Adriana Avelina Ruíz Márquez employs false eyelash adhesive to affix a small transmitter to the thorax, just behind the head, of the monarch butterfly. Weighing approximately half a gram, the monarch effortlessly carries the 60-milligram device, which incorporates a solar panel the size of a grain of rice. After the delicate operation, which necessitates a toothpick and a Q-Tip, the butterfly flutters its wings and swiftly takes flight when released by Ruíz Márquez, who serves as a deputy director for the Monarch Butterfly Biosphere Reserve.
This innovative tracking device, used for the first time in Mexico to tag monarchs before their migration back north to the U.S. and southern Canada, is believed by Ruíz Márquez to unravel some mysteries about these insects. She envisions leveraging people’s smartphones to track the butterflies’ movements, shedding light on enigmatic aspects such as their migration route, activities upon arrival and departure.
The El Rosario butterfly sanctuary, nestled in the mountainous terrain of Michoacán, plays host to the vast majority of North America’s monarchs east of the Rocky Mountains during the overwintering period. This sanctuary forms part of the Monarch Butterfly Biosphere Reserve, spanning Michoacán and the State of Mexico with a core protected area of about 135 square kilometers.
Teams from the federal Commission for National Natural Protected Areas and World Wildlife Fund (WWF) Mexico have tagged a total of 160 monarchs throughout the reserve, including 40 at El Rosario, with the new transmitters. Eduardo Rendón Salinas, a WWF Mexico biologist, asserts that these transmitters will offer unprecedented insights into the initial phase of the butterflies’ northward migration.
The monarch butterfly, which embarks on a journey spanning up to 5,000 kilometers and requires three to four generations to complete, is the sole known butterfly to undertake an annual round-trip migration. The so-called super generation of monarchs, emerging late in summer and traveling from the Prairies to the Maritimes, eventually returns southwards in one continuous flight, mating in sanctuaries like El Rosario before commencing the northward migration once more.
A cutting-edge microchipped transmitter emits a signal detectable by nearby iPhones, facilitating crowd-sourced mapping of the butterflies’ trajectory if equipped with Bluetooth and location functions. This technology enables the Project Monarch app, accessible to anyone, or a handheld receiver to trace the butterflies’ path with unprecedented spatial precision.
Previously, scientists relied on wing tags, offering limited insights into monarch movements during migration. An upgraded transmitter in development by New Jersey-based Cellular Tracking Technologies will extend compatibility to Android smartphones as passive detectors, enhancing tracking capabilities.
Following successful trials in 2023 and 2024, the transmitters were integrated into a continent-wide monarch tracking initiative involving over 20 groups, commencing in Ontario in September 2025 with the collaboration of Environment and Climate Change Canada and Birds Canada. By October, 400 monarchs had been tagged from Canada to Cuba, with their entrance into Mexico’s Monarch Butterfly Biosphere Reserve tracked in November.
During the November to March period, the reserve metamorphoses into the realm of the butterflies, with monarchs forming colossal clusters on oyamel branches. The El Rosario community, safeguarding the area from illicit logging, witnesses the sanctuary teeming with wildlife such as coyotes, armadillos, opossums, and squirrels, which vacate the area upon the monarchs’ arrival. Hummingbirds, orioles, and the round-eared mouse remain, either unaffected by monarch interactions or possessing tolerance to the toxins carried by the butterflies.
The El Rosario community sustains a thriving tourism sector during the monarchs’ winter stay, generating crucial income. Additionally, they cultivate oyamel seedlings to preserve the forest, embodying a deep sense of responsibility toward the monarch butterflies and their natural habitat.
