For the first time, scientists are tracking the migration of monarch butterflies across much of North America, actively monitoring individual insects on journeys from as far away as Ontario all the way to their overwintering colonies in central Mexico.
This long-sought achievement could provide crucial insights into the poorly understood life cycles of hundreds of species of butterflies, bees and other flying insects at a time when many are in steep decline.
The breakthrough is the result of a tiny solar-powered radio tag that weighs just 60 milligrams and sells for $200. Researchers have tagged more than 400 monarchs this year and are now following their journeys on a cellphone app created by the New Jersey-based company that makes the tags, Cellular Tracking Technologies.
Most monarchs weigh 500 to 600 milligrams, so each tag-bearing migrator making the transcontinental journey is, by weight, equivalent to a half-raisin carrying three uncooked grains of rice.
“There’s nothing that’s not amazing about this,” said Cheryl Schultz, a butterfly scientist at Washington State University and the senior author of a recent study documenting a 22 percent drop in butterfly abundance in North America over a recent 20-year period. The movements of monarchs and other flying insects are cloaked in mystery, and “now we will have answers that could help us turn the tide for these bugs.”
Tracking the world’s most famous insect migration may also have a big social impact, with monarch lovers able to follow the progress of individual butterflies on the free app, called Project Monarch Science. Many of the butterflies are flying over cities and suburbs where pollinator gardens are increasingly popular. Some tracks could even lead to the discovery of new winter hideaways.
“It’s an incredible technological advance,” said Orley Taylor, a professor emeritus at the University of Kansas who goes by Chip and has spent more time studying the migration than any other living researcher.
Monarch tracking has relied on low-tech methods ever since it began in 1935. The biggest current effort is led by Monarch Watch, founded by Dr. Taylor in 1992. The group oversees the annual sticker-tagging of more than 100,000 monarchs, though fewer than 1 percent are recovered in the winter colonies.
Even when a monarch bearing a sticker tag is found, the information it yields is limited to the butterfly’s physical characteristics and the time and place it was tagged, with no details about the long journey in between. That’s like trying to make sense of an entire book from only its first and last pages.
Now that those pages are being filled in, researchers can see that the story of the migration is even more dramatic than expected, with many monarchs blown far off course but using their strong wing muscles and remarkable navigational abilities to get back on track for Mexico.
“We can already see that a lot of things we thought we knew about how these insects move are oversimplifications,” Dr. Taylor said.
Monarchs have evolved two highly sophisticated navigational systems. Most of the time, they rely on a system that orients them in relation to the sun, keeping them pointed south throughout the day by compensating for the sun’s movements across the sky. When clouds get in the way, monarchs switch to a backup compass that relies on ultraviolet light to detect the angle of the Earth’s magnetic field.
Their twin compasses usually keep migrating monarchs headed in the correct general direction. But how the butterflies manage to locate the same isolated colonies their great-great grandparents occupied the year before is a longstanding mystery.
No more than one in four is likely to survive the journey, with the rest succumbing to unfavorable winds, hungry birds, vehicle traffic or sheer exhaustion, among other perils.
The migrators who manage to reach the colonies join a spectacle in which huge flocks circle overhead in kaleidoscopic whirls and roost so thickly on fir trees that even the sturdiest branches bend under their collective weight.
Hundreds of thousands of people annually hike up 10,000-foot mountains to witness this phenomenon, but the multigenerational migratory cycle that sustains it is under growing stress.
In the 1990s, the winter population at the Mexican colonies was regularly estimated in the hundreds of millions, but it now rarely tops 60 million. Last winter, the estimate was roughly 38 million. The much smaller West Coast monarch population is even more vulnerable. Last winter, fewer than 10,000 were seen huddling in their usual spots along the California coast.
Experts cite a host of reasons for the decline, all related to human influence.
In the fall and spring, heat and drought spurred by climate change have reduced the abundance of nectar plants along the migration routes. In the winter, deforestation, beetle infestations and climate shifts threaten the colonies. And in the summer, monarchs’ milkweed host plants have been virtually eliminated from Midwestern farm fields, once the insect’s most important breeding grounds, because of the adoption of herbicide-tolerant crops. Spring and summer are crucial because females need to lay hundreds of eggs each to make up for all the casualties in fall and winter, when most monarchs are celibate.
The 36-employee company that built the trackers, Cellular Tracking Technologies, operates out of a hangar-like space at Cape May Airport in New Jersey. Key staff members have roots in conservation biology and make tracking devices suitable for animals ranging from bumblebees to elephant seals. (They’re working on tags for beavers and platypuses, too.)
“This has been a 20-year journey for me, to create a company that I hoped one day could make major advances like this possible,” said Michael Lanzone, the founder and chief executive.
Field tests of the company’s newly enhanced insect tags began Sept. 12 in Long Point, Ontario, when two Canadian government researchers radio-tagged 30 monarchs. The next day, they checked the phone app and were shocked to see how well the monarchs were flying.
“We could see they were just bombing it across Lake Erie and then making landfall in places like Cleveland,” said Greg Mitchell, a research scientist at Environment and Climate Change Canada.
More than nine weeks later, one of those monarchs — a female designated LPM021 — has flown far beyond the known colonies and was last detected 425 miles from Guatemala.
Spurred by the success in Ontario, the company quickly gave away more than 400 tags to researchers across the Eastern United States and southern Canada. Soon, the maps on the company’s app were full of butterfly tracks, and dozens of entomologists were speculating in a group chat about which monarch would make history by being the first tracked all the way to the isolated Monarch Butterfly Biosphere Reserve west of Mexico City.
In the end, it was JMU004 that won the metaphorical gold medal in a photo finish. Tagged by Leone Brown of James Madison University in Harrisonburg, Va., he was detected 37 days later in the El Rosario colony, on Nov. 9 at 10:35 a.m. local time, just three hours ahead of a monarch tagged at Monarch Watch headquarters in Lawrence, Kan.
A disappointed but sporting Dr. Taylor broke the news on the scientists’ group chat, while Dr. Brown got to share JMU004’s triumph with her butterfly-loving 7-year-old daughter.
While the entomologists joked about the “race,” they emphasize that the real point is to see where all the radio-tagged monarchs are going, and how they get there. “It’s not about winners and losers,” Dr. Brown said. “It’s about staying in the game.”
The unorthodox paths some tagged monarchs are taking suggest they may be affected by the extra weight. But Dr. Brown, who has studied the question, thinks the impact is minimal.
“Most of them are acclimating to the tags, even if they might be moving a little slower or need a little more time,” she said.
A crucial engineering feat led to the tracking breakthrough. Earlier this year, Cellular Tracking Technologies modified its butterfly tag, which it calls BlūMorpho, in a way that allows its signals to be automatically detected by billions of Bluetooth-enabled devices, as long as a tagged butterfly passes within about 300 feet.
Its other tags are sold almost exclusively to biologists for research projects, but the company hopes BlūMorphos will also appeal to amateurs willing to pay the $200 retail price, including thousands who raise and release monarchs themselves.
Unlike bird tracking, which requires a special permit, there are no nationwide restrictions on attaching trackers to insects, although California and several other states require permits.
Still, it’s not clear that Blū tagging will be a simple process for casual users, who may end up damaging some of the butterflies.
For every monarch Dr. Brown radio-tags, she slips glassine paper envelopes over the wings to protect the scales and uses small weights to keep them from fluttering. She then brushes eyelash adhesive on its thorax and delicately applies a BlūMorpho tag, a tiny black device with a trailing antenna that isn’t much longer than the monarch’s own natural antennae.
“I don’t think these tags should be used willy-nilly — they should be used for conservation and focused research,” Dr. Brown said. “But a lot of people love monarchs, so I guess they will do what they want to do.”
Dan Fagin is a science journalism professor at New York University, where he directs the Science, Health and Environmental Reporting Program. His last book, “Toms River: A Story of Science and Salvation,” won the Pulitzer Prize for General Nonfiction.
Jonathan Corum is a Times reporter and graphics editor focusing on science and health.
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