Adriana Avelina Ruíz Márquez employs counterfeit eyelash adhesive to affix a miniature 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 equipped with a solar panel the size of a grain of rice.
After a delicate operation involving a toothpick and a Q-Tip, the butterfly takes flight as Ruíz Márquez, a deputy director for the Monarch Butterfly Biosphere Reserve, sets it free. This new tracking device, a first in Mexico for tagging monarchs before their migration back north to the U.S. and southern Canada, is expected to unravel some mysteries about the insects by utilizing smartphones to monitor their movements.
In the midst of swirling butterflies at the El Rosario butterfly sanctuary in Mexico, resembling thick clouds akin to mosquitos and blackflies during Canadian marshland dusks, Ruíz Márquez releases monarchs into patches of sunlight filtering through the towering oyamel firs.
The majority of North America’s monarchs east of the Rocky Mountains spend their winters in El Rosario, situated in the state of Michoacán about 180 kilometers west of Mexico City. This sanctuary is part of the Monarch Butterfly Biosphere Reserve, which spans across Michoacán and the State of Mexico, featuring a core protected 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 throughout the reserve, with 40 in El Rosario, using the new transmitter. According to Eduardo Rendón Salinas, a biologist with WWF Mexico, this initiative will provide the most detailed insight into the initial phase of the butterflies’ migration north.
It typically takes three to four generations of monarchs to complete the journey north, covering up to 5,000 kilometers. The super generation emerges in late summer and completes the journey south in one continuous flight, mating in sanctuaries like El Rosario before heading north again to lay eggs for the next generation.
The microchipped transmitter emits a signal detectable by nearby iPhones, allowing for crowd-sourced tracking of the butterflies’ trajectory if Bluetooth and location functions are activated. This technology enables the Project Monarch app, available for download by anyone, or a handheld receiver to map the butterflies’ path with unprecedented detail.
Previously, scientists relied on wing tags for monarch tracking, providing limited information on their movements during migration. A transmitter upgrade is in progress to enable Android smartphones to serve as passive detectors, enhancing tracking capabilities further. The transmitters were piloted successfully in 2023 and 2024 and have now been integrated into a continent-wide monarch tracking project involving over 20 groups.
The El Rosario sanctuary is guarded by a community patrol to prevent illegal logging, as the monarchs form massive clusters on the oyamel branches during their winter stay. The sanctuary, situated on collectively held land, supports a flourishing tourism industry during the monarchs’ wintering period, offering vital income to the community. Additionally, the community nurtures oyamel seedlings to maintain the forest’s health, showcasing their dedication to preserving this natural wonder.
