Mortarbugs, also known as "Bobblesaurs", are large support units that spit a line of three highly explosive projectiles over long distances, like an upgraded version of the Ranger. These projectiles do not explode instantly, but instead stick to the floor and walls, then explode after approximately three seconds, dealing significant damage to anyone nearby. They are first encountered in SynTek Residential on Easy, Normal, and Hard, and in Cargo Elevator on Insane.
- If not killed quickly, they can harass a squad and prevent them from gaining access to an area. They are essentially defenseless at short range, but like Rangers, their main strength is supporting other members of the Swarm, using their bombs to restrict marine movement or split the squad up.
- Mortarbugs actually have one of the largest health pools of all the swarm. If it is easier to avoid one than to kill it, do so to conserve ammo.
- The best way to get rid of a Mortarbug is to use powerful long-range attacks and weapons (such as the Rail Rifle or Marksman Rifle) or to rapidly close to within its minimum range and tear it apart up close and personal. Mortarbugs can move but tend to be static; darting in and out for melee attacks while avoiding projectiles is possible.
- A Mortarbug stunned with the Tesla Cannon or Tesla Sentry Coil will be rendered helpless and unable to launch its mortar attack.
- While Chainsaws can be used to quickly dispatch them, it is advisable to not get too close; although they have no melee attack, coming into direct contact with a Mortarbug will nonetheless cause damage.
- Mortarbug projectiles will harm nearby Swarm when they explode. A clever marine with a good sense of timing can lure aliens to their death this way. Even the shieldbug's limbs offer no protection against the blasts.
- Mortarbugs can destroy any kind of Sentry with enough damage inflicted, making it critical that they be killed as quickly as possible. Their firepower combined with the explosion of a Sentry can easily destroy your entire squad.