For the first time, researchers have mapped the migration of monarch butterflies across the continent over an entire breeding season. The information might help conserve a creature increasingly threatened by loss of habitat and food sources, says Tyler Flockhart, a doctoral student at the University of Guelph in Ontario.
Flockhart is lead author of a paper published recently in the Proceedings of the Royal Society B. He and his co-authors used chemical markers in butterfly wings to match monarchs with their birthplaces.
“This tells us where individuals go and where they’re coming from,” he says.
Monarchs breed across a broad area of the northern and eastern United States and southern Canada. They live up to eight weeks. The monarchs that arrive first each spring come from eggs laid by females that likely spent the winter in Mexico. These first-generation monarchs mate and lay eggs before they die, and their offspring do the same, producing three to four additional summer generations, the last of which migrates back to Mexico.
Monarch larvae eat only milkweed. The plant’s chemical signature varies from place to place, allowing scientists to pinpoint a butterfly’s birthplace by analyzing the chemical elements in its wings.
Flockhart spent the summer of 2011 following the northward migration and netting more than 800 monarchs for analysis. He began his road trip in southern Texas and logged 21,700 miles across 17 states and two provinces. The distance is equivalent to nearly four round-trips from New York to San Diego. “As far as I know, it’s the broadest sample of monarch butterflies through an entire breeding season across North America.”
Monarch colonies overwinter in Mexico. During the breeding season beginning in April, successive generations were born in Texas and Oklahoma, then in the Midwest, and then over a broad area spanning the northeast coast and the Midwest.
One key stop is the corn belt. A breeding explosion sends vast numbers of adults in several directions, including to Canada, says co-author Ryan Norris, a biology professor at Guelph.
He says loss of milkweed plants and planting of genetically modified corn and soybeans in the Midwest have affected monarch survival. “If habitats in the Midwest continue to decline, then monarchs will lose the ability to expand the breeding range, including those butterflies that end up here in Ontario.”
It’s also important to protect breeding habitat in other locations, he says, including parts of southern Texas that supply future generations to breed in the Midwest.
In 2012, the smallest-ever population of monarchs was recorded in the Mexican overwintering grounds. “They’ve been declining steadily,” says Flockhart.
“To lose monarchs would be a huge blow to the environment and to the public,” Norris says. “People can easily identify monarchs. It might be the first butterfly they see or catch as a child, and it’s often the first story they hear about how animals migrate.”
Adds Flockhart: “Every school kid knows about monarchs.” — Matt Mendenhall, Managing Editor
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