Every fall, millions of monarch butterflies engage in one of nature’s great spectacles, migrating from sites across North America to refuges in either central Mexico or coastal California, where winter temperatures are more tolerable. They fly south for thousands of miles, propelled by some innate sense of direction to places that neither they nor their parents have ever visited. But not all of them make the journey. Not all of them know the way.
Some proportion of North America’s monarchs comes from companies that breed stocks of the insect year-round and sell them to weddings, festivals, and classrooms around the United States. Others are reared by hobbyists, who collect wild eggs from their backyard and raise the butterflies in their home. These are typically well-intentioned efforts, meant to bolster the numbers of wild monarchs, which have declined by more than 80 percent in the past decade. But according to a new study, these releases might do very little to save the imperiled monarch migration.
By testing monarchs bought from a commercial supplier, Ayse Tenger-Trolander from the University of Chicago showed that they make terrible migrators. While their wild counterparts have a strong tendency to head south, the mail-order insects flew in random directions. Tenger-Trolander also found that wild monarchs became similarly inept if she raised them indoors, even if she tried her best to mimic natural conditions.
“It’s a powerful study,” says Sonia Altizer from the University of Georgia, a monarch expert who was not involved in the research. “It’s the first to definitively show that captive-bred monarchs won’t show the same orientation behavior that wild ones will.”
In recent years, growing public awareness of the monarch’s plight has led to a surge in captive rearing. But scientists and conservation groups have warned against the practice, arguing that insects housed in dense conditions would be more susceptible to diseases that they could then spread to their wild peers. Regardless, “this study shows that captive releases are unlikely to help wild monarchs,” says Karen Oberhauser from the University of Wisconsin at Madison. “It won’t make that much difference if they can’t migrate and become a normal part of the population.”
That’s not to say that all captive-reared monarchs are incompetent migrators. Last year, about 700 commercially bred individuals were tagged and released at a San Antonio, Texas, festival; five of these were later found at overwintering sites in Mexico. “But there’s a population-level difference,” says Marcus Kronforst, who led the new study. “Some individuals might be able to respond correctly, but most do not.”
The team also admits that they studied insects from just one commercial breeder. Though they suspect that other captive-reared individuals would behave similarly, they can’t say that for sure. “That’s an important caveat,” says Anurag Agrawal from Cornell University. “Nonetheless, they’re bringing scientific data to the table on a hot-button point in monarch conservation.”
Tenger-Trolander didn’t set out to wade into the debate—quite the opposite. She ordered the commercial insects in the hopes that they would be identical to their wild counterparts and provide an easy source of experimental subjects for other studies. Instead, “they turned out to be very different,” she says—a conclusion that made sense in hindsight. Mail-order butterflies come from stocks that haven’t migrated for generations, and they probably accumulated genetic changes that broke that ancestral ability.
Such changes have happened naturally elsewhere in the world. Monarchs originated in North America, but they’ve dispersed into the rest of the Americas, the Pacific Islands, and Europe. None of those other lineages migrates. Tenger-Trolander initially wondered if the commercial butterflies had come from one of these nonmigratory groups, but a genetic analysis said otherwise. They were descended directly from North American stock, but were as distinct as monarchs from other parts of the world. In other words, breeders had inadvertently created a new lineage of North American monarch that stays put.
What happens when such butterflies are released? In most cases, they probably fly around and die, with little consequence. But those that are unleashed in the summer “could potentially go out and mate with local monarchs … introducing genetic variation that is incapable of responding to the right environmental cues,” Kronforst says. “That could have consequences, but we just don’t know yet.”
“In trying to study migration, we’ve shown how fragile it is,” Tenger-Trolander says. “You can really knock out this behavior without much trouble, even by changing things you wouldn’t think are important.” For example, she and her colleagues also collected wild-monarch eggs and raised them indoors, under autumnlike temperatures and lighting schedules, much as legions of hobbyist breeders do. These insects didn’t orient south either.
“There’s these two critical parts to making a migratory insect: They have to have the correct genetics, and they have to receive the right environmental cues,” Kronforst says. “We thought we knew those cues, but our experiment suggests we don’t totally understand them.” Perhaps, aside from day length and temperature, the butterflies are also responding to the angle of the sun, or the diminishing quality of the milkweed plants they eat. Without that full suite of triggers, even those with migration-capable genes don’t set off correctly.
The briefest exposure to artificial conditions can make a difference. During her study, Tenger-Trolander moved one small group of outdoor-reared monarchs inside, to save them from a predicted overnight freeze. Those insects had lived their entire larval life outside, and were already in the pupal stage. But after just three to four days indoors, some of the adults that emerged also failed to point south. “That was the thing I was most surprised by,” Tenger-Trolander says. “I thought, What difference could these last couple of days make? And they really killed it.”
“Even with all the problems, there are some real pluses to people raising monarchs,” Kronforst adds. “One of the biggest is that it creates this love of this insect.” He hopes that hobbyists and schools will continue the practice, with a few tweaks. First, collect eggs from local habitats instead of ordering them from commercial suppliers. Second, expose them to natural outdoor conditions to “give them the best chance of developing into migratory insects.”
But if migration is so finicky, and so sensitive to environmental conditions, what will happen to the monarch’s epic journey in a rapidly changing world? Some omens come from the southern United States, where even naturally occurring monarchs are skipping the migrations, thanks to warmer year-round weather and the presence of exotic milkweeds that people have planted. “As a species, it’s likely that the monarch will survive for a long time,” Kronforst says. “But this phenomenon that we all know and love—the migration—seems to be very fragile. We’re seeing what conditions cause it to be lost, and we’re seeing those play out in front of our eyes.”
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