Friday, 20 January 2017

Screwworm Fly Eradication in Florida

Update: On March 23, 2017, the USDA Animal and Plant Health Inspection Service announced that screwworm has once again been eradicated in Florida. The release of sterile male screwworm flies continues to be an effective means of controlling this pest.

January 20, 2017


Since the spring or early summer of last year, Florida has been fighting an invasion of screwworm flies. The problem began in the Florida Keys, with Key deer being the animals worst affected, and a recent report of an infected dog in Homestead, Florida, tells us that it’s not over. This isn’t the first time screwworm has troubled Florida: history tells us it’s an old and familiar enemy. 

Decades of Battling the Screwworm Fly

   Animal and Plant Health Inspection Service
United States Department of Agriculture. "Slide of infested lamb."
  Special Collections, USDA National Agricultural Library
Accessed January 20, 2017
The gruesome habits of the New World screwworm fly, Cochliomyia hominivorax, make it an unwelcome insect everywhere it occurs. The parasitic and carnivorous maggots of the screwworm fly are responsible for illness and death in animals and humans alike, and the pest used to cause millions of dollars of lost revenues in American agriculture every year. 

In the late 1930s Edward Knipling, an American entomologist, realized that C. hominivorax had a weak spot, a vulnerability that might be exploited to control or even wipe out the fly. The female of the species will mate only once in her lifetime. Knipling thought that if he could somehow manipulate things so that matings were unsuccessful, the screw-worm fly would disappear.

The Sterile Fly Technique for Screwworm

 Special Collections, USDA National Agricultural Library.
 Accessed January 20, 2017
Subsequent to Knipling’s epiphany, Raymond Bushland successfully raised screwworm flies in the laboratory, making it possible to do research on them, and Hermun Muller used radiation to render the male flies sterile. The sterile males remained otherwise healthy and mated with female flies, but no offspring resulted. Thus, these scientists were able to break the life cycle of the screwworm fly, at least in the lab. The knowledge for an eradication program was there; all they needed was a plan and the resources to carry it out.
The first area to be tested with a screwworm fly eradication plan was southern Florida in the United States—it was warm enough there for C. hominivorax to survive the winters and spread north in the warmer months, but geographically isolated enough to be a good place for a trial run. Between 1957 and 1959 hundreds of millions of sterile male screwworm flies were released in Florida, Georgia, and Alabama. It worked. By mid 1959, there were no screwworm flies in Florida.

Screwworm Fly Eradication

Unknown. [1984].
 Special Collections, USDA National Agricultural Library.
 Accessed January 20, 2017
Efforts to wipe out the fly were then focused on California, Arizona, and Texas. This project was repeatedly thwarted, mainly because it was impossible to prevent the reintroduction of the fly across the lengthy United States-Mexico border. Realizing that the Isthmus of Tehuantepec in Mexico might provide a northern limit for the fly that would be easier to maintain, the eradication effort incorporated northern Mexico, and the effort was finally successful. By about 1985, all of Mexico north of the Isthmus of Tehuantepec, and the Continental United States - was free of C. hominivorax.
Screwworm fly eradication was so successful that it was worth continuing the effort. The US, in cooperation with Central American governments kept pushing southward, and by 1996, the screwworm fly had retreated to the Panama-Columbia border, there to be held back by the continual release of sterile male flies. Having perfected fly rearing and eradication methods, North American specialists were also able to come to the aid of Libya when the North American screwworm was accidentally introduced there in 1988.
The good news about the current problem in Florida is that we know how to deal with it. No doubt the release of sterile males will once again free the state of this horrifyingly destructive pest.

Further Reading About Screwworm

Food and Agriculture Organization of the United Nations. 1992 The New World Screwworm Eradication Programme: North Africa 1988-1992. Rome: FAO
Galvin, Thomas J., and John H. Wyss. 1996 “Screwworm Eradication Program in Central America.” Ann N Y Acad Sci. 791: 233-40.
Schmidt, Gerald D. and Larry S. Roberts. 2009 Foundations of Parasitology 8th Ed. Boston: McGraw Hill, 2009.
Delgado, Amy, Morgan Hennessey, and David Hsi. 2016 “Investigation into Introduction of New World Screwworm intoFlorida Keys.”  USDA

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