Every year, hundreds are recovered in central Mexico, the destination point of the fall migration for most of the butterflies. The monarchs then are turned over to researchers from the University of Kansas, who use the numbers on the stickers to study the butterflies' declining population and the mystery of their long-distance journey.
Betsey Ney, Tyler's director of public programs, tagged and released dozens of the insects Sunday, one at a time, as visitors watched.
Volunteer Carol Dickerson stood by in a wide-brimmed sun hat, clipboard at the ready, recording the tag number for each flight.
"¡Adios!" children and grown-ups cried as Ney released her gentle hold on each butterfly, allowing them to soar.
In between, Ney engaged visitors in a biology discussion. Among the mysteries is how monarchs born here know to set their internal compasses for Mexico, whereas their descendants, several generations later, know to head north.
"How do the great-grandchildren know to come back?" Ney asked. "It's not like a bird, where it's the same bird that comes back."
A more sobering question is what to do about the insect's decline, said Chip Taylor, a University of Kansas biology professor and director of the Monarch Watch program. Several environmental threats are to blame, he said in a telephone interview.
Among them are excessive logging of the Mexican forests that are host to the butterflies and the use of pesticides to kill milkweed - a plant that interferes with U.S. corn crops but provides food and habitat for monarchs in their caterpillar stage.
Some scientists have questioned whether the insect's numbers are dwindling, citing fairly stable populations counted each year in Cape May, among other data. But Taylor said the Cape May population represents a small portion of the total, whereas most of the migrating insects start from the agricultural Midwest.
The latest counts in Mexico - conducted by measuring the area covered by butterfly-laden trees, not by counting individual insects - reveal a steady decline, Taylor said.
"There shouldn't be any debate," he said.
One solution is to plant milkweed, which Tyler Arboretum grows in its screened butterfly enclosure. At the tagging event, Ney pointed out a few such plants stripped of their leaves by hungry caterpillars.
This is the fourth year that the arboretum has released monarchs. So far none of the Tyler insects has been identified in Mexico, as far as Ney can tell from a database maintained on the Monarch Watch website, www.monarchwatch.org. The recovery rate ranges from half of 1 percent to 5 percent, said Taylor, the program director.
Yet plenty of insects from this area have been found - 240 from Pennsylvania and 47 from New Jersey over the past dozen years, according to the website.
Typically they are found after they die, lying on the forest floor. (The tags do not harm the insects; the untagged ones die there, too.)
Taylor said the Monarch Watch program pays Mexicans 50 pesos per tagged butterfly - about $4 - a welcome source of income for many.
"Families will send all their children off in the woods to look for tags," he said. "The next time we see a kid, he'll have a new jacket and new shoes."
Butterflies are tagged and released as they begin their migration to Mexico at www.philly.com/monarchbutterflies
Contact staff writer Tom Avril at 215-854-2430 or email@example.com.