Not all monarch experts worry about tropical milkweed. “Monarchs utilize an immense landscape in the Eastern U.S., and this plant constitutes a tiny, tiny portion of the milkweeds encountered by monarchs returning in the spring,” said Chip Taylor, an ecologist at the University of Kansas who directs the conservation group Monarch Watch. “Should they be there? Probably not. But will they do immense harm? Probably not.”

If monarch populations keep falling, the coastal regions could become more important, Dr. Oberhauser said. Migration can limit disease by weeding out the sick and allowing butterflies to leave contaminated plants behind. If year-round milkweed changes the migratory behavior of enough monarchs, she said, “it could have really far-reaching impacts.”

So far, evidence that monarchs stop migrating to breed is indirect. “People plant tropical milkweed and then we see monarchs reproducing when they should be migrating or overwintering,” Dr. Altizer said. “There needs to be more experimental work done.”

And that is why Ms. Satterfield drove all night to catch butterflies in Texas.

The monarchs she collected in Dallas and at another site without tropical milkweed will help her assess the plant’s effects at four coastal sites where it is common. She plans to analyze chemicals in the butterflies’ wings to distinguish migrants from residents based partly on what they ate as caterpillars. The analyses will show if migrating monarchs pass through the sites. If they interact with resident breeders, it could help explain why disease is increasing among eastern monarchs.

To figure out if migratory monarchs are breeding at tropical milkweed sites, Ms. Satterfield will look for eggs in females and mating behavior in males. She will know if they also abandoned the migration if individuals tagged last month are still there when the migration ends.

No one disputes that loss of milkweed habitat remains the monarchs’ biggest threat. But if the population gets smaller, risks once considered less important — like severe weather and disease — could prove catastrophic.

“We’ve learned the hard way with migratory bison and whooping cranes that once we lose a migration, it is close to impossible to bring back,” Ms. Satterfield said. “Protecting the great North American journey of the monarch is crucial now, while we still have a chance.”