Screwworm Eradication: The Real Story

By: John Richard Schrock

The recent discovery that the screwworm fly has invaded southern Mexico and infected cattle (not yet at the U.S. border) has resulted in considerable misinformation from some official sources as well as a new generation of ranchers born after its prior eradication from the U.S.

Three erroneous statements are: 1) this is an invasion of a new U.S. pest coming from a foreign nation; 2) we need to develop better pesticides to treat infected cattle if the fly arrives in the U.S., and 3) our new sterile release facility being built in Texas is the first such American effort.

These statements are false. 1) The primary screwworm (species Cochliomyia hominivorax) originated in North America down into South America from before pioneer days. 2) This parasitic fly also attacks deer and other mammals, making it impossible (and a waste of time) to just treat wounds on cattle. And 3), the very first sterile release factories were part of the original effort that successfully eliminated this pest and were built across the southern U.S. starting in Florida in 1957-59.

And some printed discussions focus on blaming rather than cooperating with Mexico. But it was Mexico’s full joint cooperation with the U.S. that drove this pest from the U.S. and Mexico and down to Panama, where sterile release factories continued to maintain a barrier. With the ongoing import and export of beef up and down North and South America, it is not surprising that there have been cattle from the infected regions below Panama that have been transported to the north.

Because this parasitic fly was eliminated from the U.S. by 1981, it is not surprising that only the most elderly U.S. ranchers would have any memory of the prior U.S. eradication efforts. At the time of its eradication from the U.S., the U.S.D.A. estimated that the American livestock industry “benefits by more than $900 million a year as a result of the eradication of the screwworm.”

The screwworm fly is deadly for many cattle due to laying eggs on any cut or scratch. The fly larvae (maggots) feed on the wound and secrete chemicals that expand the wound so that more fly eggs can be laid. The wound then expands, often to a point where it is fatal to the cow. Because the fly also infests wild deer and other animals, normal chemical poisons are not effective in eliminating the pest.

In 1938, with the development of X-ray machines and the realization that radiation could cause sterility, Dr. Edward Knipling was working for the U.S.D.A. and realized that the female flies only mated once. It would be possible to raise, radiate and release sterile flies in large numbers. After several generations, the flies would go locally extinct. The system worked, preventing further generations of flies on both domestic cattle and wild deer, etc. Along with colleague Raymond Bushland, Knipling received the World Food Prize in 1992 for having increased the world food supply.

In 1988, a shipment of cattle from South America had transferred the screwworm to Libya where it began to spread on the northern coast of Africa. This would have been a disaster for a poor continent that would not only lose its cattle, but also a wide range of wildlife. Sterile flies were immediately exported from the screwworm production plant in Panama, and a disaster was averted.

A fuller detailed and illustrated story of this remarkable new insect control method—only successful with the few species that only mate once—is described in the Kansas School Naturalist on “Screwworm Eradication” published in October 2014. It is sent free to teachers and others upon request, and carries no advertisements. Readers can request a free copy of this “Screwworm Eradication” issue by writing to Kansas School Naturalist, Biology Box 4050, Emporia State University, Emporia, KS 66801 or sending an e-mail to [email protected].

Several years after this issue was written, the screwworm entered southern Florida, probably by a dog or other animal carried on the continuous small boat traffic that crosses the Carribean. The sterile flies being produced in Panama were immediately distributed in Florida and soon drove the brief Florida invasion to local extinction (extirpated). Copies of this KSN were sent to newspapers and biology teachers in southern Florida. And they got the story correct.

– John Richard Schrock is editor of the Kansas School Naturalist at Emporia State University in the United States and also teaches various classes at universities in China.