Left to Right: Rendering of a New World screwworm fly laying eggs in an open wound and footage of the USDA program that successfully expelled the pest from America in the 1960s through cobalt-60 irradiation. (Source: USDA, Atomic Energy Commission 1960 film "Round Up").

SIT drove the screwworm out of most of the United States in the 1960s, but the parasite had never been only a border problem. Although the NWS could survive winter only in the warm southern tier of the U.S. — southern Florida, south Texas, and parts of southern Arizona and California — each summer the flies fanned out northward, routinely infesting cattle and other livestock across the central U.S. and reaching as far north as Canada before autumn cold killed off the northern populations.


A simple idea, powered by physics

SIT is one of the most environmentally responsible pest-control methods ever developed.

Unlike indiscriminate bait-and-poison programs, SIT targets only the pest species while sparing beneficial organisms like bees that pollinate flowers and crops; and it leaves no chemical residue.

SIT avoids harming other living things or the environment. Independent field monitoring confirms this. During an eight-year Tsetse fly-eradication program in Senegal, non-target insect populations dipped while insecticides were in use but rebounded to pre-intervention levels as soon as sterile-fly releases began.

Through SIT programs, NWS are mass-reared through their larval and pupal stages and sterilized by irradiation—receiving a precisely controlled dose as pupae, just before they emerge as adult flies. Released by aircraft, sterile males compete with wild males to mate. Because female screwworm flies mate only once in their 10- to 30-day lifespan, a wild female that pairs with a sterile male produces no offspring. Otherwise, she can lay up to 3,000 eggs. Each generation can fan out another 50–100 miles.

All SIT programs use irradiation. The dose is calibrated to disrupt reproduction without impairing the flies' ability to fly, compete, and mate. It damages the DNA in their reproductive cells, leaving them sterile, while their more radiation-resistant (and repairable) body cells keep functioning.

The irradiated flies are not radioactive, just sterile, and safe for predators to eat.

The COPEG facility, a joint U.S.–Panama operation, mass-rears New World screwworm flies, sterilizes them via Cobalt-60 irradiation, and releases them by air to suppress wild populations. (Credit: COPEG)

SIT: Proven method to combat NWS since 1954

American entomologists Edward Knipling and Raymond Bushland pioneered the SIT concept, initially sterilizing screwworms with X-ray machines before switching to the isotope cobalt-60. X-rays worked but couldn't scale, whereas cobalt-60 was a stronger source that could sterilize flies in the bulk numbers a real eradication effort required. A decisive field test came in 1954 on the Caribbean island of Curaçao, where SIT wiped out the fly in ten weeks.

The U.S. eradication campaign against NWS, launched by the U.S. Department of Agriculture (USDA) in 1958, paired the work of its scientists and the support of affected states and ranchers with cobalt-60 gamma radiation supplied by Oak Ridge National Laboratory. A year after the program’s launch in Florida, the southeast U.S. was declared free of NWS.

SIT is the proven method that eradicated the NWS from the southwest U.S. in 1966, cross-border Texas infestations in 1982, Mexico in 1991, and ultimately all of North America in 2005.

The eradication line was steadily pushed south until the parasite was held below a narrow geographic chokepoint — Panama’s Darién Gap — where a “living firewall” of continuously released sterile flies, sustained through the binational Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG), kept it out of North America for decades.

The pesticide Screwworm Adult Suppression System (SWASS) was only first field tested in 1977 and was used to complement SIT in southern Mexico, reducing dense wild populations so SIT could finish the job.

Irradiators: ways to sterilize insects, produce, and more

Radiation is a natural part of our world, streaming from the sun and cosmos and rising from soil, rocks, and ocean water laced with naturally radioactive elements. When it is harnessed, radiation improves our quality of life in countless unseen ways. The applications of irradiation span healthcare and food safety to environmental clean-up and space research. Learn more at the International Irradiation Association.

SIT irradiators fall into two categories:

  • Gamma rays from radioactive isotopes (cobalt-60 and cesium-137), and
  • Beams generated electrically by machines (X-rays and electron beams).


Left to Right: Renderings of a typical Cobalt-60 irradiator and an E-beam irradiator., and a video on "Using Nuclear Science to Control Pests" from the International Atomic Energy Agency (Source: SQHL Radiation Engineering Technology Co., Ltd., IBA Industrial, and IAEA).

Each irradiation technology COMES WITH trade-offs:

MethodHow it WorksNotes for SIT
Gamma — Cobalt-60 (Co-60)
Penetrating high-energy photons emitted by a sealed, reactor-made source of solid cobalt metal.
The long-standing high-throughput workhorse of screwworm SIT: reliable and dose-uniform. Its deep-penetrating gamma rays treat densely packed canisters of pupae in a single load, and it has sterilized billions of flies — the Panama (COPEG) plant alone supplies about 100 million a week. Co-60 is a solid metal, so it stays in place. Downsides: the source decays (5.3-yr half-life) and must be replenished, and isotopic facilities are costlier to build and more heavily regulated than machine sources.

Being used and advanced in the U.S., Panama, and elsewhere for the screwworm, and slated for further use by USDA.
Gamma — Cesium-137 (Cs-137)
Gamma photons from a different sealed isotope (≈30-yr half-life), used as cesium chloride — a water-soluble powder.
Functionally equivalent to Co-60 for sterilizing pupae, with a longer half-life and slower decay. Comes in a soluble-powder form. Due to security concerns, Cs-137 irradiators have largely been phased out.
X-rayHigh-energy photons generated electrically when an electron accelerator drives electrons into a metal target.
Contains no radioactive source. The trade-offs are throughput and uniformity: a tube’s lower dose rate means longer exposures than gamma or e-beam, and its lower, mixed-energy photons penetrate a dense canister less evenly. While particularly well-suited to newer, smaller programs (e.g., mosquito SIT), X-ray sterilization can be scaled up with additional machines to take on larger bulk operations.

Being advanced in the U.S. for screwworm by DOE/NNSA, USDA, and University of Florida.

Electron beam

(E-beam)


A stream of high-energy electrons fired directly from an electrically powered accelerator.
Contains no radioactive source. Cheaper to build, and fewer regulatory hurdles than an isotopic facility. Its high dose rate also sterilizes each batch of pupae quickly. The trade-off is shallow penetration: pupae must be irradiated in thin, even layers rather than deep bulk canisters.

Being advanced in the U.S. for screwworm by DOE/NNSA and Texas A&M.

Left-to-Right: Co-60 harvested at Bruce Power, a Co-60 bundle loaded into a shipping flask at Bruce Power, and workers cleaning a Co-60 flask at Pickering for delivery. Ontario, Canada. (Credit: Bruce Power; Ontario Power Generation Inc.)

What cobalt-60 is, and where it comes from

Cobalt-60 (Co-60) is a manufactured radioactive isotope with a half-life of about 5.3 years; it does not occur naturally. It is made by neutron activation: stable cobalt-59 is placed in a nuclear reactor, where it absorbs a neutron to become Co-60. As the isotope decays toward stable nickel-60, it emits the penetrating gamma rays that do the sterilizing.

Most of the world’s Cobalt-60 supply comes from reactors in Canada, Argentina, and China. The same isotope sterilizes billions of medical devices, treats cancer, and helps keep produce, other foods, spices, cosmetics and pharmaceuticals safe each year from harmful microbes and pests.

From manufacture to disposal, Co-60 is heavily regulated by industry and government authorities, including the U.S. Nuclear Regulatory Commission and the Canadian Nuclear Safety Commission.

Learn more about cobalt-60 irradiators at the Gamma Industry Processing Alliance.

Left-to-Right: The Electron Beam and X-Ray Linear Accelerators and conveyer belt at the Electron Beam Research Facility at Texas A&M University; and an X-ray SIT machine in Tahiti, French Polynesia — the same X-ray irradiator used against mosquitos in Lee County, Florida. (Credit: TAMU Electron Beam Research Facility and TDR/Temoana)

How accelerators make radiation: electron beams and X-rays

Both X-ray and electron beam machine irradiators start with a particle accelerator that uses electric and radio-frequency fields to drive electrons to high energy. For an electron beam, that stream of electrons is aimed straight at the pupae; for X-rays, it is slammed into a dense, heavy-metal target — typically tungsten or tantalum — where the electrons' abrupt deceleration sheds energy as X-ray photons, a process called bremsstrahlung, or "braking radiation."

The conversion is inefficient: only about a tenth of the electron energy becomes X-rays, with the rest lost as heat, so the target must be cooled and a great deal of power is required.

Learn more about x-ray and e-beam irradiators at the Industrial Irradiation Association.

A hemisphere of partners in irradiation

Through its industrial use, radiation protects the health and safety of the public, animals, and our food supply. To expand domestic irradiation capacity, the USDA — with the U.S. Army Corps of Engineers — broke ground in 2026 on a new sterile-fly production facility at Moore Air Base in Edinburg, Texas, where it will rear and sterilize flies using established cobalt-60 irradiators. The plant is targeted to reach 100 million sterile flies per week by November 2027 and as many as 300 million as it scales. In parallel, the U.S. Department of Energy’s National Nuclear Security Administration is working with Texas A&M University's National Center for Electron Beam Research to also deploy electron-beams to combat NWS.

In Mexico, the food-safety agency SENASICA is restoring its irradiation capacity, while the International Atomic Energy Agency and the Food and Agriculture Organization coordinate efforts across Latin America and the Caribbean. The same nuclear-derived method and international cooperation underpin other pest control campaigns around the world, including an X-ray-irradiated program in Lee County, Florida to suppress the yellow-fever mosquito.

Teachers and parents, click here for a STEM project starter kit for highschoolers on the Sterile Insect Technique.

For Further Information

Media: Need to speak with a SIT expert?

To reach subject-matter experts on SIT and how nuclear is fighting the screwworm, contact media@ans.org.

Established in 1954, the American Nuclear Society (ANS) is an international professional organization of engineers, scientists, technologists, teachers, and healthcare providers devoted to the peaceful applications of nuclear science and technology. Across 60 countries, the Society’s more than 12,000 members represent government, academia, research laboratories, medical facilities, and private industry. ANS’s mission is to advance, foster, and spur the development and application of nuclear science, engineering, and technology for the benefit of humanity.


Last modified June 26, 2026, 9:38am CDT