A phorid fly larvae emerges from the empty head of its fire ant host (pictured in a file photo).

Fire ants first emigrated from Argentina to Mobile, Alabama, in the early 1930s—probably on an agricultural-produce boat. They later spread to Texas and are still widening their range.

There's a long history of efforts to remove the ants, including physical removal and pesticides. But introducing phorid flies, which do not attack native ants, may be the most promising method so far, experts say.