Rachni Workers are tiny green rachni (similar to aphids) which usually act as a first wave for Rachni Soldiers. Acting in swarms, they rush up to hostiles and explode in suicide attacks, causing heavy toxic damage which ignores shields.
Rachni Workers are encountered in a few places in the galaxy. Given their name, they likely fill the niche occupied by workers in other hive species, tending to the needs of the queen.
Tactics[]
One shot or a tech proximity mine will cause them to explode at a distance. Also since Workers are easy to kill, just shooting a few bullets into them will usually kill them. If the workers get too close they will explode, causing toxic damage to those around it. Subsequently killing them too close will have the same effect. Best to kill them at a distance.
Mass Effect 2[]
If the Rachni Queen was allowed to survive Noveria, then Shepard can meet an asari on Illium who encountered the rachni and gave her a purpose. This asari survived a ship crash: workers repaired her ship, and apparently it runs even better than before.
Mass Effect 3[]
Swarmers are miniature Reaper units suspected to have been derived from workers. The workers were spawned either from the original Noveria queen or an artificial Breeder.
Should Shepard save the queen or Breeder from Reaper captivity, she will use her workers to assist in the construction of the Crucible. However, the Breeder proves to be treacherous and directs her spawn to sabotage the work, resulting in numerous casualties before the situation is contained.