When the presence of New World Screwworm (Cochliomyia hominivorax) was confirmed in the Florida Keys in September 2016, it was the first non-isolated appearance of the parasite in the United States in over 30 years. While the devastation to endangered wildlife on this archipelago has been significant, if the fly spreads unchecked to the mainland it could result in losses approaching $1 billion annually.
The release of large numbers of sterile male flies is the only known method to eliminate established New World Screwworm (NWS) populations. Identifying the geographical extent of the invaders is the critical first step. The flies typically are not great wanderers and refuse to cross open water. But when those in Florida began turning up on nearby islands with no land links to neighboring islands, the sterile fly release campaign became that much more complicated.
The NWS flies are metallic blue-green in color and superficially resemble many other species of blow fly (family Calliphoridae) that swarm on exposed flesh in and around wooded areas. While other blow flies search out the flesh of dead animals on which to oviposit (lay) their eggs, the NWS fly is unique in that it requires the flesh of a live animal to feed the larvae, or maggots, that hatch from its eggs.
The NWS outbreak was first discovered in ailing Key deer (Odocoileus virginianus clavium), a subspecies of the larger white-tailed deer of the mainland US. Rutting season occurred simultaneously with the NWS’s arrival, and the flies have taken advantage of the antler wounds resulting from the competing bucks. Of the 120 or so known deer that have died or been euthanized due to severe infestation, only a dozen or so have been females. Gaping holes as big as a grapefruit and filled with squirming maggots (see top image) have spurred public support for the government’s effort to eliminate the fly.
Down to as few as 25 individuals in the early 1950s, the Key deer population was listed as federally endangered in 1967. With an estimated 1000 or so Key deer at the beginning of the NWS outbreak, the carrying capacity of the federal refuge established to protect the deer is believed by biologists to be around 750 individuals.
Before its elimination from the eastern US in 1959, NWS killed anywhere from 20-80% of the annual white-tailed deer crop there, laying their eggs around the wet umbilical stumps of fawns. It is hoped, and likely, that the current effort to eliminate the flies will progress quickly enough to spare the newborn fawns in April-May, the main fawning period in the Keys.
The Threat of Escape
Fortunately, the Florida Keys have virtually no livestock other than 15-20 horses and a handful of pet goats and pigs. Were the NWS fly to spread from these islands to the mainland, the livestock industry could be turned on its head.
Before NWS’s elimination from the US, cattle, sheep, and goat owners had to time births, brandings, castrations, sheerings – anything that could expose an animal’s flesh – with cooler periods when fly activity was lowest. Even then, cowboys rode the pastures inspecting individual animals every few days for wounds and treating them with insecticide when found. NWS infestation was considered by many to be the most important impediment to livestock production in the US.
When NWS was progressively eliminated from the US in the late 1950s and 1960s, the cowboy profession all but dried up. Today cow-calf herds may go weeks without getting close to a human being. Unopposed reappearance of the fly in livestock areas is estimated to result in losses of several hundred million dollars annually from deaths and lost production of infested animals and the constant monitoring and treatment of herds.
The Arsenal: Sterile Fly Release
Fortunately, we have a very effective tool for eliminating the NWS. E.F. Knipling recognized in the 1930s that female NWS flies mate only once in their 10-30 day lifetime, while males can mate repeatedly. He speculated that overwhelming the wild fly population with sterile male flies would soon ensure that all female flies only mate with sterile males. The eggs oviposited on wound edges would fail to hatch and the population would disappear.
Brief exposure of NWS pupae to high energy electromagnetic radiation in the laboratory within 2-3 days of adult fly emergence was found to render close to 100% of male NWS flies sterile, and without significantly changing their behavior.
After a more or less successful test run on Sanibel Island, Florida, the so-called Sterile Insect Technique (SIT) was used on the Caribbean island of Curaçao in 1954-55. Every week each of the island’s 440 km2 received 2600 sterile flies dropped by aircraft. Both male and females are released because it is too expensive to separate them.
After three weeks, some 70% of NWS eggs found on wounds were sterile. After 6 weeks, this percentage rose to 84%. After 9 weeks, few eggs could be found at all on the island, and ALL were sterile.
SIT worked, and it has worked time after time in all but one instance ever since. US livestock owners raised money to start their own campaign after the success on Curaçao. Between 1957-59, NWS was pushed back from Florida to the Mississippi River, and by 1966 enzootic NWS was gone from all of the US.
Mexico followed, not without some hiccups (see below), eliminating the flies by the early 1990s. The rest of Central America, from north to south, was steadily freed of the fly by 2006, when Panama was officially declared NWS-free.
Today the US government funds the majority of a sterile fly research and production facility outside Panama City, Panama. Sterile flies are regularly released around 30,000 km2 of inaccessible swamps on the Panama-Colombia border. The area forms a barrier to NWS incursions from South America and, while expensive to maintain, is considerably less expensive than dealing with the regular reappearance of the flies in Central and potentially North America in the absence of the barrier. This plant in Panama is supplying the sterile flies released in the Florida Keys today.
Despite the buffer zone in Panama, there are still breaches. We do not yet know where the NWS flies affecting the Florida Keys came from. But genetic testing is underway to compare with NWS populations in South America and the Caribbean. These may tell us their origin or, at least, where they did not come from. Either would improve our understanding of the fly and help better control it in the future.
Many Floridians speculate that the flies were introduced from Cuba, the closest country with enzootic NWS flies and only 150 km from the Florida Keys. But the fly could just as easily have arrived on a plane from any other enzootic country, probably on a pet.
One thing is relatively certain: NWS did not fly on its own from Cuba, or anywhere else, to Florida. The flies do not like to cross even small stretches of open water. The flies rarely move more than a few kilometers so long as sufficient hosts (i.e. animals with wounds) and fresh water are available. Until 1933, the Mississippi River formed an effective barrier to NWS. When severe drought in the west led to large movements of livestock to the southeast, NWS came with them for the first time.
The NWS flies recently discovered in Florida were initially limited to Big Pine Key and neighboring No Name Key, where the majority of the Key deer population is located. But within a couple of weeks, the US Department of Agriculture’s NWS expert entomologist John Welch, along with a team of experts from the sterile fly facility in Panama, detected flies on nearby islands too.
Speculating that they may have been transported as undetected hitchhikers in vehicles, the team looked for wild flies on an island some 2 km from the nearest land and inaccessible except by boat. Their effort revealed flies on this island as well. Cars were not the culprit.
Welch says he has never seen NWS flies cross such a long stretch of water. He speculates that an infested deer could have swum to the island, as they are known to do, colonizing it with the flies. But “I’ve learned to never say never, and this is another reason why,” Welch says.
While curious, this information is not likely to compromise the success of the elimination campaign in Florida. NWS is more likely to be spread by an infested pet dog or cat slipping undetected through the animal checkpoint established by the Florida and US Departments of Agriculture on the road leading off the archipelago.
Yet the fly’s willingness to cross water reinforces the fact that, in order to be efficient, sterile fly release must be accompanied by good epidemiological information, surveillance of flies and animals in infested and neighboring areas, treatment (or culling, in advanced cases) of infested animals to limit new flies, strict monitoring of animals moving out of infested areas, and communication with the public to ensure their support.
If All Goes Well…
Jamaica is the only concerted sterile fly release program (1998-2009) that has not met with success, and this was apparently due more to complicating political, economic, and social factors than to technical problems with the SIT technique.
Unexpected problems can creep up however.
When NWS was driven out of the US, a buffer zone with regular sterile fly releases was established on the 2400-km-long Mexican border. The area proved too vast and regular incursions of the fly occurred in Texas until 1982, when it was decided to move the barrier south to the Tehuantepec Isthmus (see map above), where only 220 km separate the Pacific Ocean from the Gulf of Mexico. Such great expanses of infested territory do not present a problem in the Florida Keys, with its tiny, isolated islands.
Occasional quality control issues arise too. In the early 1970s, it was discovered that sterile male flies grown at a facility in southern Mexico would mate only in the afternoon and were poor flyers, compromising their ability to compete with fertile male flies. The problem was quickly corrected.
Sterile flies are often dropped by low-flying aircraft. The islands of the Florida Keys are too small however, and winds too variable to make this a viable option. Too many of the released flies would end up in the water and drown. Instead, sterile pupae with their enclosed flies on the verge of emerging are set out at various wooded sites in and around infested areas, from which they disperse to cover infested areas.
Limited by the availability of hosts with open wounds, a population of 100 NWS flies/km2 is considered a heavy infestation. On Big Pine Key alone, with an area of about 30 km2, over 2.1 million sterile flies are being released each week. That means 70,000 released flies per km2, compared to a conservative estimate of 100 wild flies per km2, giving a ratio of 700 sterile flies to 1 wild fly. Other SIT programs have proven successful with less than a 100 : 1 ratio, so this should be more than sufficient to eliminate the flies from the Florida Keys, barring any unforeseen problems.
The sterile fly releases are scheduled to continue for at least six months, then 4-6 weeks beyond when the last known infested animal is found. Consistency is critical. It has been estimated that each week missed for releasing the flies once the campaign has begun results in up to 2 extra months of releases necessary to eliminate the flies. With the end of the hurricane season, such disruptions in the releases are unlikely. Cooler weather and an end to the rutting season in the Key deer further favor the fly elimination effort.
None of the potential problems mentioned above is insurmountable, though surprises beyond the newly discovered island-hopping abilities of these flies are always possible. While the campaign is expensive, with millions of fly pupae flown in twice a week from Panama (it is too expensive and long to establish a sterile fly facility in the US in time to address this outbreak), the costs of the fly spreading beyond the Keys would be much greater.
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