Frequently Asked Questions

Overwintering

When do monarchs arrive in Mexico and when do they leave?

Typically, monarchs arrive in Mexico around the same time as the Day of the Dead in early November. They spend the winter in the Mexican overwintering sites and then the colonies start to break up and migrate back towards the U.S. starting in March

What do monarchs do at the overwintering sites?

While overwintering, monarchs don't do much! They rest on the tree branches, and occasionally fly around if the weather allows it. Monarchs need to consume enough milkweed as caterpillars and nectar as adults before reaching the overwintering sites so that they do not need to eat to survive while they are overwintering. There is not enough nectar available at the overwintering sites to sustain that many monarchs for the entire winter, but there is typically some nectaring behavior during the winter months.

Monarchs do need water during the winter, so on warm days they find streams or water from dew or fog/clouds on the mountain. They need moisture so that they don't become dehydrated.

They remain fairly inactive clustering in trees to conserve energy/lipid reserves, but do fly some on warm days and warm their wings in the sun. Towards the end of the season, monarchs may also begin mating in Mexico as they prepare for the journey northward.

Predation by birds also happens at overwintering sites, but monarchs cannot do much to avoid this.

How many monarchs are in one overwintering colony in Mexico?

Each of the sites that monarchs occupy in the Monarch Butterfly Biosphere Reserve are different in size. Scientists measure the size of each colony or overwintering site by measuring the area of the trees/land that is occupied by monarchs. With these measurements, they use an estimate of between 10-50 million monarchs per hectare of land occupied.

How do they count butterflies at the overwintering sites?

In California, volunteers help to count the butterflies at each location by participating in the Western Monarch Thanksgiving Count. Groups of volunteers go out with a leader and estimate the number of monarchs in each cluster by counting individual butterflies. This is possible because there are many fewer butterflies at the California sites (as opposed to the Mexico sites). Check out this article by Candy Sarikonda on counting western monarchs for more information.

In Mexico, it would be impossible to count each and every butterfly at a particular overwintering site. Researchers actually measure the area occupied by monarchs and then using this measurement, estimate the number of individuals based on an estimate of between 10 and 50 million monarchs per hectare. Read this article by monarch experts Karen Oberhauser, Gail Morris and Lincoln Brower for more information on estimating the number of monarchs in Mexico.

What are the characteristics of an overwintering sites in Mexico?

Monarchs in Mexico overwinter in high elevation oyamel fir forests in the Transvolcanic mountain range of central Mexico. Typically the locations that they find favorable are about 2 miles above sea level, or about 3,000 meters. These sites are typically found on moderately steep south-soutwest facing slopes and are usually in close proximity to a stream or source of water. The temperature at these locations is fairly cool, typically just a bit above freezing. It can get warmer during nice days, but nights remain fairly cool.

Are there monarch predators in Mexico?

Yes, there are two bird species that can eat monarchs in overwintering sites in Mexico. They are the black-headed grosbeak and the black-backed oriole. There are also about 5 species of mice that prey on the monarchs on the ground.

I live near the California overwintering grounds. Should I be planting milkweed in my yard?

It is not recommended to plant milkweed within 10 miles of the coast in central and northern California where milkweed did not occur historically. Instead, plant a variety of flowers native to your region that provide nectar from early spring through fall to support monarchs and other pollinators. See the Xerces Society for Invertebrate Conservation's Priority Action Zones in California for Recovering Western Monarchs to learn which monarch conservation actions are recommended in your region.

Why is planting milkweed in coastal areas discouraged?

While monarchs require milkweed for egg laying and larval development during the late-spring and summer breeding season, during the overwintering period (approx. October-March), the butterflies are usually in reproductive diapause, which means they stop mating and laying eggs. During the winter, monarchs don't need milkweed, but they do need plants that provide nectar for energy and water to keep hydrated.

Historic records suggest that milkweed was largely absent from most coastal areas of California. Because of the mild winter temperatures, milkweed planted close to the coast can often escape hard frosts and persist through the winter instead of dying back. Persistent winter milkweed encourages monarchs to continue mating and laying eggs, which disrupts their natural annual life cycle (halts winter diapause) and comes with numerous negative consequences. This phenomenon is well documented in Florida, the Gulf Coast, and Southern California, where non-native, tropical milkweed (A. curassavica) stays evergreen and is associated with winter breeding. In coastal California, even native species may act like tropical milkweed--staying green late into the fall-- and cause similar issues as tropical milkweed.

Negative consequences of winter breeding in monarchs include the following:

OE levels in winter-breeding monarchs have been documented up to 9 times higher than OE levels in non-breeding, overwintering monarchs (Satterfield et. al., 2016). This has two primary potential outcomes to consider: Monarchs that become heavily infected with OE may die outright; and individuals that don’t die immediately have been shown to have reduced fitness but can live long enough just to further spread OE within the population – thus reducing the fitness of even more individuals.
When monarchs are congregated at an overwintering site and milkweed is present (so breeding is possible), the large number of adults can overwhelm the comparably small amount of milkweed with eggs, resulting in all or most of the larvae starving to death. In this way, milkweed near overwintering congregations can serve as a population sink (lost reproductive potential).
Another potential outcome is if milkweed near overwintering groves persists into the fall and early winter, just long enough to encourage breeding and egg-laying, and then dies off or goes dormant later in the winter, all or most larvae on that milkweed would starve.

The goal of monarch conservation is to increase the stable population size, and it is important to do so in a manner that supports a sustainable population as well as the natural life cycles of the monarch (including summer migration and winter breeding diapause). In wild populations, there are numerous natural factors at play, which tend to favor the strongest and most fit individuals – which, in turn, is important for the health of the entire population. Artificial or temporary boosts to population numbers resulting from human interventions that "save" individual butterflies that would have otherwise died can negatively affect the genetic health of the entire population.

If you'd like to learn more about monarch overwintering sites, please read our overwintering page. Explore our About Monarchs section for monarch life cycle and ecology information.

Check out Xerces Monarch Nectar Guide for coastal California for ideas of what to plant, and the State of Overwintering Sites Report for additional information.

Are there concerns with illegal logging in the monarch overwintering habitat in Mexico?

Most importantly, logging that occurs in the core zone of the Monarch Butterfly Biosphere Reserve is illegal. It is neither widespread nor common, but just as illegal things occur in the U.S., illegal things occur in Mexico when people profit economically by committing crimes. Both the local people and the government work hard to enforce the laws, but the area is huge, there is some collusion with some community members, and some large cartels are getting involved in illegal lumbering. There is also a large demand for the lumber, some of which ends up in the US. There are many organizations working together with the government and communities in the area to reduce and stop illegal logging. You can find out more about the work our partner the Monarch Butterfly Fund is doing in this area here: http://monarchconservation.org/

What does overwintering mean, and why do monarchs do it?

Insects have different strategies for surviving the winter. Some only live in places that never experience freezing temperatures, some have evolved to tolerate cold or freezing temperatures in different life stages, and some migrate to escape winter conditions. 'Overwintering' is a term used to describe how something survives the winter.

For monarchs, overwintering means migrating to another location and spending the winter there because they cannot survive the freezing temperatures in the northern parts of their range. Monarchs migrate to the mountains of central Mexico, or the Pacific Coast to overwinter in areas that are warm enough for them to spend the winter without freezing, but cool enough for them to not use all their energy when there isn't much food to eat. Monarchs are in 'reproductive diapause' when they are overwintering, which means they have paused their sexual development and will usually not reproduce until spring. By delaying reproduction and remaining relatively inactive, overwintering monarchs are able to survive up to 9 months. In contrast, breeding adult monarchs in the spring and summer months need plentiful nectar plants to fuel their reproduction, and usually live only about 1 month. For more information on monarch biology, migration and overwintering visit the Journey North and the Monarch Joint Venture websites:

Do you have tips for visiting the overwintering colonies in Mexico?

Butterflies begin arriving at the Mexican overwintering sites in early November. By December, all of the butterflies are in residence and have condensed into a smaller area, creating spectacular views. Butterflies roost in these high altitude fir forests through mid-March, with flight activity increasing around mid-February as they prepare for departure.

Mexico’s monarch sanctuaries straddle two states, the State of Mexico and Michoacán. The Cerro Pelon and Piedra Herrada sanctuaries are located in the State of Mexico, while El Rosario and Sierra Chincua are on the eastern edge of Michoacán state. Mexico City is the closest airport to the sanctuaries, and Zitácuaro, Michoacán, the closest major city. First class buses leave the Observatorio terminal in Mexico City every hour all day long and take two hours to reach the Zitácuaro bus terminal, where taxis are available. While speaking some Spanish is helpful, many manage to make the trip with minimal fluency.

All of the monarch roosts are high altitude, at about 10,000 feet. If altitude sickness is a problem for you, you may want to spend a few days acclimating in Mexico City or the surrounding area before visiting the colonies. Keep in mind that it can get chilly at night in this area and pack accordingly. Seeing monarchs is easier if you have a moderate level of physical fitness, and quite challenging if you have mobility issues. All sanctuary visits involve hiking and/or horseback riding on sometimes steep inclines. That said, about half of all foreign visitors are seniors, and most manage the trip without difficulty.

Cerro Pelon, where the monarch migration was first confirmed, is the least visited and least touristy of the sanctuaries. It features the longest and steepest trail up to the colony, as well as the most pristine forest. El Rosario has been the largest colony in recent years; this sanctuary boasts a paved trail with interpretative signs, and large crowds on the weekends. Sierra Chincua is also one of the more accessible sanctuaries, with a short and attractively paved trail. The location of the Piedra Herrada colony has shifted a lot the past few seasons, meaning that some years the ascent can be steep while others it’s relatively easy. Piedra Herrada’s proximity to Valle de Bravo and Mexico City means that weekends can get crowded here too.

There are many options for accommodations in the area. JM Butterfly B&B, run by a Mexican and American couple, is located in Macheros, a scenic rural village at the entry of Cerro Pelon. The B&B organizes tours to all four sanctuaries, and there is a family-run restaurant next door. In Zitácuaro, there are many budget accommodations located along its main drag, Revolución. A few blocks away, Casa de los Recuerdos has received favorable reviews from travelers. On the outskirts of Zitácuaro, high-end Rancho San Cayetano, run by a European couple, includes a highly praised restaurant and well-tended gardens. Another base for butterfly tourism is the mining town of Angangueo, located half way between the El Rosario and Sierra Chincua sanctuaries. In Angangueo, Hotel Don Gabino and Hotel Don Bruno are the two lodging options.

If you are only looking to do a day trip to Cerro Pelon, reference this handout.

Content above courtesy of JM Butterfly B&B.

If you would like to have the details of trip planning and transport taken care of for you, several groups affiliated with non-profits offer organized tours, including Natural Habitat/WWF and Ecolife.

Explora Latin America is another option offering guided tour options for February 2021: https://www.exploralatinamerica.com.mx/monarch-butterflies/

OPPORTUNITY FOR EDUCATORS: Natural Habitat Adventures provides a scholarship program for educators. Learn more here.

I thought monarchs were migratory, so why am I seeing monarch caterpillars during the winter?

There is a small non-migratory population of monarchs in southern Florida, but the vast majority of eastern and western monarch populations do undergo a long-distance migration to their overwintering grounds. Overwintering monarchs are typically in a state of reproductive diapause (delayed reproductive maturity) and become reproductive in early spring as they prepare for the return journey to their breeding grounds.

However, some monarchs skip the traditional long-distance migration. In parts of the southern U.S. and California, the year-round persistence of milkweed in good condition, typically the non-native tropical milkweed (Asclepias curassavica), allows monarchs to breed throughout the winter, as indicated by community science observations of eggs and larvae during winter months. Scientists don't know exactly what causes monarchs to be reproductive and to forego their long-distance migration. It could be exposure to the tropical milkweed, or some other factor, perhaps warmer conditions or even poor physiological condition in the monarchs themselves.

While we don't know why they stay, we do know that year-round tropical milkweed patches, and the year-round breeding that they allow, foster greater transmission of the protozoan Ophryocystis elektroscirrha (OE). Therefore, we recommend that tropical milkweed should be cut back in the winter and fall months in the southern U.S. and California, and should be gradually replaced with native milkweeds as they become available.

Researchers at the University of Georgia, through Project Monarch Health, have been working with community science volunteers to document OE trends in monarch populations, with a focus on how winter breeding behavior impacts disease prevalence. For more information on the parasite and how to participate in this research, visit the project website (www.monarchparasites.org). Learn more about the potential risks of growing exotic milkweeds for monarchs on this MJV handout or Expert Q&A.

What does overwintering mean, and why do monarchs do it?