Swarms of common green darner dragonflies are being seen around Chicago, and they’ll continue to be visible through the end of the month, experts say.
That’s because Chicago is a pit stop on their multigenerational mass migration across North America, which happens every spring and fall.
Stand close enough, and the dragonfly’s metallic green thorax gives way to a shimmering blue abdomen. The green darner averages about three inches long — as large as some songbirds.
Melissa Sanchez Hererra, an entomologist studying dragonflies at the University of Alabama, said the dragonflies’ migration will cover more than 900 miles.
“They’re going to start to cross the lake,” Sanchez Herrera said, “because some of these are actually coming from Canada.”
When you think of migration, birds, bats and monarchs probably come to mind for most people before dragonflies. And, of the nearly 500 species of dragonfly, only a minuscule fraction of North American species embark on long migrations, according to Herrera. The green darner typically travels to the Gulf of Mexico. Some dragonfly species, like the wandering glider, have turned up as far south as Colombia.
Sanchez Hererra said dragonflies typically aggregate the way they have near Lake Michigan for one of two reasons: eating or migrating.
Dragonflies need to find an aquatic habitat to lay their larvae, which will develop over the winter and spring until they emerge when the surrounding water temperature is suitable.
“But you want to be in a place where your larvae will not freeze,” Sanchez Hererra said. “So that’s basically the reason why they’re like looking and moving all around.”
About a week ago, Jacob Drucker, a doctoral student studying ornithology at the University of Chicago and the Field Museum, noticed the massive concentrations of dragonflies and began tracking the swarms with weather radar.
“The same weather radars that are designed to look at precipitation, rain, snow, hail, etc,” Drucker said. “They’re made for detecting objects in the sky, and so they also detect birds and insects.”
Displayed in greens and reds, Drucker pulled two maps showing the massive clusters of dragonflies moving across the lake into Illinois. The radar showed how the wind would push the dragonflies back towards the lake, only for them to return to land.
“When I was seeing high-density dragonflies on the radar the other day, I would go and kind of toggle around between the other different radar stations regionally,” Drucker said. “I was seeing that there are more in Illinois and Michigan than there were in Missouri and Indiana.”
According to Drucker, these remote sensing methods can be a tool for better monitoring biodiversity.
“As we consider this mass biodiversity loss around the globe and how to measure it … and figure out what to do about it, weather radar can be a really great tool for that,” Drucker said.
