No individual butterfly completes the entire round trip. Female monarchs lay eggs for a subsequent generation during the northward migration. Four generations are involved in the annual cycle and the generation undertaking the southbound migration live eight times longer than their parents and grandparents.
From Wikipedia on “Monarch Butterfly Migration”:
No individual butterfly completes the entire round trip. Female monarchs lay eggs for a subsequent generation during the northward migration. Four generations are involved in the annual cycle and the generation undertaking the southbound migration live eight times longer than their parents and grandparents.