Good News About Monarchs Overwintering in Mexico

Finally! There's good news from Mexico about overwintering monarch butterflies. A team of international scientists for the first time has tagged monarchs in Mexico's central highlands with electronic tracking devices. The devices allow academics, researchers and everyday gardeners to track spring migrating monarchs on devices such as bluetooth-enabled smart phones. In addition, the overwintering population in Mexico has increased about 64 percent from last year's count. Here below is my take on this good news and a primer on how monarchs find milkweed when they reach your garden.

Every spring for thousands of years monarch butterflies have migrated north from Mexico's Monarch Butterfly Biosphere Reserve. But never like this. Never like the monarchs that are moving into the US and already being spotted in several US states.

This year's migration is special thanks to a Feb. 7–14 visit by a group of US scientists who met up with Mexican collaborators at the monarchs' overwintering sites in Mexico's central highlands. The Americans were a team of scientists and technology experts from the Cape May Point Arts & Science Center and Cellular Tracking Technologies (CTT), both located in Cape May Point, New Jersey. Cape May Point is a strategically located and critical, high-density stopover on the Atlantic coast for fall migrating monarchs. It is a narrow peninsula between the Atlantic Ocean and Delaware Bay that funnels southward-migrating monarchs into significant, concentrated roosts. The Mexican collaborators were from the World Wildlife Federation; CONANP, a Mexican government institution that protects the country's natural heritage; and local representatives from the ejidos, communal owned land for the purpose of agriculture, where the reserves are located.

The Americans brought more than 170 revolutionary ultra light tracking devices developed by CTT with them for the combined group to place on overwintering monarchs. Each device consists of a tiny solar panel and short antenna and can be detected by cellular-connected devices with Bluetooth capabilities and the location function enabled. The device sends location data to a Project Monarch app that can be downloaded on Apple or Android smart phones and turns them into wildlife tracking stations. The app allows academics and scientists to search, track and study in real time how monarchs bearing the devices navigate on their migration and where they stop to feed and roost. It allows gardeners to detect tagged monarchs in or near their garden. All users, regardless of skill level or academic or scientific background or purpose, can also detect and track monarchs by name or an identification number.

The scientists place the tracking devices on the monarchs using a tagging platform that "kind of looks like a pinning board people use to preserve insects," says Sean Burcher, Ph.D., science director at the Arts and Science Center and a member of the February tagging mission. "But it's made to just kind of immobilize the wings with these two flaps that come down. And they're actually made for us by a local here in Cape May, Don Crawford."

The wooden low-tech and simple-looking, but effective, pinning device stands in stark contrast to the high tech nature of the tracking devices and the technology that enables them. Even more low tech and ordinary is how the scientists attach the tracking devices to monarchs. They use a non-toxic, fast-drying false eyelash adhesive readily available over the counter at drug stores. This technique was discovered by Leone (Lee) Brown, Ph.D., an assistant biology professor at James Madison University (JMU), located in Harrisonburg, Va. in the western part of the state. Brown is a butterfly expert and has been instrumental in partnering with CTT to develop a transmitter that is ultralight and doesn't interfere with a monarch's ability to fly. The group had not placed all the devices on monarchs during the week the Americans were in the butterfly sanctuaries. They left the remaining devices with their Mexican collaborators to continue tagging monarchs. The combined team tagged 172 monarchs with the devices.

The tracking devices were first used on 400–500 monarchs during last year's fall migration. The fall-migrating monarchs are the great, great, great grandchildren of the butterflies that left Mexico the previous spring. They are called a super generation because they live eight–nine months, whereas the monarchs that migrate north in spring and summer typically live only about 30 days. Consequently, the fall-migrating monarchs flying south to Mexico are going to a place they have never been. They spend the winter in Mexico in the biosphere reserve, a UNESCO World Heritage Site that protects millions of monarch butterflies in mountain forests in Michoacán and the State of Mexico from late October to March. Fall-tagged monarchs that made it to Mexico and survived the winter can be tracked on the Project Monarch app on the return migration. The CTT-developed devices used in last fall's migration and again this spring have led to the most comprehensive tracking study of monarch butterfly migration ever conducted, according to a CTT press release. The results provided scientists with high-resolution, near-real-time data on individual butterflies as they navigated their epic journey south. The study is being conducted by a collaboration of more than 20 research and conservation organizations across four countries: Canada, the US, Mexico and Cuba.

Amazingly, scientists only learned in 1975 just where the monarchs migrating south in the fall were headed. Since then, they've discovered much about that migration and the return migration in the spring, but realize there is still much to learn. "There's hypotheses out there. None of them are necessarily proven," says Burcher of both the navigational and conservation aspects of the two annual migrations. "There's stuff known from other insects and the way they navigate and using the sun and there's magnetic field and stuff. But really being able to look at the movement in this level of detail, it's just never been done before across this kind of scale."

Some of the things Burcher, scientists at CTT and their partners are looking at include the geographic range of monarch deployments, the time range of deployments, the impact of weather on migration — do the monarchs wait at various stops, for example, for favorable conditions such as winds moving in a certain direction to move on — and whether early season migrants versus late season migrants take different migratory paths. A hypothesis they will be able to check out is that some fall-migrating monarchs take a back door to the reserves by flying to the southern tip of Florida, jumping from Florida to Cuba, then from Cuba to the Yucatan Peninsula and finally turning north to the reserves. Last fall, for example, Burcher says scientists documented a monarch flying from Cuba to the Yucatan, but they weren't able to detect its entire path. Consequently, they don't know if this monarch was a resident in Cuba or migrating from the US with a stopover there. Obtaining more extensive data, says Burcher, will allow scientists to test their hypotheses.

From a conservation standpoint, the data will let scientists know what's important for monarchs when migrating. Information scientists will be looking for, says Burcher, will include: "Where are they stopping over? Which habitats are they using? How can we, as humans, use our limited resources for conservation efforts in habitat restoration and preservation in the places where it's going to have the greatest impact for monarch populations?" This information will further help home gardeners know what to plant and when in our gardens.

Burcher, who calls himself the technology guy, says the app is a great tool just to kind of raise awareness for this incredible phenomenon happening in everybody's backyards. "When you see that monarch on a plant in your backyard, you know it could be headed to Mexico or coming from Mexico. It's absolutely fascinating to have this tool to engage people into what's happening."

About the video: It shows monarchs in the El Rosario sanctuary. It is provided by Cellular Tracking Technologies.

Scientists couldn't have picked a better time to tag overwintering monarchs in Mexico with electronic tracking devices. The eastern monarch butterfly population overwintering in Mexico's oyamel fir forests increased about 64 percent to 2.93 hectares during the 2025–2026 count, up from 1.79 hectares in last year's count. The count, reported by Monarch Joint Venture (MJV), is based on the latest survey released by CONANP and the WWF. MJV is a partnership of federal and state agencies, non-governmental organizations, businesses and academic programs located in St. Paul, Minn. that works to protect monarch migration across the United States.

From a MJV press release: Each winter, researchers survey monarchs while they cluster together at their overwintering sites in central Mexico. Rather than counting individual butterflies, scientists estimate population size by measuring the total area of trees occupied by monarch clusters. One hectare equals approximately 2.47 acres, or just over two American football fields. Although the number of monarchs per hectare varies by year and site, estimates suggest roughly 20–30 million monarchs per hectare, with a median estimate of 21.1 million (Thogmartin et al., 2017).

Following up on the good news of the increase in the number of overwintering monarchs in Mexico, CTT reports that as of March 18 nine individual monarchs tracked by the Project Monarch Collaboration have crossed the United States border, reaching as far north as Arkansas and as far east as Louisiana, and the pace of departure from Mexican overwintering sites is accelerating rapidly. Simultaneously, according to CTT, fall-tagged monarchs that the project tracked southward to Mexico last autumn are now appearing on the northward leg of their journey, making this the first time in history that individual monarchs have been followed on a documented complete trip between the United States and their Mexican overwintering grounds and back.

Seventy of the 172 monarchs tagged at the eight overwintering sites have now departed their wintering colonies. Eighteen have crossed the Tropic of Cancer. The individual pushing farthest is ROS006, a female monarch tagged at El Rosario, one of the most celebrated overwintering sanctuaries within the Monarch Butterfly Biosphere Reserve, on February 10. She departed Mexico in mid-March, crossed into south Texas near the Gulf Coast on March 14, and covered the length of East Texas in a single day on March 15 before being last detected near Hope, Arkansas, more than 1,680 kilometers (1,044 miles) from where she spent the winter. Her track on March 15 alone, from the Texas coast to southwest Arkansas in a matter of hours, illustrates the kind of sustained, wind-assisted flight that monarchs are capable of but that has never before been documented at the level of a single known individual.

Movement is now being recorded from every one of the eight overwintering sites where transmitters were deployed: El Rosario, Sierra Chincua, Mesa, and Cerro Pelón inside the MBBR, and Oxtotilpan, El Albercon, Atlautla, and Ojo de Agua outside it. Ojo de Agua, the eighth site, had 12 individuals tagged on March 9 by Mexican partners continuing deployment operations independently after the international team had departed. That butterflies from both protected and unprotected overwintering habitat are now appearing in the United States simultaneously is among the most scientifically significant early findings of the spring season.

Alongside the Mexico-tagged monarchs, an equally remarkable story is emerging from the fall 2025 tagging data. Of the more than 400 monarchs fitted with BlūMorpho transmitters from Long Point, Ontario southward through the central and eastern United States last autumn, at least 28 have been confirmed at Mexican overwintering latitudes in early 2026. Two of them are now clearly heading north on the return leg.

One of those is JMU004, one of 20 monarchs James Madison University Professor Brown tagged with CTT tracking devices in the Harrisonburg, Va. area last fall. Brown tagged JMU004, a male, on Sept. 23 at 17:38:38 UTC (Coordinated Universal Time), which converts to 12:38:38 PM Eastern Standard Time EST (UTC-5). He is last year's Gold Star monarch. Of the 400–500 monarchs tagged in Canada and in several US locations, he was the first to be detected in the Mexican sanctuaries, El Rosario in this case. Six months later, he's begun his return migration to the US. On March 20 I detected him flying north in Texas between San Antonio and Austin. I know this because he's still wearing the tracking device and it's sending location data to the Project Monarch app. "Every detection is a window into something no scientist has ever been able to see before. These are the winter survivors, the founders of the next generation, and we're watching every step of their journey home," says Burcher.

Wouldn't it be fun if one of those steps happened with a visit by a tagged monarch to your garden!

Yes! But when monarchs arrive in Atlanta it is still early spring and the milkweed I have planted — butterfly milkweed (Asclepias tuberosa) and swamp milkweed (Asclepias incarnata) — is quite small. It's also tucked among companion plants at that point. I've often wondered because of the timing of the milkweed's growth and the arrival of monarchs if the female butterflies would even be able to find the milkweed to lay eggs on it. For an answer to my concerns, I emailed Jacob Swanson, Monarch Joint Venture program coordinator.

Not to worry, Swanson responded by email.

"Monarchs will still be able to find milkweed plants, even if they're very short," he said, to my great relief. "They use multiple visual and chemical cues to help locate milkweed plants."

Using the app to follow the monarchs' journey as they continue migrating north and search for tagged monarchs sounds like super fun, I said to Burcher.

"Yeah," he said. "It really is."