What happens when you help the "fake" Rachni queen?

At some point you will get a message that she has betrayed you and you lose the war asset. The Mass Effect 3 wiki also says she will do some damage to your Alliance Engineering Corps but I did not follow my war assets closely enough to see that happen.

Choosing to save her is a -125 war asset difference compared to not saving her.

You shouldn't have saved the artificial Rachni queen, She will betray you and kill some of you engineers working on the crucible, reducing the Alliance Engineering Corps war asset value.

If the rachni queen was killed in the first Mass Effect, then in Mass Effect 3 the Reapers will construct an artificial queen so they can have a rachni army to turn into Ravagers. If Shepard chooses to free this queen then, unlike the queen from Noveria, this artificial queen eventually proves untrustworthy after a few missions, taking away the rachni workers war asset she gave at first and sharply reducing the Alliance Engineering Corps.

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