Maintaining The 'Miraculous' Monarch Migration

Sep 18, 2017

With fall on the way, monarch butterflies are beginning their annual migration south to the mountains of Mexico. Just as scientists are beginning to understand how the butterfly knows to make a 3,000 mile journey it's never made before, that migration is under threat from loss of habitat.

Kathy Charvat is a seasonal naturalist with Great Parks of Hamilton County and a champion for monarch butterflies.
 

Caterpillars hang upside down as they pupate and go through metamorphosis. Charvat uses string to strengthen the caterpillar's bond, increasing the likelihood of it making it to adulthood.
Credit Tana Weingartner / WVXU

"I keep using the word miracle," she laughs. "It's because it's miraculous, I guess. It's just this astounding natural phenomenon that this tiny insect can travel from Toronto to Mexico by itself, overwinter there, and then start a new cycle the next spring."

That cycle is at risk because wide-scale herbicide use has decimated the milkweed plant. Monarchs almost exclusively lay their eggs on the fibrous flower.

Charvat suggests the monarch's decline is a harbinger of problems to come for other species facing extinction caused by habitat loss.

Besides milkweed, monarchs need access to lots of nectar plants to build up their body fat for the long trip south.
Credit Tana Weingartner / WVXU

"The monarch is so visible and so dramatic with its North American migration that everyone's familiar with it. Especially people of an older generation remember a childhood where the monarch was everywhere. Then since the 70s and even more in recent years there's been a huge decline in numbers."

To combat the problem, she encourages planting tropical milkweed. There have been problems farther south with a parasite infecting the plant, but Charvat says that's not an issue here because milkweed dies back in the winter.

Nectar plants are also vital. A 2016 Cornell University study suggests a major problem affecting the monarch is a lack of access to food. Just like squirrels fattening up for winter, monarch have to to build up their fat reserves before they head to Mexico. They also need to eat and drink water along the way.

"We hypothesize that lack of nectar sources, habitat fragmentation, continued degradation at the overwintering sites, or other threats to successful fall migration are critical limiting factors for declining monarchs," the authors write.

Cornell University Entomology Professor Anurag Agrawal, says, "By the time they get to Mexico, their numbers are plummeting, but at the end of the summer when they start that migration, their numbers are not down."
 

Nan Plunkett estimates she's helped hundreds of caterpillars become butterflies this summer by raising them in her home away from predators.
Credit Tana Weingartner / WVXU

Charvat's friend, Nan Plunkett, is equally concerned with the monarch's plight. She's earning a masters degree in biology from Miami University. The kitchen counter at her home on Cincinnati's West side is covered with about 130 caterpillars happily munching milkweed leaves in tiny individual condiment cup homes.

"Every once in a while one of the cats will get up [on the counter] and decide to test gravity once again," she admits with a smile.

She estimates only about 10 percent of caterpillars in the wild make it to adulthood. She's reared close to 400 since spotting the season's first monarch egg on Easter Sunday in April. She also maintains a vast flower garden - essentially her entire yard - stocked with nectar plants.

She worries the migration may become extinct. "To lose that migration, to me, would be terrible knowing that we can do something to help."

Not all of the monarchs Plunkett raises are released back into the wild. Some go to Patrick Guerra, an assistant professor in UC's Department of Biological Sciences.

UC Assistant Professor Patrick Guerra uses these refrigerator-looking units to manipulate sunlight and temperature. He'll later study how changes to those variables affect a butterfly's migration instinct.
Credit Tana Weingartner / WVXU

"What I'm trying to understand," he explains, "is how does an individual use its environment and cues it can sense in order to figure out which way to go.  For example, fall monarchs need to go south, so how does it use cues in the sky such as the sun or the earth's magnetic field, and how do they orient southward?"

Guerra uses a flight simulator, kind of like a butterfly treadmill in a barrel, to study how the butterflies know which way to fly. He's also able to manipulate their environments to see how that affects their migration instincts.

Scientists still have many unanswered questions. For Guerra, the biggest is figuring out how southbound monarchs know when to stop flying.

"It could be something from the plants - for example the oyamel fir trees they that overwinter [in]," he says. "It could be some sort of magnetic anomaly that they can key-in that's unique to those overwintering sites. It could be something like the altitude because these overwintering sites are on these high altitude mountains. So there's still lots of questions as to even figuring out how do they get there. For me, that's the holy grail of the whole enterprise right now, trying to figure out why they stop."

All this information, it's important not just for preserving the monarch and the migration, but it can be scaled up to primates, and ultimately humans. On the one hand, you might say, it's just insects, but for Guerra it isn't so far-fetched to say understanding the butterfly's story could someday help scientists solve even bigger problems, like finding a cure for cancer.