Ant brains, that is.
Fire ants are a scourge of the southern part of the United States, a pestilence that has been steadily spreading north from Alabama over the past few decades. They cause billions of dollars in damage every year to electric equipment (they are attracted to the electrical field) and wildlife and livestock, not to mention biting the hell out of my legs when I was five years old and living in North Carolina.
The tiny phorid fly, an insect native to South America, has a taste for fire ant brains. Scientists from Texas A&M's AgriLife Extension Service has run experiments introducing phorid flys to fire ant colonies. The small fly lays eggs on the ants; fly larva hatch from the eggs and craw into the ants heads where they start to eat brains.
The ants wander away from their home mound when the fly larva starts chomping. After a month or so the brains are completely eaten and the ant head falls off. A new fly crawls out and flies off to repeat the cycle.
They have released four species of phorid flies since 1999 and haven't found them to attack any other species besides fire ant. Yet.
We have a pretty terrible track record with fighting invasive species with other invasive species. There are just too many variables in the math of life to be able to predict how bringing an animal, plant, or insect from one part of the world to another is going to work out. Cane toads were introduced in Australia to fight the cane beetle, that's turned into an epic ecological disaster. There are countless other examples to be found in history.
It'd be great if the phorid fly only attacked fire ants and turned out to be a safe and effective solution, but I wouldn't bet any significant money on it in the long term. Life is pretty good at adapting and I cringe to think about having to write a story in five or ten years about problems with phorid flies and the widespread decline of ALL ant species across the South.