Scientists use tiny trackers to solve monarch butterfly mystery

Every fall, millions of monarch butterflies travel up to 3,000 miles from Canada to the forests of central Mexico. Scientists have always wondered exactly how they make that long trip — where they stop, how far they fly each day, and where they sleep at night. Now, tiny solar-powered tracking devices are helping researchers find those answers for the very first time.

Scientist Rebeca Quiñonez-Piñón grew up watching monarch butterflies and wondering about their journey. Today, she works for the National Wildlife Federation and leads a team that has attached tracking tags to nearly 1,750 butterflies this year alone. Her goal is to find the key resting spots monarchs use along the way before those places disappear.

The trackers being used are called radio tags, and each one weighs about as much as a single drop of water. They were built by a company called Cellular Tracking Technologies and cost about $200 each. Because the tags are so light, they don't slow the butterflies down or hurt them — as long as they are attached by someone who is properly trained.

Not every monarch can carry a tracker. Scientists look for butterflies that weigh more than 400 milligrams — about as much as a standard pill — so the tag won't be too heavy. Many butterflies are caught in nets and kept in cool temperatures to calm them down. Then a small dot of adhesive is carefully applied to the butterfly's abdomen, the tag is pressed on, and the insect is released.

The data coming in is already full of surprises. Scientists found the first-ever evidence of a butterfly from Utah, west of the Rocky Mountains, flying all the way to central Mexico alongside eastern monarchs. Others traveled down the East Coast to Florida, then crossed the Gulf of Mexico — a connection researchers had never seen in such large numbers before. As of this year, about 175 tagged butterflies have already been detected in Mexico.

A large number of eastern monarchs travel through what scientists call the 'Texas funnel' — a narrow path through the state — each fall and spring. Depending on the year, the butterflies pick different parts of Texas based on where they can find water, nectar, and trees to roost in overnight. Quiñonez-Piñón's study combines the tracking data with reports from everyday community scientists to better understand those choices.

Louie Yang, an insect scientist at the University of California, Davis, called the new technology 'pretty remarkable.' He said that before these tags existed, researchers had no way of seeing exactly which paths the butterflies took or how fast they moved. However, he also pointed out some limits — the tags show where a receiver picked up a signal, not an exact location, and a detected signal doesn't always mean the butterfly is still alive.

Monarchs usually live just two to six weeks, but the ones that migrate to Mexico can live up to nine months. They fly south in the fall, wait out the winter, then fly north again in the spring to lay their eggs before they die. This incredible journey has made the monarch a symbol of hope for many people, including those who support the rights of international migrants.

Sadly, monarch populations have dropped sharply in recent decades. Since the mid-1990s, the number of eastern monarchs has fallen by more than 80 percent, while western monarchs have dropped by about 95 percent. Pesticides threaten milkweed — the only plant where monarchs lay their eggs and the only food their caterpillars can eat.

Climate change is also a major threat year-round. Tierra Curry, a senior scientist with the Center for Biological Diversity, explained that rising temperatures cause problems at both ends of the journey. 'One of the problems now is that it stays too hot too long in the northern areas, and so they don't get the cues to migrate,' she said. If cold weather hits before they reach Mexico, many will not survive the trip.

Droughts dry up the water and nectar flowers they need along the way. Cars and trucks kill large numbers of butterflies on highways. In California, forests where western monarchs spend the winter are being cut down for new buildings. Late last year, a federal decision on legal protections for monarchs was delayed, though a court has ordered the government to make a final ruling by 2030.

Despite these challenges, conservationists are finding new ways to help. In Texas, special nets have been placed along some highways to guide butterflies higher into the air so they don't get hit by vehicles at night. People are also planting milkweed and other pollinator-friendly plants in gardens, along roadsides, and in parks across the country. Studies show these efforts are already helping local monarch populations grow.