<P> Monarch butterflies (Danaus plexippus) perform annual migrations across North America which have been called "one of the most spectacular natural phenomena in the world". The subspecies D. plexippus plexippus is the migratory one . Other subspecies perform minor migrations or none at all . </P> <P> Starting in September and October, eastern and northeastern populations migrate from southern Canada and the United States to overwintering sites in central Mexico where they arrive around November . They start the return trip in March, arriving around July . No individual butterfly completes the entire round trip; female monarchs lay eggs for the next generation during the northward migration and at least four generations are involved in the annual cycle . </P> <P> Similarly, the western populations migrate annually between regions west of the Rocky Mountains including northern Canada and overwintering sites on the coast of California . </P>

When do the monarch butterflies return to mexico