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Why Do You Find So Many Monarchs on Eucalyptus Trees?

by Liam O'Brien on January 16, 2018

monarchs in flight
"Monarchs in flight." (Painting by Liam O'Brien)

Monarch butterflies seem to prefer roosting in eucalyptus trees (like the wintering butterflies at Ardenwood Farm and Pacific Grove). Do they actually prefer them as a roosting spot, and if so, why? – Beth S.

First off, points to you for using the word “prefer” instead of the anthropomorphic “like.”

Even better is “show a preference for” eucalyptus trees. Beth, it’s generally believed that it comes down to their legs — long and the perfect length to hold onto the sickle-shaped eucalyptus leaf.

Researchers have concluded that Monterey pines and cypresses were probably the monarchs’ original roost trees here in California. Other native conifers as well. (I once witnessed a handful sleeping up in a palm tree! ) But if this celebrated butterfly has shown us anything, it’s that it’s got a magnificent ability to adapt. Back when blue gum (Eucalyptus globulus) flooded California and was planted as an ill-conceived lumber commodity, many of our pines had been milled away, left only in patchwork portions of the state. The butterfly made a remarkable shift then to the towering, Australian non-native.

But it’s not just the tree, it’s also the topographic lay of the eucalyptus, usually in a bowl, with a north-facing aspect. Next time you are in the Natural Bridges grove in Santa Cruz, or exploring the Rob Hill/Presidio eucalyptus clusters, note the configuration and the lay of the trees around you. All butterflies seem to put a great deal of effort in to avoid wind. (I’ve got to stop saying, “abhor” on my walks…) They thermoregulate through the day, dropping away from their overnight beds, searching for nectar food or basking on another leaf or bush at the mid-canopy. It’s hard to bask if the trees are exposed to the wind.

I’ve been monitoring roosts in San Francisco County for over a decade during the Western Monarch Thanksgiving Count and one of the sites, the Fort Mason/Youth Hostel Grove, was opened up by some severe tree-pruning and removal a few years back. I’ve never seen the clusters there like I had once — now it’s too exposed. The monarchs moved on.

I’ve learned through the years from Mia Monroe, the Bay Area Xerces Society liaison, that many roosts have absolutely no conservation protection. Obviously ones on private property are managed at the discretion of the land owner. A glorious roost out on Treasure Island that had monster trees inadvertently planted decades back in a perfect bowl-shaped aspect is now gone to development. The monarchs moved on.

Which finally, I hope, paints the picture I’ve been intending: this is a highly resistant, opportunistic creature that has shown some remarkable abilities to adapt. People get nervous when I throw out that I think if all the eucalyptus disappeared tomorrow (a tree that needs severe management), the monarch would find some other trees. We project a great deal of vulnerability on this insect. Why? Because it’s big and pretty and hits us emotionally … like pandas.

But the reverse is true as well: if the misguided eucalyptus bonanza of the turn of the twentieth century had not occurred, we as Californians may never have had such a front-row seat to this annual, eye-level phenomenon.

IMG_2344Liam O’Brien is a self-taught butterfly conservationist in San Francisco. He works diligently to restore butterflies to their rightful place: that of wildlife and an under-appreciated part of the natural food chain. Liam received the 2014 “Environmental Educator” Award from Bay Nature Magazine.

Ask the Naturalist is a reader-funded bimonthly column with the California Center for Natural History that answers your questions about the natural world of the San Francisco Bay Area. Have a question for the naturalist? Fill out our question form or email us at atn at baynature.org!

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Paul Cherubini on January 16th, 2018 at 8:12 pm

Article seems strange; e.g. the statement: “if all the eucalyptus dissappeared tomorrow”…why would the author entertain such a thought considering that the State’s blue grove eucalyptus groves have been thriving for the past 100 -150 years and counting? Similarly in regard to the authors statement eucalyptus is “a tree that needs severe management.” Why entertain such an implausible thought considering the vast majority of State’s eucalyptus groves have thrived for the past 100 -150 years with little or no management?

Crunch Hardtack on January 18th, 2018 at 9:09 am

To me, “severe management” = culling/removing non natives. Whether they thrived for 100-150 years is irrelevant. I’m not necessarily a purist or native only, but I would much rather see Monterey pine and cypress trees gracing our headlands. Maintaining euc groves in California is like planting Buddleia davidii along riparian areas in Oregon, or encouraging pampas grass (Cortaderia selloana) in the Big Sur region. It’s not to say that all eucs should be cut down immediately. Common sense would dictate removal and replacement over decades with respect to overwintering sites.

Mary McAllister on January 19th, 2018 at 9:14 am

Thanks for this acknowledgement of the fact that monarchs prefer eucalyptus for their winter roost. One study of about 180 of the over 300 sites along the coast of California where monarchs spend the winter reports that 75% of the monarchs roost in eucalyptus trees. The runners up are Monterey pine and cypress.

In addition to the physical structure of these trees and where they grow along the coast in mild temperatures, the eucalyptus has one big advantage over the runners up. Eucalyptus flowers throughout the winter months, making nectar available to the monarch during their diapause. The monarch isn’t very mobile during diapause so having their larder close at hand is a big advantage to them.

Eucalyptus is being eradicated all over California, so the speculation that monarchs will lose their winter roost is realistic and credible. When a eucalyptus grove in Monterey was radically thinned, the monarchs left. When the trees were planted by activists (against the wishes of the nativists), the monarchs returned.

Those who think that Monterey pine and cypress are a substitute for eucalyptus should think again. Both of those species, which are native to California, are also being eradicated in the Bay Area. There is fossil evidence that Monterey pine has lived in the Bay Area several times in the distant past. Yet because of the absurd definition of “native” being used by nativists, it is being eradicated here because it wasn’t here in 1769, the magical year used to confer native status. “Native” is being defined as “prior to the arrival of Europeans,” based on the mistaken belief that Native Americans had not altered the land. We now know that isn’t true, but nativism has not yet acknowledged that flaw in their “reasoning.”

So, Monterey pine and cypress are not substitutes for eucalyptus because they aren’t considered native here. They are native about 100 miles away, but that doesn’t satisfy the purists who are controlling our public lands.

Another advantage that eucalyptus has over Monterey pines is that the blue gum lives in Australia from 300-500 years and arborists with no nativist prejudice say they are likely to live that long here. The Monterey pine is one of our shortest lived trees. If it escapes pitch canker or bark beetle, it will live about 150 years at most.

Don’t get me wrong. I like Monterey pine and cypress just as much as eucalyptus and I would be happy to have more of them. That’s not going to happen here in the Bay Area as long nativism continues to have a death grip on our public lands.

Nativism is destroying some of our most valuable environmental assets, including the winter roost of monarch butterflies.

Bev Jo on January 19th, 2018 at 8:19 pm

I so agree about the horrific damage nativism (primarily promoted by non-native privileged people) is doing to our Bay Area environment.

I love all three of those trees, but it’s true that Eucalyptus are the most likely to survive our increasingly warm climate. Plus, in spite of nativist propaganda, they are resistant to fire and help keep our urban forests cool and wet, as well as create the firebreaks so needed in the North Bay and Southern California Fires.

They also are loved (why on earth should we edit from our minds the idea that animals do love, like, and appreciate the trees who give them shelter or food or ideal nests, just as we would, other than to distance ourselves from nature?) by the hummingbirds who drink their nectar, and the large raptors such as eagles, buteos, owls who prefer Eucs for nesting. (I think their tall and open growth makes it much safer for the larger raptor fledgings to learn to fly in.)

Fifty percent of our native oaks, bays, etc. are dying from Sudden Oak Death, so we need every tree we can get, and the most maligned, Eucalyptus, are the healthiest and most disease resistant. Why not learn from the native animals, which trees to value and protect?

Julie Gallegos on January 19th, 2018 at 9:08 pm

The much-maligned eucalyptus is the workhorse of our 7-Million-strong Bay Area. A “Native” greenscape will never 1. Survive and 2. Provide carbon sequestration or habitat so badly needed in the 21st Century. We should celebrate the tall, hardy, and cooling euc forests we have and admit that their presence is one of the reasons we love our Bay Area. Grown-ups admit the truth.

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