Anurag Agrawal with butterfly

New Yorkers witness the monarch butterfly’s epic migration to Mexico every September—but these days, fewer of the colorful travelers pass through. Nobody is sure why. Many have blamed a decline in milkweed 

Monarch Research Takes Flight

February 6, 2017

Anurag Agrawal with butterfly

New Yorkers witness the monarch butterfly’s epic migration to Mexico every September—but these days, fewer of the colorful travelers pass through. Nobody is sure why. Many have blamed a decline in milkweed, an essential food source, for the iconic butterfly’s dwindling numbers.

Not so fast, says Cornell ecologist Anurag Agrawal.

With Academic Venture Fund support, Agrawal and his team tracked monarch migration patterns over the past two decades, pulling in population data from citizen scientists and conservation organizations. Their ground-breaking study found that monarch numbers are actually stable through the summer, when the breeding butterflies and caterpillars rely on milkweed. Instead, the problem starts when the butterflies set out on their fall migration.

“Planting native milkweeds won’t hurt, but it also won’t fix the problem,” Agrawal explains. “Our findings point to a lack of flower nectar sources during the southern migration. The butterflies also seem to be having trouble finding and settling in at the remote, degraded overwintering sites.”

In Mexican sanctuaries, where the monarchs hibernate in spectacular clusters on mountain fir trees, butterfly numbers have been dwindling over the past 20 years. The downward trend continues in the southern United States when the monarchs make their return journey—but then the population bounces back during the milkweed season.

Monarchs and Milkweed BookAgrawal's new book, Monarchs and Milkweed, will be available in April, 2017

Over several summer generations, monarchs today build up surprisingly large populations before migration, comparable to past levels that once held relatively steady through the year. This new pattern of bigger ups and downs is a puzzle, Agrawal says, that may provide clues for future conservation efforts. A major report released by the U.S. National Academies of Sciences in May 2016 cited the researchers’ findings.

The monarchs’ story will reach a broader audience of butterfly lovers when Agrawal’s new book, Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution, comes out in April.

It’s a story all of us need to take to heart. “Monarchs are sentinels for the health of our continent,” Agrawal says. “They travel from Canada to Mexico, tasting milkweed and nectar as they move through the landscape, so they are exposed to a lot. If they are declining, that spells bad news for the health of our ecosystems.”

 

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