Science & Nature
Swarms of Dragonflies Are on the Move Across Illinois, Part of a Great and Mysterious Migration
A male green darner dragonfly with his signature bright blue abdomen. (emprised / iStock)
“It’s the most exciting time of the year if you love dragonflies.”
And Cindy Crosby does.
The author of “Chasing Dragonflies” and the blog “Tuesdays in the Tallgrass” is talking about the start of the annual southern migration of green darners (Anax junius), one of the largest and most common dragonfly species.
“This is the time of year right now, this is the thick of it. It’s just so exciting to see hundreds of them aloft,” said Crosby, who coordinates dragonfly monitoring at Morton Arboretum and Nachusa Grasslands. “We’ll see 100, we’ll see 300 in a migrating swarm. You’ll see them move across interstates — I’ve seen them move across I-90.”
Yes, dragonflies migrate — some species at least — a phenomenon Chicagoans were treated to at the lakefront during the recent Air & Water Show.
Video: Dragonflies are on the move along Lake Michigan in Chicago. (Patty Wetli / WTTW News)
The darners darted through the air, executing dazzling loop-di-loops and other aerobatic maneuvers to rival the Blue Angels. But unlike the Navy fliers, the insects’ performance is likely to repeat throughout late summer into fall, and not just along the shoreline.
“Reports have been popping up all over the place. It’s all over northeastern Illinois at the moment that this is happening,” and swarms have also been spotted north in Wisconsin, said Doug Taron, chief curator emeritus at Peggy Notebaert Nature Museum. “At any given point you may see them, they may vanish, you may see more of them that probably won’t be the same ones. I’m aware of significant swarms having been seen in the Chicago area as late as early October, so this may be on and off for a while.”
What people are witnessing isn’t necessarily the act of migrating itself, per se, but rather the dragonflies’ feeding and mating frenzies during stopover breaks.
“If they’re migrating, they’re all going to be flying in the same direction, and you don’t tend to see them at quite the density because they’re a little bit more spread out,” Taron said. “Then when they form the feeding swarm, they’re kind of going every which way. And that’s when people are more likely to notice.”
Green darners often reach sexual maturity as they migrate, so plenty of mating and egg-laying (ovipositing) is also taking place during the journey. Congregations near water tend to be engaged in mating, while feeding activity generally occurs away from water, according to Jason Bried, a researcher with the Illinois Natural History Survey and Prairie Research Institute at the University of Illinois Urbana-Champaign.
“I’ve stood mesmerized by one of these (swarms) for another migratory dragonfly — the wandering glider (Pantala flavescens) — as countless individuals flowed like a river past my vantage point for several minutes,” said Bried.
Migratory Mystery
Green darners are some of the largest dragonflies, measuring up to three inches long. (U.S. Fish and Wildlife Service)
As migration patterns go, the green darner’s has more in common with monarch butterflies than, say, birds.
Where the same bird will make the trip north and south — think Monty and Rose flying to Texas and Florida, and then returning to Chicago — multiple generations of dragonflies and butterflies are involved in migration. But the monarch’s movements are far better documented than the dragonfly’s.
“We’re kind of where monarchs were 50 to 60 to 70 years ago when people were like, ‘Well, where do they go?’” Crosby said. “I did a lot of work for ‘Chasing Dragonflies’ on the migration chapter — I talked to experts, I read a lot — and came away going, ‘Wow, there’s so much we don’t know.’ So if somebody wants to do their Ph.D. in dragonfly migratory behavior, so much of it is still a mystery.”
Scientists suspect cooler temperatures trigger the onset of migration — perhaps a slight dip of a few degrees had some jumping the gun in the past week — and wind speed also seems to play a role. Once they take flight, the dragonflies typically follow linear landforms, like mountain ranges, rivers or shorelines as they head south.
“It seems risky along large lakeshores because of strong winds pushing them out over the water, and often they’re in swarms making easy targets for falcons, kestrels and other predators that may be fueling their own migrations. Yet they manage!” Bried said.
“It’s like the Old Country Buffet. They’re getting eaten as they go,” Crosby joked.
While monarchs have very specific roosting sites in Mexico on the southern end of their migration — the discovery of which was a game-changer for monarch research — no such location has been found for green darners.
The dragonflies generally turn up along the U.S. Gulf Coast and Mexico’s Caribbean coast (Vera Cruz is a known hot spot) — flying up to 800 miles — but they’re not concentrated in anything approaching the monarch’s roosting numbers.
“The dragonfly swarms seem big, but roosting monarchs are orders of magnitudes bigger,” said Taron. “What that means is it allows tagging to be used more effectively as a tracking mechanism. Because you know where to look for (monarchs) at the end of migration, you can recover some of the tags and we know (the butterfly) started in upstate New York, or Chicago, or wherever the tag was affixed.”
Dragonflies are much more dispersed, greatly reducing the chances of finding a tag, he added, so organized tagging programs are scarce.
As technology advances, new methods for tracking dragonflies are becoming available. Scientists now have the capability to examine a dragonfly’s wings or the shedded exoskeletons of the nymph stage (the exuviae, similar to a cicada’s) and identify the latitude of the original waters from which the insect emerged.
“It just completely blows my mind,” said Crosby. “Every day, every year, we’re learning something new. Information is constantly changing, as all good science does. We’re always learning.”
‘They’re Around Us All the Time’
Dragonfly swarm at Montrose Beach, Aug. 11, 2024. (Patty Wetli / WTTW News)
Contrary to a couple of popular myths, dragonflies don’t sting and they don’t bite.
“They’re not particularly interested in people,” Taron said. “There have been cases where swarms can get so dense that they are just randomly bumping into people. We’ve had people get startled by that. But they’re not trying to get people.”
That doesn’t mean dragonflies are harmless — if you’re another insect.
A dragonfly’s main role in the ecosystem is as a predator, throughout its lifecycle. Nymphs (technically naiads) will even feed on fish fry, according to Ken Johnson, an educator with University of Illinois Extension.
Adults are particularly valued for chowing down on mosquitoes, though they’ll also eat flies, moths and occasionally other dragonflies, Johnson said.
They owe much of their success as hunters to their speed, maneuverability (they can fly backwards) and amazing eyesight: Dragonflies have two large compound eyes composed of tens of thousands of facets, and three simple eyes pointing forward, giving them an almost 360-degree field of vision.
There’s so much to appreciate about these creatures— and if the phenomenon of migrating swarms draws more attention to these oft-overlooked insects, Crosby is all for it.
They teach us about climate change, because they’re temperature and water sensitive; they teach us about aerodynamics, and through biomimicry we can enhance our technology; and they have spikes on their wings that repel viruses, which is teaching us how to make hospitals safer, Crosby said, ticking off some of dragonflies’ amazing attributes.
“But the No. 1 reason I love them is because they are so beautiful and different and gorgeous and ... gosh don’t we need more beauty right now in the world,” Crosby said.
“So many people don’t see them until there’s a big group,” she continued. “But they’re around us all the time: They’re in parking lots, they’re at stop lights, they’re in Wrigley Field when you go to a ballgame, and once you start paying attention to them, you see them everywhere.
“They open our eyes to some of the smallest, most magical parts of the natural world. And what a delight that is.”
Want To Be a Dragonfly Monitor?
All dragonflies start out as aquatic creatures, but green darners are among the species that tend to fly further afield from water. (U.S. Fish and Wildlife Service)
The Peggy Notebaert Nature Museum manages a program called the Illinois Odonate Survey, which monitors dragonflies and their close relatives, the damselflies.
“This is a program anybody can participate in,” said Doug Taron, the survey’s longtime coordinator.
Application forms are available online, and people who are accepted will be assigned a monitoring location.
“This place that you’re assigned has a walking route mapped out on it. And you go out a variety of times during the summer and you walk the route; you count all of the dragonflies that you see within 20 feet of yourself while you’re walking,” Taron explained.
It’s this sort of observational, community-based science that helped cracked the code on monarch’s migration patterns, and it could do the same for dragonflies.
This article originally published on Aug. 15. It has been updated to correct the spelling of Jason Bried's name. We regret the error.
Contact Patty Wetli: @pattywetli | (773) 509-5623 | [email protected]
