States was cleared of screwworm. The clearing did not hold on its own, because infections kept cropping up as screwworms from Mexico kept entering the country, so millions of sterile flies had to be dropped continuously along the border.

The next factory was larger still. A joint U.S.-Mexican insectary near Tuxtla Gutierrez started turning out flies in 1976 and eventually exceeded 400 million sterile flies each week. After a reorganization, managers drew a critical line across Mexico in 1981 and pushed it steadily south to concentrate resources against infections north of it. The line arrived at the Isthmus of Tehuantepec during 1984, a year ahead of schedule, where a new barrier was established.