Carnivorous parasite A fly that poses a solemn threat to livestock has returned to the United States after 60 years. This week, the US Department of Agriculture confirmed presence of the Fresh World snail in a calf in south Texas.
Eliminated in the US in 1966 and as far south as possible Panama by 2006, its recent re-emergence in Mexico increased the likelihood that the snail would eventually re-enter the country, and modeling showed it could arrive as early as summer 2025. It took a little longer, but the snail arrived. To prevent an outbreak, officials are using a proven technique: releasing masses of adult snail flies.
A worm infection occurs when a female fly lays eggs in open wounds or other body parts of warm-blooded animals. Once the eggs hatch, worms emerge and feed on living tissue before turning into flies. As adults, snail flies do not bite or feed on flesh. Scientists in the 1930s and 1940s thought that if they could prevent female flies from breeding, they could break the cycle. At the time, Fresh World snails were killing hundreds of thousands of cattle each year, mostly in the American South and Southwest.
In the 1950s, USDA researchers made a breakthrough by administering radiation to male snails and rendering them sterile. Once released into an infected area, sterile males mate with wild female insects and lay nonviable eggs. No offspring are born and the population collapses. Known as the sterile insect technique, it was first successfully used on the island of Curaçao off the coast of Venezuela. It took just seven weeks to eradicate the pest, and the effort saved the island’s goat herds, which were a vital food source.
This technique takes advantage of the fact that female Fresh World snails mate only once in their lives. “The sterile insect technique is perhaps the most telling example of a completely effective biological control mechanism,” says Sally DeNotta, associate professor of veterinary medicine at the University of Florida. “The life cycle stops. No offspring are produced. It was very successful.”
For years, a dense stretch of rainforest between Panama and Colombia known as the Darién Pass served as a biological barrier through which sterile flies were released to prevent the snails from spreading north. However, insects began to break through this barrier in 2022.
To prevent an outbreak in south Texas, the USDA has sealed off an approximately 12-mile area around the infected calf and is conducting targeted releases of sterile snail flies from trucks. This is in addition to the 4 million sterile flies a week already dropped in the area. The agency predicts the snail will move north in February moved its efforts to disperse 100 million sterile flies per week to focus on an area along the U.S.-Mexico border.
“While this development poses a serious threat to our livestock and wildlife, it does not surprise us,” USDA Secretary Brooke Rollins said during a hearing of the House Agriculture Committee meeting on Thursday.
She said it takes about 400 million flies a week to repel the bugs. Currently, only about 100 million flies can be produced per week in the United States facility located in Panama.
A sterile insect facility in Mexico closed in 2012, but the USDA did investing $21 million Assisting in the renovation and conversion of an existing fruit fly facility in Metapa, Mexico to produce an additional 60-100 million sterile flies per week. According to the USDA, the facility is expected to be operational this summer.