Part 1: Migration Magic
It’s not easy being a monarch butterfly these days. Your caterpillar’s essential milkweed food is no longer abundant; illegal logging and beetle infestations threaten your main winter roosts in Mexico, and climate change is playing havoc with nectar plants on your migration routes. Where hundreds of millions of monarchs roosted annually in the mountain forests of Central Mexico in the 1980s, only about 38 million made it there last winter. Over a 15-year period, the U.S. Fish and Wildlife Service estimates that 1 billion fewer Eastern population individuals returned to their overwintering sites. For the much smaller western populations, the situation is even worse, their numbers being barely one percent of what it been in the 1980s.
Monarch butterflies go through four life stages: egg, larva (caterpillar), pupa and adult. Each female lays up to a few hundred eggs, always on milkweed plants. After a few days, the caterpillars hatch and feed ravenously on the milkweed stalks before pupating. Ten to 14 days later, the actual butterflies emerge as hardy orange and black flying machines.
East of the Rockies, monarchs come in two lifetimes, short-lived (up to two months) for those flying north and long-lived (about eight months) for southbound ones. Coming north, each individual’s actual flying lifetime is a mere three weeks or so before stopping to reproduce, passing on genes to their progeny like a relay runner’s baton before dying. Two or three generations after leaving their winter haven in Mexico (and, to a lesser extent, Florida), most of them they arrive in the northern U.S. or southeast Canada, where the long-lived generation is born, destined to flap their way south to their grand-, or great-grandparents’ winter roost.
The challenge for these southbound fliers begins in the fall. Incredibly, their inherited genes include a map of the route — up to 3,000 miles — to their destination. That’s not all: The creature’s tiny brain includes a navigation system that uses the sun for orientation (seeing in blue, green and ultraviolet) along with an internal clock to compensate, hour-by-hour, for the sun’s daily movement across the sky. Flying at about 6 miles per hour, fewer than one in five completes the journey, the rest having fallen prey to hungry birds, strong headwinds or fatigue. Upon arrival in the Mexican states of Michoacan and Mexico, they swirl in great clouds above the high-elevation Oyamel firs before settling down, each clinging to its neighbor above in long thick chains of orange and black. There, they spend four to five months in semi-hibernation, typically fluttering around seeking nectar and water when the air warms up during the day.
At the end of their time in the high-elevation groves and before starting to head north, they mate, with the male typically forcing the female to the ground or on a tree branch where they copulate. Then the monarch’s annual cycle starts anew. In their millions, they head 1,000 miles or so to the southern U.S., where the females lay their eggs on milkweed plants — and die, having passed on their genes to the next generation.
Next week: How revolutionary tiny transmitters are helping conserve the remaining monarchs.
Barry Evans (he/him, barryevans9@yahoo.com, planethumboldt.substack.com) and his wife have been enraptured while visiting several of the high-altitude monarch reserves west of Mexico City.
This article appears in King Salmon Recovery, Egg-citing Condor News + Black Heritage Ball.