Adriana Avelina Ruíz Márquez uses fake eyelash glue to attach a tiny transmitter to the thorax, just behind the head, of the monarch butterfly.
The monarch, which weighs about half a gram, easily carries the 60-milligram device, which includes a solar panel the size of a grain of rice.
The butterfly flaps its wings and quickly gets airborne after Ruíz Márquez, a deputy director for the Monarch Butterfly Biosphere Reserve, releases it following the delicate operation, which also requires a toothpick and a Q-Tip.
Ruíz Márquez said she believes this new tracking device — used in Mexico for the first time to tag monarchs before they migrate back north to the U.S. and southern Canada — will help solve some mysteries about the insects, in part by using people’s smartphones to track their movements.
“There is a lot of mystery to their route of migration, their activity when they arrive, when they leave,” she said.
Around her, butterflies swirled, thick like mosquitos and blackflies during Canadian marshland dusks, in patches of sunlight pouring through the branches of the towering oyamel firs of the mountainous El Rosario butterfly sanctuary.
The vast majority of North America’s monarchs east of the Rocky Mountains overwinter in El Rosario, which is located in the state of Michoacán about 180 kilometres west of Mexico City.
It’s one of six sanctuaries that make up the Monarch Butterfly Biosphere Reserve, which is spread across Michoacán and the State of Mexico and has a core protected area of about 135 square kilometres.
A total of 160 monarchs across the reserve, including 40 in El Rosario, have been tagged with the new transmitter by teams with the federal Commission for National Natural Protected Areas and World Wildlife Fund (WWF) Mexico.
They will help provide the most detailed look to date at the first leg of the butterflies’ migration north, said Eduardo Rendón Salinas, a biologist with WWF Mexico.
While the clouds of monarchs arrive around the beginning of November, they leave in dribs and drabs throughout March, he said.
“It is very important to do this new type of tagging in hibernation sites, to determine how they finish hibernating in Mexico,” said Rendón Salinas.
He said they will now be able to track the movement of monarchs between colonies — which had been previously only theorized.
Millions of monarch butterflies spend the winter in the El Rosario butterfly sanctuary in Mexico’s Michoacán state.
It takes three to four generations of monarchs to complete the journey north, which can span up to 5,000 kilometres.
The so-called super generation of monarchs, which emerges in late summer from the Prairies to the Maritimes and down through the U.S. Northeast and Midwest, completes the journey south in one shot. They mate in sanctuaries like El Rosario before heading north again, laying the next generation of eggs on the way.
The monarch, which cannot survive the cold winters of northern climates, is the only butterfly known to make an annual two-way migration.
Crowd-sourced tracking
The new microchipped transmitter sends a signal that can be detected by nearby iPhones to crowd-source the butterflies’ trajectory, if the devices have Bluetooth and location functions turned on.
This allows the Project Monarch app, which anyone can download, or a handheld receiver to help chart the path of the butterflies.
“Literally, [monarchs] are flying over roads and people are driving down the road and it’s giving us a location. It’s a fantastic level of spatial detail that was never before possible,” said David La Puma, director of global market development with New Jersey-based Cellular Tracking Technologies, which developed the device.
Previously, scientists relied on tags stuck to monarch wings, but they essentially only revealed the point where the butterfly was tagged and where they were eventually found, he said.
“We really didn’t get the nuance of that movement that’s happening during the migration period,” said La Puma.
La Puma said his firm is currently developing an upgrade to the transmitter that will also allow Android smartphones to be used as passive detectors.
After successful pilots in 2023 and 2024, the transmitters were used as part of a continent-wide monarch tracking project, involving over 20 groups. It launched in Ontario in September 2025 with the involvement of Environment and Climate Change Canada and Birds Canada.
Thirty monarchs were tagged with the transmitters at the Long Point, Ont., UNESCO biosphere reserve on the shores of Lake Erie, about 180 kilometres south of Toronto.
“I saw the data of the monarchs crossing the lake and pinging off boats in the lake and then pinging on the south shore, going through urban areas in Ohio,” La Puma said.
“Watching them move through this urban matrix and then eventually passing through, into the Midwest, was spectacular.”
By October, a total of 400 monarchs had been tagged from Canada to Cuba. They were tracked entering Mexico’s Monarch Butterfly Biosphere Reserve in November.
Protected by community
Between November and March, the reserve becomes the empire of the butterflies.
The monarchs swarm and form massive clusters on the oyamel branches, which sometimes snap under their weight, said Horacio Cruz Guzmán, who is part of a community patrol that protects the area from illegal logging.
Many of the animals in the El Rosario sanctuary — including coyotes, armadillos, opossums, squirrels, rabbits and most birds — flee the area when the monarchs arrive, Cruz Guzmán added.
Hummingbirds, orioles and the round-eared mouse remain because they either don’t interact with monarchs or they have built up a tolerance to the toxins carried by the butterflies — and their caterpillar form — due to the milkweed they feed on in Canada and the U.S., he explained.
Even after the monarchs leave in March, the animals don’t return until rain and time have washed away all traces of the butterflies, he said.
The sanctuary sits in the El Rosario ejido — land that is collectively held by the community, which cares for and protects the area.
The people of El Rosario have developed a thriving tourism business that operates during the roughly five months the monarchs winter in the sanctuary, providing the community with a vital source of income.
The people also grow oyamel seedlings that they plant in the sanctuary to keep the forest healthy.
“This is truly a great task that we carry out as en ejido, and as people native to this community,” said Cruz Guzmán.
“We feel fortunate to belong here, and to have this natural wonder that is the monarch butterfly.”
