Adriana Avelina Ruíz Márquez employs counterfeit eyelash adhesive to affix a miniature transmitter to the monarch butterfly’s thorax, just behind its head. Weighing in at approximately half a gram, the butterfly effortlessly carries the 60-milligram device, complete with a solar panel the size of a grain of rice. Following a delicate procedure that necessitates a toothpick and a Q-Tip, Ruíz Márquez, a deputy director at the Monarch Butterfly Biosphere Reserve, sets the butterfly free as it takes flight with a flutter of its wings. This new tracking device, utilized for the first time in Mexico to tag monarchs before their migration back north to the U.S. and southern Canada, is anticipated to unravel some enigmas about the insects, leveraging people’s smartphones to monitor their movements.
“There is a lot of mystery surrounding their migration route, their behavior upon arrival, and departure,” Ruíz Márquez stated.
Surrounded by swirling butterflies, reminiscent of the density of mosquitos and blackflies in Canadian marshlands at dusk, patches of sunlight filter through the branches of the towering oyamel firs in the mountainous El Rosario butterfly sanctuary. The majority of North America’s monarchs east of the Rocky Mountains seek refuge in El Rosario during winter, situated in Michoacán, about 180 kilometers west of Mexico City. It is one of the six sanctuaries constituting the Monarch Butterfly Biosphere Reserve, spanning Michoacán and the State of Mexico, with a protected core area of roughly 135 square kilometers.
Teams from the federal Commission for National Natural Protected Areas and World Wildlife Fund (WWF) Mexico have affixed the new transmitters to a total of 160 monarchs across the reserve, including 40 in El Rosario. Eduardo Rendón Salinas, a biologist with WWF Mexico, emphasized that these transmitters will offer the most detailed insight to date into the monarchs’ initial migration northward. While the monarch clouds typically arrive in early November, they depart sporadically through March.
“It is crucial to implement this new form of tagging in hibernation sites to determine their final hibernation process in Mexico,” remarked Rendón Salinas.
The tracking of monarch movement between colonies, previously theoretical, is now feasible. The annual two-way migration of the monarch, the only known butterfly undertaking such a journey, spans up to 5,000 kilometers and requires three to four generations to complete. The super generation of monarchs, emerging in late summer from the Prairies to the Maritimes and traversing down through the U.S. Northeast and Midwest, accomplishes the southward journey in one continuous flight. They mate in sanctuaries like El Rosario before embarking northward again, laying the next generation of eggs en route.
The new microchipped transmitters emit signals detectable by nearby iPhones, enabling crowd-sourced tracking of the butterflies’ trajectory if Bluetooth and location functions are enabled. The Project Monarch app or a handheld receiver can then help map the butterflies’ path.
David La Puma, director of global market development at New Jersey-based Cellular Tracking Technologies, the developer of the device, expressed excitement about the enhanced spatial detail provided by the monitoring system. Previously, scientists relied on wing tags that indicated only the point of tagging and eventual discovery of the butterfly, lacking nuanced movement insights during migration. Plans are underway to upgrade the transmitter to enable Android smartphones as passive detectors.
Following successful pilots in 2023 and 2024, the transmitters were integrated into a continent-wide monarch tracking initiative involving over 20 groups. Launching in Ontario in September 2025 with the participation of Environment and Climate Change Canada and Birds Canada, the project saw 30 monarchs tagged with transmitters at the Long Point, Ont., UNESCO biosphere reserve.
By October, 400 monarchs had been tagged from Canada to Cuba, ultimately tracked entering Mexico’s Monarch Butterfly Biosphere Reserve in November. The reserve transforms into the realm of butterflies between November and March, with monarchs clustering on oyamel branches, causing some to snap under their weight. Community patrols safeguard the sanctuary from illegal logging, ensuring protection for the area and its inhabitants.
The El Rosario community has cultivated a thriving tourism sector that operates during the monarchs’ winter stay, providing essential income. They also nurture oyamel seedlings to preserve the forest’s health, demonstrating a profound commitment to the monarch butterflies’ natural wonder.