Cryogenically Frozen Flies Are Being Revived in Uruguay to Fight Screwworm Plague

The flies - bred in a lab - will be imported in "a dormant state,” revived and released from airplanes to combat the destructive screwworm.
Frontal close-up of a Sarcophaga fly, domestic species considered a pest by most people. Stock photo from Getty.

Millions of frozen flies will soon be on their way to Uruguay, which will then be reanimated and unleashed into the countryside to combat a plague of a parasitic fly species in the country: the screwworm, known locally as la bichera.

The screwworm fly, which is native to the Americas, gets its name for “screwing” into the flesh of warm-blooded mammals, then laying eggs which hatch larvae that eat living tissue. While most fly species larvae eat mostly dead flesh, the screwworm’s larvae attaches to living creatures and can cause severe tissue damage and death to its host, making them particularly dangerous for livestock like cows and goats.


In Uruguay, livestock production is a major industry, and the screwworm reportedly causes losses of $40 million dollars a year, according to the Uruguayan government.

Fernando Mattos, Uruguay’s Livestock, Agriculture, and Fishing Minister, announced the purchase of the frozen flies from Panama in a press conference, saying that it was time to “liberate ourselves from this great plague to the production system.” He also said that around 1000 Uruguayans are bitten each year by the screwworm, sometimes leading to grave health effects.

Uruguay will import the army of cryogenic flies “in a dormant state” and will “revive” the insects in a lab, Mattos said, then release them from airplanes. The flies were bred in a Panamanian laboratory and sterilized, unable to produce larvae. Beginning in September next year, officials will disperse millions of flies per week in defined zones that will build a “containment barrier” around the small South American nation along its borders with Argentina and Brazil.

Then, in four years, once the sterilized flies have settled in the region, the government will conduct a nationwide eradication of the screwworm within the country. The sterilized flies, now naturalized around the border, will create a natural blockade stopping new screwworm flies from entering the country. Essentially, the primarily male sterilized flies will compete to mate with the female screwworms in the area. Due to their sterilization, the females will not become pregnant. Sterilized flies do not reproduce, and as more and more are continually released, the remaining screwworm population will decline rapidly.


This tactic has proved successful in the past.

The technique first began in the 1950s in the United States and was slowly introduced throughout North and Central America over the ensuing decades. Collectively, nations from the U.S. to Panama eliminated the screwworm fly, creating a final barricade of sanitized flies in the Southern tip of Panama, on a border river with Colombia that separates Central and South America.

Mattos said that the plan is “not copy and paste, but it's a proven technique that we will have to adapt for the characteristics of Uruguay.”

The minister specifically pointed out that the length of their borders, 10 times that of the area that Panama still uses to keep the screwworm at bay, is also with two countries who are not implementing the technique. He said hopes in the future that soon this will become a “regional campaign” similar to the success in North and Central America.

“For now, Uruguay is alone, but we are raising the issue with our neighbors, to eventually achieve an agreement with the neighbors, which is fundamentally the same biological containment tactic to stop the reintroduction (of the screwworm).”