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Fighting Flies With Flies: An In-Depth Look at the 2016 Screwworm Outbreak Response in Florida

screwworm fly - Cochliomyia hominivorax

Cochliomyia hominivorax is not just any ordinary fly. Mother screwworm flies lay their eggs in open wounds, and the maggots that hatch then gorge on exposed flesh. The pest was eradicated in North America in the 20th century via the sterile insect technique, and it hadn’t made any new appearance in the U.S. until an outbreak in the Florida Keys in late 2016. A new article in the Journal of Medical Entomology details the response to the outbreak by entomologists and officials at local, state, national, and international levels. (Photo credit: Judy Gallagher/Flickr, CC BY 2.0)

On October 10, 2016, a plane landed in Marathon, in the Florida Keys, with 2,736,000 passengers. They were an army called in from our allies in Panama, sent to swarm the archipelago and defend it from invaders. Of course, the soldiers knew nothing of their mission, nor did any of the ensuing waves of nearly 200 million reinforcements.

They were just flies, after all.

Well, Not Just Any Flies

Until about 10 days earlier, almost no one had seen Cochliomyia hominivorax in the wild in the continental United States in a generation. The eradication of the screwworm fly in the late 1950s has stood as a towering achievement in the history of insect pest management, in no small part because of how devastating C. hominivorax can be. It is decidedly not just any old fly. Mother screwworm flies lay their eggs in open wounds, and the maggots that hatch then gorge on exposed flesh. Wildlife, pets, and even humans can be targets, but the flies’ potential effect on livestock is most substantial; the United States saves an estimated $1.3 billion in damage to livestock every year by keeping the pest at bay.

The flies sent from Panama weren’t just any old flies, either. They were screwworm flies, too, with one crucial difference: They were all sterilized. Pioneered by Edward F. Knipling and Raymond Bushland at the U.S. Department of Agriculture in the 1930s, the sterile insect technique (SIT) is simple in concept: Release an overwhelming number of infertile insects (of whatever species is targeted) in an area where a pest species occurs, and they’ll mate with the wild ones but produce no offspring. With sustained releases of sterile specimens, the pest population will eventually crash. Once effective means for mass-rearing and sterilizing screwworm flies (via irradiation in a lab) were developed, the gambit was put into use. Trials began in the early 1950s, and by 1959 the screwworm was eradicated in the southeastern United States. Eventually, all of North America down to the Panama-Columbia border was cleared of C. hominivorax.

Until 2016. On September 30 that year, entomologists Steven Skoda, Pamela Phillips, and John Welch received an email at their posts at the U.S. Department of Agriculture (Skoda and Phillips at the Agricultural Research Service’s Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas, and Welch at the Animal and Plant Health Inspection Service in College Station). The email came from officials in the Florida Keys, with images of a deer corpse with a rather gruesome head wound infested by suspicious insect larvae. Later that day, a sample that had been sent to the National Veterinary Services Laboratories was confirmed to be Cochliomyia hominivorax. The screwworm had returned.

sterile screwworm fly ground release chamber setup

Steven Skoda, Pamela Phillips, and John Welch spearheaded the U.S. Department of Agriculture’s emergency response to the outbreak of screwworm flies in the Florida Keys in 2016. The outbreak was eradicated through use of the sterile insect technique, in which millions of sterilized screwworm flies (Cochliomyia hominivorax) were released from 30 ground release chambers (such as shown here) over the course of several weeks. (Image originally published in Skoda et al 2018, Journal of Medical Entomology)

Over the ensuing weeks and months, Skoda, Phillips, and Welch led a team of federal, state, local, and international officials in response to the outbreak, which made national headlines that fall. Today, their detailed account of the response effort, along with lessons learned through the experience, is published in the Journal of Medical Entomology. [Editor’s note: The journal article contains a couple images of severely infested wounds on Key deer. Squeamish readers beware.]

In the end, the re-eradication was swift, with the outbreak declared over by the following March. Welch credits that success to preparedness. He and his colleagues were ready to spring into action when that email arrived.

“My first thought,” he says, “was that I needed to be in Florida as soon as possible!”

The Screwworm Factory

Somewhere in the Keys earlier that summer, likely on Big Pine Key, a pair of male Key deer met in a mating-season clash, and one emerged with an injury from the other’s antlers. Unfortunately for the deer, a screwworm fly had made its way back to the Keys somehow—a year and a half later, scientists are still unsure of the origin of the outbreak—and the deer’s injury turned into an infestation.

More screwworm flies emerged after infesting the deer, and they sought out more injured deer, and the cycle grew. By the time the USDA team arrived, 47 Key deer had been euthanized due to severely infested wounds (myiasis), with the first case dating all the way back to early July. Making matters worse, local officials had been disposing of the infested deer in a single outdoor “boneyard.” Third instar screwworms can successfully pupate after leaving a wound on a dead animal, and so the disposal practice “effectively created a ‘screwworm factory’ for fertile flies,” the USDA team recounts.

This was a clear breakdown in intended protocol, which Welch attributed to a lack of on-the-ground awareness, because screwworm hadn’t been seen in so many years. “As the demographics changed with time, subsequent ranchers, farmers, and pet owners had not experienced the devastation that screwworm causes. In this aspect, the Screwworm Eradication Program is a victim of its own success,” he says.

sterile screwworm fly ground release chamber refill

The chambers for releasing sterile screwworm flies during the 2016 Florida Keys outbreak were filled with about 76,000 pupae about twice weekly. (Image originally published in Skoda et al 2018, Journal of Medical Entomology)

The Response Plan

After the notification, the USDA team set into motion their rapid response plan, elements of which had been in place at the ARS Screwworm Research Unit since at least the 1980s. The Panama – United States Commission for the Eradication and Prevention of Screwworm, known by the acronym COPEG, was notified immediately, as were colleagues at Florida Department of Agriculture and Consumer Services, USDA-ARS, USDA-APHIS-International Services, and USDA-APHIS-Veterinary Services. COPEG is home to the Screwworm Barrier Maintenance Program, a joint effort of the U.S. and Panama through which sterile screwworm flies are continually released to prevent fertile flies from drifting back into North America. It also serves as the primary producer of sterile screwworm flies for outbreak response elsewhere.

Welch and Skoda arrived in Florida on October 3. Phillips followed on October 9. “I felt prepared although a bit overwhelmed,” Phillips says.

None the less, they got to work. The team’s step-by-step process is narrated in fine detail in their account in the Journal of Medical Entomology, but the primary actions over the following days and weeks included:

  • A new protocol for disposal and decontamination of infested animals was installed by October 4: freezing in a mobile freezer unit.
  • Also by October 4, a mandatory checkpoint was established on Florida’s Overseas Highway to inspect all animals leaving the Keys. It remained in operation until late March, eventually inspecting more than 17,000 animals.
  • Fertile screwworm flies were collected (attracted with rotting liver) and identified, and some were shipped to Panama to be tested for mating compatibility with the sterile fly population.
  • Locations were scouted and ground release chambers (30 in all), were installed on 13 keys where screwworm flies had been identified, plus Marathon to the east, as a preventive barrier.
  • On October 10, the first shipment of COPEG’s sterile flies arrived on the plane from Panama. They came in pupal form, chilled so they wouldn’t emerge until being placed in the ground release chambers.
  • Releases of sterile flies, eventually totaling 188.4 million, continued twice weekly until April 25, 2017, though no fertile screwworm flies were found in the Keys after November 7.
screwworm outbreak response - ground release chamber locations

Florida’s Lower Keys was the focus of the sterile fly release (in zone bordered in orange) in response to the screwworm fly outbreak in 2016. A total of 30 ground release chambers (noted with orange symbols) were placed throughout the islands. Four chambers were placed on Marathon (bordered in blue) as a preventive effort to reduce the chance that the outbreak would spread toward the Florida mainland. (Image originally published in Skoda et al 2018, Journal of Medical Entomology)

For the most part, the process all went according to plan. Collections of fertile screwworm flies showed the population being brought into check in a matter of weeks, and by all indications the outbreak was contained by year’s end. Knipling and Bushland’s sterile insect technique triumphed yet again.

But there was one last surprise, which had the potential to blow the outbreak wide open again just when it seemed to be over. On January 6, a dog with a screwworm infestation was reported in Homestead, Florida, just south of Miami on the mainland. It had been taken to a veterinarian on December 19, but once again not reported immediately. Welch, Skoda, Phillips, and colleagues jumped back into action.

They traveled to Homestead and quickly established ground release chambers for sterile flies, while developing plans for aerial releases from planes. In examining the area where the dog had been housed, they found 19 empty puparia and 13 dead adult screwworms, confirming that at least a small number of fertile flies had emerged into the area. Luckily, the worst-case scenario did not come to pass. No additional fertile screwworm flies were collected over several weeks of monitoring, and the ground releases of sterile flies ended in mid-March. On March 23, 2017, the USDA declared the outbreak eradicated.

Lessons Learned

It remains unclear how the screwworm made its way back to Florida or where it came from. Genetic analysis of captured specimens has not yet identified similarity with populations of the flies from endemic areas. The USDA team notes that, for future potential outbreaks, “a larger library of genetic ‘fingerprints’ should be developed, so that, if needed, there is a higher probability of determining a region from which and outbreak originated.”

screwworm monitoring with liver

Throughout the response to the 2016 screwworm outbreak in the Florida Keys, USDA entomologists monitored and tracked the levels of fertile screwworm flies. An effective attractant was rotting liver, often from deceased Key deer that had fallen to screwworm infestations during the outbreak. (Image originally published in Skoda et al 2018, Journal of Medical Entomology)

Welch says the lag time in reporting was a major risk factor, though perhaps increased awareness after the 2016 outbreak may help in the future. Skoda adds that prompt reporting from the local level “could be the difference in rapid eradication in a rather small, contained area versus a long, expensive project covering a wider area with the prospect of domestic animal and human infestation.”

Despite those problems, though, Welch rates the outbreak response and re-eradication a clear success, noting it was “a team effort by people and support from international, federal, state, county, and city agencies; non-government agencies; and private individuals.”

But all involved will remain on watch. “Education about and vigilance for screwworm must be continued and improved,” Welch says. “As long as screwworm populations remain, international travel and commerce makes reinfestation of previously eradicated areas and introduction into areas where screwworm never existed a real and present danger.”


  1. There is no such place as “Marathon Key.” Marathon is a city composed of the following islands: Boot Key, Knight’s Key, Hog Key, Vaca Key, Stirrup Key, Crawl Key, Little Crawl Key, East and West Sister’s Island, Deer Key, Fat Deer Key (excluding the portion in Key Colony Beach), Long Point Key, and Grassy Key. The historic village of Marathon occupied part of Vaca Key. So-called “Marathon Key” does not exist.

    • Thanks Larry. We apologize for the inaccuracy. The post has been updated to correct the error. Thanks again!

  2. A landscape technician in the Leesburg area, (Lake County, Florida), is being treated for an active case…Florida Hospital is overseeing the treatment

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