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Migration or expansion, monarch butterflies make us aflutter<br><br>
Ron Sullivan and Joe Eaton<br><br>
Wednesday, December 12, 2007<br><br>
They're back! <br>
<br>
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. <br>
<br>
"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."<br>
<br>
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. <br><br>
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.<br>
<br>
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."<br><br>
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. <br>
<br>
Bletz says integrated pest management using parasitic wasps has the
beetle under better control.<br>
<br>
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.
<br><br>
There's evidence they get their bearings from the Earth's magnetic
field.<br><br>
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." <br>
<br>
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.<br>
<br>
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. <br>
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.<br>
<br>
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.<br>
<br>
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. <br>
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.<br>
<br>
A related puzzle: Are western and eastern monarch populations linked?
<br><br>
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. <br>
<br>
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.<br>
<br>
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. <br>
<b> <br>
Where to spot them <br>
Ardenwood Historic Farm:</b>
<a href="http://www.ebparks.org/parks/ardenwood"><i>
www.ebparks.org/parks/ardenwood</a></i>, (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.<br>
<b> <br>
Monarch Bay Golf Course: </b>(510) 577-6085. Tours on Saturdays through
January, 10:30 a.m. and 1:30 and 2:30 p.m.<br>
<b> <br>
Monarch Watch:
</b><a href="http://www.monarchwatch.org"><i>www.monarchwatch.org</a></i>
. Excellent source on monarch migration and wintering.<br>
<b> <br>
Monarch Butterfly Wintering Sites in California:
</b>
<a href="http://www.xerces.org/Monarch_Butterfly_Conservation/sites_to_visit.htm">
<i>www.xerces.org/Monarch_Butterfly_Conservation/sites_to_visit.htm</a>
</i>. A page maintained by the Xerces Society, dedicated to the
conservation of butterflies and other invertebrates.<br>
<b> <br>
Books <br>
</b>-- "Chasing Monarchs: A Migration With the Butterflies of
Passage," by Robert Michael Pyle. (Mariner Books; 2001; $14.)<br>
-- "Four Wings and a Prayer: Caught in the Mystery of the Monarch
Butterfly," by Sue Halpern. (Vintage Books; 2002; $13.)<br>
<i>Joe Eaton and Ron Sullivan are freelance nature and garden writers in
Berkeley. E-mail them at
<a href="mailto:home@sfchronicle.com">home@sfchronicle.com</a>.</i>
<br><br>
<br>
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*************************<br>
Scott Hoffman Black<br>
Ecologist/Entomologist<br>
Executive Director<br>
The Xerces Society for Invertebrate Conservation<br>
4828 SE Hawthorne <br>
Portland, OR 97215 <br>
Direct line (503) 449-3792<br>
sblack@xerces.org<br><br>
<i>The Xerces Society is an international, nonprofit organization that
protects wildlife through the conservation of invertebrates and their
habitat. <br>
</i> <br>
To join the Society, make a contribution, or read about our work, <br>
please visit
<a href="http://www.xerces.org/">www.xerces.org</a>.<br><br>
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