he Monarch Butterfly (Danaus plexippus) is a widespread tropical insect that ranges as far north as Canada. It cannot withstand freezing winter temperatures nor can it last in the high extremes of summer’s heat. To survive, the Monarch travels to safe stayover sites that are neither too cold nor too warm in what may be the most unique migration in nature.
The Monarchs hatched from the matings at the Pacific Grove site are capable of mating as soon as they emerge from the crysalis. They fly north as the lands warm up, to Oregon. They lay their eggs for the next generation and die, having lived only around four to six weeks. The Oregon hatched Monarchs, capable of mating with a short life span will fly further north to Washington, lay eggs and die once more. The Washington generation moves the farthest north into Canada, where the last milkweed is growing, usually within 100 miles of the US-Canadian border. Here they lay the last eggs of the summer.
Why is this migration so unique?
In many species, such as birds and whales, the same individuals travel the same routes year after year. However, the monarchs that migrate to Pacific Grove have never been here before. In fact, it was the great-great-grandparents of these monarchs from Canada who left Pacific Grove four generations before!
Monarchs east of the Rocky Mountains spend the winters in high mountains in centeral Mexico.
Monarchs west of the Rocky Mountains come to Pacific Grove and several other locations on the central California coast. Recent research indicates that monarchs from Arizona also travel to California.
Overwintering sites are found from just north of the San Francisco Bay Area to as far south as San Diego and Baja, California. En route, they may travel as far as 2,500 miles, covering perhaps a hundred miles a day, and flying as high as 10,000 feet. This is a mighty achievement for a creature much smaller and more fragile than the tiniest bird!
How do they find their way?
Scientists think that the monarchs may rely on the Earth’s magnetic field, the position of the sun and the polorization of the sun’s rays. These butterflies, hatched around the beginning of September, on the most northern milkweeds in Canada, with nothing but instinct to guide them, take five to six weeks to travel to Pacific Grove.
We begin to see them arriving during the first two weeks of October. They go immediately to the locations in town where their ancestors were sheltered the previous year. It is a mystery how these new Monarchs know where they should spend the winter.