[Pollinator] Migration or expansion, monarch butterflies make us aflutter

[Scott Black]

Migration or expansion, monarch butterflies make us aflutter

Ron Sullivan and Joe Eaton

Wednesday, December 12, 2007

They're back!

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: 
<http://www.ebparks.org/parks/ardenwood>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>www.monarchwatch.org. 
Excellent source on monarch migration and wintering.

Monarch Butterfly Wintering Sites in California: 
<http://www.xerces.org/Monarch_Butterfly_Conservation/sites_to_visit.htm>www.xerces.org/Monarch_Butterfly_Conservation/sites_to_visit.htm. 
A page maintained by the Xerces Society, dedicated to the 
conservation of butterflies and other invertebrates.

Books
-- "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 <mailto:home at sfchronicle.com>home at sfchronicle.com.



*************************
Scott Hoffman Black
Ecologist/Entomologist
Executive Director
The Xerces Society for Invertebrate Conservation
4828 SE Hawthorne
Portland, OR 97215
Direct line (503) 449-3792
sblack at xerces.org

The Xerces Society is an international, nonprofit organization that 
protects wildlife through the conservation of invertebrates and their habitat.

To join the Society, make a contribution, or read about our work,
please visit <http://www.xerces.org/>www.xerces.org.


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