Va'eira

Rashi on 7,19 says Aharon had to do the first two Makkos because Moshe had been saved by water when he was put in the river as a baby. Rashi on 8,12 says Aharon had to do the third Makka because the dirt saved him when he buried the Egyptian he killed. But why did it have to be Aharon who turned the stick into a snake in 7,9-10? Sticks have never saved Moshe's life, why couldnt Moshe do it?