A Scourage Difficult to Defeat

A SCOURAGE DIFFICULT TO DEFEAT

ARTICLE BY JANICE BRYSON

Who would have thought a female fly could be “A Scourge Difficult To Defeat”? The screwworm fly was reported in the American West as early as 1825. She was potentially fatal to any warm-blooded animal, including humans, but most particularly cattle.

The female fly, after a one-time mating, deposits eggs in wounds and abrasions. When the eggs hatch within approximately twenty-four hours depending on the weather, the larvae sustain themselves by eating only live flesh. These larvae can grow as large as one half inch long. Dropping to the ground after four to seven days, they search for a suitable pupation site – usually within the first inch of topsoil or under leaf litter, rocks, or fallen limbs. During this time, the larval skin shrinks and hardens to form the puparium, which is dark brown. This stage may last seven to ten days depending on the temperature. Adult flies will emerge to mate and the cycle is repeated. In five to seven days, a full grown steer can die if the infestations are untreated.

Southern Arizona rancher Stuart Krentz wrote in his memoirs that the screwworm “altered the cattle business….from the first day of June, when the first rain started, until the cold weather - - all we ever did was watch for screwworms...You had to get all your branding work done ….before the screwworms started.”

Keepers of The Range – The History of the Arizona Cattle Growers relates the history of the screwworm problem in cattle. In the early 1930’s, the screwworm was costing livestock producers in the Southeast $400 million annually. Agricultural Research Scientists Edward F. Knipling and Raymond C. Bushland began to look further than topical wound applications for screwworms. In 1946, Knipling, now working at the USDA, read a paper discussing radiation’s use to alter the genetic material of insects. He became convinced the radiation could sterilize male screwworm eggs. Sterile flies would mate with native females, the eggs would not hatch and no offspring would be produced. The screwworm problem would eventually be solved.

Airplanes dropped sterilized male flies and by 1959 the Southeast was free of screwworms. America’s Southwest was far more extensively infested and the screwworms there were the next target. The area was cleared for a time; but with cattle and flies traveling between the United States and Mexico the Southwest began to reinfect.

Arizona rancher John Hays as well as ranchers from Texas testified before a congressional agricultural subcommittee advising that Arizona and the Southwest were again suffering from cattle-killing screwworms. Today, in 2019, discussions are being held on a border fence between the U.S. and Mexico; and so it was in the 1960’s when Davis and Hays testified. They argued that although a permanent 2,000 mile fence might prevent an influx of screwworm-inflected cattle for a while, they had another suggestion. A better solution was a subsidized program set up to introduce irradiated male flies into Mexico which would push a buffer line back to just north of Guatemala.

Arizona cattlemen had levied one dollar a head on their range stock and contributed $350,000 matched by the state legislature to the control effort. The Senate agriculture committee approved the joint U.S. and Mexico program winning praise from Arizona Senator Carl Hayden. Then Senator Lyndon B. Johnson of Texas supported the Texas sterile fly program from the beginning and obtained funding as well as dedicating the opening of the Mission, Texas plant himself. By 1963, Arizona became part of the Southwest Animal Health Research Foundation joining cattlemen from Texas, New Mexico and Louisiana in an effort to promote private funding to match federal dollars.

The November, 1965 Convention Issue of the Arizona CattleLog includes a President’s Letter from outgoing Arizona Cattle Growers Association president Ray Cowden. He writes that Congress required a commitment of $300,000 to match the Federal appropriation for the screwworm project. Only $124,000.00 had been raised so far. The issue includes a picture of Susan Krueger, Miss SWEEP of 1965. Arizona sportsmen were urged to support the Arizona Game Protective Association screwworm fund-raising project. Ten thousand donation requests had been mailed to Arizona sportsmen and 200 collection canisters had been placed in sporting goods outlets. If you made a donation; you received a SWEEP decal for your bumper.

The screwworm population disappeared for a couple of years but did not fade away. In 1967, Hays reported in the Arizona CattleLog that funds were exhausted and the USDA was considering cutbacks. Arizona’s Senator Carl Hayden remembered seeing the threat based on cases he saw while growing up along the Salt River in the Tempe area and he was a powerful ally. Through his influence, funds became available and the war continued.

In the early 1970’s weather conditions with periodic high rainfall, flooding and humid weather, began to work against ranchers. The condition were ideal for the screwworms. Cases were reported in Arizona, Texas and New Mexico. The USDA sparked a new program and created a fiercer, still sterile “Super Fly” called 009. New sterile screwworm centers were opened in Mexico. Texas was given two million dollars to continue their work on the sterile fly program. The barrier was advanced all the way to Panama against a fly invasion from South America. No other program has resulted in such dramatic results in the southern half of the United States. In 2016 screwworm was found in a small population of deer in Florida but they have been contained. Screwworm eradication is one of the greatest entomological success stories of all times and also one of the least well-known peaceful uses of atomic energy: perfection of an effective control for screwworm, Cochliomyia hominivorax (Coquerel), using x-radiation.