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.”