Adriana Avelina Ruíz Márquez delicately affixes a miniature transmitter to the thorax of a monarch butterfly using faux eyelash adhesive. The transmitter, weighing 60 milligrams and equipped with a rice grain-sized solar panel, is effortlessly borne by the butterfly, which weighs a mere half a gram. Following the precise operation, which involves a toothpick and a Q-Tip, Ruíz Márquez, a deputy director at the Monarch Butterfly Biosphere Reserve, releases the butterfly, allowing it to swiftly take flight.
This innovative tracking device, a first-time implementation in Mexico to tag monarchs before their migration back to the U.S. and southern Canada, is anticipated to unravel some of the mysteries surrounding these insects. Ruíz Márquez expressed optimism that leveraging people’s smartphones will aid in tracking the butterflies’ movements, shedding light on their migration patterns and behaviors.
The El Rosario butterfly sanctuary, nestled in the mountainous terrain of Michoacán, Mexico, serves as a winter haven for the vast majority of North America’s monarchs east of the Rocky Mountains. This sanctuary, one of six comprising the Monarch Butterfly Biosphere Reserve, spans approximately 135 square kilometers across Michoacán and the State of Mexico.
Teams from the federal Commission for National Natural Protected Areas and World Wildlife Fund (WWF) Mexico have outfitted a total of 160 monarchs, including 40 in El Rosario, with the cutting-edge transmitters. This initiative, spearheaded by Eduardo Rendón Salinas, a WWF Mexico biologist, aims to provide unprecedented insights into the initial phase of the monarchs’ migration northward.
The monarch butterfly’s remarkable journey spans up to 5,000 kilometers and requires three to four generations to complete. The “super generation” of monarchs, emerging from regions like the Prairies to the Maritimes, undertakes the southbound migration in a single flight, mating at sanctuaries such as El Rosario before commencing the northward journey once more to lay the next generation of eggs.
The microchipped transmitters emit signals detectable by nearby iPhones, enabling crowd-sourced tracking of the butterflies’ trajectories when Bluetooth and location services are enabled. The Project Monarch app, accessible to all, or a handheld receiver can be utilized to map the butterflies’ route, providing a level of spatial detail previously unattainable.
Previously, researchers relied on wing tags for monarch monitoring, offering limited insights into their migratory movements. Cellular Tracking Technologies, the developer of the device, is currently enhancing the transmitter to enable Android smartphones to function as passive detectors, expanding the tracking capabilities.
After successful pilot programs in 2023 and 2024, the transmitters were integrated into a continent-wide monarch tracking initiative involving over 20 groups. This project kicked off in Ontario in September 2025 with the participation of Environment and Climate Change Canada and Birds Canada. The monarchs tagged in different regions, including Long Point, Ont., were tracked entering Mexico’s Monarch Butterfly Biosphere Reserve by October.
During the winter months, El Rosario becomes the realm of the butterflies, with monarchs clustering on oyamel branches, sometimes causing them to break under the sheer weight. The community diligently safeguards the sanctuary from illegal logging, ensuring the preservation of this natural wonder that sustains vital tourism and environmental conservation efforts.
