no such ability as "locust" anywhere in Warcraft III TFT or otherwise, that's why i assumed you were looking for Locust Swarm.
if ur looking for what ramza said, that is a part of the Locust Swarm ability and is inseparable from it.
if you changed the unit that was used, there would be a bunch of those units going around invulnerable and unselectable. the field for it is Data - Swarm Unit Type. see how it says Locust? under Module -> Object Editor F6 -> Units -> Standard Units (834) -> Undead (77) -> Special (12) there is a unit at the bottom called a Locust. there is no way to make units unselectable in the way that those Locusts are when called upon by Locust Swarm, except for having a dummy unit with one REALLY DAMN LONG LASTING Locust-Swarm-based ability.
excuse me for being right about where to find what you're looking for.