Solar Tags Track 1,750 Monarchs to Map Declining Migration Routes

Scientists attached solar-powered radio tags to nearly 1,750 monarch butterflies this year to solve a decades-long puzzle: where the insects stop to rest and feed during their 3,000-mile migration. The new data aims to map critical stopover locations before habitat loss, pesticides, and climate change erase them.

The tracking effort is a response to a severe population decline. Since the mid-1990s, the eastern monarch population has dropped more than 80%, while the western population has fallen by approximately 95%. Rebeca Quiñonez-Piñón, senior scientist at the National Wildlife Federation, leads the study that combines real-time tracking with community science to monitor routes through the "Texas funnel," a narrow corridor used by monarchs east of the Rocky Mountains.

The devices, developed by Cellular Tracking Technologies, cost $200 and weigh about as much as a drop of water. David La Puma, vice president of global market development for Cellular Tracking Technologies, said most tagged butterflies weigh more than 400 milligrams to ensure the tag is not burdensome. Researchers catch the insects with nets, keep them cool, and apply the transmitters to their abdomens with adhesive before releasing them.

So far, the data has revealed unexpected connections. Scientists have detected the first evidence of a butterfly from Utah, west of the Rockies, joining eastern monarchs in central Mexico. Other tagged insects traveled down the Eastern Seaboard to Florida, crossed the Gulf of Mexico, and are now moving across Mexico. About 175 tagged butterflies have been detected in Mexico since the migration began this year. Emma Pelton, senior conservation biologist with the Xerces Society, said the team is thrilled by these new population links that previous technology could not prove.

Louie Yang, an insect ecologist at the University of California, Davis, described the technology as remarkable because it reveals movement speeds and paths. He noted, however, that the tags indicate where a receiver detected the insect, not its exact location, and detection does not confirm the butterfly is still alive. Further studies are needed to determine if the tags alter butterfly behavior.

The stakes are high as environmental pressures mount. Tierra Curry, senior scientist at the Center for Biological Diversity, said human-caused climate change is a primary threat. Prolonged heat in northern areas can delay migration cues, while late freezes can kill butterflies before they reach the oyamel fir forests in central Mexico. Pesticides also threaten milkweed, the exclusive food source for monarch caterpillars.

Conservationists are implementing physical protections while the data is gathered. In Texas, nets on some highways encourage butterflies to fly higher to avoid vehicle strikes. Other efforts include mass plantings of milkweed in gardens and along transportation corridors. Late last year, the Trump administration delayed a decision on federal protections for the species. A court has ordered the government to make a final decision by 2030.