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 Edit
If the Rachni Queen was allowed to survive Noveria, then Shepard meets an asari on Illium who encountered the rachni and gave her a purpose. The Workers worked tirelessly to repair the asari's ship, and apparently it runs even better than before.
Mass Effect 3 Edit
If the Rachni Queen survived the events on Noveria, Shepard has the opportunity to rescue her in 2186. If Shepard chooses to rescue the queen, sacrificing the krogan Aralakh Company in the process, she will use her workers to assist in the construction of the Crucible.