Poison is the Hornet's secondary attack and is first seen in Satellite. This attack causes a puddle of poison to appear which could drain a Lyoko Warrior's life points. X.A.N.A. improves it in Swarming Attack by allowing the hornets to spit a cloud that can take half of a character's life points.
Poison did not appear after season 1 in the show, but it still was a part of the franchise.