The screwworm fly, Cochliomyia hominivorax was once a common pest in the United States, throughout Central America and the Caribbean, in most of South America, and even Libya, Africa. Today, it is confined to South America and a few Caribbean islands. The fly has retreated because we have found a way to eradicate it. Now it is under siege in South America.
Why eradicate screwworm?
Cochliomyia hominivorax is a large fly that has the unfortunate need to lay its eggs on live warm blooded animals. The female fly deposits her eggs near a break in the skin; hatching maggots invade the live tissue through the opening and feed on healthy tissue. Feeding screwworm maggots cause horrendous wounds, which are vulnerable to secondary infection.
Screwworm infestations are often fatal. People who survive may endure scarring and disfigurement, especially when the maggots have infested the nasal sinuses. In livestock, scarring and tissue damage ruin hides.
While the immediate effects of screwworm in a human infection are memorable, the major impact of this pest comes from losses in livestock rearing. In Brazil alone, the estimated annual losses due to screwworm myiasis are US 1770 million (Vargas-Terán).
The sterile insect technique
Insecticides are the first line of defense against screwworm. However insects tend to develop resistance to these chemicals, and resistance has arisen in C. hominivorax, suggesting that insecticides are already becoming less effective.
The sterile insect technique takes advantage of a curious fact about C. hominivorax reproductive strategy: the female only mates once. If she mates with a sterile male, no offspring result; therefore, if we can get all female screwworm flies to mate with sterile males, the insect will be eradicated.
Rearing massive numbers of screwworm flies in captivity, then sterilizing the males with radiation and releasing them into the wild, has resulted in the eradication of C. hominivorax in the Americas all the way south to the border between Panama and Colombia. The same technique eradicated the fly in Libya after it was accidentally introduced there.
Today, South American countries are cooperating and experimenting with the sterile fly technique to determine whether it can be used successfully on that continent, where the C. hominivorax is present in every country except Chile. The widespread distribution of the fly and the size of the landmass together mean that new strategies will be needed, but if South America is successful, the last remaining populations of screwworm flies on Caribbean will inevitably follow, and we will have deliberately wiped a nasty species off the globe.
It’s fair to ask whether exterminating C. hominivorax could have any unanticipated negative consequences for the natural environment. The Mexican American Commission for the Eradication of Screw-Worm (COMEXA) is satisfied that the radiation used to sterilize the flies poses no environmental risk, and that “there has been no evidence of any impact at all on biodiversity, perhaps because a multitude of fly species occupy the same biological niche” (qtd. in Godoy).
Sources:
da Silva, N. M. and A. M. L. de Azeredo-Espin. 2009. "Investigation of mutations associated with pyrethroid resistance in populations of the New World Screwworm fly, Cochliomyia hominivorax (Diptera: Calliphoridae)." Genet Mol Res 8 (3)
Godoy, Emilio, and Raúl Pierri. 2010. “Latin America: Radioactive Attack on Flesh-Eating Screw-Worm.” Inter Press Service
Vargas-Terán, M., H. C. Hofmann, and N. E. Tweddle. 2005. “Impact of Screwworm Eradication Programmes using the Sterile Insect Technique.” In: V.A. Dyck, J. Hendrichs and A.S. Robinson (eds.), Sterile Insect Technique. Principles and Practice in Area-Wide Integrated Pest Management, Netherlands: Springer.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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