Adriana Avelina Ruíz Márquez utilizes adhesive from fake eyelash glue to affix a miniature transmitter to the thorax of a monarch butterfly just behind its head. The monarch, weighing approximately half a gram, effortlessly carries the 60-milligram device, which features a solar panel no larger than a grain of rice.
Once the delicate operation is completed, with the assistance of a toothpick and Q-Tip, the butterfly flutters its wings and swiftly takes flight after being released by Ruíz Márquez, who serves as the deputy director of the Monarch Butterfly Biosphere Reserve. This innovative tracking device, deployed for the first time in Mexico to tag monarchs before their migration back to the U.S. and southern Canada, is anticipated to unravel some mysteries about these insects by leveraging people’s smartphones to monitor their movements.
According to Ruíz Márquez, there is much ambiguity surrounding the migration route of the monarchs, their behavior upon arrival, and departure. In the lush El Rosario butterfly sanctuary, butterflies swarm around like mosquitoes and blackflies during Canadian marshland dusks, basking in patches of sunlight filtering through the towering oyamel firs.
The majority of North America’s monarchs east of the Rocky Mountains choose El Rosario as their wintering ground in Michoacán, located about 180 kilometers west of Mexico City. This sanctuary is one of six within 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 tagged a total of 160 monarchs across the reserve, including 40 in El Rosario, with the new transmitters. This initiative will offer unprecedented insights into the initial phase of the butterflies’ northern migration, as highlighted by Eduardo Rendón Salinas, a biologist with WWF Mexico.
The tracking of monarchs between colonies, previously only a theoretical concept, can now be realized with this new tagging method at hibernation sites, as explained by Rendón Salinas. The monarch’s biannual migration, covering up to 5,000 kilometers and requiring three to four generations to complete, culminates in the super generation making a direct journey south from the Prairies to the Maritimes and back north after mating in sanctuaries such as El Rosario.
The novel microchipped transmitter emits a signal detectable by nearby iPhones to crowdsource the trajectory of the butterflies, provided the devices have Bluetooth and location services activated. This enables the Project Monarch app, accessible to everyone, or a handheld receiver to map the butterflies’ path with remarkable precision, a capability previously unavailable.
David La Puma, the director of global market development at New Jersey-based Cellular Tracking Technologies, the developer of the device, lauds the level of spatial detail achievable with this technology, surpassing the limited information obtained from tags affixed to monarch wings. Plans are in progress to enhance the transmitter to support Android smartphones as passive detectors.
Following successful trials in 2023 and 2024, the transmitters were integrated into a continent-wide monarch tracking project involving over 20 organizations. The project was initiated in Ontario in September 2025 with the participation of Environment and Climate Change Canada and Birds Canada, tagging thirty monarchs with transmitters at the Long Point UNESCO biosphere reserve.
The monarchs, totaling 400 from Canada to Cuba by October, were monitored as they entered Mexico’s Monarch Butterfly Biosphere Reserve in November. During the winter months, the reserve transforms into the monarchs’ realm, attracting tourists and generating vital income for the community of El Rosario, which collectively manages and safeguards the area.
The people of El Rosario foster a thriving tourism industry during the monarchs’ winter sojourn, sustaining the community economically. They also cultivate oyamel seedlings to preserve the forest’s health, reflecting their commitment to protecting the natural wonder that is the monarch butterfly.