Scabaras locusts are encountered during the Dealing with Scabaras quest. They are basically giant locusts without riders, unlike the locusts that Locust lancers and Locust rangers ride. These locusts, although part of the Scabarites race, cannot be used for Scabarites slayer assignment.
The swamp area (Used to, in the First Age be Ullek) east of the Dealing with Scabaras dungeon. These are found just east from the Archaeologist camp, after going down the stairs and climbing over the Fallen Pillar. They are in an area along with Locust lancers, Locust rangers, Scabaras lancers, and Scabaras rangers. The area is considered part desert, and the player may require water (or Enchanted water tiara) due the desert heat effect. Three tiles south of the statue, and all north of this line, are desert areas. Staying south of this line will negate the desert heat effect.