Migration or expansion, monarch butterflies make us aflutter
Ron Sullivan and Joe Eaton
Wednesday, December 12, 2007
The monarch butterflies have returned to their favored Bay Area winter roosting spots, festooning the trees with orange and black. Ardenwood Historic Farm near Fremont had about 600 last week, according to supervising naturalist Ira Bletz.
“That’s a typical number for this time of year,” he says. “They continue arriving into December. Last year we had close to 2,000. It goes up and down; every year is different.”
The butterflies have also returned to Monarch Bay Golf Course in San Leandro, which had higher numbers than Ardenwood on the Thanksgiving count; to Fort Mason, the Gill Tract in Richmond and other traditional sites. Muir Woods National Monument ranger Mia Monroe, who also runs the Xerces Society’s California Monarch Campaign, reports “generally low numbers” along the Central Coast.
Most of those roosts are in eucalyptus groves, not the pines and cypresses used historically. “It is likely that, had ‘eucs’ not been introduced, the phenomenon of mass-wintering monarchs would not exist in California today,” Monroe and lepidopterist Robert Michael Pyle wrote in 2004. Unlike native conifers, eucalyptus provides nectar as well as shelter.
The butterflies’ predilection for eucs has occasioned some tension between monarchists, as they call themselves, and native plant advocates. Pyle and Monroe recommend “nurtur[ing] tolerance for essential stands of eucalyptus – sometimes even refreshing them – until we can restore mature groves of native species.”
Ardenwood had up to 6,000 monarchs before the late 1990s, when 2,500 of the park’s eucalyptus trees succumbed to a wood-boring beetle from their native Australia.
Bletz says integrated pest management using parasitic wasps has the beetle under better control.
You probably know the basic monarch story: Hatch out on a milkweed plant, eat a lot, emerge from a green-gold chrysalis, migrate south to the wintering grounds, then back north to found a new chain of generations. It’s been assumed that monarchs east of the Rockies headed to the mountains of central Mexico and their western counterparts to coastal California, and that both these flights were directed migrations: these insects, with their poppy-seed-sized brains, somehow orienting and navigating on their journeys, as Arctic terns and green turtles do.
There’s evidence they get their bearings from the Earth’s magnetic field.
But not all the monarchs at Ardenwood are long-distance travelers. “We’ve found a few tagged butterflies here,” Bletz explains. “Our prizewinner was one tagged in British Columbia. But some were tagged in the Bay Area.”
None so far has come from the vast area between the coast and the Rockies, where there’s plenty of milkweed for caterpillars (the source of adults’ vivid colors and bitter taste). In fact, only a handful of tagged Great Basin monarchs have ever been recovered in California.
The best-known critic of the western migration model is a retired UC Santa Barbara entomologist named Adrian Wenner, known to some as Lord of the Gadflies.
Wenner, who is also a beekeeper, has argued for years that Karl von Frisch was all wrong about the dance language of honeybees, the phenomenon that nature writers and filmmakers are so fond of.
Based on observations of monarch roosts in Santa Barbara County, Wenner proposes that what’s happening is less a true migration than an annual range expansion and contraction. He argues that when the local roosts break up in February, it’s still too cold for the butterflies to survive the northward passage through the Sierra Madre and San Rafael Mountains, let alone the Sierra Nevada and Cascades. He’s also seen late-winter monarchs flying in all directions – not just northward – and females laying eggs on coastal milkweed in February.
Wenner also points out that the fall flight is against the prevailing wind and only at midday, which wouldn’t allow for the vast distances putative migrants would have to cover. And he has never seen migrating adults in the Central Valley in fall.
In his model, monarchs leaving the roosts just fan out into the immediate neighborhood, with subsequent generations working their way inland; at the end of the year, their descendants drift back to the coast. It’s unclear whether Bay Area butterflies show similar behavior.
A related puzzle: Are western and eastern monarch populations linked?
Pyle spent the fall of 1996 tracking monarchs through the Great Basin; he saw most flying a southeasterly vector, as if headed for Mexico. Scientists report no significant genetic differences between eastern and western monarchs.
The quick recovery of California coastal monarch numbers in 1996-97 following a disease outbreak the previous year suggests reinforcement from the east, possibly related to a shift in prevailing winds over the Gulf of Mexico.
As far as we can tell, the jury is still out on these issues. Analyzing the chemical fingerprints of the different milkweed species monarch caterpillars eat might provide some clues. Meanwhile, the mysteries needn’t detract from the beauty of the winter roosts.
Where to spot them
Ardenwood Historic Farm: www.ebparks.org/parks/ardenwood, (510)796-0663. Guided tours of the monarch roost Saturdays and Sundays at 1 and 2 p.m.; after New Year’s Day, on Tuesdays, Thursdays and Fridays at 2:15 pm. Also educational programs at 11 a.m. on weekends.
Monarch Bay Golf Course: (510) 577-6085. Tours on Saturdays through January, 10:30 a.m. and 1:30 and 2:30 p.m.
Monarch Watch: http://www.monarchwatch.org Excellent source on monarch migration and wintering.
Monarch Butterfly Wintering Sites in California: www.xerces.org. A page maintained by the Xerces Society, dedicated to the conservation of butterflies and other invertebrates.
— “Chasing Monarchs: A Migration With the Butterflies of Passage,” by Robert Michael Pyle. (Mariner Books; 2001; $14.)
— “Four Wings and a Prayer: Caught in the Mystery of the Monarch Butterfly,” by Sue Halpern. (Vintage Books; 2002; $13.)
Joe Eaton and Ron Sullivan are freelance nature and garden writers in Berkeley. E-mail them at email@example.com.