Check this out:
Arizona State University biologist Adrian Smith drops a few army ants into an Aphaenogaster harvester ant nest and mayhem ensues. Within seconds, the harvesters mass-evacuate with their queen and all the brood they can grab.
A remarkable aspect of this behavior is its predictability. Almost every colony will bail at the mere scent of an army ant. And it’s not just this species, either. I’ve seen Pheidole, Camponotus, Nylanderia, and other ants react the same way. A complete freak-out is apparently a successful strategy to counter army ant raids, and its ubiquity among target ants testifies to just how much pressure army ants place on the survival of other colonies.