Scientists are using tiny, solar-powered trackers to follow the epic journeys of monarch butterflies, shedding light on the critical stopover locations needed for their survival. These dazzling orange-and-black insects undertake a migration of up to 3,000 miles (4,800 kilometers) from as far south as Canada to spend winters in central Mexico.
For decades, researchers have struggled to pinpoint the exact routes, daily distances, and nighttime resting spots of these migrating monarchs. The new radio tags, weighing about as much as a drop of water, are attached to nearly 1,750 butterflies this year, offering unprecedented insights. The public can follow the butterflies' progress on the free app Project Monarch Science.
Rebeca Quiñonez-Piñón, senior scientist at the National Wildlife Federation and a leader of the tracking effort, explained that the research is vital in the face of declining monarch populations threatened by habitat loss, pesticides, and climate change. "Pollinators are crucial to protect and ensure that our ecosystems remain healthy," she said.
While not all monarchs migrate, a significant portion east of the Rocky Mountains pass through a narrow corridor known as the “Texas funnel” during their fall and spring journeys. Their specific routes within this region can vary annually, depending on the availability of water, nectar, and trees for overnight shelter.
Initial findings from the tracking project have revealed surprising migratory paths. Scientists have documented a monarch west of the Rockies reaching central Mexico, a route previously unconfirmed for eastern migrants. Other butterflies have traveled down the Eastern Seaboard to Florida, crossed the Gulf of Mexico, and tracked across Mexico, revealing connections between populations that were previously only hypothesized. "We're a year into it, and I think we're all just thrilled," said Emma Pelton, senior conservation biologist with the Xerces Society for Invertebrate Conservation.
The development of these lightweight tags by Cellular Tracking Technologies has overcome limitations of previous tracking devices, which were often too large or heavy. The tagging process requires careful training and delicate handling to ensure the butterfly's safety. Butterflies weighing over 400 milligrams are typically selected for tagging, and they are calmed in cool temperatures before the adhesive and transmitter are applied to their abdomen.
Despite the technological advancements, limitations remain. The tags do not provide exact locations but rather indicate where a receiver has detected the butterfly. Further studies are also needed to understand any potential impacts of the tags on butterfly behavior.
Monarch populations have seen drastic declines, with eastern monarchs dropping more than 80% and western populations by about 95% since the mid-1990s. Pesticides targeting weeds, including milkweed—the sole food source for monarch caterpillars—and human-caused climate change pose significant threats. Extreme temperatures can disrupt migration cues, while droughts can diminish vital nectar sources. Development also continues to threaten wintering grounds for western monarchs.
Conservation efforts are underway, including highway nets to encourage butterflies to fly higher and avoid traffic, as well as widespread plantings of milkweed and pollinator-friendly plants. These initiatives aim to bolster local populations and support the monarchs' remarkable, yet increasingly fragile, migration.