Will History Repeat Itself?

The Culprit

The Culprit

Flying amongst the humid air of southern Florida, zipping its way through citrus groves and grazing cattle, was a winged pest that served as quite the detriment to Florida agriculture producers in the not too distant past. This small, voracious villain was an innocuous looking fly that cost the state an average of $20 million dollars in economic losses a year (http://flaentsoc.org/webbaum/baumhover.html) due to the trail of destruction it left in its wake across the whole state and virtually all of the southern United States. No, I’m  not referring to aphids or whiteflys that affect citrus, and I’m definitely not referring to the current problems presented by psyllids as they spread greening disease throughout citrus groves, but rather I am referring to screwworms.

The USDA has recently pledged 1 million dollars and is attempting to bring together both private and public entities (http://southeastfarmpress.com/orchard-crops/usda-syncs-agencies-citrus-greening) to help combat the massive problem that faces Florida agriculture in the manifestation of greening disease among the state’s numerous citrus groves. However, instead of lamenting and writing a doom and gloom piece about greening, I wanted to go over a past successful collaboration between government entities such as USDA and private growers as they faced another winged pest that was a scourge of farmers, and whose front lines of the battle occurred in Florida.

Depiction of wound inoculation by screwworm

Depiction of wound inoculation by screwworm

Screwworms (Cochliomyia hominivorax) were a major setback to livestock growers in both North and South America before their eradication, with the life cycle of the fly consisting of females laying eggs in any sort of open wound (cuts, scrapes, punctures) on livestock that may have resulted from thorns, birthing, eye infections, insect bites, or normal cattle handling procedures such as branding or dehorning. Even newborn calves were susceptible to infestation of screwworms in their navels shortly after being born. Once a female lays her eggs in the wound, the larvae hatch and then begin to feed on the healthy flesh of the infected animal, beginning a process of eating the animal alive. This feeding subsequently attracts other females to lay their eggs in the same wound, and if this cycle is not abated, an animal may die in 5-10 days from the affliction. Their prevalence to infect livestock with such voracity often caused livestock ranchers to restrict calving and working of animals during the winter months when the flies were not present.

Screwworm infestation on cow eye

Screwworm infestation on cow eye

Successful wound treatment during regular screenings of livestock could control the hatched larvae, and this was demonstrated in Florida during the 1930’s, but this was only effective in controlling the fly with intensive management techniques that would be hard to implicate in every herd present in the state. The Florida demonstration was also flawed in the sense that wildlife and other nearby untreated livestock could serve as an infinite repository for wave after wave of flies that could reinfest a recently treated animal. A broader, more drastic approach was needed, and a couple of USDA scientist led the charge by proposing a “sterile male” control theory. This theory required the release of male screwworm flies that had been rendered sterile via irradiation (x-ray machines) out into the wild that would then subsequently mate with the endemic females. Females only mate once in their lifetime, and they retain the sperm from the one time mating event for the production of multiple egg layings during their lifespan. This resulted in females that had mated with the sterile males to lay eggs that were inviable.

Screwworm larvae

Screwworm larvae

After the initial lab tests proved promising, it was time to prove the theory out in the field, and the USDA did so by stocking the sterile males on Sanibel Island on the west coast of Florida. Within months, eggs layed on livestock were only hatching at an 80% rate, but complete eradication could not be achieved or proven due to wild impregnated females flying in to the island from the mainland. To prove the model even further, the researchers next introduced the lab reared male flies onto an island off the coast of Venezuela. This time the experiment was a resounding success, and screwworms were eradicated within 6 months from the island after the implementation of the sterile male stocking.

With confidence in the methodology, the eradication program moved back to Florida and was implemented on a large-scale in conjunction with the Florida Livestock Board in 1955. The program continued successfully  throughout the Southeast in the late 1950’s and finally moved on to its toughest challenge yet in 1962 when USDA researchers in Kerrville, TX begun the sterile male stocking program to alleviate the state of its yearly 1 million cases of screwworm infestations. Texas’ ideal environmental conditions for screwworm proliferation, high cattle numbers, and proximity to Mexico and its infected cattle population served as potential stumbling blocks to the continued success of the program, but within a year, both Texas and New Mexico were declared officially clear of endemic infestations of screwworms. After the continental U.S., the program then spread into the rest of North America, Central, and South America, eventually being implemented everywhere in the world where such pests were a problem.

To this day, the complete eradication of the screwworm has been declared for the U.S., Mexico, and most of Central America. In the U.S. alone, it is estimated that eradication of this pest results in benefits of nearly $900 million dollars. What started out as a small test on a Florida island culminated into a worldwide collaboration between governments and private citizens that resulted in designation of the program being “the most efficient and successful international animal health program in the history of the United Nations Organisation (http://en.wikipedia.org/wiki/Cochliomyia).” Perhaps this example of such a success will prove as inspiration or at least as hope for the continued struggle (and future struggles) of Florida as it battles the latest citrus affliction, with an eye towards what great achievements can be made when both private and public entities dedicate themselves towards a cooperative stance in dealing with agricultural issues.

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