Darner’s green dragonflies migrate over Lake Michigan

Flocks of common green dragonflies will be visible near Chicago through the end of the month. Chicago is the latest stop on a dragonfly’s journey — part of its multi-generational mass migration — across North America that occurs every spring and fall.

Stand close enough, and the common green bird’s namesake becomes apparent: its metallic green chest gives way to a shimmering blue belly. The green runner averages about three inches in length, and is as large as some songbirds.

The dragonfly will travel more than 900 miles, said Melissa Sanchez Herrera, an entomologist who studies dragonflies at the University of Alabama.

“They will start crossing the lake,” Sanchez Herrera said. “Because some of these actually come from Canada.”

When you think of migration, birds, bats and monarchs may come to mind. Of the approximately 500 dragonfly species, a small percentage of North American species also make long migrations, Herrera said. The green dragonfly typically travels as far as the Gulf of Mexico, but Sachez-Herrera adds that some dragonfly species, such as the Wandering Glider, have appeared as far south as Colombia.

Dragonflies must travel these long distances out of necessity. Dragonflies gather the way they do near Lake Michigan, typically, for one of two reasons: to eat or to migrate, Sanchez-Herrera said.

Dragonflies must find an aquatic habitat to lay their larvae, where they will develop over the winter and spring until they emerge when the surrounding water temperature is right.

“But you want to be somewhere where your larvae don’t freeze,” Sanchez-Herrera said. “This is the main reason why they like to look and move around.”

Weather radars detect dragonflies

Last week, Jacob Drucker, a doctoral student who studies ornithology at the University of Chicago and the Field Museum, noticed massive gatherings of dragonflies and began tracking the swarms using weather radars.

“It’s the same weather radars that are designed to look at precipitation, sleet, snow, hail, etc.,” Drucker said. “It is designed to detect objects in the sky, so it also detects birds and insects.”

Displayed in green and red, Drucker pulled out two different maps showing huge schools of dragonflies moving across the lake into Illinois. The radar was able to show how the wind would push the dragonfly toward the lake, then back to land.

“When I was seeing high-density dragonflies on the radar that day, I would go back and forth between different other radar stations regionally,” Drucker said. “I thought there were more people in Illinois and Michigan than in Missouri and Indiana.”

According to Drucker, these remote sensing methods could be used to monitor biodiversity on a scale never before seen.

“As we think about this massive loss of biodiversity around the world and how to measure it … and figure out what to do about it, weather radar can be a really great tool for making that happen,” Duker said.

In the meantime, sit back and enjoy the show. Don’t worry too much if you miss it this year, Sanchez Herrera said. “You can wait for next year, maybe you’ll see them then.”

Juanpablo Ramirez Franco covers climate change and the environment for WBEZ and Grist. Follow him on Twitter at @__jonbab.