A monarch butterfly carrying a tiny tag developed by Cellular Tracking Technologies in Cape May Point, N.J., Nov. 12, 2025. Scientists used tiny new sensors to follow the insects on journeys that take thousands of miles to their winter colonies in Mexico. (Hannah Beier/The New York Times)
Amid the eucalyptus groves at Natural Bridges State Beach in Santa Cruz, local bug enthusiast Sasha Hernandez squinted through the draping green leaves, searching for flashes of black and orange — the wings of migratory monarch butterflies traveling from west of the Rocky Mountains.
This year, the butterflies’ annual migration will carry a new kind of buzz as smartphones sync with the motions of the monarchs. For the first time in California, citizen scientists like Hernandez can join the effort to track monarch butterflies thanks to Blu+, a new generation of ultra-light tags that communicate using Bluetooth technology.
Through the Project Monarch app, available for iPhone or Android, anyone with a smartphone can assist researchers in monitoring migration patterns by scanning their surroundings. If a Blu+ tagged butterfly flutters within a 100-yard range, the phone detects the signal and uploads data to a central database. Those data points will help unveil how monarchs move about in overwintering sites like Santa Cruz.
“We want to identify what a monarch’s overwintering path looks like,” said Ashley Fisher of the Xerces Society, which works on the conservation of insects and other invertebrates. Fisher and her colleagues are hoping to better understand what prompts monarchs to abandon or remain in overwintering sites
Unlike their eastern counterparts that travel thousands of miles to Mexico, western monarchs follow a more fragmented migration route. Most journey from inland valleys and mountain ranges west of the Rockies to coastal California, where groves of eucalyptus, cypress and Monterrey pine provide shelter from winter storms. Because the western migration is smaller and more dispersed, it is especially vulnerable to local habitat changes, making widespread community tracking efforts even more valuable.
For decades, monarch tracking relied on old-school methods: writing numbers on wings and using spotting scopes to re-identify tagged butterflies. But it is now benefiting from a technological revolution that has already transformed studies of bird migration. Since 2014, researchers in the Motus network have used radio towers to track birds and other flying creatures fitted with tags that transmit radio signals.
“These tags started out big enough for turkey vultures. Then songbirds. Now, they’re light enough for a monarch,” Fisher said. Each device weighs just two-thousandths of an ounce and is about the size of a grain of rice; for the monarch, it’s the equivalent to a human wearing a 22-pound backpack. It is attached to the monarch’s thorax using eyelash glue, a gentler alternative to toxic super glues.
The ultra-small Blu+ system marks a technological leap that will reach citizen scientists like Hernandez. Developed by Cellular Tracking Technologies and Cape May Point Arts & Science Center in New Jersey, the tags are powered by miniature solar panels and emit radio pings every three seconds, transmitting data such as location, battery level and time.
Those signals can be detected by stationary receivers like Motus towers, or by smartphones using the Project Monarch app. Because the radio waves operate on the same frequency as Bluetooth connections, even a casual hiker with the app open might record a monarch’s movement, filling in data gaps for researchers.
For Hernandez, a second-year neuroscience student at UC Santa Cruz, the technology offers a hands-on extension of her biology classes. “The hardest thing about science is that it’s not always accessible, so it’s great to have science that I can easily access on my phone,” Hernandez said.
Blu+ has already been implemented during eastern monarch migrations to Mexico. As of July 2025, almost 360,000 detections had been uploaded from receiving towers and crowdsourced detections using the phone app. By mid-December, the tags will be activated and ready for detection on the West Coast.
Each tag represents both promise and risk. While their design balances monarch survival, battery life and data quality, the researchers remain cautious. If tagged monarchs were to be recovered in a state of discomfort, it would be reason to discontinue tagging them, Fisher said.
Another challenge lies in the solar-powered battery, which serves as an alternative to heavier rechargeable ones. As they roost in the trees, monarchs cluster together for warmth. But this behavior could shield the tiny solar panel from sunlight, resulting in data collection gaps. Still, the butterflies need sunlight to warm their bodies to fly. So any gaps should be short.
Getting more citizen scientists involved in the annual survey of western monarchs is important, given the huge areas over which they migrate. “I hear stories about people who grew up in areas where they migrate who would describe these huge aggregations in ways that, even as a professional monarch butterfly biologist, I’ve probably never seen,” Fisher said.
Western monarch populations reaching coastal California have declined by roughly 95 percent over the past few decades, driven by factors like habitat loss and pesticide use, Fisher said. Where coastal California once hosted thousands, counts in recent years have barely broken a thousand, according to Charis van der Heide, senior biologist at the environmental consulting firm Althouse and Meade and regional coordinator for the Santa Barbara Western Monarch Thanksgiving Count.
That’s why even small contributions from citizen scientists matter, said van der Heide. Each migration season, volunteers fan out across California’s coast from October to January to tally butterflies. This year, Xerces is encouraging them to run the Project Monarch app in the background while they count.
Van der Heide compared the data collection methods to a treasure hunt. While the Blu+ tags are not replacing existing methods, it “adds an extra element to the hunt.”
The data that volunteers collect could help pinpoint what makes a good overwintering site beyond what is already known about the importance of proximity to the coast, low elevation and diverse nectar sources. Understanding which groves monarchs linger in and which they abandon could guide future restoration efforts, giving volunteers a deeper role in shaping conservation science.
For app users like Hernandez, it’s the link to the iconic insects that is most meaningful. “When I see a monarch fly away, the first thing I think is: Where did it go?” Hernandez said. “It’s incredible that I can maintain a connection with something so small.”
