Monarch butterflies in California need eucalyptus trees for their winter roost

Monarchs are probably the best-known butterfly in North America, partially because they are distinctively beautiful, but also because of their epic migration.  East of the Rocky Mountains, monarchs migrate from the Canadian border and the Atlantic Coast to spend the winter months in fir trees in Michoacan State in Mexico.  West of the Rockies, monarchs migrate from the Canadian border and the Pacific Coast to overwinter along the coast of California from Mendocino County to San Diego County, near the Mexican border.

Monarch Butterfly.  Creative Commons
Monarch Butterfly. Creative Commons

No single monarch makes the entire journey.  It takes two to three generations of monarchs to make the entire round trip.  How each successive generation knows the route remains largely a mystery, although theories exist.  There are a couple of fascinating books about the migration that we recommend to our readers.  Four Wings and a Prayer is a book about the 38-year effort of Canadian entomologists, Fred and Norma Urquhart, to understand the migration.  It reads more like a suspenseful mystery than the non-fiction book that it is.  Flight Behavior is by Barbara Kingsolver, one of our favorite novelists because nature is often the subject of her work.  Although it is fiction, it has been carefully researched by Kingsolver who studied biology before becoming a writer.  It is engaging both as a cautionary tale for environmentalists and as a personal redemption story.

The western migration of the monarch

Monarch butterflies roosting in eucalyptus tree.
Monarch butterflies roosting in eucalyptus tree.

We will focus on the western migration of the monarch because that’s our neck of the woods, but also because this migration is one of the reasons why many people who care about nature and wildlife object to the destruction of eucalyptus trees.  Eucalypts are the preferred trees for over-wintering monarchs“Three types of trees were used most frequently by roosting monarchs:  eucalyptus (75% of the habitats primarily Eucalyptus globulus), pine (20% of the habitats primarily Pinus radiata), and cypress (16% of the habitats Cupressus macrocarpa).  Twelve other tree species were identified…with a combined prevalence of only 10%…habitats had smaller populations when the roosting tree type was a species other than eucalyptus, pine, or cypress.” (Three different studies by different authors are the source of these data, therefore they don’t add up to 100%.) (1)

For those who may not know the botanical names, that’s Monterey pine and Monterey cypress that are the runners-up to eucalypts as the most popular trees for over-wintering monarchs.  Although monarchs roost in those trees in their native range on the Monterey peninsula, they also use those species outside their native range.  Unfortunately, just as the eucalyptus is a target of native plant advocates who demand their destruction because they are not native, both Monterey pine and Monterey cypress are targeted for destruction outside their native range.  For example, both Monterey pine and Monterey cypress will be eradicated from hundreds of acres of public land if the FEMA grants are funded in the East Bay.  This is just 150 miles away from where those trees are native and there is fossil evidence that they existed in the East Bay in the distant past.  In other words, most of the trees used by monarchs for their winter homes are in jeopardy of being destroyed by the native plant movement.

Another nativist myth BUSTED!

One of the reasons why we are telling this story is that it is a tidy little example of the justifications fabricated by native plant advocates to support their destructive agenda.  In the case of the monarch, native plant advocates claim that prior to the arrival of Europeans, before eucalypts were planted and Monterey pines and cypresses were planted outside their native range, the monarch used native trees for their over-wintering habitat.  The “assessment form” used by the California Invasive Plant Council to classify Blue Gum eucalyptus as invasive says,  “[The Blue Gum] provides roost sites for migratory monarch butterflies…ecological niches for butterflies and raptors probably formerly filled by native plant species.”  No evidence is provided in support of that statement.  We have also read that claim in comments of native plant advocates on internet articles in response to those who defend eucalypts because they are needed by monarchs.

Like many of the “cover stories” of native plant advocates, this is just not true.  A search of the scientific literature about monarchs enables us to bust this particular myth to smithereens.  It would be simple enough for native plant advocates to look at the evidence before spinning their tales, but it is apparently easier to make it up, especially when they are rarely questioned.  Million Trees exists to fill this informational void.

The historical record of the western migration of monarchs

The earliest record of over-wintering monarchs in California is from 1864, when monarchs were observed over-wintering in Monterey pines in their native range.  Richard Vane-Wright, the scientist who reports this record, explains why he believes it is probably the first incidence of over-wintering monarchs in California:

“’Previous to that, no mention has been found of this interesting phenomenon…The early Spanish chronicles and traditions make no mention of it, although Monterey, a scant three miles distant, was gay with life when the last century came in…even David Douglas, the world famed botanist, and the keenest-eyes of all the strangers who came [to California] is silent regarding it.’…Douglas, the indefatigable fir tree collector, appears to have made no mention of the phenomenon in 1830-1832, despite spending two winters at Monterey.” (2)

Vane-Wright believes the eastern monarch migration to Mexico also began around the same time.  His theory is that the agricultural practices of early settlers, which cleared trees, created a population explosion of the milkweed that is the host plant of monarchs.  More milkweeds resulted in more monarchs and monarchs began to migrate in response to population pressure, he believes.  He calls this the “Columbus Hypothesis.”  (2)

Biological facts explain why monarchs choose these species of trees

Aside from the historical record, the biology of monarchs and the physical characteristics of the trees in which they over-winter explain why these species of trees are required by the over-wintering monarch.  During the late fall and winter, monarchs enter a dormant phase called diapause.  They continue to need nectar and moisture during that period, but they are not very active, so these resources must be close by.  Although they migrate to the coast from Mendocino to Mexico, they are most abundant around the mid-point of that range, where temperatures and rainfall are moderate.  Most of the approximately 250 roosting sites are within 2.4 kilometers of the ocean, so wind protection is important for them while they are roosting.  All of these factors predict the ideal conditions provided by eucalyptus trees:

  • Monarchs need tall trees (of at least 60 feet) because they roost in the intermediate level of the canopy where wind protection is greatest (3)
  • The forest must be dense enough to provide wind protection,
  • The tree canopy must be open so that the roosting monarchs receive filtered sunlight to keep their bodies warm enough.
  • The monarchs need enough moisture for hydration, but not so much that they are soaked and lose their body heat.  So, dew and/or fog provide the ideal amount of moisture.   (1 & 4)

All of these requirements for the monarch’s winter roost point to their dependence on eucalyptus, pines and cypress.  The trees that are native to the narrow strip of the coast of California do not meet these criteria.  They are not tall enough and they do not grow that close to the ocean because they do not tolerate wind.  The native vegetation of that narrow strip of California coast is predominately dune scrub and coastal grassland prairie.  And these are the vegetation types that the ecological “restorations” in the Bay Area are trying to re-create.  These vegetation types will not be suitable habitat for over-wintering monarchs.  Furthermore, plans to drastically thin eucalyptus forests on hundreds of acres of the East Bay Regional Park District will render those habitats useless for over-wintering monarchs.

In addition to the physical properties of eucalyptus, the monarch benefits from the fact that it is flowering from about December to May, while the monarch is roosting in the tree.  The flowers of eucalyptus contain a copious amount of nectar which is also important to the honeybee because it is flowering at a time when there are few other sources of nectar.  One study reported observing monarchs feeding on the flowers of Eucalyptus globulus. (5)

Risky Business

We have mixed feelings about reporting this research about monarchs to our readers because there is some risk to the monarchs in doing so.  The evidence suggests that monarchs did not over-winter in California prior to 1864, after the magical date that nativists have selected to freeze-frame California’s landscape to their nativist ideal.  Here in the San Francisco Bay Area, this magical date is 1769, when the expedition of Portola laid eyes on the San Francisco Bay.  Will nativists declare the monarch an alien invader to be eradicated along with the hundreds of plants and animals they claim “don’t belong here?”  This may seem a far-fetched conjecture, but keep in mind that the European honeybee is being eradicated in some “restorations” because it is not native.  The honeybee is essential to the survival of American agriculture, yet its existence is threatened by the radical agenda of the native plant movement.

That’s the risk we take in reporting this evidence because we hope that it helps our readers to understand the absurdity of the nativist agenda.

Update:  Monarchs have returned to Natural Arches State Beach in Santa Cruz in big numbers.  Here is a link to a report that includes a lovely video of the roosting Monarchs.  


(1)    Dennis Frey and Andrew Schaffner, “Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(2)    Richard Vane-Wright, “The Columbus Hypothesis:  An Explanation for the Dramatic 19th Century Range Expansion of the Monarch Butterfly,” in Biology and Conservation of the Monarch Butterfly, Natural History Museum of Los Angeles County, 1993.

(3)    Andres Kleiman and Miguel Franco, “Don’t See the Forest for the Butterflies:  The Need for Understanding Forest Dynamics at Monarch Overwintering Sites,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(4)    Kingston Leong, et. al., “Analysis of the Pattern of Distribution and Abundance of Monarch Overwintering Sites along the California Coastline.” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.

(5)    Susan Chaplin and Patrick Wells, “Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California,” Ecological Entomology, 7:249-256, 1982

25 thoughts on “Monarch butterflies in California need eucalyptus trees for their winter roost”

  1. Very good and concise information on the Monarch. I have never heard of honeybees being removed from an area. Wow, that is news to me. Now what kind of agenda is this? Do people even think about what they are doing in these removals? I like your photos of the Monarchs BTW, especially grouped in the eucalyptus.

    1. Honeybees are being eradicated on the Nature Conservancy properties on the Channel Islands, off the coast of California. That is one of the most extreme “restoration” projects. Many non-native animals have been exterminated there. There may be other projects that are killing honeybees, but that is the only documented case of which we are aware. As you know, we stick very close to the documented public record to maintain credibility and reduce flak.

      1. Thank you. My readers should be aware of this too. I will post your link on Slow it Down Sunday’s post. I have many followers that raise honeybees that will surely take offense to this. A few also raise Monarchs for release too.

  2. You seem to be overly harsh on people promoting native species. These are the people
    promoting the planting of milkweed and biodiversity. Are you so focused on a single species
    that you literally can’t see the forest for the trees? The space these non-native trees are occupying are harming the needed habitat of how many other species? Until you step back and answer this question it might be better to gain promoters of native habitat and biodiversity as willing allies in the support of monarchs and other deserving species.

    1. If this is the only article you have read on Million Trees, you might get the mistaken idea that we are solely interested in how eucalyptus benefits monarch butterflies. Here are a few more articles on Million Trees about the biodiversity of the eucalyptus forest:

      Empirical studies do not support the assumptions of native plant advocates about the inferiority of non-native plants and trees as habitat for wildlife. More importantly, the widespread use of herbicides to eradicate non-native species are harmful to wildlife. These projects are doing far more harm than good. We support the planting of milkweed and any other plant you wish. We are only opposed to the needless destruction of plants and trees that are performing valuable ecological functions and the methods used to destroy them.

      1. Forgive me for disagreeing with you, but your statements here regarding the biological “value” of eucalyptus are very short-sighted and fairly misinformed. While eucalyptus do provide suitable roosting habitat for monarch butterflies, and suitable nesting habitat for various birds including both passerines and raptors, their effects on ecological function as a whole are undeniably negative. In southern California eucalyptus windrows adjacent to irrigation ditches disperse voluminous amounts of seed downstream during runoff events, where it readily invades our limited areas of remaining riparian habitat.

        Webmaster: The point of the article on which you are commenting is that eucalyptus not only provide suitable roosting habitat for monarchs along the coast of California, but more importantly native trees cannot substitute for them because they won’t grow where the eucalypts are growing and/or they do not provide the same type of habitat because they are not tall enough or, in the case of redwoods, they have a dense canopy that cannot keep the monarchs warm enough. Likewise, raptors require tall trees where native trees will not grow.

        This website is devoted to the urban forest in the San Francisco Bay Area which was virtually treeless prior to settlement.

        The predominant species of eucalyptus in California is the Blue Gum. Although it is generally not invasive, when planted beside streams and irrigation swales, its seeds can spread. So, your observation of eucalyptus spreading is consistent with the scientific literature. Such a scenario is rare in California and is not a legitimate reason for eradicating eucalyptus in the many places where it is not spreading.

        In the absence of its native competition it becomes easily established. Within a matter of years a few seedlings can develop into dense monotypic stands (the antithesis of biodiversity) that distribute copious amounts of leaf litter, which further inhibit beneficial understory growth and the development of a stratified riparian canopy that many bird and animal species depend on. This reduction in vegetation variety causes an observable shift in avifaunal diversity, away from gleaners and cavity nesters towards more opportunistic species. This shift is extremely evident in obligate understory nesters, such as the federally endangered least Bell’s vireo, that has become virtually absent from historically occupied habitat that has been invaded by aggressive non-natives such as eucalyptus or tamarisk. The reduced understory growth and modified canopy structure has detrimental effects on myriad other species, including small mammals such as wood rats and long-tailed weasels that depend on a dense understory for forage, nesting, and cover, and even larger cryptic mammals, such as the mountain lion that also depend on dense understory vegetation for concealment during hunting and movement in general.

        Webmaster: There are four empirical studies conducted in the Bay Area that found equal numbers of plants and animals, including insects, birds, amphibians and mammals in eucalyptus forest and native woodlands. You will find citations of these studies in many articles on this website. The understory in the eucalyptus forest varies, but is often dense and diverse, including many native plants, some rare. You will find photos of these forests on this website.

        We have coyotes in the Bay Area and they are often found in the eucalyptus forest. Mountain lions are rare, but we are a densely populated urban area, so it is probably not suitable habitat for them.

        When direct impacts to those species that may occupy eucalyptus trees or groves are appropriately addressed and avoided (i.e. bird nests, monarch winter roosts), and subsequent, immediate planting involves large native trees, eucalyptus removals should be promoted, not eschewed.

        1. The primary reason that we are opposed to destruction of eucalyptus is because of the damage that is done to the environment when they are destroyed. They release tons of carbon dioxide into the atmosphere after they are destroyed and their wood chips decay on the ground. The stumps of the trees must be repeatedly sprayed with Garlon to kill the roots of the tree so that it will not resprout. Garlon is toxic to aquatic life and moderately toxic to adult bees. One study reported that Garlon significantly reduced the reproductive success of birds. Garlon kills mycorrhizal fungi in the soil. Eucalyptus also provides a windbreak where other trees cannot grow. When eucalypts are clear-cut from hillsides, erosion occurs. The projects that destroy non-native trees in the Bay Area rarely have resources to replant after the trees are removed. The result of tree removal is often just weedy messes, not native plants, let alone trees that rarely grow where eucalyptus now grow.

          We do not advocate for planting eucalyptus. However, they are expected to live about 300 more years and we believe that less damage will be done to the environment if they are allowed to die a natural death.

  3. How do Oaks compare with Euc? Oaks are coined “keystone” trees, as they are key in supporting thousands of different species of our biological diversity. Are they not as favored by Eucs? June Louks

      1. As this article explains, monarchs use non-native trees for their over-wintering roost in California for the following reasons: (1) they over-winter very close to the ocean where the climate is moderate (not too warm and not too cold) where oak trees do not grow because they do not tolerate wind; (2) monarchs roost in the mid range of tall trees (taller than oaks) because they are sheltered from the wind in that range; (3) monarchs choose a tree with a more open canopy than an oak because the dappled sunlight provides the warmth they need; (4) monarchs need access to nectar during their over-wintering roost which they find in the winter-blooming eucalyptus, but would not find in oaks.

  4. This website is not based on scientific evidence. The 2013 Eucalyptus White paper by Griffiths and Villablanca studied the wintering habits of monarchs and they preferred native trees over eucalyptus trees. Each year the trees chosen were different depending on the climate conditions, but if monarchs were wintering in eucalyptus trees they generally switched to native trees for part of the season. Mixed forests were best because the climate conditions that are best for the monarch butterfly vary and each type of tree can be beneficial depending on the climate conditions.

    1. The Griffiths & Villablanca study was conducted in the native range of Monterey pines, cypress, and redwoods. The study is therefore not representative of the 300+ locations where monarchs overwinter along the entire coast of California.

      The native ranges of these tree species are small. Monterey pines are native in “three disjunct populations in San Mateo and Santa Cruz counties, Monterey County, and San Luis Obispo County. The native population of Monterey cypress is significantly smaller: “The native range of the species was confined to two small relict populations, at Cypress Point in Pebble Beach and at Point Lobos near Carmel, California.”

      Where Monterey pine and cypress have been planted outside their native range, they are being eradicated by the same public land managers who are eradicating eucalyptus. For example, when UC Berkeley destroyed approximately 18,000 non-native trees over 10 years ago, many were Monterey pines. Their plans to eradicate 80,000 more trees include all Monterey pines in the project area. In San Francisco, the plans (SNRAMP) of the Natural Areas Program propose to destroy many Monterey cypresses on Mount Davidson. The GGNRA has destroyed about 500 Monterey pines on Hawk Hill in Marin County and many Monterey cypresses throughout their properties.

      Furthermore, both Monterey pine and Monterey cypress have much shorter lives than eucalyptus. Monterey pine lives at most 150 years and Monterey cypress about 250 years, compared to E. globulus, which lives in its native range from 200-500 years. Therefore, even where they are not being eradicated, they will die long before E. globulus and are unlikely to be replanted outside their small native range by public land managers who are committed to a “natives-only” policy.

      We are unaware of any attempts to eradicate redwoods outside their native range, in the few locations where they still exist. They seem to have escaped the wrath of nativism. However, the range of redwoods is very narrow: “The redwoods occupy a narrow strip of land approximately 450 miles (724 km) in length and 5 to 35 miles (8-56 km) in width. The northern boundary of its range is marked by two groves…within 15 miles (25 km) of the California-Oregon border. The southern boundary of redwood’s range is marked by a grove…Monterey County, California.”

      Although native plant advocates may be willing to plant redwoods outside their native range, they do not have that option because of the horticultural requirements of redwood. Redwoods require more water than Monterey pine and cypress and they do not tolerate wind, which prevents them from being successful in many coastal locations, where monarchs overwinter. Redwoods cannot be successfully grown south of Monterey County where the climate is warmer and drier than its native range.

      In other words, monarchs do not have the option of roosting in native trees in most of the places in California where they overwinter. This is a more accurate description of the behavior of overwintering monarchs and the alternatives that are available to them in about 300 locations along the entire coast of California, where they have overwintered in the past:

      “Three types of trees were used most frequently by roosting monarchs: eucalyptus (75% of the habitats primarily Eucalyptus globulus), pine (20% of the habitats; primarily Pinus radiata), and cypress (16% of the Cupressus macrocarpa). Twelve other tree species were identified…with a combined prevalence of only 10%…The negative sign for this indicator means that habitats had smaller populations when the roosting tree type was a species other than eucalyptus, pine, or cypress…our long-term analysis showed that abundance has historically been greater at habitats dominated by eucalyptus, pines, or cypress than at those with ‘other’ species. Stands of these three signature taxa may be more likely to produce a community structure and associated microclimate that increases the residence time of monarchs. Furthermore, these taxa may produce a more attractive landscape architecture in terms of sensory cues to migratory monarchs arriving in a certain region.” (Dennis Frey and Andrew Schaffner, “Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America,” in The Monarch Butterfly Biology and Conservation, Cornell University Press, 2004.)

      For the record, we will add that we would be happy to have more Monterey pines and cypress and if public land managers would quit destroying them, we would consider them attractive alternatives to eucalyptus. However, for the moment, we must assume that the crusade against all non-native trees will continue unabated.

      1. Nice article; I found the “Columbus Hypothesis” mentioned very interesting! While I guess I might consider myself a “nativst”, I believe there is some substance in valuing native/non-native species based on ecolgoical function and not by chance origins as is suggestive of the article. I was rather put off by the tone of this article, however. Strident phrases such as, “Another nativist myth BUSTED!” come off as emotional and really lessen the credibility of the attempts at scientific explanations put forward, and really the entire message that it seems Million Trees is professing. I value Eucalyptus trees as Monarch havens and for their beauty, but I also understand the importance of native ecosystems in California which I guess makes me a nativist. I think you might win over more people, such as myself, if you do not draw such a hard line in the sand.

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