Every autumn, millions of monarch butterflies migrate south to the forested uplands of central Mexico’s Sierra Madre Occidental range.
Historically, so many butterflies make the journey — some from as far north as Canada — that researchers cannot possibly count them all.
Instead, they count the number of acres covered with butterflies. They estimate that an acre can hold about 25 million butterflies. In 1996, the butterflies in central Mexico occupied about 45 acres, the highest number ever recorded.
Chip Taylor, founder and director of Monarch Watch, an organization that helps track the migration, has traveled to Mexico many times to see it.
“The branches are bending with the weight of these butterflies,” he said.
Sometimes, a gust of wind blows them from the trees, and “you end up standing in a blizzard of butterflies,” he said. “The predominant sound you get when this happens is the sound of rustling wings.”
But monarch numbers have fallen sharply in recent years. Monarch Watch has a nationwide network of volunteers who tag the insects during the fall migration, and those volunteers had little to report last year. Taylor said several volunteers reported tagging none. Taylor, who usually tags 40 monarchs, tagged just one.
In Mexico, the most recent migration covered just 1½ acres.
“We have two choices: One, we can fight this, or we can lose the migration. That’s what it comes down to,” Taylor said.
The decline has numerous reasons: climate change, deforestation and habitat loss, and agricultural use of pesticides and herbicides.
Monarchs are pollinators, which makes them important for plant reproduction. They also share habitat with small birds and animals, which feed larger birds and animals, Taylor said. Monarchs reflect ecosystem health and biodiversity.
“Monarchs are symbolic of what’s happening on a larger scale,” Taylor said. “You eliminate monarchs, and you eliminate everything else that shares that habitat.”
Monarch migration is complex, and researchers are trying to fully understand it.
Migration path mysterious
Researchers needed decades to trace the monarch migration.
In 1940, Canadian zoology Professor Fred Urquhart developed a tag that would stick to butterfly wings, but still it was difficult to follow the insects’ progress.
Urquhart and his wife, Norah, organized volunteers, including citizen scientists in Mexico. Volunteers who found tagged butterflies would send them to Urquhart’s lab, but he could not pinpoint where they had spent the winter. By the mid-1970s, the Urquharts had discovered a major migration site in the mountains of Michoacan, Mexico, where researchers found a butterfly tagged in Minnesota. The discovery is the subject of a 2012 film called Flight of the Butterflies.
In late summer and early fall, butterflies in the United States and Canada start flying toward Mexico, said Gary Nabhan, the W.K. Kellogg endowed chair in sustainable food systems at the University of Arizona Southwest Center. Many make it all the way to the Sierra Madre, and after wintering in the mountains, those butterflies head north again.
During the journey north, monarchs produce four generations. So, a butterfly might get as far as Texas, for example, where it lays eggs and dies. The next generation might get as far as Kansas, the one after that, Minnesota.
In the fall, the migration south begins again. The butterflies in the fourth generation live eight or nine months and can make it as far south as Mexico.
But not all butterflies wind up in Mexico. Some go to Florida or the California coast. Others go to western Mexico. Researchers divide the migrations into various zones and classify populations as eastern and western, but the butterflies are genetically the same.
“The east and west populations are identical genetically,” said Gail Morris of the Southwest Monarch Study, which tracks the butterflies in Arizona and the West.
What moves monarchs from one migration corridor to another may be as simple as a strong wind. It also may be nature’s way of improving their odds of survival.
Migration carries risk. Cold winters and predators can be hard on the population in a given year. Having multiple migration corridors lowers the risk.
The plight of the monarchs is attracting political attention.
During their summit meeting in Toluca, Mexico, in February, the leaders of Mexico, the United States and Canada pledged to support the monarch. Mexican President Enrique Peña Nieto, President Barack Obama and Canadian Prime Minister Stephen Harper called it a “landmark species” and said they hope to form a task force to address the problem.
Monarchs in rapid decline
Monarch butterflies once were so common that some people find it hard to believe that they are declining so rapidly.
“If you tell an old-timer that, they say, ‘Well, that can’t be, because when I was a kid they were everywhere,’ ” Nabhan said.
That was the case when Taylor, who grew up in Minnesota, was a child. He would put a caterpillar in a peanut-butter jar with a leaf and watch as it became a butterfly. It was a common experiment for kids who grew up in the Midwest and had an interest in nature.
The key to the butterfly experiment was simple: The leaf had to be a milkweed leaf. That’s all the caterpillar needed to survive to adulthood. Monarchs lay their eggs on milkweed and nothing else.
Researchers believe the loss of milkweed plants is a big reason for the monarchs’ decline.
Milkweed is not a cash crop and so has been eliminated from many Midwestern farm fields to make room for more corn and soybeans. Nabhan and others believe that simply planting milkweed in the path of the migration can increase the monarchs’ numbers.
“We have to get the message out,” she said.
Gardeners can plant milkweed
Morris said milkweed can be planted in home gardens, greenbelts, parks, schools churches and at businesses.
“There’s no place that’s too small,” she said. “Just incorporating some of … native milkweeds could make a difference.”
Milkweed is available from botanical gardens and other suppliers, but it can be tough to find in local nurseries because gardeners haven’t asked for it. Monarch advocates are working with nurseries and botanical gardens to make the plants more available.
“We’re really set on having this not be another doomsday story,” Nabhan said.
Monarch advocates believe the insects still have time to recover. Anyone who has a patch of land can help.
“People don’t want to despair about the problems anymore. They want to work toward solutions,” Nabhan said.
The fact that monarch recovery was discussed at the North American summit was a hopeful sign, he said. Leading monarch advocates recently asked the Obama administration to establish a program for monarch recovery.
“Human migration has been an incredibly divisive thing,” Nabhan said. “And monarch butterfly migration has been kind of a hands-across-the-border thing,” Nabhan said.
In the meantime, monarch lovers have a message: plant milkweed.
7 facts about monarch butterflies
1. The butterflies’ distinctive coloring warns predators that the monarch is foul tasting and poisonous.
2. In their larval stage, monarch caterpillars feed on milkweed. Adult monarchs get their nutrients from flower nectar.
3. Monarchs can travel 50 to 100 miles a day. It can take up to two months to complete their southward journey to winter habitats.
4. Monarchs can produce four generations during one summer. The first three generations live two to six weeks and continue moving north. The fourth generation can live up to nine months. These are the butterflies that migrate south for winter.
5. The butterflies cannot fly if their body temperature is less than 86 degrees. They will sit in the sun or “shiver” their wings to warm up.
6. Monarchs can survive below-freezing temperatures if they stay dry. If they get wet and the temperature drops, they will freeze to death.
7. The monarch butterflies’ fall migration pathways lead southward mostly to Mexico but also to Florida and California.