Monarch butterfly.
Photograph by: Blair Gable, Windsor Star
Thirty-five years after Ontario scientists Fred and Norah Urquhart famously discovered the monarch butterfly's winter refuges in the mountains of Mexico, another team of Canadian researchers has unlocked one of the last lingering mysteries surrounding the insect's annual migration.
Experts from the University of Guelph and Environment Canada, probing the remains of dozens of monarchs and milkweed samples from 17 sites in the eastern U.S., have identified a previously unknown flight path over the Appalachian Mountains that reinforces the importance of protecting the species' habitats in the Great Lakes region of the northern U.S. and southern Canada.
"It's a groundbreaking finding," U of G biologist Ryan Norris said in a summary of the study, published in the latest issue of the journal Biology Letters.
"It solves the long-standing mystery of why monarchs always show up later on the east coast compared to the interior," he noted. "Importantly, it means that the viability of east coast populations is highly dependent upon productivity on the other side of the mountains."
Key to the finding was the identification of isotopes from the butterfly and milkweed specimens. These distinctive chemical signatures can help scientists reconstruct the life histories of individual monarchs, pinpoint their birthplaces and retrace the routes they've travelled during their migrations.
The team was trying to understand why butterflies in the eastern U.S. between Maine and Virginia arrived at their summer destinations several weeks later than other populations in central Canada and the U.S. Midwest.
What the scientists found is that most of the monarchs arriving in July in the eastern U.S. were the newborn offspring of certain butterflies that had migrated that spring from Mexico to sites west of the Appalachians before laying eggs in the Great Lakes region.
The second-generation butterflies, born west of the Appalachians in June, then flew eastward across the mountains to continue their life cycles along the U.S. east coast.
"Ours is the first proof of longitudinal migration," said the study's lead author, U of G graduate student Nathan Miller.
Co-authors Leonard Wassenaar and Keith Hobson, both Saskatchewan-based scientists with Environment Canada, have pioneered isotope-analysis methods of mapping butterfly birthplaces through trace elements found in their wings.
The research team noted that the monarch is considered a species of "special concern" in Canada, and that the new findings should prompt modifications to conservation efforts throughout the butterfly's North American range.
Until now, they stated, protection efforts have focused largely on key breeding sites along the monarch's northward route from Mexico.
"Our results suggest that the most productive breeding region for monarchs found along the east coast is the northwest region, which produced nearly 90 per cent of the monarchs," the Biology Letters paper concludes.
"Thus, conservation planning will require targeting multiple regions to maintain all life-history strategies, emphasizing the importance of targeting the Great Lakes region for conservation efforts."
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