A universal banner of spring throughout much of North America, here in Central California the monarch butterfly’s black-lined, autumn orange wings also herald the fall, for they roost here in winter. And who could blame them?
With perfectly protected eucalyptus havens like Monarch Grove in Natural Bridges State Beach and thousands of fellow monarchs for company, why not settle in? Especially if every autumn, when you return to the park, you get a Welcome Back Monarchs homecoming party?
Two species of monarch exist, one native to North America and the other to South America. During the North American summer, the northern monarchs live across the continent, drinking flower nectar and searching for mates. When a male spies a lovely lass he flirts, bumping into her until she lands on the ground to continue their amour. The female then lays eggs, one at a time, on the milkweed plants that the larvae munch. Each female lays 300 to 400 eggs in her lifetime. Toxins in the milkweed mean those gorgeous auburn wings also serve as a warning to potential predators: monarchs are poisonous.
After increasing in size up to 2,000 times, larvae cocoon up in a chrysalis and metamorphose. Three to four such summer generations live for two to five weeks each. Those born in the late summer, however, will migrate and overwinter, living for up to nine months. Most of their lives, this “Methuselah” generation exists in a kind of adolescent limbo brought on by the shortening days, with their reproductive organs remaining undeveloped until the onset of spring.
When the weather turns chilly, this long-lived generation heads for warmer climes, traveling between 50 and 100 miles a day on air currents and thermals. Those living across on the eastern side of the Rocky Mountains travel up to 3,000 miles to converge upon a 20-hectare spot in central Mexico. Monarchs west of the Rockies begin arriving in groves of eucalyptus, Monterey pine or cypress near San Diego, Pacific Grove or Santa Cruz in mid-October, heading out again around Valentine’s Day.
Because no individual monarch ever completes the migration, the question of how these butterflies know when and where to migrate is a subject of great interest. The flight patterns may be inherited, and the butterflies may use photoreceptive proteins in their antennae to detect the earth’s magnetic fields.
UPDATE Dec. 4, 2024: Monarch butterflies are expected to soon be recommended for addition to the threatened or endangered species list by the U.S. Fish and Wildlife Service based on the latest scientific information (NPR, 11/19/24). Though they do have some natural predators, loss of habitat (i.e., milkweed), a few hot and dry years (climate change) and the use of herbicides and pesticides are the real threats to monarch populations (Hilltromper 2013).
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