For decades, the most effective tool against them was widespread dispersal of sterile screwworm flies, which are now mostly produced at the facility in Panama. Opened in 2006 on the narrow isthmus, the plant was intended to maintain a barrier to stop wild screwworm flies from marching north into Central America and North America. But now wild screwworm flies have spread throughout Mexico and in June infested livestock on Texas farms for the first time in decades. Once the male offspring are sterilized, they could be released to mate with wild female screwworm flies, which then lay unfertilized eggs. Wild female screwworm flies normally mate only once, so sterile male flies can make the screwworm population progressively decline until it is eradicated.