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That Strange Cloud Over St. Louis Last Week? Migrating Monarch Butterflies

Lincoln Brower

Last Friday, the St. Louis office of the National Weather Service picked up something pretty unusual on its radars.

As first reported by Citylab’s John Metcalfe, meteorologists detected a cloud-like formation that kept moving around and changing into odd shapes. After some analysis, they concluded that the “cloud” was in fact a giant swarm of monarch butterflies, headed south on its annual migration to Mexico.

“A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape!” the weather service explained on its Facebook page. On radar, the formation actually looked something like a giant butterfly centered somewhere above St. Charles, with its wings spreading more than 250 miles into Missouri and Illinois.

We’ve reported on the monarch’s remarkable migration before ― you can find out more about it here. And this past spring, the City of St. Louis launched an initiative to encourage residents to plant milkweeds ― the monarch caterpillar’s only food source.

Since the mid-1990s, monarch butterfly populations have decreased by about 90 percent. Scientists like Lincoln Brower attribute their precipitous decline to habitat loss due to illegal logging in Mexico, and to the widespread use of herbicides in U.S. agriculture.

Credit U.S. National Weather Service
The St. Louis office of the National Weather Service posted this radar image of the monarch butterfly cloud on its Facebook page on Friday, Sep. 19.

But for now, the monarchs appear to be holding on. As the weather service put it, “NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!”

Here at St. Louis Public Radio, we second that.

Follow Véronique LaCapra on Twitter@KWMUScience