The South American phorid fly attacks foraging fire ants, injecting eggs into the ant with a needle-like appendage. As the larvae mature, they attack and destroy the brain, causing the ant to wander aimlessly like a zombie. After two weeks or so, the ant’s head falls off and a new fly emerges, ready to attack other ants. The phorid flies have never been found to attack native ants, preferring to dine only on their South American hosts.

Scientists have previously released other species of phorid flies in Texas, but those species attack only ants in disturbed nests, and thus make only a minor contribution to controlling the invader. The new species is the first to attack foraging ants outside the nest.

The ants quickly become aware of the new flies and retreat into their nests, said entomologist Rob Plowes of the university. “It’s kind of like a medieval activity where you are putting a castle under siege,” he said. That retreat allows native ants better access to food sources, he added.