Maggot factory offers hope against flesh-eating parasites

A factory in the southeastern Mexican city of Tuxtla Gutierrez is hard at work churning out an unusual product: bugs.

Sterile screwworm flies (Cochliomyia hominivorax) have been bred here since 1976 as part of a US Department of Agriculture (USDA) eradication program.

In the wild, female screwworm flies lay as many as 400 eggs near open wounds on animals. The maggots that develop use two needle-sharp, hook-shaped mouthparts to burrow deep into the animal’s flesh. The swarm of screwworms can consume so much tissue that the victim is virtually eaten alive from within.

Screwworm flies, which are found in most tropical regions around the world, were first documented on Devil’s Island off the coast of French Guiana in 1858. From there they spread throughout the warmer regions of the Americas, and by the first half of the 1900s, the parasites were wreaking havoc on livestock. In the southern US, Texas alone suffered $11.2 million in losses in 1935.

By the late 1940s, scientists were aggressively seeking a method to control the flies. In the early ’50s, USDA entomologists Raymond C. Bushland and Edward F. Knipling developed a technique to irradiate the flies, rendering them sterile.

View more: Click the images below to learn more about Screwworm breeding.

Sterile males released into the larger population would mate — but not breed — with fertile females. Female screwworms mate only once, so with no offspring, the species would gradually die out. At first, Bushland and Knipling’s theory faced widespread skepticism.

But experiments such as a 1953 trial on the Caribbean island of Curaçao, in which the screwworm was successfully eradicated after the introduction of sterilised males, vindicated them.

Sterile males were introduced in Florida in 1957, and by 1966, the pests were eradicated in the US. To keep the flies from reentering the South from Mexico, the Agricultural Research Service (ARS) branch of the USDA opened the Tuxtla Gutierrez factory, which at its peak produced billions of sterile flies every year.

By 1991, screwworms were wiped out in Mexico and, since then, in all of Central America. (In South America, the financial toll of screwworms is still more than a billion dollars annually.) Last May, the ARS opened a new factory in Pacora, Panama. The plant produces 40 million flies a week to maintain the Central American eradication, while the Mexican factory serves as a backup source.

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