Fresh News About Monarch Butterflies


Monarch butterfly (Danaus plexippus). Photo: Kenneth Dwain Harrelson, Wikimedia Commons

I’ve written about the monarch butterfly (Danaus plexippus), the world’s best known migrating lepidoptera, whose annual 3,000 mile/4,800 km migration from the mountains of Mexico to edge of the boreal forest in Canada is the stuff of legend, more than once in this blog (Monarch Butterflies Are Back!Plant More than Milkweed to Save Monarchs and Monarchs Arrive in Europe!, for example), but there is much research going on about them and scientists learn more about monarchs each year. 

Here are some interesting recent discoveries:

Winter Numbers Down

Some monarchs are tagged so their migration can be followed. Photo: Katja Schulz,

Sadly, Mexican authorities report that the number of monarch butterflies overwintering in Mexican forests was down by half during the 2019–2020 winter season. Just 7 acres (2.83 hectares) was covered, a 53% decrease from the 2018–2019 season, when monarchs covered 15 acres (6.05 hectares) of forest.

Jorge Rickards, the managing director of WWF-Mexico, noted that this is not necessarily a cause for alarm, but added that “we must remain vigilant and not allow it to become a trend in the coming years. Conservation is a long-term job.”

New Colony of Monarch Butterflies Discovered in Mexico

Butterflies at the newly discovered Nevado de Toluca colony. Photo:

For a long time, it was thought that there was only one colony of monarchs in Mexico, at what is now the Monarch Butterfly Biosphere Reserve in the state of Michoacán, but a second one was recently discovered near the Nevado de Toluca volcano, a good 4-hour drive to the southeast. 

For years, locals had remarked the presence of limited numbers of monarchs in the vast (53,419 ha) Nevado de Toluca National Park during the winter, but no one was able to find whether they were just stragglers or if there was a colony hidden there. Just before Christmas in 2018, however, a routine park patrol finally did find a large colony clinging to oyamel firs (Abies religiosa) at an altitude of more than 3,000 meters. In 2020, researchers returned to study the situation and calculated that there were about 20 million monarchs in the new colony, enough so tree branches bend under their weight.

Curiously, while the colony at the Monarch Butterfly Biosphere Reserve stays on the same trees all winter, which has made it into a tourist attraction, the new colony at Nevado de Toluca moves every night, so is harder to find and study.

Local governments currently have no plans to open this new colony to mass tourism.

Plant the Right Milkweed

We all know that monarch caterpillars will only feed on milkweeds (Asclepias spp.) and a few other closely related species, but it turns out that the milkweed species you plant to feed visiting monarchs is very important. 

The most commonly grown garden milkweed, butterfly weed (Asclepias tuberosa), turns out to be a bit of a dud when it comes to feeding monarch caterpillars. Photo: H. Zell, Wikimedia Commons

Butterfly weed (Asclepias tuberosa), the most common milkweed offered in garden centers and a popular garden perennial, is actually the least favorite milkweed species and monarch caterpillars rarely feed on it. It doesn’t have the milky sap of other milkweeds and there may be something in that sap that female monarchs seek when looking for a place to lay their eggs. 

Monarchs seem to like common milkweed best (A. syriaca). Photo: Stefan.lefnaer, Wikimedia Commons

The species most frequented by monarchs is the common milkweed (A. syriaca), a weedy species rarely grown in gardens. One study showed that 85% to 92% of monarchs overwintering in Mexico had fed on common milkweed as caterpillars. That said, most other species studied so far are good host plants as well, including another fairly popular garden species, swamp milkweed (A. incarnata). Apparently, it’s only butterfly weed (Asclepias tuberosa) that isn’t really worth planting if your goal is to provide a host plant for monarch caterpillars..

Monarch caterpillars like hybrid plants as well as species. Photo: Maria L. Evans, Wikimedia Commons

Another study challenges the commonly repeated belief that only wild species of milkweed should be used in butterfly gardens. It found that hybrid milkweeds are just as attractive to egg-laying females as straight species. 

Cut Your Milkweeds Back 

Cutting back milkweeds before they bloom makes them more attractive hosts for caterpillar larvae. Photo:

Research by Nate Haan of Michigan State University shows that female monarchs prefer to lay their eggs on fresh young stems that haven’t flowered yet rather than tougher, aging ones. That’s not a problem early in the season, but Haan discovered that cutting back a third of a garden’s milkweeds in June and another third in July resulted in more monarch butterfly eggs being laid on the resprouting plants. This heavy pruning doesn’t harm the milkweed plants and they quickly rebound.

The tropical milkweed (A. curassavica) may actually disrupt monarch mutation if it isn’t cut back. Photo: Renjusplace, Wikimedia Commons

And cutting back would be a good solution to another dilemma. The tropical milkweed (A. curassavica) is the most popular garden milkweed in the southern U.S., but its habit of continuous bloom well into fall and even winter can be harmful to migration. It’s believed monarchs tend to stop on patches of this plant rather than continue their trip south in a timely manner, disrupting migration. Also, a debilitating protozoan parasite (Ophryocystis elektroscirrha) tends to build up in tropical milkweed, at least in areas where it isn’t killed back by frost in winter, and can weaken caterpillars and cripple adult butterflies. 

Both these flaws can be easily mitigated by cutting tropical milkweed back hard in the fall. With the plant absent from the landscape, the butterflies will continue their migration normally and won’t be as likely to become infected with the disease.

Let Them Be Free

Mass butterfly releases aren’t helping save monarchs! Photo:

Apparently, monarchs raised indoors as caterpillars largely fail to migrate. They just don’t seem to pick up on the clues (colder weather, dieback of blooming plants) that tell wild-born monarchs it’s time to head south, according to biologists Ayse Tenger-Trolander and Marcus R. Kronforst of the University of Chicago. 

That means the thousands of monarchs sold by butterfly farms to schools, weddings, funerals and other events for mass butterfly releases may be good publicity for the monarch cause, but don’t directly help monarch survival in any way, as they simply are not flying back to Mexico. 

Planting Flowers to Feed Adults

The common belief that planting milkweeds (Asclepias spp.) is all you need to do to save monarchs is incorrect. True enough, female monarchs will only lay their eggs on milkweeds and their caterpillars will only feed on those same plants, but adult monarchs feed on nectar of a wide range flowers. In fact, don’t even particularly like milkweed flowers (they are instead largely pollinated by bees and wasps). 

Why not create a monarch way station? But drop letter B (Asclepias tuberosa) from the list of good monarch butterfly plants. (Read above to see why.) Photo:

If you want to create a garden for monarchs (and why not officially start a monarch waystation?), you need to grow more than milkweeds. Instead, try to provide a wide range of flowers, making sure that there are some in bloom throughout the entire growing season (read Plant More than Milkweed to Save Monarchs), especially at the extreme ends of the gardening season, that is, early spring and late fall.

Also, it’s turning out that mixed plantings actually confuse butterflies. They use sight to find their food, relying on plant shapes. So, a “butterfly meadow,” with flowering plants mingling any which way, might not be the best solution. Instead, you’ll get more positive results by growing single plants isolated from others by mulch. And planting in a north-south pattern is best. 

So, now you know what to do to make your garden more monarch butterfly friendly!

Help Conserve Monarch Butterflies by Being Part of a Monitoring Network across North America

A composite of five different views of a monarch emerging from its chrysalis. Photo: Commission for Environmental Cooperation

Follow the Blitz on social media at #MonarchBlitz

Join hundreds of volunteers in Canada, Mexico and the United States, from July 27 to August 4, for the 2019 International Monarch Monitoring Blitz (the Blitz) and be part of this regional initiative to help conserve the monarch butterfly (Danaus plexippus). By participating, you can help monarch experts gain more information to understand the distribution of the migratory monarch butterfly in North America.

“Observations from the public can help scientists gain valuable information that will support regional efforts to protect the monarch butterfly and its habitat all along its migratory flyways,” said André-Philippe Drapeau Picard, Mission Monarch coordinator at the Insectarium/Montréal Space for Life.

For one week, the Blitz invites people across North America to go out to gardens, parks and green areas and monitor milkweed plants for monarch eggs, caterpillars, chrysalises and butterflies. This information will help researchers identify priority areas for monarch conservation actions. Data gathered during the Blitz will be uploaded to the Trinational Monarch Knowledge Network, where they will be accessible for anyone to consult and download.

To take part in the Blitz, go to Mission Monarch page if you are in Canada. If you are east of the Rocky Mountains in the United States, follow the Monarch Larva Monitoring Project link, and if you are west of the Rocky Mountains, use the link for the Western Monarch and Milkweed Mapper. In Mexico, you can go to Naturalista. Or, simply follow the Blitz on social media, using the hashtag #MonarchBlitz.

Monarch butterfly overwintering sites were first recorded by scientists in California over 200 years ago and in Mexico in 1975. Since then, the monarch has become an emblematic species for North America. After an alarming decrease in its populations over the last 20 years, the eastern monarch population overwintering in central Mexico showed a significant increase this past winter. However, the population is still well below historic levels, which inspires questions about what conservation efforts are needed to continue this positive trend.

Meanwhile, the western monarch overwintering population along coastal California hit an all-time low this winter, with less than 1% of the historic population size remaining. Public participation in community science in the West is more important than ever to help understand and reverse this population’s dramatic decline.

“The majestic monarch butterfly, a flagship North American pollinator and symbol of international cooperation, needs your help with its spectacular annual migration across the continent. Join us by contributing to the International Monarch Monitoring Blitz,” says Cora Lund Preston, Communications Specialist at the Monarch Joint Venture.

The Blitz is an initiative of the Trinational Monarch Conservation Science Partnership, created through the Commission for Environmental Cooperation (CEC). Through the Blitz, scientists from the Insectarium/Montréal Space for LifeEnvironment and Climate Change Canada (ECCC), Monarch Joint VentureJourney North, the US Fish and Wildlife Service, the Xerces Society for Invertebrate Conservation, and Mexico’s Comisión Nacional de Áreas Naturales Protegidas (Conanp) are asking the public to help them understand monarch and milkweed distribution throughout North America.

Media Contact – CEC
Sarah Julien
514 781 2781
Monarch Project Lead – CEC
Georgina O’Farrill
Blitz Coordinator – Insectarium/Montréal Space for Life
André-Philippe Drapeau Picard


  • Community science, also called citizen science and participatory science, is the process by which non-scientists contribute actively and voluntarily to research projects.
  • 486 participants across Canada, Mexico, and the United States
  • 1,323 observations
  • 53,588 milkweed plants monitored
  • 13,796 monarchs observed
  • 6,905 eggs
  • 4,900 caterpillars
  • 470 chrysalises
  • 1,521 butterflies


  • Monarch butterflies weigh less than a gram.
  • There are two recognized migratory routes in North America: Eastern and Western.
  • Migration covers 3,000-5,000 km (2,000 to 3,000 miles) that span over three countries.
  • The Eastern migratory population has declined by more than 80% in 20 years, while the Western population has declined by more than 90%.
  • Everyone can help the monarch by participating in community science, creating habitat and spreading the word.

Texte adapted from a press release by the Commission for Environmental Cooperation

Monarch Butterflies are Back!



Monarch butterfly numbers are up this year.

Congratulations, butterfly gardeners! You’re obviously doing a good job, since the monarch butterfly (Danaus plexippus), whose numbers crashed drastically in 2013, are up… in a big way.

You may recall the glum news in 2013-2014. Never had so few monarch butterflies reached their wintering grounds in the fir and pine forests of Central Mexico. Only 0.67 hectares of forest were covered in monarchs that year, for a total of some 33 million monarchs, the lowest number ever, and experts were concerned that the era of the migrating monarch was nearing an end.

20160324CEnglish.png Fortunately, numbers picked up considerably in 2014-2015 and jumped massively this winter (2015-2016), covering 4.01 hectares for an estimated 140 million monarchs. While this is still 30% below the long-term average of 6 hectares, it is still a massive improvement and corresponds to about the average number this century.

What Happened?

The crash of the 2013 was due largely to poor weather. Drought conditions, first in Mexico, then in the Southwestern US, drastically cut back on the number of monarchs that were able to reproduce. (Monarchs move northward generation by generation from spring through mid-summer and at each stage along their route, they need a healthy food source of their only food plant, the milkweed [Asclepias spp.] and drying, desiccated plants won’t host many larvae!) Therefore, fewer monarchs made it past the drought zone to more northern climes…

… And those that did found less food for their caterpillars than ever. The increasing efficiency of modern farming, which leaves no space for native plants, including milkweeds, is seriously hampering the creature’s ability to reproduce. Their caterpillars can only live on milkweeds and if the number of available milkweeds continues to drop, the monarch’s survival as a migrating species will remain critical.

20160324F.pngLast summer (2015), the weather couldn’t have been better. Where milkweeds still do grow, they were well-watered and healthy all across the monarch’s route north, just what they need to reproduce abundantly. Plus the news that monarchs need milkweeds as a food source is becoming common knowledge, thanks to programs like the Million Pollinator Garden Challenge, and gardeners across North America are planting milkweeds, many of which are attractive garden plants, in increasing numbers. Governments are helping too. The US, for example, has restored over 100,000 hectares of fields without pesticides in order to save the monarchs, at a cost of over $20 million.

Goal of 250 Million Monarchs

The monarch population has always been unstable. Numbers go up and down according to all sorts of factors. However, if we’re congratulating ourselves about 140 monarchs this year, that number would have seemed disappointing 20 years ago, when 250 million monarchs was considered the average. Many associations from Mexico to Canada have made that their goal for 2020: to reach an average overwintering population of 250 million monarchs again.

Much still has to be done… and not all of it is within the grasp of home gardeners. Illegal logging in the monarch’s wintering grounds in Mexico remains a threat you can’t do much about, for example. And it’s hard to convince farmers to leave space for few native plants. But at least anyone can plant milkweeds.

Milkweeds for Your Garden


Asclepias tuberosa

Butterfly weed (A. tuberosa) is a stunningly beautiful milkweed with bright orange to yellow flowers that you could be growing in your garden, for example. It likes full sun, well-drained to dry soils and will grow in USDA hardiness zones 3 to 9 (AgCan zones 4 to 9).


Asclepias incarnata

It doesn’t thrive in my rainy climate, but the swamp milkweed (A. incarnata), with pink or white flowers, certainly does. It is just as hardy, perhaps even moreso… and in spite of its name, it doesn’t need to grow in a swamp, but is instead perfectly happy growing in full sun under average garden conditions. And yes, I did have monarch caterpillars on mine last summer after two years in which I saw no monarchs whatsoever.

Note that neither of these plants are invasive. They’ll grow in a dense clump and stay where you plant them. They are each, in their own way, ideal plants for laidback gardeners.


Asclepias curassavica

You don’t have a garden? Well, perhaps you have a balcony. If so, you could grow plants of tropical milkweed (A. curassavica). This plant, grown as an annual in the North, is actually the monarch’s main food source in Mexico, where it grows wild as a perennial. It has bright orange flowers with a yellow center. It grows readily from seed and will bloom the first summer if you start it indoors. After a summer outside, you can then bring it indoors as a houseplant in the fall if you want to. If you can’t find seeds locally, I know of at least two mail order sources, William Dam Seeds and Richters Herbs, that sell them.


Monarch caterpillar

If you find leaves being chewed on your garden’s milkweeds, you’ll know you’ll have succeeded. Look for a rather bizarrely striped caterpillar: a future monarch butterfly. Congrats! You’ll have done your part to save this fascinating creature!

From the Far End of Its Range

Where I live in Quebec City is pretty much as far north as monarchs ever go. When they’ve made it here, you know it’s been a good year. I’ve also been lucky enough to have seen the monarchs overwintering in a Mexican forest, a sight (and sound, as the millions of fluttering wings give off a sort of soft murmur) I’ll never forget. So I encourage every North American gardener reading this blog to plant at least one Asclepias this summer. It’s just one more plant to add to your garden palette, yet such a simple choice can have a huge impact on the monarch’s survival.