Monarch butterfly populations are studied and quantified during the winter, when they are roosting in the shelter of trees, and during the summer breeding season in warmer climates. These studies tell different stories. The breeding population in North America seems to be holding steady since the 1990s in many parts of the country, but the over-wintering population has been steadily dwindling during the same period. As an academic ecologist recently told the New York Times, “’So it’s not really a production problem,’ said Dr. Davis, an author of the new paper. ‘We don’t have fewer monarchs. We have fewer monarchs reaching the wintering colonies.’”
The most recent study of the breeding population of monarchs is based on a huge data set of 135,000 observations in 403 different sites in North America, partly collected by volunteers of the North American Butterfly Association (NABA) annual summer butterfly count since 1993. The analysis of current population trends reveals interesting clues about the future of monarchs and probably many other butterfly species:
- The study “used federal data to estimate how much glyphosate was being used in the area around each survey site. They found that in some regions, especially in parts of the Midwest, glyphosate use was associated with declines in abundance.”
- “But they also documented a countervailing force: climate change. In the northern part of the United States, increasing temperatures were correlated with increases in monarch abundance. This effect was especially pronounced in the Midwest, suggesting that the warming climate might have partly offset the effects of glyphosate in that region.”
The study of monarch breeding populations in North America found that the Southwest was one of the regions in the US where monarch population declines were greatest. That finding is consistent with the study of academic entomologists, Matt Forister and Arthur Shapiro, of butterfly populations in the West. They analyzed data from over 40 years of counting butterfly populations (including NABA data) to learn that 450 butterfly species in western states have declined 1.6% per year in the past 20 years, for a cumulative total of 25% fewer butterflies. Although there are several factors—such as habitat loss and pesticides—their study determined that the strongest factor was climate change, particularly warmer temperatures in the fall.
Professor Shapiro explained during an interview on KALW why extreme heat is harmful to butterflies, although the reasons have not been proven yet. Monarchs are one of the butterfly species that is dormant during winter months. They breed in spring when temperatures begin to warm and days become longer. Warmer winter temperatures are reducing the length of dormancy, which increases their need for year-around food and weakens them if there is inadequate food. Extreme heat and drought have an impact on plants, reducing available food for all butterflies.
Studies of Migrating Monarchs
How do studies of migrating monarchs compare to studies of breeding populations in North America? There are two major migrations of monarchs in North America. The migration east of the Rocky Mountains spends the winter in Mexico and the migration west of the Rockys spends the winter on the coast of California. Both of the overwintering populations have plummeted since the 1990s until the winter of 2022 when the population stabilized in Mexico and increased substantially in California.
The increase in the California monarch migration was described by Jessica Griffiths in an article published by the Sierra Club’s national magazine. That article is significant for several reasons. The particular roosting site where the population increase was greatest was a eucalyptus grove in Pismo Beach, California: “We are standing in a eucalyptus grove on a small patch of undeveloped land bordered by farms near the town of Pismo Beach, on the central California coast. The air smells faintly of brussels sprouts and compost, with an overlay of something like Vicks VapoRub—the distinct scent of eucalyptus. Griffiths gazes up at the branches and smiles. There are so many butterflies.” The irony is that Jessica Griffiths is the author of a deeply flawed study that claims that monarchs prefer native conifer trees to eucalyptus trees for their winter roost. One wonders if Jessica Griffiths experienced cognitive dissonance as she counted 17,845 monarch butterflies roosting in a eucalyptus grove where only nine monarchs roosted the previous year.
Jessica Griffiths provides an important clue to changes in the monarch migration in the Sierra Club article. She says monarchs roost in the trees until the temperature rises to about 55⁰ Fahrenheit, when their body temperatures rise enough that they can actively seek the nectar they need to survive. She says, “They are basically solar powered,” which is another way of saying they are cold-blooded animals that require the heat of the sun to be active. In the eucalyptus groves that monarchs prefer as their winter roost in California, nectar is close at hand because eucalyptus blooms during winter months, at a time when little else is blooming.
When the climate changes, entire ecosystems change with it
When days become shorter in the fall, monarchs in California stop breeding and begin their migration to the coast. Breeding resumes when days become longer in the spring. But hours of daylight are not the only determinant of the monarch breeding season. Warmer temperatures at night are triggering the monarch breeding season earlier than in the past. In fact, some entomologists hypothesize that many monarchs are now breeding year around. The presence or absence of milkweed does not trigger the breeding season, which is determined by hours of daylight and temperature.
If the warming climate enables monarchs to breed year around, why would be object? The more monarchs, the merrier, right? Unfortunately, hobbyist naturalists DO object to altering the timing and location of the breeding of monarchs. This is a Letter to the Editor of the Yodeler, the newsletter of the San Francisco Bay Chapter of the Sierra Club:
The author of the letter to the editor of the Yodeler asks us NOT to plant milkweed near the coast or monarch overwintering sites, presumably because she doesn’t want the monarch’s breeding season to begin when and where it has not occurred in the past. The fact is, the climate has changed and monarchs are responding to those changes. Who are we to argue with monarchs about what they need to do to survive?
Bay Nature has published an article about monarchs seen in Marin County during their breeding season, where they have over-wintered in the past, but not bred historically. The warming climate and the availability of perennial tropical milkweed is making Marin County suitable breeding habitat: “A lot of people have this feeling that without the migration, the monarch is nothing,” says James. “That’s not necessarily true. If we got rid of the migration, the butterflies could still continue. For humans, that would be a pity. But in the ecology of things…it’s not that bad.” The author of the article welcomes monarchs to Marin County, “A new Bay Area neighbor, adapting to a changing world, making do with what is available, as we all must.”
The monarch migration is not sacrosanct. Monarch butterflies also live in Central and South America, in the Caribbean, in Australia, and even in parts of Europe and New Guinea. But all of these monarch populations are sedentary, meaning they stay in one place and don’t migrate. If changes in climate enable monarchs to live and breed year around, why would we want to prevent them from doing so?
If monarchs can find what they need year around, why should they be forced to migrate? Migration is physically demanding, depleting the physical resources of an animal. If survival of a species doesn’t require migration, more physical resources are available for other functions, such as increased reproduction or less need for food to fuel the migration. Images of struggling human migrants come to mind. Wouldn’t they all be better off if circumstances at home would enable them to stay home?
A comparable change has occurred in the life cycle of Anise Swallowtail butterflies. Prior to the introduction of non-native fennel to California, Anise Swallowtails bred only once each year because its native host plant—closely related to non-native fennel–was not available during most of the year. Non-native fennel is a perennial plant that is available year-around, making it possible for Anise Swallowtails to breed throughout the year. Thanks to non-native fennel, we enjoy the company of many more Anise Swallowtails. We should not think of the life cycles of plant and animal species as immutable. Rather, they are constantly changing to adapt to changes in their environment and adaptation is what will ensure their survival.
Native vs. Non-native Milkweed?
Hobbyist naturalists also ask that we plant only native milkweed, the host plant for monarch caterpillars. Such restrictive advice is not beneficial to the survival of monarch populations. Although a popular opinion among hobbyists, advice against planting non-native milkweed for monarchs is contradicted by scientific sources:
- “…there is little evidence to support the idea that planting Tropical Milkweeds will weaken Monarch populations and NO evidence to support the idea that Tropical Milkweeds are “trapping” Monarchs and stopping them from migrating…” American Butterflies, magazine of the North American Butterfly Association
- A study of lifespan of monarchs breeding on non-native milkweed compared to native milkweed found that monarchs raised on tropical milkweed (A. curassavica) lived as long or longer than monarchs raised on other species of milkweed. They were less likely to be infected, and once infected, tolerated the infection well. (Leiling Tao et.al., “Disease ecology across soil boundaries: effects of below-ground fungi on above-ground host—parasite interactions,” Proceedings of Royal Society of Britain, 282: 2015.1993.)
- An article from the UC Davis Bug Squad says they plant tropical milkweed and two species of native milkweed. Monarchs have a strong preference for tropical milkweed: “In July, we collected 11 caterpillars from the narrowleaf [native] milkweed; we rear them to adulthood and release them into the neighborhood. But in the numbers game, the tropical milkweed won. From July through today, we have collected a whopping 43 eggs or caterpillars from [non-native] A. curassavica. How many from [native] A. speciosa? Sadly, none.”
Hobbyists theorize that tropical milkweed harbors more parasites than native milkweed because tropical milkweed is a perennial plant, which suggests that parasites could accumulate from one year to the next. If gardeners are concerned about the potential for accumulation of parasites, they are advised to cut tropical milkweed back during winter months. Because tropical milkweed is a perennial, it is available for monarch breeding earlier in the spring than annual native milkweed. If monarchs breed earlier in the spring, tropical milkweed accommodates earlier breeding.
How to help monarchs
The future of monarchs is uncertain, just as the future of all life in our changing climate is uncertain. I am betting that monarchs have a future partly because they have survived many changes in the environment for some 50 million years since butterflies evolved from moths. We can best help monarchs by staying out of their way. They would also probably benefit if we would stop destroying their habitat, particularly eucalyptus trees and tropical milkweed.
Vegetation changes as the climate changes and animals follow the vegetation they need as they must to survive. Breeding season of butterflies and other wildlife is also likely to change with the climate. The rebounding monarch population is probably another case of animals moving to find what they need. We should not stand in their way. They know what they need better than we do.