I agree and disagree with the person below. "On" would work in this sentence, and "to" could work sometimes; however, not in this sentence. Wreak havoc to pedestrians doesn't work. The key to this, slighly advanced quesiton, is understanding prepositions and objects. You could never do something ON a meeting, rather IN a meeting. Therefore, Wreak Havoc in the meeting, works. You can't do something IN a pedestrian, reather on a pedestrian, so wreak have on pedestrians works.